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An ultralow lattice thermal conductivity of 0.14 W$\cdot$ m$^{-1} \cdot$K$^{-1}$ along the $\vec b$ axis of As$_2$Se$_3$ single crystals was obtainedat 300 K by first-principles calculations involving the density functionaltheory and the resolution of the Boltzmann transport equation. This ultralowlattice thermal conductivity arises from the combination of two mechanisms: 1)a cascade-like fall of the low-lying optical modes, which results in avoidedcrossings of these with the acoustic modes, low sound velocities and increasedscattering rates of the acoustic phonons; and 2) the repulsion between thelone-pair electrons of the As cations and the valence $p$ orbitals of the Seanions, which leads to an increase in the anharmonicity of the bonds. Thephysical origins of these mechanisms lie on the nature of the chemical bondingin the material and its strong anisotropy. These results, whose validity hasbeen addressed by comparison with SnSe, for which excellent agreement betweenthe theoretical predictions and the experiments is achieved, point out thatAs$_2$Se$_3$ could exhibit improved thermoelectric properties.

Ultralow and Anisotropic Thermal Conductivity in Semiconductor As2Se3