Phase Equilibria in Systems Involving the Rare-Earth Oxides. Part I. Polymorphism of the Oxides of the Trivalent Rare-Earth Ions

The polymorphic relationships of the pure rare-earth oxides have been reinvestigated using X-ray diffraction methods for identification of phases. The oxides of the trivalent rare earth ions crystallize in three different types: A, B, and C. Each oxide has only one truly stable polymorph: La(2)O(3), Ce(2)O(3), Pr(2)O(3), and Nd(2)O(3) belong to the A type; Sm(2)O(3), Eu(2)O(3), and Gd(2)O(3) to the B type; Tb(2)O(3), Dy(2)O(3), Ho(2)O(3), Er(2)O(3), Tm(2)O(3), Yb(2)O(3), and Lu(2)O(3) to the C type. In addition Nd(2)O(3), Sm(2)O(3), Eu(2)O(3), and Gd(2)O(3) have low-temperature, apparently metastable, C-type polymorphs. The low-temperature form inverts irreversibly to the stable form at increasingly higher temperatures for decreasing cation radius.