Frustration in Coupled Rattler System: KOs2O6

Abstract

The phenomenon of frustration, which gives rise to many fascinating phenomena, is conventionally associated with the topology of non-bipartite lattices, where nearest-neighbor (nn) interactions and global connectivity compete in the lowering of energy. The issue of rattling atoms in spacious lattice sites is a separate occurrence that can also lead to a high density of low energy states (unusual low temperature thermodynamics) and to practical applications such as in improved thermoelectric materials. In this letter we address a unique situation where both phenomena arise: a four-fold single-site instability leads to rattling of cations on a diamond structure sublattice where nn interactions frustrate simple ordering of the displacements. The system deals with this coupling of rattling+frustration by commensurate ordering. Such a disorder-order transition may account for the second phase transition seen in KOs2O6 within the superconducting state, and the unusual low-energy dynamics and associated electron-phonon coupling can account for the qualitative differences in physical properties of KOs2O6 compared to RbOs2O6 and CsOs2O6, all of which have essentially identical average crystal and electronic structures.