POLYMORPHISM OF R ₂ O ₃ . WO ₃ (R equals RARE EARTH) COMPOUNDS AT HIGH TEMPERATURES.

R//2O//3. WO//3 compounds (R equals La, Ce, Nd, Sm, Dy and Y) were studied by different thermal treatments including fusion and splat-cooling. In addition to three known modifications, two new modifications were found. One of them, found for all compounds studied, was the stable phase at high temperatures. It was identified to be tetragonal. The phase transition temperatures between this phase and the low temperature phases were 1,410 degree C for La//2O//3. WO//3, 1,330 degree C for Ce//2O//3. WO//3, and 1430 degree C for Nd//2O//3. WO//3. Lattice parameters of these tetragonal phases are given. The other phase found in R//2O//3. WO//3 (R equals Ce - Y) seemed to be metastable. Another phase reported as a modification of La//2O//3. WO//3 in literatures could be identified as a stoichiometric compound 7La//2O//3. 8 WO//3. The phase relations between these six phases are discussed.