PHASE-DIAGRAM OF URANIUM AT HIGH-PRESSURES AND TEMPERATURES

The phase diagram of uranium has been studied to 100 GPa by in situ di amond-anvil cell x-ray/laser-heating experiments. The gamma (bcc) phas e is discovered at high pressures, and the melting curve is presented to 100 GPa, The gamma phase, B=113.3 Cpa, is approximately 20% softer than the a(orthorhombic), B=135.5 GPa. The volume change in the alpha/ gamma transition shows a strong pressure dependence, ranging from 6% a t ambient pressure to less than 1% at 80 Gpa. Free-energy calculations , using Debye-Gruneisen quasiharmonic theory, show that the softer bul k modulus of the gamma phase, compared to the alpha phase, stabilizes the gamma phase at high temperatures. [S0163-1829(98)10817-2].