Ultralow and Anisotropic Thermal Conductivity in Crystalline As₂Se₃

Among renewable sources of energy, thermoelectricity has been in recent years the subject of intensive research. Thermoelectric applications depend fundamentally on materials. Therefore, research on thermoelectricity has been, to a large extent, the search for or development of materials for that purpose. In this work we studied the lattice thermal conductivity of crystalline As₂Se₃, through state-of-the-art calculations that combine density functional theory and semi-classical Boltzmann transport equation. This chalcogenide compound is a semiconductor that crystallizes in a layered monoclinic structure. The calculations show an ultralow lattice thermal conductivity of 0.14 W m⁻¹ K⁻¹ along the b axis at 300 K. The origin of this ultralow thermal conductivity arises from a cascade-like fall of the low-lying optical modes and from the avoided crossing of these with acoustic modes. These results, whose validity has been addressed by comparison with the compound SnSe, for which excellent agreement between the theoretical predictions and the experiments is achieved, point out that As₂Se₃ could exhibit improved thermoelectric properties.