Selected mixtures in 69 binary systems involving Ab03, Ga20 a, CrZ0 3, Fe20 a, SczOa, In20 3, Y20 3, and the rare earth oxides were stud ied by X-ray diffraction techniques after heat treatment at various temperatures. A plot of the radii of the A +3 cations versus the radii of B+3 cations shows the regions of stability for the different structure types found for Ithe double oxides of the trivalent cations. The following structure types were encoun tered: A, B, and C-type rare earth oxide; corundum, beta gall ia; kappa alum ina; garnet; perovskite; and several types which could not be d efinite ly re lated to known structures. The majority of A+3 B+3 0 3 compounds have the perovskite structure. Several phases, including (1-x)Fez0 3·xAh03 ., and (1-x) Fe,03·xGa20 3 ." appear to have structures s imi lar to kappa alumina. Solid solution dcfin itely occurs in many of t he garnet type compounds which contain gailia. Based on the data collected in this survey, the s ubsolidu s phase equi libria re lationships of 79 binary systems were drawn.

Th e polymorphi c relatioll ships of t he pure rare-carlh oxides have been r ei nves tigated us ing X-ray diffrac tion methods for ident ificat ion of phases. The ox ides of the t rivalen t rare eart h ions crystalli ze in t hree different typ es: A rt B, and C. Each oXIde has o nly one t ruly stab le polymorph: La203, Ce20 3, Pr20 3, a nd Nd2 u 3 belong to the A type ; Sm 20 3, Eu20 3, a nd Gd20 3 to t he B type; Tb20 3, Dy20 3, H020 3, E1"20 3, Tm20 3, Yb20 3, and LU 20 3 to t he C ty pe. In addition Nd20 3, Sm20 3 EU 20 3, and Gd20 3 have .low-te mpcrature, appare ntly metastable, C-type pol ymo rphs. 1.~h e low-te mperat ure fOt"lTI lI~verts IITcvers lbl.v to the stab le form at in creas in gly hi gher temperatures fo r dec rea sIn g catIOn radlLl s.

Se lecte d mixtures i n 2 l bi nary a nd 9 ternary rare-earth oxide syste ms were studied by X-ray diffraction after h eat treatment aL 1,650° C a nd above. Two graphs were drawn to show specific r egions of stability for t he various strucLure types. Each givcs t he a verage ionic rad ius of constiLuent cations versus the mole percent of the smaller cation. One diagram is es~e ntiall :-' divided inLo areas of solid solution of t he A, B, and C rare-eart h oxide struct ure typ es. The other indicaLes a fi eld of p erovskite-type compou nds bordered by regions of A, B , or C so lid soluLions. These diagrams were used to predict Lhe subsolidus phase diagrams of a number of syste ms. A total of forty-one subsolidu s binar y a nd on e tema r.v rare-earth oxide sy Lems were given. A tolerance fac tor equal to 0.77 was assigned as the minimum value for the form aLion of a perovskite-type compound.

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