BINARY SYSTEM HFO2-GD2O3

Phase relationships in the HfO2Gd2O3 system have been studied by thermal and X-ray analysis and the phase diagram of the system has been plotted. The existence of a compound Gd2Hf2O7, with a pyrochlore structure [a = 10.490 A ̊], has been established. This compound melts congruently at 2440 °C. Solid solutions based on the pyrochlore-type compound are, on each side of the stoichiometric composition, transformed into fluorite-type solid solutions. Two hexagonal compounds of an M7O11.5- and M7O11-type have been found in the Gd2O3-rich region (>50 mol.% Gd2O3), viz., Gd5.25Hf1.75O11.375, which possesses a very narrow range of homogeneity, is formed at about 60 mol.% Gd2O3,and Gd6HfO11 which is formed within the Gd2O3 concentration range between 75 and 90 mol.% Gd2O3. The first of these compounds undergoes a transformation at 1500 °C into a cubic solid solutions of fluorite and C-type, and the second above 1700 °C into a cubic solid solution in C-Gd2O3. © 1968.