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Title: Novel K rattling: A new route to thermoelectric materials?

We have performed ab initio molecular dynamics simulations to study the alkali-metal dynamics in the Al-doped (KAl{sub 0.33}W{sub 1.67}O{sub 6} and RbAl{sub 0.33}W{sub 1.67}O{sub 6}) and undoped (KW{sub 2}O{sub 6} and RbW{sub 2}O{sub 6}) defect pyrochlore tungstates. The K atoms exhibit novel rattling dynamics in both the doped and undoped tungstates while the Rb atoms do not. The KAl{sub 0.33}W{sub 1.67}O{sub 6} 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 reductionmore » 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.« less
Authors:
; ; ;  [1] ;  [2]
  1. Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales 2232 (Australia)
  2. Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581 (Japan)
Publication Date:
OSTI Identifier:
22275825
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 115; Journal Issue: 3; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 36 MATERIALS SCIENCE; ACTIVATION ENERGY; ALUMINIUM COMPOUNDS; COMPUTERIZED SIMULATION; DIFFUSION; DOPED MATERIALS; MOLECULAR DYNAMICS METHOD; PHONONS; POTASSIUM COMPOUNDS; POTENTIALS; RUBIDIUM COMPOUNDS; TEMPERATURE DEPENDENCE; THERMAL CONDUCTIVITY; THERMOELECTRIC MATERIALS; TUNGSTATES