Das system UO 2UO 3LaO 1.5

Abstract

At 1250°C, the following five single-phase regions were found in the UO 2UO 3LaO 1.5 region of the ternary uranium-lanthanum-oxygen system: 1. (a) β-U 3O 8: no solubility for LaO 1.5 could be observed. 2. (b) Fluorite phase: concerning the oxygen:metal ratio this phase may exist superstoichiometrically as (U, La)O 2+ x(ss) (0 ⩽ x ⩽ 0.25), substoichiometrically as (U,La)O 2− x(ss) (0 ⩽ x ⩽ 0.40) and stoichiometrically as (U 1− y ,La y )O 2(ss) (0 ⩽ y ⩽ 0.67). 3. (c) Rhombohedral phase I: as concerns the U:La ratio this ordered phase occurs stoichiometrically at the limiting compositions UO 2·6LaO 1.5 (Me 7O 11) with tetravalent uranium and UO 3.6LaO 1.5 (Me 7O 12) with hexavalent uranium, showing a maximum of phase width with the limiting compositions UO 2.5·5LaO 1.5 and UO 2.5·7LaO 1.5. 4. (d) Rhombohedral phase II: this phase extends from 71.5 to 76.5 mole-% LaO 1.5 for U(VI) and shows only little phase widths concerning the O:Me ratio. Above 1310°C this ordered phase transforms reversibly to a disordered phase having fluorite structure. 5. (e) LaO 1.5: no solubility for UO 2+ x could be observed. In the pseudo-binary system UO 2+ x -LaO 1.5 (p(O 2) = 1 atm) a third rhombohedral phase R III (Me 8O 16) occurs between 55 and 66.7 mole-% LaO 1.5 (T ⩽ 1220°C). The limiting composition of this phase regarding the highest content of lanthanum as well as that of oxygen is ( U 1 3 , La 2 3 ) O 2 . Below 1550°C, the mean valency of uranium in UO 2+ x -LaO 1.5 mixed oxides at p(O 2) = 1 atm is rather independent of the annealing temperature but markedly increases with rising content of lanthanum in the samples. A valency of exact 6.00 is observed for mixed oxides containing 66.7 mole-% LaO 1.5 or more.