Phase diagram of uranium at high pressures and temperatures

Abstract

The phase diagram of uranium has been studied to 100 GPa by in situ diamond-anvil cell x-ray/laser-heating experiments. The \ensuremath{\gamma} (bcc) phase is discovered at high pressures, and the melting curve is presented to 100 GPa. The \ensuremath{\gamma} phase, $B=113.3\mathrm{GPa},$ is approximately 20% softer than the \ensuremath{\alpha}(orthorhombic), $B=135.5\mathrm{GPa}.$ The volume change in the $\ensuremath{\alpha}/\ensuremath{\gamma}$ transition shows a strong pressure dependence, ranging from 6% at ambient pressure to less than 1% at 80 GPa. Free-energy calculations, using Debye-Gr\"uneisen quasiharmonic theory, show that the softer bulk modulus of the \ensuremath{\gamma} phase, compared to the \ensuremath{\alpha} phase, stabilizes the \ensuremath{\gamma} phase at high temperatures.