A Possible Isomorphic Transition in β-Pyrochlore Compounds

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 β-pyrochlore AOs₂O₆ (A=K, Rb or Cs). We find the line of the first order phase transition from large to small oscillation amplitude phases as temperature decreases. Our theory can explain the origin of the mysterious first order transition in KOs₂O₆. We also discuss the possibility of a phase with local electric polarizations.