The recurrence of dense face-centered cubic cesium

Abstract

At ambient conditions, alkali metal cesium (Cs) owns a body-centered cubic phase, and this phase will transform to a face-centered cubic (fcc) phase at a pressure of 2.3 GPa. Under stronger compression, Cs will transform to oC84, tI4, oC16, and double hexagonal close-packed (dhcp) phases in sequence. Here, using first-principles structure searching prediction and total-energy calculation, we report that the Cs will re-transform to the fcc phase as the post-dhcp phase above 180 GPa. The transition state calculations suggest that the phase transition takes place by overcoming an energy barrier (144 meV/atom at 200 GPa) and finishes within a volume collapse of 0.3%. The electronic states at Fermi level are derived mainly from d electrons and there is a large overlap between inner core electrons, making the high-pressure fcc Cs distinguished from the first one at low pressure. The same phase transition also occurs in potassium and rubidium but with higher pressures.

Liquid structure under extreme conditions: high-pressure x-ray diffraction studies

“…After the initial bcc-fcc transition, in a narrow region of stability below the melting-curve minimum in the range 4.2-4.3 GPa, solid Cs undergoes an isomorphic fcc-fcc transition into a complex incommensurate host-guest structure with 84 Cs atoms in the unit cell [388]. On further compression, Cs transforms to a bcc (Cs-IV) structure, with further transitions occuring at higher-p [389]. Similarly, solid Rb transforms to an orthorhombic stacking structure with 52 atoms in the unit cell at 13 GPa [390], followed by a transition to an incommensurate structure at 16 GPa [391].…”