Band Structures and Anomalous Softening of Graphite Intercalation Compounds C₈K_1−xRb_x

Abstract

The band structures of C₈Rb, C₈K_1−xRb_x and a new superlattice C₂₄KRb₂ are calculated for the first time using the self-consistent pseudopotential method and a mechanism of the anomalous softening observed in this mixed crystal at x=2⁄3 is investigated from the electronic standpoint. In the band structure calculation of the mixed crystals C₈K_1−xRb_x, the virtual crystal approximation is used. It is shown that x=2⁄3 is a magic number in a sense that a saddle point type van Hove singularity in the density of states of three-dimensional interlayer bands touches just at the Fermi level and that this van Hove singularity gives rise to softening in the elastic constant.