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We have applied ab initio molecular dynamics (MD) simulations to studymetal-metal coupling on the alkali-metal sublattice in the {\beta}-pyrochloreosmates, AOs2O6 (A = K, Rb, Cs) at 300 K. We find that the alkali-metal atoms(rattlers) couple to each other more strongly than they couple to the cageatoms, and that, at 300 K, this coupling is strongest for Cs. We show that thiscoupling controls the dominant dynamics in the rattling of these atoms. Weprovide preliminary evidence that the rattlers couple to each other primarilythrough the T2g mode whereas their coupling to the cage modes occurs throughthe T1u mode. Rattler coupling through the T2g mode provides insight into thetrend in spectral broadening from Cs to K. The spectral broadening is inverselyproportional to the strength of the dynamical correlations on the alkali-metalsublattice which in turn depend on the atomic size of the rattler, decreasingfrom Cs to K. Thus, the broadest spectrum exhibited by the K is partly aconsequence of the small size of this rattler which permits a greater range ofmotions involving combinations of both correlated and anti-correlated dynamics.We emphasize that the identification of the somewhat distinct roles of the T1uand T2g modes in rattler coupling reported in this work is a significant steptowards a complete fundamental mechanism of rattler dynamical coupling in theseosmates. We believe that such a mechanism will have profound implications for abroad class of cage compounds, including clathrates and skutterudites.

Towards a Mechanism of Rattler Coupling in the {\beta}-Pyrochlores AOs2O6 (A = K, Rb, Cs)