Phasengleichgewichte in den systemen uranoxid-oxide der seltenen erden (Ho, Er, Tm, Yb, Lu)

Abstract

The subliquidus equilibria of the systems uranium oxide-oxides of the rare earths (RE = Ho,Er,Tm,Yb,Lu) were investigated in detail between 1100 and 1550°C at p(O 2) = 1 atm. It has been found that all systems possess four one-phase regions separated by the corresponding two-phase regions. The one-phase regions are: U 3O 8, fluorite, rhombohedral, and C-type phase. No solubility of REO 1·5 in U 3O 8 could be detected. In the uranium-rich part the extension of the fluorite phase increases with temperature, whilst, in the rare-earth-rich part, the limiting value around 64 mole % REO 1·5 shows no dependence on temperature. The 1:1 reaction products (U 0·5, RE 0·5)O 2·00 have a remarkable thermal stability. A rhombohedral phase with hexavalent uranium exists from about 72 mole % REO 1·5 up to the respective 1:6 compound UO 3·6REO 1·5. The solubility of uranium oxide in the rare earth oxides is very small and decreases with increasing temperature. IR-spectra taken from different phase regions give no reference to UO 2 2+-groupings in the mixed oxides.