STRUCTURAL STABILITY IN URANIUM

Diamond-anvil cell experiments and first-principles theory have been u sed to investigate the structural stability of uranium up to 1 Mbar in pressure. Experiments and theory agree; there is no phase transition in uranium below 1 Mbar. Previous speculations about a crystallographi c phase transition in uranium below this pressure are thus shown to be incorrect. In this regard, uranium is exceptional in the series of li ght actinides, where pressure-induced phase transitions typically occu r at pressure below 1 Mbar. The ground-state crystal structure of uran ium is orthorhombic with three structural parameters: the axial ratios b/a and c/a, and an internal parameter y measuring the displacement, along the b-axis, of alternate planes. The experimental and theoretica l results reported here indicate that one of these parameters, c/a, is substantially more sensitive to pressure than the other two, changing by as much as 5%, while b/a and y are constant within 1% within the p ressure range studied. This flexibility in the alpha-U structure facil itates this structure over a wide pressure range. Theory suggests that electrostatic contributions to the total energy drive the variation i n the c/a ratio as a function of pressure, and a simple model is utili zed to show this.