Abstract
We investigate the temperature response of the alkali-metal rattling modes in beta-pyrochlores, AOs(2)O(6) (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20 K, 100 K and 300 K. Our results show that the temperature response of the T-1u mode is clearly different from that of the T-2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T-1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T-2g mode, and (2) the T-1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T-1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T-2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potentials reveals that, for K, the potential at a higher temperature is not a simple extrapolation to higher energy of that at a lower temperature. Instead, we find significant reconstruction of the potential at different temperatures. Finally, we explore the temperature response of the coupling between the alkali metals and find a complex temperature dependence which suggests that the origin of the coupling may be more complex than a pure Coulomb interaction. We also find an unexpected increase in the static disorder of the system at low temperatures for the K and Rb pyrochlores.