Structural behavior of α-uranium with pressures to 100 GPa

The structural behavior of α-uranium at room temperature has been studied up to 100 GPa in diamond anvil cells using angle-dispersive x-ray diffraction at a synchrotron source. The orthorhombic Cmcm structure is stable to at least 100 GPa. We present details of the variation of all structural parameters, derived from Rietveld analysis of the diffraction patterns: the lattice parameters a, b, and c, and the positional parameter y, as a function of pressure. Precise data, as well as the use of different media for the pressure transmission, allow us to question previous values of several parameters deduced by x-ray diffraction, especially the bulk modulus, and we obtain a revised bulk modulus of B₀=104(2)GPa with B₀^′=6.2(2). We have also performed accurate (zero-temperature) electronic structure calculations with full structural relaxation up to 100 GPa to test theory against our experimental results. The magnitude and trends of the calculated structural parameters are in reasonable agreement with experiment. In contrast, our bulk modulus calculated at our zero-pressure volume is B₀=136GPa and B₀^′=5.07, in agreement with previous calculations, differing markedly from experimental values.