Phase Equilibria in Systems Involving the Rare-Earth Oxides. Part II. Solid State Reactions in Trivalent Rare-Earth Oxide Systems

Selected mixtures in 21 binary and 9 ternary rare-earth oxide systems were studied by X-ray diffraction after heat treatment at 1,650° C and above. Two graphs were drawn to show specific regions of stability for the various structure types. Each gives the average ionic radius of constituent cations versus the mole percent of the smaller cation. One diagram is essentially divided into areas of solid solution of the A, B, and C rare-earth oxide structure types. The other indicates a field of perovskite-type compounds bordered by regions of A, B, or C solid solutions. These diagrams were used to predict the subsolidus phase diagrams of a number of systems. A total of forty-one subsolidus binary and one ternary rare-earth oxide systems were given. A tolerance factor equal to 0.77 was assigned as the minimum value for the formation of a perovskite-type compound.