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)215-217cubic–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