Phasengleichgewichte in den Systemen Thoriumoxid-Lanthanidenoxide

Abstract

In the thorium oxide-rare earth oxide systems a great solubility of REO 1.5 in ThO 2 can be observed. The width of this fluorite phase (Th, RE)O 2− x increases with increasing temperature and strongly depends on the ionic radius of the lanthanide element. A maximum solubility exists in the ThO 2NdO 1.5 system where the difference | r Th 4+ − r RE 3+ | between the ionic radii is a minimum. The smallest phase width occurs in the ThO 2LuO 1.5 system, where the difference between the ionic radii is greatest. The hexagonal (A-REO 1.5) and monoclinic (B-REO 1.5) rare earth oxides do not take up ThO 2 into solid solution, in contrast to the cubic (C-REO 1.5) oxides which show a small solubility (< 5 mol %) for ThO 2. The incorporation of ThO 2 in C-REO 1.5 to give a C-type phase (RE, Th)O 1.5+ x stabilizes the cubic rare earth oxides against transformation to B-REO 1.5. Two new ordered phases, ThO 2·3REO 1.5 and ThO 2·11REO 1.5, were found in the systems with the light rare earth elements La, Pr, and Nd. These compounds are stable only at high temperatures, but can be obtained in a metastable state by rapid quenching. It has been shown by density measurements that in the fluorite phase (Th,RE)O 2− x as well as in the C-type-phase C-(RE, Th) 1.5+ x a completely filled cation lattice exists with vacancies [in (Th,RE)O 2− x ] or interstitials [in (RE, Th)O 1.5+ x ] in the anionic part of the lattice. Detailed phase diagrams showing phase equilibria for temperatures between 1250°C and 2100°C are given.