We provide direct evidence to understand the origin of low thermal conductivity of SnSe using elastic measurements. Compared to state-of-the-art lead chalcogenides $\mathrm{Pb}Q(Q=\mathrm{Te}$, Se, S), SnSe exhibits low values of sound velocity $(\sim 1420\mathrm{m}/\mathrm{s})$, Young's modulus $(E\sim 27.7\mathrm{GPa})$, and shear modulus $(G\sim 9.6\mathrm{GPa})$, which are ascribed to the extremely weak Sn-Se atomic interactions (or bonds between layers); meanwhile, the deduced average Grüneisen parameter γ of SnSe is as large as ∼3.13, originating from the strong anharmonicity of the bonding arrangement. The calculated phonon mean free path (l ∼ 0.84 nm) at 300 K is comparable to the lattice parameters of SnSe, indicating little room is left for further reduction of the thermal conductivity through introducing nanoscale microstructures and microscale grain boundaries. The low elastic properties indicate that the weak chemical bonding stiffness of SnSe generally causes phonon modes softening which eventually slows down phonon propagation. This work provides insightful data to understand the low lattice thermal conductivity of SnSe.

• Received 2 April 2016
• Revised 22 July 2016

DOI:https://doi.org/10.1103/PhysRevB.94.125203