E.R. Andrievskaya/ Journal of the European Ceramic Sociery 28(2008)2363-2388 2072600 Nd+1207 2400 27 Eu2H1207 2200 1.04 0.1050.1100.115 2000 0.1000.105 0.11 L+,nm Fig 9. The lattice parameter of the ordered phases Ln2Hf2O(O)Ln2Zr2O(o)(a)and melting temperatures corresponding to the compositions 33.3 mol% Ln2O3 66.7 mol% ZrO,(b)vs ionic radius of lanthanide. The ordered phase Lazzr207 is stable up to its melting tem- form of REO, which has not been found in pure oxides. 223 225 perature,the Nd2zr2or exists at approximately 2300C and In the systems with REO from the middle of the series(Sm2O3 lower, while the Gd2Zr2O7 is stable at temperatures less than Eu2O3, Gd2O3, Dy2O3), the widening of homogeneity fields 1500C. In this series of pyrochlore-type hafnate and zir- was revealed for C-type of solid solutions. In the systems with conates we found a linear increasing of their lattice parameters, heavy REO, the solid solutions of C-tyr and linear decreasing of their melting temperatures with increas- solubility in the wide range of temper. pe demonstrate unlimited ing of ionic radius of Ln*as shown in Fig9 High-temperature phase H is known in several REO and The most thermodynamically stable phase is the lanthanum yttria. In all studied systems, the continuous solubility in the irconate(hafnate), having minimal melting temperature in the H-Ln2O3 has been observed. Polymorphous modification X, eries Ln2Zr207 (Ln2Hf2 07) and the loosest lattice(the largest contrary to form H, was found for lanthanides of cerium sub- lattice parameter). Lanthanides from the end of the series do not group, and extension of the homogeneity fields for the X-phase form ordered compounds with pyrochlore-type lattice, because decreases from La2O3 to Gd2O3 Solid solutions basedon X-and the spatial factor is not appropriate to order ions in the lattice. H-forms of rEO cannot be quenched from high temperatures Recently the phenomenon of ordering/disordering was studied Equimolar compositions of rEo are able to be ordered and in the pyrochlore phases by molecular static methods. It intermediate phases of perovskite type remains stable if cation was shown, that the enthalpy of ordering quickly decreases with radii difference is greater than or equal to 0.022 nm. Amid sev ion radii increase in site B(B corresponds to Zr**and Hf+)and eral systems Ln203-Y203, this condition is satisfied for the decrease of Frenkel's pair formation energy(oxygen vacancy diagram La2O3-Y2O3, where this phase was definitely found in the site 48f+ interstitial ion in position &a of the pyrochlore (LaYO3) However, as this ratio is equal to the boundary value, lattice). This is clear, that the ionic and electronic conductivity the LaYO is stable in solid state. In the other systems, the can be increased.241,242 This rule will be working in the ternary LnYO, were not found. In the ternary systems, the substitu- systems, if high-enthalpy phases of pyrochlore would be doped tion of large size cation Ln+ by Zr*+ of smaller size can be a with disordering additives reason for stabilization of inyo 3. General characteristics and phase reactions in the 3.1. Stability intermediate phases and prognosis phase systems Ln O3-Y2O3 equilibria in the systems Ln2O3-Y203 Existing data 66, 243-256 and results of the present research Following the regularities of constitution, one can forecast low several conclusions concerning regularities in phase dia- the constitution of unknown six diagrams. Such a prognosis is grams of the systems REO-yttria In these systems, the solid important for defining the surface topology of the liquidus su solutions based on polymorphous modification of REO were face, because the elements of ternary diagrams situated close to found(Fig. 10). The fields of mutual solubility of components the bounded binary systems Ln2O3-Y2O3, strongly contribute defined by the size of cations and structure of rEO. The coor- of invariant points. Temperature of invariant points and extreme dinates of invariant points in the phase diagrams of the systems points on liquidus curves, points on solidus, X-H transition Ln2O3-Y2O3 are presented in Table 6 and Fig. 10. and solid-state reactions in the binary systems Ln2O3-Y203 The additives of Y2O3 to light lanthanides(La2O3, Pr203, change linearly with lanthanides ionic radius(Figs. 1l and 12) Nd2O3)promote formation of the solid solutions based on B- These dependences work the best for oxides of cerium sub-E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 2373 Fig. 9. The lattice parameter of the ordered phases Ln2Hf2O7 () Ln2Zr2O7 (♦) (a) and melting temperatures corresponding to the compositions 33.3 mol% Ln2O3 66.7 mol% ZrO2 (b) vs. ionic radius of lanthanide. The ordered phase La2Zr2O7 is stable up to its melting tem￾perature, the Nd2Zr2O7 exists at approximately 2300 ◦C and lower, while the Gd2Zr2O7 is stable at temperatures less than 1500 ◦C.238 In this series of pyrochlore-type hafnates and zir￾conates we found a linear increasing of their lattice parameters, and linear decreasing of their melting temperatures with increas￾ing of ionic radius of Ln3+ as shown in Fig. 9. The most thermodynamically stable phase is the lanthanum zirconate (hafnate), having minimal melting temperature in the series Ln2Zr2O7(Ln2Hf2O7) and the loosest lattice (the largest lattice parameter). Lanthanides from the end of the series do not form ordered compounds with pyrochlore-type lattice, because the spatial factor is not appropriate to order ions in the lattice. Recently the phenomenon of ordering/disordering was studied in the pyrochlore phases by molecular static methods.239,240 It was shown, that the enthalpy of ordering quickly decreases with ion radii increase in site B (B corresponds to Zr4+ and Hf4+) and decrease of Frenkel’s pair formation energy (oxygen vacancy in the site 48f + interstitial ion in position 8a of the pyrochlore lattice). This is clear, that the ionic and electronic conductivity can be increased.241,242 This rule will be working in the ternary systems, if high-enthalpy phases of pyrochlore would be doped with disordering additives. 3. General characteristics and phase reactions in the systems Ln2O3–Y2O3 Existing data 66,243–256 and results of the present research allow several conclusions concerning regularities in phase dia￾grams of the systems REO–yttria. In these systems, the solid solutions based on polymorphous modification of REO were found (Fig. 10). The fields of mutual solubility of components in these systems are defined by steric factor, which in turn, is defined by the size of cations and structure of REO. The coor￾dinates of invariant points in the phase diagrams of the systems Ln2O3–Y2O3 are presented in Table 6 and Fig. 10. The additives of Y2O3 to light lanthanides (La2O3, Pr2O3, Nd2O3) promote formation of the solid solutions based on B￾form of REO, which has not been found in pure oxides.223,225 In the systems with REO from the middle of the series (Sm2O3, Eu2O3, Gd2O3, Dy2O3), the widening of homogeneity fields was revealed for C-type of solid solutions. In the systems with heavy REO, the solid solutions of C-type demonstrate unlimited solubility in the wide range of temperature.246,247 High-temperature phase H is known in several REO and yttria. In all studied systems, the continuous solubility in the H-Ln2O3 has been observed. Polymorphous modification X, contrary to form H, was found for lanthanides of cerium sub￾group, and extension of the homogeneity fields for the X-phase decreases from La2O3 to Gd2O3. Solid solutions based on X- and H-forms of REO cannot be quenched from high temperatures. Equimolar compositions of REO59 are able to be ordered and intermediate phases of perovskite type remains stable if cation radii difference is greater than or equal to 0.022 nm. Amid sev￾eral systems Ln2O3–Y2O3, this condition is satisfied for the diagram La2O3–Y2O3, where this phase was definitely found (LaYO3). However, as this ratio is equal to the boundary value, the LaYO3 is stable in solid state. In the other systems, the LnYO3 were not found. In the ternary systems, the substitu￾tion of large size cation Ln3+ by Zr4+ of smaller size can be a reason for stabilization of LnYO3. 3.1. Stability intermediate phases and prognosis phase equilibria in the systems Ln2O3–Y2O3 Following the regularities of constitution, one can forecast the constitution of unknown six diagrams. Such a prognosis is important for defining the surface topology of the liquidus sur￾face, because the elements of ternary diagrams situated close to the bounded binary systems Ln2O3–Y2O3, strongly contribute to the topology of the liquidus surface, existence and position of invariant points. Temperature of invariant points and extreme points on liquidus curves, points on solidus, X H transition and solid-state reactions in the binary systems Ln2O3–Y2O3 change linearly with lanthanides ionic radius (Figs. 11 and 12). These dependences work the best for oxides of cerium sub-