Structural mechanisms and order-parameter symmetries for the high-pressure phase transitions in alkali metals

Abstract

The structural transitions occurring between the high-pressure phases of alkali metals are described by displacive mechanisms. The different structures are associated with critical instabilities of the ambient-pressure body-centered-cubic structure. Unifying mechanisms are found for the transitions occurring in (Li and Na) and (K, Rb and Cs), respectively. The RbIII-RbIV-RbV and CsIII-CsIV phase sequences are interpreted as reflecting the hybridization process of the $s$- and $d$-wave functions, which induces a deformation of the atomic shells, allowing a closer packing of the KIII, RbIV, and RbV structures. The RbIII and CsIII structures are proposed to represent commensurate approximations of incommensurately modulated structures locking in at the RbV and CsIV phases.