Theoretical study of the pressure-concentration diagram for the Ce-Th alloy system

The high pressure and low temperature phase diagram of CeTh3, CeTh, and CeT h3 compounds has been investigated and compared to experimental data for th ree CexTh1 - x alloys. At higher pressures, the theoretical calculations co mpare very well with experimental observations whereas at lower pressures, the agreement is less accurate. The general pressure behavior of the CexTh1 - x is, however, in agreement between theory and experiment. Analysis of t he theoretical model reveals that the phase stability in these alloy system s is driven by electronic structure effects and in particular an increased f-electron character with increasing pressure. Density functional theory sh ows that the Ce-Th alloy systems will undergo crystallographic phase transi tions from fce to bet at elevated pressures. The transition pressures are s hown, in agreement with experiment, to increase with Th content in the Ce-T h alloy in a nonlinear fashion. At very high pressures, above 200 GPa, the CexTh1 - x alloys display a unified picture with a saturated c/a axial rati o close to 1.65. Both these features are shown to be related to the increas ed f-band character with pressure and the preference for distorted structur es which comes with increasing f-electron dominance. Specifically, our firs t-principle calculations show that the f-band population saturates to about 1.5 f electron at high pressures over 200 GPa explaining the saturation of the c/a axial ratio for the Ce-Th alloys. Simple model calculations, utili zing unhybridized and pure canonical f bands in conjunction with Madelung e nergy corrections, show that an f-electron metal with about 1.5 f electrons , stabilizes in the bct structure with an axial c/a ratio close to 1.65, in accordance with the first-principle calculations and available experimenta l data.