Rare‐Earth Polymorphism and Phase Equilibria in Rare‐Earth Oxide‐Water Systems

Abstract

A high‐water‐pressure high‐temperature study has been conducted of the systems Y 2 O 3 ‐H 2 O, Sm 2 O 3 ‐H 2 O, Nd 2 O 3 ‐H 2 O, and La 2 O 3 ‐H 2 O. The trihydroxides La(OH) 3 , Nd(OH) 3 , and Sm(OH) 3 are isomorphous, but Y(OH) 3 does not belong to this group. On the other hand, the oxyhydroxides YOOH, SmOOH, and NdOOH are isomorphous; LaOOH, if it exists, cannot be stable in the presence of water at room temperature. Pressure‐temperature curves for the equilibrium between the trihydroxide, oxyhydroxide, and a new hydroxy compound have been determined for each of the three smaller ions. The reversibility of the transition between the A‐, B‐, and C‐type rare earths has been established, and new X‐ray data on the B structure are listed. The relations between the polymorphs as a function of ionic size are changed from those given by Goldschmidt. The very marked difference in stability in air between the A‐ and C‐type structures suggests the obvious desirability of converting rare‐earth oxide ceramics into the C structure with proper compositional variation and thermal treatment.