《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Stress-induced cubic–tetragonal transformation in partially

Availableonlineatwww.sciencedirect.com SCIENCE DIRECT非 JOURNAL OF PHYSIC EMISTRY OF SOLIDS ELSEVIER Journal of Physics and Chemistry of Solids 65(2004)1103-1112 www.elsevier.com/locate/jpcs Stress-induced cubic-tetragonal transformation in partiall stabilized zro2: Raman spectroscopy study A.A. Sobol, Yu.K. Voronko Laser Materials and Technology Research Center, General Physics Institute, Russian Academy of Sciences, Vavilov street 38, Building 'D, Moscow 119991, Russia Received 9 May 2003: revised 7 October 2003: accepted 10 November 2003; available online 7 February 2004 Abstract Regularities of the cubic-tetragonal transformation(C-t)in partially stabilized zirconia were studied with Raman spectroscopy and high-temperature Raman spectroscopy techniques. New ' low temperature mechanism of tetragonal nanoparticles formation in a volume of cubic solid solution was revealed in ZrO2-GdO3(Eu,O3)(6-8 mol%) single crystals. This mechanism includes nucleation of the tetragonal nanoparticles due to diffusionless C-t phase transformation at the first stage and gradual decrease of the stabilizer concentration insidet'. domains after subsequent low-temperature annealing. Predominant orientation of tetragonal domains due to the stress-induced C transformation was registered in ZrO2-Gd2O3(8 mol%)single crystals. C 2004 Elsevier Ltd. All rights reserved. Keywords: D. Phase transformations; C. Raman spectroscopy 1. Introduction ZrO2-based ceramics and single crystals [6-12. Raman spectroscopy was useful in studies in situ m+t transform Partially stabilized zirconia(PSZ) has great potentialities ations in the heating-cooling process [7-9, 13, 14]. Several as engineering and refractory materials [1]. The stress- experiments were carried out for studying transformations induced martensitic tetragonal to monoclinic(t-m)phase of ZrOz monoclinic phase into orthorhombic structures transition in PSZ ZrOz-Y2O3 solid solution was studied under high pr ressure [516] As to t→ C andt… C phase previously [2, 3]. However, there is another diffusionless transformations, there is a small experimental information cubic-tetragonal(C-t) transformation intrinsic to this on the nature of this phenomenon. Cubic-tetragonal phase system. This transformation almost was not studied due to transition at heating-cooling was studied only for Zro2 enormous experimental difficulties caused by the high Yb2O3(Eu2O3)(6-8 mol%) single crystals by high- temperature of the C-t phase transition temperature Raman spectroscopy technique [17]. Raman There exist different opinions on the nature of the C!' spectra of nanometric-size tetragonal zirconia under high transformation in PSZ(ZrO2-Y2O3) According to Ref [4], pressure up to 40 GPa were studied in Ref. [18] this transformation was displacive but nonmartensitie The goal of this paper is the application of the polarized Other authors described the transformation as martensitic Raman spectroscopy method for studying the stress-induced and similar to the t-m phase transition [5]. Both models C-t phase transformation of PSz single crystals suggest a possibility to induce the C-t transformation by deformation of PSz samples. However, up to now, the existence of such phenomenon in PSZ has not been proved 2 Experimental procedure Raman spectroscopy was shown to have advantage Single crystals of Zro2-Gd2O3(Eu2O3)(6-8 mol%) in studying the phase transformation and the structure under study were grown by cold-container technique [7 Plate-shaped 7X7X 3-mm' samples were cut and then Corresponding author. Tel. +7-95-135-03-01; fax: +7.95-135-02-70. polished. The samples were oriented along the three four E-mail address: sobol lst gpi. ru(AA. Sobol). fold axes of the cubic structure by means of the X-ray 0022-3697/S- see front matter o 2004 Elsevier Ltd. All rights reserved. doi:10.1016jpcs.2003.11.038

Stress-induced cubic–tetragonal transformation in partially stabilized ZrO2: Raman spectroscopy study A.A. Sobol*, Yu.K. Voronko Laser Materials and Technology Research Center, General Physics Institute, Russian Academy of Sciences, Vavilov street 38, Building ‘D’, Moscow 119991, Russia Received 9 May 2003; revised 7 October 2003; accepted 10 November 2003; available online 7 February 2004 Abstract Regularities of the cubic–tetragonal transformation (C ! t 0 ) in partially stabilized zirconia were studied with Raman spectroscopy and high-temperature Raman spectroscopy techniques. New ‘low temperature’ mechanism of tetragonal nanoparticles formation in a volume of cubic solid solution was revealed in ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) single crystals. This mechanism includes nucleation of the tetragonal nanoparticles due to diffusionless C ! t 0 phase transformation at the first stage and gradual decrease of the stabilizer concentration inside t0 - domains after subsequent low-temperature annealing. Predominant orientation of tetragonal domains due to the stress-induced C ! t 0 transformation was registered in ZrO2 –Gd2O3 (8 mol%) single crystals. q 2004 Elsevier Ltd. All rights reserved. Keywords: D. Phase transformations; C. Raman spectroscopy 1. Introduction Partially stabilized zirconia (PSZ) has great potentialities as engineering and refractory materials [1]. The stress￾induced martensitic tetragonal to monoclinic (t ! m) phase transition in PSZ ZrO2 –Y2O3 solid solution was studied previously [2,3]. However, there is another diffusionless cubic–tetragonal (C ! t 0 ) transformation intrinsic to this system. This transformation almost was not studied due to enormous experimental difficulties caused by the high temperature of the C ! t 0 phase transition. There exist different opinions on the nature of the C ! t 0 transformation in PSZ (ZrO2 –Y2O3). According to Ref. [4], this transformation was displacive but nonmartensitic. Other authors described the transformation as martensitic and similar to the t ! m phase transition [5]. Both models suggest a possibility to induce the C ! t 0 transformation by deformation of PSZ samples. However, up to now, the existence of such phenomenon in PSZ has not been proved experimentally. Raman spectroscopy was shown to have advantages in studying the phase transformation and the structure of ZrO2-based ceramics and single crystals [6–12]. Raman spectroscopy was useful in studies in situ m $ t transform￾ations in the heating–cooling process [7–9,13,14]. Several experiments were carried out for studying transformations of ZrO2 monoclinic phase into orthorhombic structures under high pressures [15,16]. As to t $ C and t0 $ C phase transformations, there is a small experimental information on the nature of this phenomenon. Cubic–tetragonal phase transition at heating–cooling was studied only for ZrO2 – Yb2O3 (Eu2O3) (6–8 mol%) single crystals by high￾temperature Raman spectroscopy technique [17]. Raman spectra of nanometric-size tetragonal zirconia under high pressure up to 40 GPa were studied in Ref. [18]. The goal of this paper is the application of the polarized Raman spectroscopy method for studying the stress-induced C ! t 0 phase transformation of PSZ single crystals. 2. Experimental procedure Single crystals of ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) under study were grown by cold-container technique [7]. Plate-shaped 7 £ 7 £ 3-mm3 samples were cut and then polished. The samples were oriented along the three four￾fold axes of the cubic structure by means of the X-ray 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.11.038 Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112 www.elsevier.com/locate/jpcs * Corresponding author. Tel.: þ7-95-135-03-01; fax: þ7-95-135-02-70. E-mail address: sobol@lst.gpi.ru (A.A. Sobol)

l104 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 Load ds of the as- received zro -Ln,o Number of Type of F the sample Undoped uuuu 246888 Sample 100] C, hydroxides coprecipitation; m, rapidly quenched melt; s.c., single ig. 1.(a) Scheme of the experiment on the sample deformation during th stal grown by cold-container technique C-I phase transformation. (b)Orientation of the sample under study; 1 and 2 denote the non-deformed and deformed zones, respectively technique. We also analyzed samples obtained by water quenching ZrO2-Eu2O3(2-8 mol%)melts, which had been previously investigated in Ref. [17]. Undoped powder 3. Potentialities of Raman spectroscopy in studying tetragonal ZrO2-sample was synthesized by hydroxides the c-t phase transformation co-precipitation technique [19]. A list of the sample under study is presented in Table 1. Raman spectra of the samples The Raman spectrum of tetrag were investigated at 300 K with Spex-Ramalog-1403 group) consists of six lines of the Aig 2B1g +3Es pectrometer using conventional back scattering geometry ymmetries [10]. This spectrum is characteristic of both under Ar(488 nm)excitation. High-temperature Raman pure ZrOz and ZrOz-Ln2O3 solid solutions (Ln implies the spectroscopy studies up to 1900 K were carried out with the lanthanide series and Y) in the region of 0-8 mol% of original method described in Ref. [20]. In contrast to Ln2 O3 [19]. The Raman spectrum of the tetragonal phase conventional Raman spectroscopy technique, we used the was shown to essentially differ from those for cubic and laser monoclinic zirconia [19]. Intensities of the Raman lines of operated in a pulse repetition regime at a frequency of different symmetries in the polarized Raman spectra 10 kHz and the power pulse duration of 10 ns. The laser depended on the orientation of tetragonal fragments. The average power was 1-3 W at the peak pulse power of about C-d phase transformation in the absence of deformation 10-30 kW.High peak pulse power of the laser resulted in a stimulated nucleation of three types of tetragonal domains high contrast between the Raman scattering signal and the (B, D and F)in the volume of primary cubic structure(O hermal-radiation background. We also used a signal gating space group)(Fig. 2). According to Ref [21 ], the tetragonal circuit with the signal duration time of 11 ns, which locked z-axes of the domains should be oriented along the three C4 out the registration system for the time of the absence of an axes, and vectors x, y are rotated through the angle of 45 exciting laser pulse, the thermal irradiation background with respect to the cubic axes of the primary fluorite being suppressed by a factor of 10 structure. The calculated intensities of the a,.B, and Fig. la displays the scheme of the stress-induced C-I modes for B, D and F tetragonal domains are displayed phase transformation experiments. A single crystal was separately in Table 2. The exciting beam and the laced into the furnace between two sapphire rods. The polarization vector Eex were directed along the [001] and upper rod had a sharp end of 1 mm in diameter. The sample [100] axes, respectively (Er-position in Fig. 2). Two was gradually heated for 3 h up to the temperature of scattering geometries were used in calculations. The 1800 K. A load(30 kg/mm) was applied to the sample analyzer was aligned in parallel and perpendicularly to the after holding at 1800 K for 30 min. The loading was realized Eex-vector in the cases of the ll and 1 geometries(Fig. 2) in the direction along one of the three cubic axes of the The Raman tensors corresponding to the Alg, BIg and e sample as shown in Fig. 1b. After keeping under loading modes are shown in the bottom of Table 2. Intensities of (15 min) at 1800 K, the sample was rapidly cooled to the plarized lines calculated for the sum of all three domains temperature below the C-t transformation (1350 K). are displayed in the >B, D, F column and for the sum of D Then, the sample was cooled for 2 h without loading to and F domains--in the 2D, F column. The data of the 300 K. Polarized Raman spectra were registered at 300K >B,D, F column correspond to equiprobable orientations of for both nonstressed (1)and stressed(2) zones of the the z axes of the tetragonal domains along the three cubic C4 crystals(Fig. 1b). Registration of the Raman spectra was axes. Only one line of the Ag -mode could be registered in carried out through the 40-um holes burned in the carbon the Raman spectrum for the geometry while the rest film by a focused excitation laser beam. This film was 2Blg+ 3Eg-modes appeared in the spectrum for the 1 preliminarily deposited on the surface of the single crystal geometry in this case

technique. We also analyzed samples obtained by water quenching ZrO2 –Eu2O3 (2–8 mol%) melts, which had been previously investigated in Ref. [17]. Undoped powder tetragonal ZrO2-sample was synthesized by hydroxides co-precipitation technique [19]. A list of the sample under study is presented in Table 1. Raman spectra of the samples were investigated at 300 K with Spex-Ramalog-1403 spectrometer using conventional back scattering geometry under Ar (488 nm) excitation. High-temperature Raman spectroscopy studies up to 1900 K were carried out with the original method described in Ref. [20]. In contrast to conventional Raman spectroscopy technique, we used the copper vapor laser as an excitation source. This laser operated in a pulse repetition regime at a frequency of 10 kHz and the power pulse duration of 10 ns. The laser average power was 1–3 W at the peak pulse power of about 10–30 kW. High peak pulse power of the laser resulted in a high contrast between the Raman scattering signal and the thermal-radiation background. We also used a signal gating circuit with the signal duration time of 11 ns, which locked out the registration system for the time of the absence of an exciting laser pulse, the thermal irradiation background being suppressed by a factor of 104 . Fig. 1a displays the scheme of the stress-induced C ! t 0 phase transformation experiments. A single crystal was placed into the furnace between two sapphire rods. The upper rod had a sharp end of 1 mm in diameter. The sample was gradually heated for 3 h up to the temperature of 1800 K. A load (<30 kg/mm2 ) was applied to the sample after holding at 1800 K for 30 min. The loading was realized in the direction along one of the three cubic axes of the sample as shown in Fig. 1b. After keeping under loading (15 min) at 1800 K, the sample was rapidly cooled to the temperature below the C ! t 0 transformation (1350 K). Then, the sample was cooled for 2 h without loading to 300 K. Polarized Raman spectra were registered at 300 K for both nonstressed (1) and stressed (2) zones of the crystals (Fig. 1b). Registration of the Raman spectra was carried out through the 40-mm holes burned in the carbon film by a focused excitation laser beam. This film was preliminarily deposited on the surface of the single crystal. 3. Potentialities of Raman spectroscopy in studying the C ! t 0 phase transformation The Raman spectrum of tetragonal zirconia (D15 4h space group) consists of six lines of the A1g þ 2B1g þ 3Eg symmetries [10]. This spectrum is characteristic of both pure ZrO2 and ZrO2 –Ln2O3 solid solutions (Ln implies the lanthanide series and Y) in the region of 0–8 mol% of Ln2O3 [19]. The Raman spectrum of the tetragonal phase was shown to essentially differ from those for cubic and monoclinic zirconia [19]. Intensities of the Raman lines of different symmetries in the polarized Raman spectra depended on the orientation of tetragonal fragments. The C ! t 0 phase transformation in the absence of deformation stimulated nucleation of three types of tetragonal domains ðB; D and FÞ in the volume of primary cubic structure (O5 h- space group) (Fig. 2). According to Ref. [21], the tetragonal z-axes of the domains should be oriented along the three C4 axes, and vectors x, y are rotated through the angle of 458 with respect to the cubic axes of the primary fluorite structure. The calculated intensities of the A1g; B1g and Eg modes for B; D and F tetragonal domains are displayed separately in Table 2. The exciting beam and the polarization vector Eex were directed along the [001] and [100] axes, respectively (E1-position in Fig. 2). Two scattering geometries were used in calculations. The analyzer was aligned in parallel and perpendicularly to the Eex-vector in the cases of the k and ’ geometries (Fig. 2). The Raman tensors corresponding to the A1g; B1g and Eg modes are shown in the bottom of Table 2. Intensities of polarized lines calculated for the sum of all three domains are displayed in the PB; D; F column and for the sum of D and F domains—in the PD; F column. The data of the PB; D; F column correspond to equiprobable orientations of the z axes of the tetragonal domains along the three cubic C4 axes. Only one line of the Ag-mode could be registered in the Raman spectrum for the k geometry while the rest 2B1g þ 3Eg-modes appeared in the spectrum for the ’ geometry in this case. Fig. 1. (a) Scheme of the experiment on the sample deformation during the C ! t 0 phase transformation. (b) Orientation of the sample under study; 1 and 2 denote the non-deformed and deformed zones, respectively. Table 1 The compositions and synthesis methods of the as-received ZrO2 –Ln2O3 solid solutions Number of the sample Type of Ln Ln2O3-concentration (mol%) Synthesizing method 1 – Undoped c 2 Eu 2.5 m 3 Eu 4 m 4 Eu 6 m 5 Eu 8 m 6 Eu 8 s.c. 7 Gd 8 s.c. c, hydroxides coprecipitation; m, rapidly quenched melt; s.c., single crystal grown by cold-container technique. 1104 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112

A.A. Sobol, Y.K. Voronko Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 1105 Load domains results in the appearance of only lines of the 3E modes in the Raman spectrum for the 1 geometry (D, F column in Table 2). Thus, registration of polarized Raman spectra is the reliable method of revealing results of the stress- [001] induced C-I transformation under loading However, at first sight, it is impossible to carry out above-mentioned experiments. Bulk tetragonal single crystals of own. as to crystals of Y-PSZ synthesized by cold container technique, they appear as milky color due to the presence of large dimension tetragonal precipitates and are inadequate for the [010] polarized Raman spectroscopy studies The application of the polarized Raman spectroscopy for studying the C-t transformation in PSZ single crystals is possible due to the specific mechanism of tetragonal phase formation in ZrO2-Gd2O3(Eu2O3)(6-8 mol%)[17, 22) solid solutions. which will be described belot [100] Ea 4. Formation of the tetragonal phase in ZrOz-Gd2O3 (Eu2O3)solid solutions Fig. 2. Orientation of the isolated tetragonal domains B, D and F in the volume of ZrO2-Gd2 O3(Eu,O3)( 8 mol%)cubic solid solution. En and E, are the directions of Eex-electric vector of the excitation A nature of t-domain formation can be qualitatively explained for ZrO2-rich region of the equilibrium ZrO2 Gd2O3(Eu2O3) phase diagram(Fig 3). The diagram of the Deformation along the definite CA axis in the process of description of the systems under study. The temperature T1 C-d transformation created the different conditions for separates the cubic and the cubic +tetragonal (C+t) nucleation of B, D and F types of t' domains(Fig. 2) regions, whereas To denotes the temperature of the C-t Loading can induce the predominant formation of either b phase transformation. These temperatures were determined or D+ Domain. The predominant formation of the B domain to be in the region of 1400-1700 K for ZrO2-Gd2O3 would result in decreasing the intensity of the Alg mode in the (Eu2O3)(6-8 mol%)samples [17]. There was an essential conditions of the gec omer difference in the phase formation at different annealin component of the Alg Raman tensor from the summarized temperatures(points 2 and 3 in Fig 3). The temperature of intensity equation of the 2B, D, F column (Table 2). More- the point 2 lies between T1 and To. The usual decomposition over,only lines of the 2B,g modes should be registered in the of the C-solid solution due to diffusion-controlled reaction spectrum of the 1 geometry at the predominant B domain must occur at this temperature [22]. Decomposition formation. In contrast, the predominant formation of D+F products are the low-Gd2O3(Eu2O3) t-phase(=2.5 mol% Table 2 Calculated intensities of the tetragonal vibrational modes in the parallel (ID)and the crossed( l )scattering geometries for three types of domains according to Fg.2(Eax‖E1) The domain SD.F E(1) 000 E

Deformation along the definite C4 axis in the process of C ! t 0 transformation created the different conditions for nucleation of B; D and F types of t0 domains (Fig. 2). Loading can induce the predominant formation of either B orD þ F domain. The predominant formation of theB domain would result in decreasing the intensity of the A1g mode in the conditions of the k geometry due to canceling the azz component of the A1g Raman tensor from the summarized intensity equation of the PB; D; F column (Table 2). More￾over, only lines of the 2B1g modes should be registered in the spectrum of the ’ geometry at the predominant B domain formation. In contrast, the predominant formation of D þ F domains results in the appearance of only lines of the 3Eg modes in the Raman spectrum for the ’ geometry (PD; F column in Table 2). Thus, registration of polarized Raman spectra is the reliable method of revealing results of the stress￾induced C ! t 0 transformation under loading. However, at first sight, it is impossible to carry out the above-mentioned experiments. Bulk tetragonal single crystals of a pure ZrO2 cannot be grown. As to single crystals of Y–PSZ synthesized by cold container technique, they appear as milky color due to the presence of large￾dimension tetragonal precipitates and are inadequate for the polarized Raman spectroscopy studies. The application of the polarized Raman spectroscopy for studying the C ! t 0 transformation in PSZ single crystals is possible due to the specific mechanism of tetragonal phase formation in ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) [17,22] solid solutions, which will be described below. 4. Formation of the tetragonal phase in ZrO2 –Gd2O3 (Eu2O3) solid solutions A nature of t0 -domain formation can be qualitatively explained for ZrO2-rich region of the equilibrium ZrO2 – Gd2O3 (Eu2O3) phase diagram (Fig. 3). The diagram of the ZrO2 –Y2O3 system [23] was used as a prototype for the description of the systems under study. The temperature T1 separates the cubic and the cubic þ tetragonal (C þ t) regions, whereas T0 denotes the temperature of the C ! t 0 phase transformation. These temperatures were determined to be in the region of 1400–1700 K for ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) samples [17]. There was an essential difference in the phase formation at different annealing temperatures (points 2 and 3 in Fig. 3). The temperature of the point 2 lies between T1 and T0: The usual decomposition of the C-solid solution due to diffusion-controlled reaction must occur at this temperature [22]. Decomposition products are the low-Gd2O3 (Eu2O3) t-phase (<2.5 mol% Fig. 2. Orientation of the isolated tetragonal domains B; D and F in the volume of ZrO2 –Gd2O3 (Eu2O3) (8 mol%) cubic solid solution. E1 and E2 are the directions of Eex-electric vector of the excitation beam. Table 2 Calculated intensities of the tetragonal vibrational modes in the parallel (k) and the crossed ( ’ ) scattering geometries for three types of domains according to Fig. 2 (EexkE1) The mode The domain BDF PB; D; F PD; F k ’ k ’ k ’ k ’ k ’ A1g a 2 0 a 2 0 b 2 0 2a 2 þ b 2 0 a 2 þ b 2 0 B1g 0 c 2 00 00 0 c 2 0 0 Eg 00 0 e 2 0 e 2 0 2e 2 0 2e 2 A1g ¼ a 0 0 0 a 0 0 0 b ; B1g ¼ c 0 0 0 2c 0 000 ; Egð1Þ ¼ 000 0 0 e 0 e 0 ; Egð2Þ ¼ 0 0 2e 000 2e 0 0 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112 1105

A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 (8 mol%)crystals can be repeated using the subsequent low- temperature annealing [17]. By contrast, the particles of e t-phase formed in the diffusion-controlled reaction are retained even after a long-time(20-30 h) annealing at T>T 5. Raman spectroscopy study of the formation of different tetragonal phases in ZrO2-Gd,O3(Eu2O3) solid solutions The above-mentioned two mechanisms of phase for- mation in the ZrO2-Gd2O3(Eu2O3) system are easily 1000 registered by the Raman spectroscopy technique. Fig. 4 displays the Raman spectra at 300 K for a series of rapidl Eu2O3(Gd2O3)-mol% quenched ZrO2-Eu2O3 melts under the variation of the Eu2O3 concentration from 2.5 up to 8 mol%. The Raman t trum of a is also shown in Fig 4. In this figure, all six Raman lines of tetragonal D4, phase are presented in the Raman spectra of the samples. A Fig. 3. Equilibrium ZrO2-Gd2O3(Eu 03)phase diagrams. Ti is the continuous change in the line frequencies is observed with o the cr bet ween turma nd t apd s e egte s nd e i h e temper atse the growth of Eu2O3 concentration, a shift of theRamanline in the experiments. Bottom-the boundaries of the t' and C metastable position within the range 240-270 cm being the most phases at 300K remarkable(Fig. 4). Redistribution of intensities and broadening of the Raman lines followed a growth in the stabilizer concentration. Thus. the most intensive narrow line in the 240-270 cm spectral range for a pure ZrO2 and 9mol% of Gd, 03(Eu,,). This process resulted in the ZrO2-EzO3(2.5 mol% )is registered as the weakest broad formation of large-size t-particles and the milky single band in the spectrum of the ZrO2-Eu2O3(8 mol%)sample crystals. In turn, there is a growth of relative intensities of the two Q The temperature at the point 3 is lower than To of the lines in the ptin o im me srd s ion of 600 cm with increasing Eu2O3 content. These tw merged practically into a unified is similar to the Gd2O3(Eu2O3) cubic phase should nucleate and dominating in the spectrum of the ZrO2-Eu2O3 in the crystal volume as a result of this transformation. (8 mol%)sample(Figs. 4 5). Previously, the band with Diffusion processes at T T(point 1 in Raman shift (cm") Fig 3). Then, the process of formation of the low-Gd2O (Eu2O3)t'-domains in the volume of ZrO2-Gd2O3(Eu2O3) Fig 4. Raman spectra at 300 K of the rapidly quenched ZrO2-(Eu203 melts at variation of Eu,O3-conter

stabilizer) plus the cubic solid solution containing <9 mol% of Gd2O3 (Eu2O3). This process resulted in the formation of large-size t-particles and the milky single crystals. The temperature at the point 3 is lower than T0 of the C ! t 0 phase transition. The t0 -particles whose composition is similar to the Gd2O3 (Eu2O3) cubic phase should nucleate in the crystal volume as a result of this transformation. Diffusion processes at T , T0 for ZrO2 –Gd2O3 (Eu2O3) (8 mol%) was shown to proceed very slowly [17]. They interfere with the nucleation of t-precipitates due to the diffusion-controlled reaction in the case of annealing at T , T0 [17]. For the solid solution with Ln2O3 stabilizers of small Ln-ions (Y2O3, Yb2O3, Lu2O3), this phenomenon resulted in the freezing stabilizer concentration in t0 domains. In contrast, the primary concentration of Ln2O3 oxides with large Ln cations (Gd2O3 and Eu2O3) did not retain in supersaturated t0 -domains even at low-temperature annealing. Gradual decrease in the concentration of Gd2O3 (Eu2O3) inside the t0 -domains occurred, the volume around these domains being fertilized by Gd2O3 (Eu2O3) oxides. The Gd2O3 (Eu2O3) concentration in t0 -particles can be reduced to 2–3 mol% due to long-time low-temperature annealing [22]. The low-temperature phase formation mechanism was reversible for ZrO2 –Gd2O3 (Eu2O3) (8 mol%) system. The composition of the material can be rapidly homogenized by annealing at T . T1 (point 1 in Fig. 3). Then, the process of formation of the low-Gd2O3 (Eu2O3) t0 -domains in the volume of ZrO2 –Gd2O3 (Eu2O3) (8 mol%) crystals can be repeated using the subsequent low￾temperature annealing [17]. By contrast, the particles of the t-phase formed in the diffusion-controlled reaction are retained even after a long-time (20–30 h) annealing at T . T1: 5. Raman spectroscopy study of the formation of different tetragonal phases in ZrO2 –Gd2O3 (Eu2O3) solid solutions The above-mentioned two mechanisms of phase for￾mation in the ZrO2 –Gd2O3 (Eu2O3) system are easily registered by the Raman spectroscopy technique. Fig. 4 displays the Raman spectra at 300 K for a series of rapidly quenched ZrO2 –Eu2O3 melts under the variation of the Eu2O3 concentration from 2.5 up to 8 mol%. The Raman spectrum of a pure ZrO2 ceramic is also shown in Fig. 4. In this figure, all six Raman lines of tetragonal D15 4h phase are presented in the Raman spectra of the samples. A continuous change in the line frequencies is observed with the growth of Eu2O3 concentration, a shift of the Raman line position within the range 240–270 cm21 being the most remarkable (Fig. 4). Redistribution of intensities and broadening of the Raman lines followed a growth in the stabilizer concentration. Thus, the most intensive narrow line in the 240–270 cm21 spectral range for a pure ZrO2 and ZrO2 –Eu2O3 (2.5 mol%) is registered as the weakest broad band in the spectrum of the ZrO2 –Eu2O3 (8 mol%) sample. In turn, there is a growth of relative intensities of the two lines in the region of 600 cm21 with increasing Eu2O3 content. These two lines merged practically into a unified band dominating in the spectrum of the ZrO2 –Eu2O3 (8 mol%) sample (Figs. 4 and 5). Previously, the band with the frequency of 600 cm21 was assigned to F2g mode of the O5 h fluorite structure on the basis of the polarized Raman Fig. 3. Equilibrium ZrO2 –Gd2O3 (Eu2O3) phase diagrams. T1 is the boundary between cubic and C þ t phases regions and T0 is the temperature of the C ! t 0 transformation. 1,2 and 3 denote the annealing regimes used in the experiments. Bottom—the boundaries of the t0 and C metastable phases at 300 K. Fig. 4. Raman spectra at 300 K of the rapidly quenched ZrO2 –(Eu2O3) melts at variation of Eu2O3-content. 1106 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112

A.A. Sobol, Y.K. Voronko Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 V-cm ZrO2-Eu2O3(& mol%) melt after several annealing cycles of with different time duration at To <T<T. This figure Eg displays the process of a nucleation and the volume growth 600Bg of the low-Eu2O3(2.5-3 mol%)t-phase with increasing the F2(C) annealing time. The Raman spectrum of the sample after the longest annealing(& h)revealed an intense broad line in he region of 600 cm-I and a narrow Raman line at 400 265 cm. This demonstrates the presence of the low-Eu2O3 t-phase in addition to a considerable quantity of the Eu2O3- B1 rich phase in the sample after long-time annealing at To A1 T<71(Fig.6) 200 The presence of two phases in the sample under study Eg resulted in the doublet form of lines in the tetragonal raman spectra. The positions of the Raman lines of a certain phase practically were not shifted with variation of the annealing time. This fact showed the absence of considerable change in the composition of these two phases during the thermal treatment as exemplified by the phase diagram. It should be Eu2O3- mol% noted that it takes a long annealing time(above 1 h)to initiate ig. 5. Variation of the tetragonal modes frequencies in the rapidly the decomposition process via the diffusion-controlled reac quenched ZrO2-Eu2O3 melts versus the Eu2O3 concentration. tion(Fig. 6) study of the cubic ZrO2-Eu2O3(12 mol%) single crystals Evolution of the Raman spectra at 300 K for the rapidl [11]. The frequency of the F2g mode for ZrO2-Eu2 0 quenched Z1O2-Eu2O3(8 mol%)melt after annealing at T (12 mol%)also points out in Fig. 5. Thus, Figs. 4 and 5 To(point 3 of the diagram in Fig 3)essentially differed from demonstrate the evolution of the ran those considered above. Changes in the Raman spectra tetragonal phase while moving from a pure ZrO, with the became noticeable already after 5-min annealing(Fig. 7) highest tetragonality(the cla ratio) to the practically owever, they are not associated with the appearance of stabilized cubic solid solution (cla- 1) in ZrO2-Eu2O3 narrow Raman lines of the low-Eu2O3 t-phase. Gradual (8 mol%) variations in positions and intensities of the Raman lines with The position and width of the Raman lines in the Raman growing annealing time were observed(Fig. 7). The spectra shown in Fig. 4 allow us to reliably indicate the regularities in transformations of the Raman spectra shown of a stabilizer. Thus. it was not difficult to register the changes of Raman spectra, which were caused by decreasing process of the decomposition of initially quenched ZrO Eu2O3(8 mol%) melt due to the diffusion-controlled 42cm1265cm1 reaction at To <T<Ti(point 2 in Fig. 3). Fig. 6 illustrates the Raman spectra at 300 K of the preliminary quenched as-quenched 5 min 20 55 min 0 200400600 0200400600800 Raman shift(cm) Fig. 6. Raman spectra at 300 K of the rapidly quenched ZrO2-(Eu2O3) Fig. 7. Raman spectra at 300 K of the rapidly quenched ZrO2-(Eu2O3 (8 mol%) melt after repeated annealing at 1700 K(above the C (8 mol%) melt after repeated annealing at 1350 K(below the C transformation, point 2 in Fig. 3)with different time expositions. transformation, point 3 in Fig. 3)with different time expositions

study of the cubic ZrO2 –Eu2O3 (12 mol%) single crystals [11]. The frequency of the F2g mode for ZrO2 –Eu2O3 (12 mol%) also points out in Fig. 5. Thus, Figs. 4 and 5 demonstrate the evolution of the Raman spectra for the tetragonal phase while moving from a pure ZrO2 with the highest tetragonality (the c=a ratio) to the practically stabilized cubic solid solution ðc=a ! 1Þ in ZrO2 –Eu2O3 (8 mol%). The position and width of the Raman lines in the Raman spectra shown in Fig. 4 allow us to reliably indicate the presence of the tetragonal phase with a certain concentration of a stabilizer. Thus, it was not difficult to register the process of the decomposition of initially quenched ZrO2 – Eu2O3 (8 mol%) melt due to the diffusion-controlled reaction at T0 , T , T1 (point 2 in Fig. 3). Fig. 6 illustrates the Raman spectra at 300 K of the preliminary quenched ZrO2 –Eu2O3 (8 mol%) melt after several annealing cycles of with different time duration at T0 , T , T1: This figure displays the process of a nucleation and the volume growth of the low-Eu2O3 (2.5–3 mol%) t-phase with increasing the annealing time. The Raman spectrum of the sample after the longest annealing (8 h) revealed an intense broad line in the region of 600 cm21 and a narrow Raman line at 265 cm21 . This demonstrates the presence of the low-Eu2O3 t-phase in addition to a considerable quantity of the Eu2O3- rich phase in the sample after long-time annealing at T0 , T , T1 (Fig. 6). The presence of two phases in the sample under study resulted in the doublet form of lines in the tetragonal Raman spectra. The positions of the Raman lines of a certain phase practically were not shifted with variation of the annealing time. This fact showed the absence of considerable change in the composition of these two phases during the thermal treatment as exemplified by the phase diagram. It should be noted that it takes a long annealing time (above 1 h) to initiate the decomposition process via the diffusion-controlled reac￾tion (Fig. 6). Evolution of the Raman spectra at 300 K for the rapidly quenched ZrO2–Eu2O3 (8 mol%) melt after annealing at T , T0 (point 3 of the diagram in Fig. 3) essentially differed from those considered above. Changes in the Raman spectra became noticeable already after 5-min annealing (Fig. 7). However, they are not associated with the appearance of narrow Raman lines of the low-Eu2O3 t-phase. Gradual variations in positions and intensities of the Raman lines with growing annealing time were observed (Fig. 7). The regularities in transformations of the Raman spectra shown in Fig. 7 at low-temperature annealing are similar to changes of Raman spectra, which were caused by decreasing Fig. 5. Variation of the tetragonal modes frequencies in the rapidly quenched ZrO2 –Eu2O3 melts versus the Eu2O3 concentration. Fig. 6. Raman spectra at 300 K of the rapidly quenched ZrO2 –(Eu2O3) (8 mol%) melt after repeated annealing at 1700 K (above the C ! t 0 transformation, point 2 in Fig. 3) with different time expositions. Fig. 7. Raman spectra at 300 K of the rapidly quenched ZrO2 –(Eu2O3) (8 mol%) melt after repeated annealing at 1350 K (below the C ! t 0 transformation, point 3 in Fig. 3) with different time expositions. A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112 1107

A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 the Eu2O3-content in the series of rapidly quenched melts presented in Fig 4. Finally, the Raman spectrum of the ZrO2 A1(265cm) mol%)quenched melt after the I h low-temperature annealing was practically similar to that of the quenched melt with Eu2O3 (2.5 mol%)(Figs. 4 and 7). Thus, the low- (2) temperature mechanism gives an opportunity to create a large number of t' particles with a variable concentration of Eu20 up to 2.5 mol% in the volume of the primary ZrO2-Eu2O3 (8 mol%)solid solution. The small intensity of the broad band in the region of 600 cm for the Raman spectra of the sample after 1-hour low-temperature annealing(Fig. 7)indicates a retention of only small quantity of the Eu2O3-rich volume in his case. The presence of the Eu2O3-rich phase in the sample 200400600800 subjected by 1-hour low-temperature annealing could be registered also according to the low-intensity A band at A(242cm) (b) 80cm in the Raman spectrum(Fig. 7). This band was shown to be the 'Boson peak'appeared in the Raman spectra of disordered ZrO2-Ln-O3 with the Ln, O3 content above 6 8 mol% 11 Different mechanisms of the tetragonal phase formation in ZrO2-Gd,O3(Eu,O3)(6-8 mol%) solid solutions essentially influence on the transparency of single crystals synthesized by cold-container method. The formation of the large-sized t-particles with 2.5-3 mol% stabilizers due to 2B1g+3Eg ⊥ the diffusion-controlled reaction resulted in the milky single crystals. These crystals stayed nontransparent after anneal 0200400600800 ing at T>T for a long time( several days)[17]. In turn, Raman shift(cm") low-temperature mechanism retained the single crystals totally transparent in spite of the presence of t-domains with Fig.. The polarized Raman spectra at 300 K of the as-grown Zro2 This implies that the size of t'-particles formed in the course EalIE, and elIE, respectively (according to Fi. on ckoi ystals I the same low concentration(2.5-3 mol %)of the stabilizer. (Eu] 03)(8 mol%)(a) and Zr0z-(Gd2O3)(8 mol%)(b) single and 2 are the nc vector of the C-t transformation in ZrO2-Gd2O3(Eu2O3)(6- 8 mol%)crystals was essentially smaller than the wave- (2.5 mol%)t- domains in the as-received ZrO2-Eu2O3 length of visible light. Low-temperature annealing of such (8 mol%) single crystal(Fig. 8a). This implies that the samples did not result in a growth in the dimensions of t'. C-d transformation accompanied by reducing the Eu2O3 nanodomains and only caused decreasing the stabilizer content in t'-domains occurred in the process of Zro2 concentration [17]. Thus, there was a possibility to create Eu2O3 (8 mol%)crystal growth. The spectrum of this crystal oriented nanoparticles of the tetragonal phase with stabilizer co pletely polarized lines(Fig. &a, 1) concentration on the order of 2.5-8 mol% in the volume of when(according to Fig. 2)the electric vector(Ecx) of 6)(6-8 mol%) single crystals. exciting light was parallel to the vector El. By contrast, the Such single crystals were quite suitable for the polarized Raman spectra were completely depolarized(Fig. &a, 2)at Raman spectroscopy studies EexllEy(Fig. 2). This depolarization of exciting and scattered light beams was caused by birefringence phenom enon due to the presence of anisotropic tetragonal domains 6. Polarized Raman spectra of tetragonal phase with the z axes forming the angle of 45 with the eex lle, in nonstressed and stressed PSz single crystals direction(Fig. 2)[12]. Thus, only the EexlIEI geometry could be used for polarized Raman spectroscopy exper Fig. 8 demonstrates the polarized Raman spectra tals. These single crystals were totally transparent due to the one 265-cm line was registered in the ll spectrum,while absence of decomposition processes at such growth the rest five tetragonal modes were observed in the geometry. This is consistent with the calculation results The position of the most intense band at 265 cm and presented in Table 2 for the 2B, D, F column. Thus, the he shape of narrow lines of the Raman spectrum Raman spectra shown in allowed us to reliably demonstrate the predominant existence of Eu2O3 divide the Raman line of the Alg-mode(265 cm )from

the Eu2O3-content in the series of rapidly quenched melts presented in Fig. 4. Finally, the Raman spectrum of the ZrO2– Eu2O3 (8 mol%) quenched melt after the 1 h low-temperature annealing was practically similar to that of the quenched melt with Eu2O3 (2.5 mol%) (Figs. 4 and 7). Thus, the low￾temperature mechanism gives an opportunity to create a large number of t0 particles with a variable concentration of Eu2O3 up to 2.5 mol% in the volume of the primary ZrO2–Eu2O3 (8 mol%) solid solution. The small intensity of the broad band in the region of 600 cm21 for the Raman spectra of the sample after 1-hour low-temperature annealing (Fig. 7) indicates a retention of only small quantity of the Eu2O3-rich volume in this case. The presence of the Eu2O3-rich phase in the sample subjected by 1-hour low-temperature annealing could be registered also according to the low-intensity A band at 80 cm21 in the Raman spectrum (Fig. 7). This band was shown to be the ‘Boson peak’ appeared in the Raman spectra of disordered ZrO2–Ln2O3 with the Ln2O3 content above 6– 8 mol% [11]. Different mechanisms of the tetragonal phase formation in ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) solid solutions essentially influence on the transparency of single crystals synthesized by cold-container method. The formation of the large-sized t-particles with 2.5–3 mol% stabilizers due to the diffusion-controlled reaction resulted in the milky single crystals. These crystals stayed nontransparent after anneal￾ing at T . T1 for a long time (several days) [17]. In turn, low-temperature mechanism retained the single crystals totally transparent in spite of the presence of t0 -domains with the same low concentration (2.5–3 mol%) of the stabilizer. This implies that the size of t0 -particles formed in the course of the C ! t 0 transformation in ZrO2 –Gd2O3 (Eu2O3) (6– 8 mol%) crystals was essentially smaller than the wave￾length of visible light. Low-temperature annealing of such samples did not result in a growth in the dimensions of t0 - nanodomains and only caused decreasing the stabilizer concentration [17]. Thus, there was a possibility to create oriented nanoparticles of the tetragonal phase with stabilizer concentration on the order of 2.5–8 mol% in the volume of cubic ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) single crystals. Such single crystals were quite suitable for the polarized Raman spectroscopy studies. 6. Polarized Raman spectra of tetragonal phases in nonstressed and stressed PSZ single crystals Fig. 8 demonstrates the polarized Raman spectra obtained at 300 K in the case of rapidly grown ZrO2 – Eu2O3 (8 mol%) and ZrO2 –Gd2O3 (8 mol%) single crys￾tals. These single crystals were totally transparent due to the absence of decomposition processes at such growth conditions. The position of the most intense band at 265 cm21 and the shape of narrow lines of the Raman spectrum demonstrate the predominant existence of Eu2O3 (2.5 mol%) t0 -domains in the as-received ZrO2 –Eu2O3 (8 mol%) single crystal (Fig. 8a). This implies that the C ! t 0 transformation accompanied by reducing the Eu2O3 content in t0 -domains occurred in the process of ZrO2 – Eu2O3 (8 mol%) crystal growth. The spectrum of this crystal consisted of narrow completely polarized lines (Fig. 8a, 1) when (according to Fig. 2) the electric vector ðEexÞ of exciting light was parallel to the vector E1: By contrast, the Raman spectra were completely depolarized (Fig. 8a, 2) at EexkE2 (Fig. 2). This depolarization of exciting and scattered light beams was caused by birefringence phenom￾enon due to the presence of anisotropic tetragonal domains with the z axes forming the angle of 458 with the EexkE2 direction (Fig. 2) [12]. Thus, only the EexkE1 geometry could be used for polarized Raman spectroscopy exper￾iments with tetragonal single crystals. Polarized spectra in Fig. 8a, 1 proved the equiprobable orientations of t0 domains along the three C4 cubic axes in nonstressed samples. Only one 265-cm21 line was registered in the k spectrum, while the rest five tetragonal modes were observed in the ’ geometry. This is consistent with the calculation results presented in Table 2 for the PB; D; F column. Thus, the Raman spectra shown in Fig. 8a, 1 allowed us to reliably divide the Raman line of the A1g-mode (265 cm21 ) from Fig. 8. The polarized Raman spectra at 300 K of the as-grown ZrO2 – (Eu2O3) (8 mol%) (a) and ZrO2 –(Gd2O3) (8 mol%) (b) single crystals. 1 and 2 are the scattering geometries with the excitation electric vector EexkE1 and EexkE2; respectively (according to Fig. 2). 1108 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112

A.A. Sobol, Y.K. Voronko Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 lines of the 2BIg+ 3Eg manifold. In turn, it was impossible to separate the Raman lines of the Big and Eg modes using the EexlIEt geometry for the model involving three types of B, D and F domains. Application to this aim of the eexllE geometry was useless due to birefringence effect considered 1500K The polarized Raman spectrum at 300 K of the as-received ZrO2-Gd2O3(8 mol%)single crystal differed from those mentioned above by both the lower frequency of the Alg mode (245 cm)and the broad bands Raman spectrum(Fig. 8b) ooK after homogenization These results showed that only the C-t transformation took place in the crystal, while decreasing the Gd2O3-content inside t-phase domains due to the low-temperature annealing was as-grown bstructed by the growth conditions for the ZrO2-Gd2O3 (8 mol%)single crystal 200 The polarized Raman spectrum obtained allowed select Raman shift cm ng the Alg-mode lines from those of 2B1]+ 3E, manifold Fig. 9. A change of the Raman spectra of the ZrO2-(Eu203)( 8 mol%) d-domains with the primary Gd203(8 mol%)content single crystal at different annealing regimes and temperature The values obtained were used for new experiments devoted induced vibrations (L-lines)in addition to the six tetragonal to studying the stress-induced C-d transition as-grown Zro2-Gd2O3( 8 mol%)(Fig. 8b). These L-lines It should be noted that the single crystals under examination retained its transparency after thermal treatments mentioned are a result of a presence of large amount of stabilizer ions above provided that the temperature increase was sufficiently and oxygen vacancies into the t' phase with Gd2O3(8 mol%) content.The nature the L -Raman lines were studied in rapid, the ZrO2-Gd203( 8 mol%)sample admitting a slower detail in the ZrO2-based cubic solid solutions with heating regime than the ZrO2-Eu2O3(8 mol%)crystal. This disordered structure [11]. Presence of the L-lines, as well as the ZrO2-Gd2O3( 8 mol%)sample that was used for the as the intensive A-Boson peak in the Raman spectra of the C-t transition studies under loading grown ZrO2-Gd2O3(8 mol%)O2 supported the disor- The results of the experiments are shown in Fig. 10 dered tetragonal structure of t' domains formed in this ig. 10a displays the results of comparison of the polarized sample. Thus, it is possible to freeze the t-phase with initial Raman spectra at 300 K for nonstressed (1)and stressed ( 2) id,0,(8 mo1%)concentration in ZrOx-based single zones of the ZrO2-Gd2 03(8 molo) crystal that prelimina- crystals without using rapidly quenching conditions rily underwent the deformation in the process of C-t' The long-time low-temperature annealing for 2-3 h of transformation according to the procedure described abov the as-grown ZrO2-Gd203( 8 mo1%)crystal stimulated the Intensities of the lines in the spectra shown in Fig.10 were process of decreasing Gd2O3 content inside t'-domains up to normalized to ntensity of the Raman band in the 2.5 mo1%. The initial broad Raman line spectrum of the region of 320 cm. The position of the A ig line(257cm ZrO2-Gd2O3(8 mol%) crystal, which is shown in Fig. 8b, n Fig. 10a demonstrates a decrease in the Gd,O3 was transformed after such treatment into a narrow-line concentration in t'-domains compared to the primary spectrum similar to those of the as-received ZrO2-Eu2 0: composition(the peak position at 242 cm )due to the (8 mol%)displayed in Fig. 8a. In turn, the ZrO2-Eu20 low-temperature annealing in course of the experiment mol%)crystal corresponded to the low Eu2O3-content t'- According to the data presented in Figs 4 and 5, the Gd2O3 domains could be homogenized by annealing at T>T concentration in the sample subjected to the stress-induced (point 1 in Fig 3). The Eu203-rich t'-domains formed by experiment could be evaluated as 4-5 mol% Gd2 O3. The such treatment could be frozen in the homogenized sample difference between the Raman spectra for stressed and using subsequent rapidly quenching. This phenomenon is nonstressed zones is registered in Fig. 10a. The intensity of illustrated by transformation of the narrow-line Raman the Alg line in the l geometry was essentially lower in the spectrum for the as-received ZrO2-Eu2O3( 8 mol%)sample case of registration in the 2-zone than in the l-region. The into the broad-band spectrum of the homogenized crystal spectra for the I geometry displayed reduced intensities of Then, it was possible to trace the transformation of three from five lines of the 2B1gt3Eg manifold in the same the Raman spectrum of the homogenized ZrO2-Eu2O3 registration scheme(Fig. 10a). These regularities should be (8 mol%)at heating through the t-C phase transition revealed if the formation of the B-type domains occurred region using high-temperature Raman spectroscopy method predominantly under loading(Table 2 and Fig. 2). Thus, the (Fig 9). With the help of this technique, it was possible to deformation of the homogenized ZrO2-Gd2O3(8 mol%) evaluate To-temperature within the accuracy of 10-15 K for crystal at cooling through the c-t transformation the ZrO2-Gd2O3(Eu2O3)(8 mol%) crystals under stud temperature region resulted in the predominant nucleation

lines of the 2B1g þ 3Eg manifold. In turn, it was impossible to separate the Raman lines of the B1g and Eg modes using the EexkE1 geometry for the model involving three types of B; D and F domains. Application to this aim of the EexkE2 geometry was useless due to birefringence effect considered above [12]. The polarized Raman spectrum at 300 K of the as-received ZrO2–Gd2O3 (8 mol%) single crystal differed from those mentioned above by both the lower frequency of the A1g mode (245 cm21 ) and the broad bands Raman spectrum (Fig. 8b). These results showed that only the C ! t 0 transformation took place in the crystal, while decreasing the Gd2O3-content inside t 0 -phase domains due to the low-temperature annealing was obstructed by the growth conditions for the ZrO2–Gd2O3 (8 mol%) single crystal. The polarized Raman spectrum obtained allowed select￾ing the A1g-mode lines from those of 2B1g þ 3Eg manifold in t0 -domains with the primary Gd2O3 (8 mol%) content using the EexkE1 geometry (Fig. 8b). A series defect￾induced vibrations (L-lines) in addition to the six tetragonal lines were registered in the polarized Raman spectra of the as-grown ZrO2 –Gd2O3 (8 mol%) (Fig. 8b). These L-lines are a result of a presence of large amount of stabilizer ions and oxygen vacancies into the t0 phase with Gd2O3 (8 mol%) content. The nature the L -Raman lines were studied in detail in the ZrO2-based cubic solid solutions with disordered structure [11]. Presence of the L-lines, as well as the intensive A-Boson peak in the Raman spectra of the as-grown ZrO2 –Gd2O3 (8 mol%)O2 supported the disor￾dered tetragonal structure of t0 domains formed in this sample. Thus, it is possible to freeze the t0 -phase with initial Gd2O3 (8 mol%) concentration in ZrO2-based single crystals without using rapidly quenching conditions. The long-time low-temperature annealing for 2–3 h of the as-grown ZrO2 –Gd2O3 (8 mol%) crystal stimulated the process of decreasing Gd2O3 content inside t0 -domains up to 2.5 mol%. The initial broad Raman line spectrum of the ZrO2 –Gd2O3 (8 mol%) crystal, which is shown in Fig. 8b, was transformed after such treatment into a narrow-line spectrum similar to those of the as-received ZrO2 –Eu2O3 (8 mol%) displayed in Fig. 8a. In turn, the ZrO2 –Eu2O3 (8 mol%) crystal corresponded to the low Eu2O3-content t0 - domains could be homogenized by annealing at T . T1 (point 1 in Fig. 3). The Eu2O3-rich t0 -domains formed by such treatment could be frozen in the homogenized sample using subsequent rapidly quenching. This phenomenon is illustrated by transformation of the narrow-line Raman spectrum for the as-received ZrO2 –Eu2O3 (8 mol%) sample into the broad-band spectrum of the homogenized crystal (Fig. 9). Then, it was possible to trace the transformation of the Raman spectrum of the homogenized ZrO2 –Eu2O3 (8 mol%) at heating through the t0 ! C phase transition region using high-temperature Raman spectroscopy method (Fig. 9). With the help of this technique, it was possible to evaluate T0-temperature within the accuracy of 10–15 K for the ZrO2 –Gd2O3 (Eu2O3) (8 mol%) crystals under study. The values obtained were used for new experiments devoted to studying the stress-induced C ! t 0 transition. It should be noted that the single crystals under examination retained its transparency after thermal treatments mentioned above provided that the temperature increase was sufficiently rapid, the ZrO2–Gd2O3 (8 mol%) sample admitting a slower heating regime than the ZrO2–Eu2O3 (8 mol%) crystal. This was the ZrO2–Gd2O3 (8 mol%) sample that was used for the C ! t 0 transition studies under loading. The results of the experiments are shown in Fig. 10. Fig. 10a displays the results of comparison of the polarized Raman spectra at 300 K for nonstressed (1) and stressed (2) zones of the ZrO2 –Gd2O3 (8 mol%) crystal that prelimina￾rily underwent the deformation in the process of C ! t 0 transformation according to the procedure described above. Intensities of the lines in the spectra shown in Fig. 10 were normalized to the intensity of the Raman band in the region of 320 cm21 . The position of the A1g line (257 cm21 ) in Fig. 10a demonstrates a decrease in the Gd2O3 concentration in t0 -domains compared to the primary composition (the peak position at 242 cm21 ) due to the low-temperature annealing in course of the experiment. According to the data presented in Figs. 4 and 5, the Gd2O3 concentration in the sample subjected to the stress-induced experiment could be evaluated as 4–5 mol% Gd2O3. The difference between the Raman spectra for stressed and nonstressed zones is registered in Fig. 10a. The intensity of the A1g line in the k geometry was essentially lower in the case of registration in the 2-zone than in the 1-region. The spectra for the ’ geometry displayed reduced intensities of three from five lines of the 2B1g þ 3Eg manifold in the same registration scheme (Fig. 10a). These regularities should be revealed if the formation of the B-type domains occurred predominantly under loading (Table 2 and Fig. 2). Thus, the deformation of the homogenized ZrO2 –Gd2O3 (8 mol%) crystal at cooling through the C ! t 0 transformation temperature region resulted in the predominant nucleation Fig. 9. A change of the Raman spectra of the ZrO2 –(Eu2O3) (8 mol%) single crystal at different annealing regimes and temperatures. A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112 1109

l110 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 A1g(257cm") (B and >B, D, F columns). Furthermore, an essential decrease in the intensity of the Aig-line in the Raman petra of the zone 2 for the lI-geometry indicated the following relation: axr az between the components of the Alg-mode Raman tensor. Rather interesting results were obtained after repeated annealing at T< To of the ZrO2-Gd,O3(8 mol%)single crystal previously subjected to the deformation. The (1)|(2) ⊥ ample(Fig. 10b) showed that predominant orientation of t domains along the stressed C4 axis was retained after the repeated low-temperature annealing without loading, the 200 400 600 character of the polarized spectra in Figs. 10a and b being the same. The repeated annealing resulted in narrowing A1g(265cm) shift and redistribution of intensities of Raman lines(Fi 10b), which are a result of decreasing the stabilizer concentration in t' domains due to ' low-temperature 2B1 lechanism'of the tetragonal phase formation. The position of the Alg mode(264 cm )in the Raman spectra presented in Fig. 10b evaluates the Gd2O3 content in t' domains as 2.5-3 mol%. Essentially narrowing Raman lines resulted in more noticeable difference in the polarized Raman spectra (1)(2) registered in the zones I and 2 in the crystal subjected to the ⊥ repeated annealing. Thus, the phenomenon of decreasin intensities of the 3Eg modes, when changing exciting-beam positions from the zone l to 2 of the crystal, was registered 200 600800 very distinctly, namely, in the I spectrum of the Raman shift(cm") repeatedly annealed crystal(Fig. 10b) Fig. 10. The polarized Raman spectra at 300 K of ZrO2-Gd2O3( 8 mol%) The assignment of the symmetry of six Raman modes of single crystal for the non-deformed(1)and deformed (2)zones(according the tetragonal form in Zro -based solid solutions, which to Fig. Ib).(a)after annealing at 1800 K and cooling through the C-t were determined in our experiments, is shown in Figs. 5 and phase transformation temperature region under loading, (b) after the 10 and in Table 3 repeated annealing at 1350 K(20 h) without loading. of t' domains with the z axis parallel to the stressed C, cubic 7. Discussion axis. It should be noted that a redistribution of the raman line intensities in 1 and 2 zones allowed us to separate The experiments mentioned above revealed a possibility Raman lines of the B, and Eg modes in the Raman spectra of formation of oriented tetragonal domains in ZrO2-Gd2O3 for the I geometry using the calculation results of Table 2 8 mol%)solid solution because of the stress-induced Table 3 Frequencies(cm )at 300 K and symmetry assignments of the Raman bands of the tetragonal Zro, and ZrO2-based solid solutions Symmetry of the modes Method of assignments Ig [12」 (146,319,469,608,638) ZrO2-3 mol% Eu, O3(sc. This work 323,607 147462,640 ZrO2-3 mol% Gd,O3(.c) This work 150.476.620 ZrO2-8 mol% Gd,O3(sc) 146,458648 ZrOz-undoped (e f) 20 326.616 155476645 ZrO2-undope 14 6206662 266,474645 ZrOz-4.6 mol% Y2O3(sc) 266,474645 PPPPcNccc 18,28 149,269,461 ZrO2-undoped (powder) 27 5,595 257,410,645 ZrO2-undoped (powder) P, polarized study; C, calculated; N, not determined; s.c., single crystal; e f, epitaxial film. Concentration of Ln2O3 in the tetragonal nanodomains was evaluated according to the position of the Alx raman mode (see the text

of t0 domains with the z axis parallel to the stressed C4 cubic axis. It should be noted that a redistribution of the Raman line intensities in 1 and 2 zones allowed us to separate Raman lines of the Bg and Eg modes in the Raman spectra for the ’ geometry using the calculation results of Table 2 (B and PB; D; F columns). Furthermore, an essential decrease in the intensity of the A1g-line in the Raman spectra of the zone 2 for the k-geometry indicated the following relation: axx p azz between the components of the A1g-mode Raman tensor. Rather interesting results were obtained after repeated annealing at T , T0 of the ZrO2 –Gd2O3 (8 mol%) single crystal previously subjected to the deformation. The polarized Raman spectra for the zones 1 and 2 of the sample (Fig. 10b) showed that predominant orientation of t0 domains along the stressed C4 axis was retained after the repeated low-temperature annealing without loading, the character of the polarized spectra in Figs. 10a and b being the same. The repeated annealing resulted in narrowing, shift and redistribution of intensities of Raman lines (Fig. 10b), which are a result of decreasing the stabilizer concentration in t0 domains due to ‘low-temperature mechanism’ of the tetragonal phase formation. The position of the A1g mode (264 cm21 ) in the Raman spectra presented in Fig. 10b evaluates the Gd2O3 content in t0 domains as 2.5–3 mol%. Essentially narrowing Raman lines resulted in more noticeable difference in the polarized Raman spectra registered in the zones 1 and 2 in the crystal subjected to the repeated annealing. Thus, the phenomenon of decreasing intensities of the 3Eg modes, when changing exciting-beam positions from the zone 1 to 2 of the crystal, was registered very distinctly, namely, in the ’ spectrum of the repeatedly annealed crystal (Fig. 10b). The assignment of the symmetry of six Raman modes of the tetragonal form in ZrO2-based solid solutions, which were determined in our experiments, is shown in Figs. 5 and 10 and in Table 3. 7. Discussion The experiments mentioned above revealed a possibility of formation of oriented tetragonal domains in ZrO2 –Gd2O3 (8 mol%) solid solution because of the stress-induced Fig. 10. The polarized Raman spectra at 300 K of ZrO2 –Gd2O3 (8 mol%) single crystal for the non-deformed (1) and deformed (2) zones (according to Fig. 1b). (a) after annealing at 1800 K and cooling through the C ! t 0 phase transformation temperature region under loading, (b) after the repeated annealing at 1350 K (20 h) without loading. Table 3 Frequencies (cm21 ) at 300 K and symmetry assignments of the Raman bands of the tetragonal ZrO2 and ZrO2-based solid solutions Ref. Symmetry of the modes Object Method of assignments A1g B1g Eg [12] 262 (146,319,469,608,638) ZrO2 –3 mol% Eu2O3 (s.c.)a P This work 265 323,607 147,462,640 ZrO2 –3 mol% Gd2O3 (s.c.)a P This work 242 343,620 150,476,620 ZrO2 –8 mol% Gd2O3 (s.c.)a P [29] 270 318,602 146,458,648 ZrO2-undoped (e.f.) P [26] 266 326,616 155,476,645 ZrO2-undoped C [14] 616 155,326 266,474,645 ZrO2 –4.6 mol% Y2O3 (s.c.) N [25] 326 155,616 266,474,645 ZrO2-undoped C [18,28] 602 319,648 149,269,461 ZrO2-undoped (powder) C [27] 630 155,595 257,410,645 ZrO2-undoped (powder) C P, polarized study; C, calculated; N, not determined; s.c., single crystal; e.f., epitaxial film. a Concentration of Ln2O3 in the tetragonal nanodomains was evaluated according to the position of the A1g Raman mode (see the text). 1110 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112

A.A. Sobol, Y.K. Voronko Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 cubic-tetragonal phase transformation. Deformation of a in Table 3. The methods of assignments and the objects crystal while cooling in the C-t phase-transition studied are also presented in this table. It should be temperature region stimulated the predominant nucleation noted that polarized Raman study is a sole experimenta of' domains with the tetragonal axis parallel to the stressed means for reliable symmetry assignments for vibrational C4 cubic axis. Retention of this predominant orientation of t modes. Only three papers (including this one)involved domains after subsequent annealing without loading shower the polarized Raman spectroscopy studies(Table 3). The the impossibility of nucleation of new t domains, while frequencies of six tetragonal bands, which were deter holding the sample at a temperature below the C-t mined in these papers, slightly differ because of different transition. If this phenomenon took place, new t domains Ejects under study (ZrOx-epitaxial films and bulk pSz would have three equiprobable orientations, and the single crystals with different stabilizer concentrations) polarized Raman spectra was similar to those of nonstressed Nevertheless the symmetry assignments of all six mode sample. Thus, the amount and the size of t' particles formed for Refs. [12, 29] and the results of this paper completely below the C-t transition was determined basically by the coincide with each other. The specialty of these assign conditions of t'-particle nucleation due to the diffusionles ments is the position of the Alg mode in the region of phase transformation. The size of t -domains formed due to near 260 cm-1 The condition axr < az for the Ais the C-e transformation are many times smaller than the mode Raman tensor determined in Ref. [29 was also wavelength of visible light(probably tens of nanometers), confirmed in our study. The experimental assignments of which results in the absolute transparency of the Zro2 Ref [29] and of this study were supported by the model Gd2O3(Eu2O3)(6-8 mol%) single crystals. Meanwhile, calculations only in Ref. [26]. Other model calculations low-temperature annealing resulted in reducing the concen- performed in Refs. [25, 27, 28] implied the quite different tration of the stabilizer inside t' domains from 8 to 2.5 assignments and proposed the Alg-mode position in the 3 mol% depending on the holding time. This phenomenon region of 600 cm. There are no experimental confir occurred independently of the fact whether the samp mations for such assignments. The data of Ref. [14]on underwent deformation or not. Thus, interesting he Alg-mode frequency laying at 600 cm should not peculiarities of the ZrO2-Gd2O3(Eu2O3)(6-8 mol %) be considered as true ones. The authors of Ref. [14] solid solutions allow a low stabilizer concentration (2- could not use polarized Raman spectra of the ZrO2- 3 mol%)tetragonal nanodomains to be formed predomi- Y203(4.6 mol%)bulk single crystals mentioned in this nantly oriented along the certain cubic axis in transparent study because of their opacity and birefringence effects The phenomenon of intermode interaction, assigned to Our studies of Raman spectra for the ZrO2-Gd2O3 the t-C transition in Zro2 in high pressure experiments (Eu,O3)(6-8 mol%)solid solutions show that the cubic [18], proposed the presence of the Eg mode at 260cm tetragonal phase transformation did not occur completely, instead of the Alg one. However, this effect could be related and a certain volume of the solid solution retained the cubic to other type of pressure-induced transformation rather than structure. Such model explains the formation of t' particles to the t-C transition. The nature of this phenomenon can with different stabilizer concentration via the low-tempera be clarified with the polarized Raman spectroscopy study in high pressure experiments. It should be noted that this phenomenon occurred only in Thus, the data of Ref. [12, 29 and of our study on the ZrO2-Ln203(6-8 mol%)with oxides of Ln-metals in the symmetry assignments of the Raman modes for DAS onset and in the middle of the Lanthanide series and is tetragonal phase of Zro2 can be considered as only results, impossible for well-known Y-stabilized PSz[17, 22]. In this reliably confirmed by experimental studies connection, our results concerning the nature of the C-t transformation in ZrO2-Gd2O3(Eu2O3)(6-8 mol%) samples do not contradict to conclusions obtained pre Acknowledgements viously for Y-doped PSz (4, 24 The results obtained by the polarized Raman The authors are grateful to EE. Lomonova for providing troscopy for the stressed C-t transformation in bulk the samples and L.L. Ershova for orientating single crystals ZrO2-Gd,03(8 mol%)single crystals allowed us to by X-ray technique. This work was supported by the claim the symmetry of all six vibrations for DAS Russian Foundation for Basic Research, project no 01-02 tetragonal phase. There were lively debates on determi 16098 nation of the symmetry of these vibrations in metastable tetragonal zirconia and PSz [12, 14, 18, 25-29. Interest in these studies was caused by calculations of possible References models of the C-t phase transition in ZrO2 and the role of the Ale soft mode in driving the transformation. [1 A. Heuer, L. Hobb(Eds ) Advances in ceramics, Science and Technology of Zirconia, 1981, p 3. a lot of different variants for the tetragonal raman bands [2 D B. Marshall, M.R. James, Reversible stress induced martensitic assignments were suggested. Some of them are indicated transformations in ZrO, J. Am. Ceram Soc. 69(1986)215-217

cubic–tetragonal phase transformation. Deformation of a crystal while cooling in the C ! t 0 phase-transition temperature region stimulated the predominant nucleation of t0 domains with the tetragonal axis parallel to the stressed C4 cubic axis. Retention of this predominant orientation of t0 domains after subsequent annealing without loading showed the impossibility of nucleation of new t0 domains, while holding the sample at a temperature below the C ! t 0 transition. If this phenomenon took place, new t0 domains would have three equiprobable orientations, and the polarized Raman spectra was similar to those of nonstressed sample. Thus, the amount and the size of t0 particles formed below the C ! t 0 transition was determined basically by the conditions of t0 -particle nucleation due to the diffusionless phase transformation. The size of t0 -domains formed due to the C ! t 0 transformation are many times smaller than the wavelength of visible light (probably tens of nanometers), which results in the absolute transparency of the ZrO2 – Gd2O3 (Eu2O3) (6–8 mol%) single crystals. Meanwhile, low-temperature annealing resulted in reducing the concen￾tration of the stabilizer inside t0 domains from 8 to 2.5– 3 mol% depending on the holding time. This phenomenon occurred independently of the fact whether the sample underwent deformation or not. Thus, interesting peculiarities of the ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) solid solutions allow a low stabilizer concentration (2– 3 mol%) tetragonal nanodomains to be formed predomi￾nantly oriented along the certain cubic axis in transparent single crystals. Our studies of Raman spectra for the ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) solid solutions show that the cubic ! tetragonal phase transformation did not occur completely, and a certain volume of the solid solution retained the cubic structure. Such model explains the formation of t0 particles with different stabilizer concentration via the low-tempera￾ture mechanism. It should be noted that this phenomenon occurred only in ZrO2 –Ln2O3 (6–8 mol%) with oxides of Ln-metals in the onset and in the middle of the Lanthanide series and is impossible for well-known Y-stabilized PSZ [17,22]. In this connection, our results concerning the nature of the C ! t 0 transformation in ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) samples do not contradict to conclusions obtained pre￾viously for Y-doped PSZ [4,24]. The results obtained by the polarized Raman spec￾troscopy for the stressed C ! t 0 transformation in bulk ZrO2 –Gd2O3 (8 mol%) single crystals allowed us to claim the symmetry of all six vibrations for D15 4h tetragonal phase. There were lively debates on determi￾nation of the symmetry of these vibrations in metastable tetragonal zirconia and PSZ [12,14,18,25–29]. Interest in these studies was caused by calculations of possible models of the C ! t 0 phase transition in ZrO2 and the role of the A1g soft mode in driving the transformation. A lot of different variants for the tetragonal Raman bands assignments were suggested. Some of them are indicated in Table 3. The methods of assignments and the objects studied are also presented in this table. It should be noted that polarized Raman study is a sole experimental means for reliable symmetry assignments for vibrational modes. Only three papers (including this one) involved the polarized Raman spectroscopy studies (Table 3). The frequencies of six tetragonal bands, which were deter￾mined in these papers, slightly differ because of different objects under study (ZrO2-epitaxial films and bulk PSZ single crystals with different stabilizer concentrations). Nevertheless the symmetry assignments of all six mode for Refs. [12,29] and the results of this paper completely coincide with each other. The specialty of these assign￾ments is the position of the A1g mode in the region of near 260 cm21 . The condition axx p azz for the A1g mode Raman tensor determined in Ref. [29] was also confirmed in our study. The experimental assignments of Ref. [29] and of this study were supported by the model calculations only in Ref. [26]. Other model calculations performed in Refs. [25,27,28] implied the quite different assignments and proposed the A1g-mode position in the region of 600 cm21 . There are no experimental confir￾mations for such assignments. The data of Ref. [14] on the A1g-mode frequency laying at 600 cm21 should not be considered as true ones. The authors of Ref. [14] could not use polarized Raman spectra of the ZrO2 – Y2O3 (4.6 mol%) bulk single crystals mentioned in this study because of their opacity and birefringence effects. The phenomenon of intermode interaction, assigned to the t ! C transition in ZrO2 in high pressure experiments [18], proposed the presence of the Eg mode at 260 cm21 instead of the A1g one. However, this effect could be related to other type of pressure-induced transformation rather than to the t ! C transition. The nature of this phenomenon can be clarified with the polarized Raman spectroscopy study in high pressure experiments. Thus, the data of Ref. [12,29] and of our study on the symmetry assignments of the Raman modes for D15 4h tetragonal phase of ZrO2 can be considered as only results, reliably confirmed by experimental studies. Acknowledgements The authors are grateful to E.E. Lomonova for providing the samples and L.I. Ershova for orientating single crystals by X-ray technique. This work was supported by the Russian Foundation for Basic Research, project no. 01-02- 16098. References [1] A. Heuer, L. Hobb (Eds.), Advances in ceramics, Science and Technology of Zirconia, 1981, p. 3. [2] D.B. Marshall, M.R. James, Reversible stress induced martensitic transformations in ZrO2, J. Am. Ceram. Soc. 69 (1986) 215–217. A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112 1111

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