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We investigate the temperature response of the alkali-metal rattling modes in{\beta}-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initiomolecular dynamics (MD) simulations performed at 20 K, 100 K, and 300 K. Ourresults show that the temperature response of the T1u mode is clearly differentfrom that of the T2g mode for all three pyrochlores. In this regard, twofeatures are of particular note for both K and Rb; (1) the T1u mode exhibits adistinctly stronger softening response with decreasing temperature compared tothe T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasingtemperature. These two findings suggest that the T1u mode is significantly moreanharmonic and sensitive to the cage dynamics than the T2g mode. Examination ofthe local potentials around the alkali-metal atoms reveals that K has theflattest and most anharmonic potential at all temperatures while Cs exhibitsthe narrowest potential. The temperature dependence of the local potentialsreveals that, for K, the potential at a higher temperature is not a simpleextrapolation to higher energy of that at a lower temperature. Instead, we findsignificant reconstruction of the potential at different temperatures. Finally,we explore the temperature response of the coupling between the alkali metalsand find a complex temperature dependence which suggests that the origin of thecoupling may be more complex than a pure Coulomb interaction. We also find anunexpected increase in the static disorder of the system at low temperaturesfor the K and Rb pyrochlores.

Temperature Dependence of Alkali-Metal Rattling Dynamics in the {\beta}-Pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD Simulation