Lattice dynamical study of alkaline atom vibrations with anharmonic interactions in β-pyrochlore oxides

Vibrations of alkaline guest atom A in β -pyrochlore oxides AOs2O6 (A = K, Rb, and Cs) are analyzed by a simple interaction model, where interatomic force constants between A atoms k ( AA ) and between A atom and cage k ( AC ) are employed. The obtained coefficients show that A atoms vibrate in highly anharmonic potential with strong interactions between A atoms. Especially for K atoms in KOs2O6, its potential can be approximately described only by fourth-order term.