Toward a mechanism of rattler coupling in the β-pyrochlores AOs2O6 (A = K, Rb, Cs)

We have applied ab initio molecular dynamics simulations to study metal-metal coupling on the alkali-metal sublattice in the beta-pyrochlore osmates, AOs(2)O(6) (A = K, Rb, Cs) at 300 K. We find that the dynamics of the alkali-metal atoms (rattlers) exhibit stronger rattler-rattler correlations than rattler-cage correlations, and that, at 300 K, this correlation is strongest for Cs. We show that the rattler-rattler correlations control the dominant dynamics in the rattling of these atoms. We provide preliminary evidence that the rattler correlated motion occurs primarily through two somewhat distinct vibrational modes: a high-energy mode (peak A) couples the rattlers to each other and a low-energy mode (peak B) couples the rattlers to the cage modes. Rattler-rattler correlated motion through the high-energy mode provides insight into the trend in spectral broadening from Cs to K. The spectral broadening is inversely proportional to the strength of the dynamical correlations on the alkali-metal sublattice which in turn depend on the atomic size of the rattler, decreasing from Cs to K. Thus, the broadest spectrum exhibited by the K is partly a consequence of the small size of this rattler which permits a greater range of motions involving combinations of both correlated and anti-correlated dynamics. We emphasize that the identification of the somewhat distinct roles of the high-energy (peak A) and low-energy (peak B) modes in rattler coupling reported in this work is a significant step toward a complete fundamental mechanism of rattler dynamical coupling in these osmates. We believe that such a mechanism will have profound implications for a broad class of cage compounds, including clathrates and skutterudites.