Functional Materials for Energy

Origin of anomalous atomic vibrations in efficient thermoelectrics revealed

Figure 1: Comparison of S(Q,E) measured with INS (left) and calculated (right) with density functional theory (DFT). SnTe results are the top and PbTe results at the bottom. White arrows point out anomalous double-peak in PbTe, which is absent in SnTe.

Thermoelectric SnTe and PbTe compounds were investigated with inelastic neutron scattering (INS) and first-principles calculations to understand the basis of their anharmonic lattice dynamics. The phonon anharmonicity of these materials is of both fundamental importance and of practical interest.  Specifically, this knowledge can be used to improve the energy efficiency of thermoelectric materials that convert heat to electrical energy because anharmonicity suppresses lattice thermal conductivity.

Our experiments showed that, although SnTe is closer to a ferroelectric lattice instability, which can promote anharmonicity, the phonon spectra in PbTe exhibits an even more anharmonic character. Greater anharmonicity is revealed by splitting of the transverse-optic phonon branch at the zone center, in the case of PbTe (in Figure 1 bottom left see the split intensity at the bottom of the upper “V” curve pointed out by a white arrow). This unusual behavior was clearly reproduced in our first-principles calculations (Figure 1 bottom right). Most importantly, our simulations also revealed the details of how the nesting of phonon dispersions leads to the observed effect. This discovery provides a fundamentally new understanding of atomic vibrations in important energy materials, and provides a guideline for designing materials with tailored atomic dynamics.

C. W. Li, O. Hellman, J. Ma, A. F. May, H. B. Cao, X. Chen, A. D. Christianson, G. Ehlers, D. J. Singh, B. C. Sales, and O. Delaire, “Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics,” Phys. Rev. Lett. 112, 175501 (2014).    DOI: 10.1103/PhysRevLett.112.175501  

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