Phase equilibria in the system HfO2-Y2O3-La2O3 AT 1900°C

Abstract

621.762We have used x-ray phase analysis, electron-probe microanalysis, petrography, and electron microscopy on annealed specimens to study phase equilibria in the ternary system HfO 2 − Y 2 O 3 − La 2 O 3 at 1900°C over the entire concentration range. We have plotted the isothermal cross section of the phase diagram for this system at the indicated temperature. We found 23 phase regions. A typical feature of the system is formation of solid solutions based on different crystal modifications . We did not observe new phases in the system. The nature of the phase equilibria in the system is consistent with the high relative thermodynamic stability of lanthanum hafnate (ΔH°L a 2 Hf 2 O 7 ≈ 100 kJ/mole) compared with LaYO 3 . We established that adding a third component extends the thermal stability region for the ordered phase of LaYO 3 toward higher temperatures.Keywords: phase equilibria, hafnium oxide, yttrium oxide, lanthanum oxide, isothermal cross section, solid solution, ordered phase, pyrochlore type phase, phase diagram of a ternary system.The HfO 2 − Y 2 O 3 − La 2 O 3 system is of interest from the standpoint of designing refractory, structural, optical, and other materials that can operate at high temperatures and have unique properties due not only to the features of the original oxides (high melting points, thermodynamic stability, martensitic nature of the series of transformations) but also the effect of complex alloying additives [1][2][3]. The aim of this work was to study the interactions in the HfO 2 − Y 2 O 3 − La 2 O 3 system at 1900°C.We previously studied [4,5] the phase equilibria in the system under consideration at 1250°C and 1600°C, and we made a mathematical prediction of the solid-phase transformations at 1900°C [6,7]. In the literature [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] we find descriptions of the structural characteristics of the limiting binary phase diagrams of the systems HfO . Analysis of the literature data on the limiting two-component systems and the results of a study of the phase equilibria in the ternary system showed that at 1900°C, the phase equilibria in the system will be complex. This is determined to a significant extent by the polymorphism of the starting components and formation of two intermediate phases in the limiting systems: La 2 Hf 2 O 7 (Py) and LaYO 3 (R), crystallizing in a cubic structure of the pyrochlore type and the perovskite type with rhombic distortions.In the La 2 O 3 − Y 2 O 3 system, broad regions of solid solutions are formed based on different crystal modifications of the starting components [8][9][10][11][12][13][14][15]. The temperatures of the polymorphic transformations decrease for La 2 O 3 (X H, H A) with yttrium oxide additives and for Y 2 O 3 (H C) with lanthanum oxide additives. The extent of the regions of solid solutions increases as the temperature increases. About 83 mole% Y 2 O 3 is dissolved in X-La 2 O 3 . The solid solutions based on the H phase of rare earth oxides form a continuous series. Th...