Lattice dynamics of the tin sulphides SnS₂, SnS and Sn₂S₃: Vibrational spectra and thermal transport

We present an in-depth first-principles study of the lattice dynamics of the tin sulphides SnS₂, Pnma and π-cubic SnS and Sn₂S₃. An analysis of the harmonic phonon dispersion and vibrational density of states reveals phonon bandgaps between low- and high-frequency modes consisting of Sn and S motion, respectively, and evidences a bond-strength hierarchy in the low-dimensional SnS₂, Pnma SnS and Sn₂S₃ crystals. We model and perform a complete characterisation of the infrared and Raman spectra, including temperature-dependent anharmonic linewidths calculated using many-body perturbation theory. We illustrate how vibrational spectroscopy could be used to identify and characterise phase impurities in tin sulphide samples. The spectral linewidths are used to model the thermal transport, and the calculations indicate that the low-dimensional Sn₂S₃ has a very low lattice thermal conductivity, potentially giving it superior performance to SnS as a candidate thermoelectric material.