1, showing the low-temperature distortion of the Sn coordination polyhedron from displacements along. The Pnma and Cmcm structures are both orthorhombic and are illustrated in Fig. Both compounds crystallize in a Pnma phase (space group 62) at low temperature, and transform to a higher symmetry Cmcm structure (space group 63) when heated above T C = 807 K (SnSe) or 878–887 K (SnS) 10, 11, 13, 14. SnS and SnSe are chemically and structurally similar. Supplementing our INS data with high-resolution Raman spectroscopy and anharmonic first-principles simulations, we demonstrate that drastic, anisotropic, and mode-dependent anharmonic phonon renormalizations occur near a phase transition, and that these impact thermal conductivity, by altering both group velocities and the phase space for phonon-phonon interactions. We also succeed in mapping dispersions across the phase transition in SnSe, extending our previous results that were limited to Pnma 12. Further, we find that the transition is accompanied by a striking extensive anharmonic collapse of the transverse acoustic (TA) and transverse optical (TO) branches along extended portions of the dispersion, as opposed to a proposed condensation at a single high-symmetry wavevector Q = Y (refs. Using high-resolution inelastic neutron scattering (INS) on single-crystals, we chart the evolution of phonons in SnS from 150 K to 1050 K and clearly reveal the soft-mode nature of the Pnma–Cmcm phase transition. Critically, the lattice dynamics of both systems remain insufficiently understood, especially at high temperatures where their crystal structure undergoes a structural phase transition known to be associated with very strong anharmonicity but whose underlying mechanism remains controversial. The sister compound SnSe has been one of the most intensely studied thermoelectric materials in recent years and has set records for thermoelectric conversion efficiency, in part because of its ultralow thermal conductivity 4, 9. Its bulk electronic structure is also attracting strong interest for valleytronics applications 8. 5), and has recently demonstrated fast improvements in thermoelectric efficiency 6, 7. SnS is particularly suitable as an absorber in solar cells due to its high optical absorption coefficient and direct bandgap E g ≃ 1.3 eV (ref. It exhibits strong anharmonicity and a structural phase transition at high temperature, making it ideal to investigate the fundamental effects of anharmonicity 1, 2, 3, 4. SnS is a candidate for both solar cells and thermoelectric devices. In particular, fundamental studies of how anharmonicity in the interatomic potential energy surface impacts both phase stability and thermal transport are required. Improving renewable energy technologies such as photovoltaics and thermoelectrics requires a deeper understanding of the atomistic processes underlying their energy conversion and thermal behavior. 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. 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. Further, our results solve a prior controversy by revealing the soft-mode mechanism of the phase transition that impacts thermal transport and thermoelectric efficiency. 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. 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.
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