Strong anharmonicity and high thermoelectric efficiency in high-temperature SnS from first principles

SnS and SnSe are isoelectronic materials with a common phase diagram. Recently, SnSe was found to be the most efficient intrinsic thermoelectric material in its high-temperature C m c m phase above 800 K. Making use of first-principles calculations, here we show that the electronic and vibrational properties of both materials are very similar in this phase and, consequently, SnS is also expected to have a high thermoelectric figure of merit at high temperature in its C m c m phase. In fact, the electronic power factor and lattice thermal conductivity are comparable for both materials, which ensures a similar figure of merit. As in the case of SnSe, the vibrational properties of SnS in the C m c m phase are far from trivial and are dominated by huge anharmonic effects. Its phonon spectra are strongly renormalized by anharmonicity and the spectral functions of some particular in-plane modes depict anomalous non-lorentzian profiles. Finally, we show that non-perturbative anharmonic effects in the third-order force-constants are crucial in the calculation of the lattice thermal conductivity. Our results motivate new experiments in the high temperature regime to measure the figure of merit of SnS.