Lattice dynamics and vibrational entropy in the PbSe/SnSe thermal switch

Solid-state thermal switches enable programmable heat-flow control, by coupling or isolating devices from their heat sinks on demand, improving efficiency, reliability for example for high-power electronics. IV–VI semiconductor alloys are promising platforms for solid-state thermal switches based on temperature-driven structural changes. In Pb_0.5Sn_0.5Se a ~3× variation in thermal conductivity was observed [Nishimura et al. Adv. Electron. Mater. 8 2200024 (2022).] when the material transitions from a high-temperature rock-salt structure to a low-temperature layered polymorph. Here we report on the investigation of the microscopic origin of the switching by combined inelastic neutron scattering and 119-tin Mössbauer spectroscopy. Analysis of the phonon density of states shows a pronounced vibrational entropy difference between phases. Spectral changes concentrate in the low-energy vibrations of the heavy-atom sublattice and there is a significant jump in the Sn/Pb atomic displacements upon switching. Overall, there is a 0.4 k_B/atom difference in vibrational entropy between phases, which indicates a significant role of vibrations in driving the switching behavior.