Frustration in Coupled Rattler System: KOs2O6

TL;DR

This paper explores how frustration and rattling atoms in KOs2O6 lead to unique disorder-order transitions and influence its physical properties, distinguishing it from similar compounds.

Contribution

It introduces a combined frustration and rattling model explaining the second phase transition and low-energy dynamics in KOs2O6.

Findings

Frustration and rattling coexist in KOs2O6 leading to a disorder-order transition.

The transition explains the second phase transition within the superconducting state.

Differences in physical properties between KOs2O6 and similar compounds are linked to this mechanism.

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.