  We have performed ab initio molecular dynamics (MD) simulations to study the
alkali-metal dynamics in the Al-doped (KAl0.33W1.67O6 and RbAl0.33W1.67O6) and
undoped (KW2O6 and RbW2O6) defect pyrochlore tungstates. The K atoms exhibit
novel rattling dynamics in both the doped and undoped tungstates while the Rb
atoms do not. The KAl0.33W1.67O6 experimental thermal conductivity curve shows
an unusual depression between ~ 50 K and ~ 250 K, coinciding with two
crossovers in the K dynamics: the first at ~ 50 K, from oscillatory to
diffusive, and the second at ~ 250 K, from diffusive back to oscillatory. We
found that the low-temperature crossover is a result of the system
transitioning below the activation energy of the diffusive dynamics whereas the
high-temperature crossover is driven by a complex reconstruction of the local
potential around the K atoms due to the cage dynamics. This leads to a
hardening of the K potential with increasing temperature. This unusual
reconstruction of the potential may have important implications for the
interpretation of finite-temperature dynamics based on zero-temperature
potentials in similar materials. The key result is that the novel K rattling,
involving local diffusion, leads to a significant reduction in the thermal
conductivity. We suggest that this may open a new route in the phonon
engineering of cage compounds for thermoelectric materials where the rattlers
are specifically selected to reduce the lattice thermal conductivity by the
mechanism of local diffusion.
