Abstract
The lattice dynamics and high-temperature structural transition in SnS and SnSe are investigated via inelastic neutron scattering, high-resolution Raman spectroscopy and anharmonic first-principles simulations. We uncover a spectacular, extreme softening and reconstruction of an entire manifold of low-energy acoustic and optic branches across a structural transition, reflecting strong directionality in bonding strength and anharmonicity. Further, our results solve a prior controversy by revealing the soft-mode mechanism of the phase transition that impacts thermal transport and thermoelectric efficiency. Our simulations of anharmonic phonon renormalization go beyond low-order perturbation theory and capture these striking effects, showing that the large phonon shifts directly affect the thermal conductivity by altering both the phonon scattering phase space and the group velocities. These results provide a detailed microscopic understanding of phase stability and thermal transport in technologically important materials, providing further insights on ways to control phonon propagation in thermoelectrics, photovoltaics, and other materials requiring thermal management.
Highlights
The lattice dynamics and high-temperature structural transition in SnS and SnSe are investigated via inelastic neutron scattering, high-resolution Raman spectroscopy and anharmonic first-principles simulations
From our complementary heat capacity measurements (Supplementary Fig. 18), the phase transition in SnS occurs at 875 K which is consistent with our neutron data and prior results[24]
Spectacular softening is observed for extended regions of the TAc and TOc dispersions along [100] in Pnma below TC, and an extremely soft TAc branch persists in the Cmcm phase
Summary
The lattice dynamics and high-temperature structural transition in SnS and SnSe are investigated via inelastic neutron scattering, high-resolution Raman spectroscopy and anharmonic first-principles simulations. Our simulations of anharmonic phonon renormalization go beyond low-order perturbation theory and capture these striking effects, showing that the large phonon shifts directly affect the thermal conductivity by altering both the phonon scattering phase space and the group velocities. Based on the continuous evolution of lattice parameters and bond lengths measured with neutron diffraction, and the group–subgroup symmetry relation of Cmcm and Pnma, Chattopadhyay et al characterized the transition as second-order, and predicted a soft phonon mode distortion at the Y zone-boundary point of the Cmcm phase, re-emerging as an Ag zone-center optic phonon mode in the Pnma phase[10,11] This was never directly confirmed, as no phonon dispersions have been reported for SnS at all, while only limited low-resolution INS measurements were performed in SnSe for T reaching near TC Mapping phonon dispersions at T > 800 K in either SnS or SnSe has proved challenging owing to sample sublimation, but the current measurements circumvented this difficulty (see “Methods”) and achieved a comprehensive view of the lattice dynamics across the structural phase transition into the Cmcm phase
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