Charles A. Sorrell

Thermal expansion and phase transformations of the U3O8−z phase in air☆

Abstract Detailed thermal expansion data have been obtained by high temperature X-ray diffractometry of U3O8 and U3O8−z in air between 22 and 1100°C. Stoichiometric U3O8 changes continuously, reversibly and anisotropically above room temperature with expansion along the a-axis and contraction along the b-axis from orthorhombic to hexagonal symmetry at 350 ± 10°C. A small but continuous contraction along the c-axis occurs up to 1100°C. Both parameters of the hexagonal phase expand smoothly and reversibly to approximately 875°C and exhibit no discontinuity as a result of loss of oxygen which begins near 600°C. At 875–925°C the structure changes to lower symmetry by contraction along the a-axis and expansion along the b-axis. This structural change is accompanied by a more extensive loss of oxygen and is usually irreversible unless the crystallite size is sufficiently small, ∼0.05 μm for which both the compositional and structural changes are reversible with temperature. Calculated unit cell volumes undergo slight expansion and contraction up to 350°C, with a broad maximum at approx. 200°C and a smooth expansion at a decreasing rate from 350–875°C. The irreversible transition beginning at 875°C is ostensibly associated with an expansion-contraction anomaly in the temperature range 875–925°C, followed by smooth expansion at a decreasing rate to 1100°C. The thermal expansion characteristics have been interpreted in terms of rotation of octahedral linkages in the orthorhombic and hexagonal structures and the irreversible transition has been tentatively explained in terms of two U3O8−z phases, one orthorhombic and one monoclinic. The apparently contradictory structural analyses for the U3O8 and U3O8−z phases in the literature can be reconciled on the basis of the thermal expansion results which involve both changes in temperature and composition.