Modulation of the electronic structure and thermoelectric properties of orthorhombic and cubic SnSe by AgBiSe₂ alloying

Abstract

Recently, single-crystals of tin selenide (SnSe) have drawn immense attention in the field of thermoelectrics due to their anisotropic layered crystal structure and ultra-low lattice thermal conductivity. Layered SnSe has an orthorhombic crystal structure (<i>Pnma</i>) at ambient conditions. However, the cubic rock-salt phase (<i>Fm</i>3̄<i>m</i>) of SnSe can only be stabilized at very high pressure and thus, the experimental realization of the cubic phase remains elusive. Herein, we have successfully stabilized the high-pressure cubic rock-salt phase of SnSe by alloying with AgBiSe₂ (0.30 ≤ <i>x</i> ≤ 0.80) at ambient temperature and pressure. The orthorhombic polycrystalline phase is stable in (SnSe)_1-<i>x</i> (AgBiSe₂) _<i>x</i> in the composition range of 0.00 ≤ <i>x</i> < 0.28, which corresponds to narrow band gap semiconductors, whereas the band gap closes upon increasing the concentration of AgBiSe₂ (0.30 ≤ <i>x</i> < 0.70) leading to the cubic rock-salt structure. We confirmed the stabilization of the cubic structure at <i>x</i> = 0.30 and associated changes in the electronic structure using first-principles theoretical calculations. The pristine cubic SnSe exhibited the topological crystalline insulator (TCI) quantum phase, but the cubic (SnSe)_1-<i>x</i> (AgBiSe₂) _<i>x</i> (<i>x</i> = 0.33) showed a semi-metallic electronic structure with overlapping conduction and valence bands. The cubic polycrystalline (SnSe)_1-<i>x</i> (AgBiSe₂) _<i>x</i> (<i>x</i> = 0.30) sample showed n-type conduction at room temperature, while the orthorhombic (SnSe)_1-<i>x</i> (AgBiSe₂) _<i>x</i> (0.00 ≤ <i>x</i> < 0.28) samples retained p-type character. Thus, by optimizing the electronic structure and the thermoelectric properties of polycrystalline SnSe, a high <i>zT</i> of 1.3 at 823 K has been achieved in (SnSe)_0.78(AgBiSe₂)_0.22.