Proximity to structural phase transition is one of the key ingredients in suppressing the thermal conductivity of known good thermoelectric materials like PbTe. Our goal is to engineer the properties of PbTe and related materials to tune their proximity to phase transitions, thereby substantially suppressing the thermal conductivity and further improving their thermoelectric performance.

The major obstacle in the design of materials with low lattice thermal conductivity is the difficulty in efficiently scattering phonons across the entire frequency spectrum. We show that driving PbTe materials to the brink of the soft optical mode phase transition via strain or alloying is a powerful strategy to solve this problem. We predict that PbTe, Pb(Se,Te) and (Pb,Ge)Te alloys driven near the phase transition will have considerably lower lattice thermal conductivity than that of PbTe (by a factor of 2 − 3).

Reduction of phonon lifetimes across the entire spectrum by driving PbTe near the phase transition via tensile (001) strain.

R. M. Murphy, E. D. Murray, S. Fahy, and I. Savić, "Broadband Phonon Scattering in PbTe-based Materials Driven Near the Ferroelectric Phase Transition by Strain or Alloying", Physical Review B 93, 104304 (2016)

We show how tuning the proximity to the soft optical mode phase transition via chemical composition affects the lattice thermal conductivity of Pb1-xGexTe alloys. Using first-principles virtual-crystal simulations, we find that the anharmonic contribution to thermal conductivity is minimized at the phase transition due to the maximized acoustic-optical anharmonic interaction. Our results suggest that tuning soft optical modes in Pb1-xGexTe and similar alloys may be a promising strategy to enhance their thermoelectric figure of merit.

Total lattice thermal conductivity of Pb1-xGexTe alloys (blue circles) and its anharmonic contribution (black circles) as a function of the alloy composition.

R. M. Murphy, É. D. Murray, S. Fahy, and I. Savić, "Ferroelectric Phase Transition and the Lattice Thermal Conductivitiy of Pb1-xGexTe Alloys", Physical Review B 95, 144302 (2017).