High-pressure phases of the light alkali metals

Abstract

Quantum-theoretical calculations are used to study high-pressure phases of sodium up to 500 GPa. Sixteen structures, mainly selected on the basis of experience obtained from experimental and theoretical high-pressure studies of other alkali metals, Li, Rb and Cs, are examined with respect to stability and pressure-induced structural transformations. Up to ≈80 GPa the bcc structure is favored, whereas Na-fcc exists between ≈80 and ≈130 GPa, where it becomes dynamically unstable and undergoes a rhombohedral distortion to hR1. At this pressure a multitude of lower coordinated structures have very similar enthalpies. The cubic cI16 structure (as found for Li) may exist between 130 and 170 GPa. Among the structures examined, the calculations show that the Na phase with the lowest enthalpy has the CsIV structure between 170 and 220 GPa. Beyond 220 GPa a structure, oC8, with Cmca symmetry may be stable over a wide pressure regime. Whereas the s→d transition determines the structural changes of Cs, the behavior of the light alkali metals under pressure is associated with an s→p transition. Although the emphasis is on sodium, new results for lithium are also presented, illustrating similarities and differences between the high-pressure behaviors of these two light alkali metals. At very high pressures Na may become an insulator, whereas so far we did not find any possible high-pressure Li-phases with a non-zero band gap.