High-pressure structures of lithium, potassium, and rubidium predicted by anab initioevolutionary algorithm

Abstract

We have extensively explored the high-pressure structures of lithium (Li), potassium (K), and rubidium (Rb) using an ab initio evolutionary algorithm. For Li, an unexpected cubic $P{4}_{1}32$ structure containing sixfold coordinated lithium atoms and ${\text{Li}}_3$ equilateral triangles was discovered to be stable above 300 GPa. This structure is reported for the first time in the elements and shows charge accumulation in the voids of the structure, rather than within the ${\text{Li}}_3$ triangles. At pressures above the stability field of complex incommensurate phases, the heavier elements K and Rb were predicted to adopt the sequence of $I{4}_1/amd\to Cmca\to \text{double-hexagonal-close-packed}$. This sequence parallels the experimentally known structural sequence in Cs, which can be explained by the predominant $d$ character of the valence electrons in K, Rb, and Cs at high-pressures. The major $p$ character at the Fermi level in Li makes it distinct from K, Rb, and Cs.