Recently, single crystals of SnSe have drawn an immense attention in the field of thermoelectrics owing to its anisotropic layered crystal structure and ultra-low lattice thermal conductivity. Layered SnSe attains an orthorhombic crystal structure (Pnma) at ambient conditions. However, the cubic rock-salt phase (Fm-3m) of SnSe can only be stabilized at very high pressure and thus 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≤ x ≤0.80) at ambient temperature and pressure. Orthorhombic polycrystalline phase is stable in (SnSe)1-x(AgBiSe2)x for composition range of 0.00≤x&lt;0.28 and these are measured to be narrow band gap semiconductors, whereas the band gap closes upon increasing the concentration of AgBiSe2 (0.30≤x&lt;0.70) with the cubic rock-salt structure. We confirm stabilization of cubic structure at x = 0.30 and associated changes in electronic structure using first-principles theoretical calculations. Pristine cubic SnSe exhibits topological crystalline insulator (TCI) quantum phase, but the cubic (SnSe)1-x(AgBiSe2)x (x = 0.33) possesses semimetallic electronic structure with overlapping conduction and valence bands. Cubic polycrystalline (SnSe)1-x(AgBiSe2)x (x = 0.30) sample shows n-type conduction at room temperature while the orthorhombic (SnSe)1-x(AgBiSe2)x (0.00≤x&lt;0.28) samples retain its p-type character. Thus, by optimizing the electronic structure and the thermoelectric properties of polycrystalline SnSe, a high zT of 1.3 at 823 K has been achieved in (SnSe)0.78(AgBiSe2)0.22.

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

ACS Spring 2022

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