A Possible Phase Transition in beta-pyrochlore Compounds

TL;DR

This paper models beta-pyrochlore compounds as anharmonic oscillators, revealing a first-order phase transition and potential local electric polarization, explaining experimental observations in KOs_2O_6.

Contribution

It introduces a five-state hopping model with intersite repulsions to explain phase transitions in beta-pyrochlore compounds.

Findings

First order phase transition from large to small oscillation amplitudes.

Possible emergence of local electric polarizations.

Model explains the transition observed in KOs_2O_6.

Abstract

We investigate a lattice of interacting anharmonic oscillators by using a mean field theory and exact diagonalization. We construct an effective five-state hopping model with intersite repulsions as a model for beta-pyrochlore AOs_2O_6(A=K, Rb or Cs). We obtain the first order phase transition line from large to small oscillation amplitude phases as temperature decreases. We also discuss the possibility of a phase with local electric polarizations. Our theory can explain the origin of the mysterious first order transition in KOs_2O_6.