Phase diagram of uranium at high pressures and temperatures

The phase diagram of uranium has been studied to 100 GPa by in situ diamond-anvil cell x-ray/laser-heating experiments. The {γ} (bcc) phase is discovered at high pressures, and the melting curve is presented to 100 GPa. The {γ} phase, B=113.3GPa, is approximately 20% softer than the {α}(orthorhombic), B=135.5GPa. The volume change in the α/γ 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 {γ} phase, compared to the {α} phase, stabilizes the {γ} phase at high temperatures.