Extended anharmonic collapse of phonon dispersions in SnS and SnSe

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.

Direct visualization of polaron formation in the thermoelectric SnSe

“…Previous work has identified a number of soft and strongly coupled optical phonons in the zone-center region of the Pnma phase ( 15 , 17 , 20 , 38 ). As k approaches zone-center, the PDS contribution overlaps with the Bragg peak line shape.…”

“…Unlike most previous studies, which have attempted to understand the enhancement of thermoelectric properties in SnSe in terms of lattice anharmonicity and the phase transition ( 17 – 20 ), in this work, we focus on the momentum dependence of electron–phonon coupling in the Pnma phase specifically ( 21 ). We seek to develop a full understanding of the carrier–lattice interactions that may also contribute to thermoelectric performance.…”

Slowing down the heat in thermoelectrics

“…In fact, atomic bonding in most materials is anharmonic with various strengths of anharmonicity 48–55 . Strong anharmonicity causes intrinsic phonon scattering due to Umklapp processes (U‐processes) and determines the intrinsically low κ lat in materials with multidimensional crystal structures 28,29,34,51,56–62 …”

Phase Stability of the Tin Monochalcogenides SnS and SnSe: A Quasi-Harmonic Lattice-Dynamics Study

“…Previous theoretical studies and experimental INS measurements have shown that the Pnma ↔ Cmcm phase transition is driven by condensation of a "soft" TO phonon mode of Ag symmetry found at the Brillouin zone center in the Pnma phase and the Y wavevector in the Cmcm phase. 65 As noted previously, these soft modes manifest as negative eigenvalues in the dynamical matrix of the equilibrium structure, corresponding to imaginary harmonic frequencies, and indicate the structures to be local maxima on the structural PES. By "following" the eigenvectors corresponding to those modes, one can locate the nearest lower-energy stationary point, which may be a minimum or another maximum.…”