Abstract
The systems neptunium oxide - oxides of the rare earths (RE Ho, Er, Tm, Yb, Lu) investigated between 1250°C and 1550°C at p(O 2) = 1 atm have three one-phase and two two-phase regions. In the neptunium-rich part a solid solution with fluorite structure is found which increases slowly with rising temperature. The 1:1 reaction products (Np 0,5, RE 0,5)O 2 show less thermal stability than the analogous double oxides with uranium. The rhombohedral region is identical in all systems with the 1:6 compound NpO 3·6REO 1,5, because no rhombohedral phase range is observed. The maximum solubility of neptunium oxide in the rare earth oxides is 2 mole% and, similarly, increases slowly with temperature. In all mixed oxides investigated, neptunium is oxidized more or less to oxidation states higher than four, independent of the various phase regions.
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