The design of better thermoelectric materials would enable the reduction of energy consumption in cars by converting waste heat in exhausts into useful electrical power, as well as cooling hot spots on computer chips using solid state refrigerators. However, the need for both high electrical conductivity and low thermal conductivity creates a design conflict. Here we show how rattling modes suppress the thermal conductivity in the electrically conducting thermoelectric oxide, sodium cobaltate.

The so-called square superstructure of Na0.8CoO2 shown in Figure 35 comprises arrays of tri-vacancy clusters [1]. We have performed first-principles density-functional calculations for the lattice dynamics of this large-period superstructure on the UK’s national supercomputer facility HECToR using the CASTEP code, and the results are presented as a colour contour diagram in Figure 36. The calculated spectrum differs markedly from that of the stoichiometric compound at energies below E ~ 20 meV. In particular, there is an additional narrow mode at E ~ 13 meV in the superstructure that is completely absent for NaCoO2.

The supercell of Na0.8CoO2 showing the Einstein-like rattling mode at E ~ 13 meV

Fig. 35: The supercell of Na0.8CoO2 showing the Einstein-like rattling mode at E ~ 13 meV comprising mainly displacements of the (red) Na 2b ions inside tri-vacancy clusters. Na 2d ions are blue, Co is green and O is gold.

Inelastic X-ray scattering (IXS) measurements were performed on a single crystal of Na0.8CoO2 of a few hundred micrometres in size with the square superstructure using the ID28 spectrometer. Scans of variable energy transfer were performed at a number of settings of m thend > 1Qa aloas c00p-symed mng mampus-diTEP code, fit calpeakectsit2d i2 shown in Figure 36ctional calcuP code, ter">Ask ific Dl dataen aS) iff of >2.

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Fig. 35:0.8CoObsp;meV th the square supers. Fformed first-principles density-functional calculations th thempriss arrays of tri-vacancy cm> i2 i2E ~is gold.