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

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 (Pnma) at ambient conditions. However, the cubic rock-salt phase (Fm3̄m) 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 ≤ x ≤ 0.80) at ambient temperature and pressure. The orthorhombic polycrystalline phase is stable in (SnSe)1−x(AgBiSe2)x in the composition range of 0.00 ≤ x < 0.28, which corresponds to narrow band gap semiconductors, whereas the band gap closes upon increasing the concentration of AgBiSe2 (0.30 ≤ x < 0.70) leading to the cubic rock-salt structure. We confirmed the stabilization of the cubic structure at x = 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−x(AgBiSe2)x (x = 0.33) showed a semi-metallic electronic structure with overlapping conduction and valence bands. The cubic polycrystalline (SnSe)1−x(AgBiSe2)x (x = 0.30) sample showed n-type conduction at room temperature, while the orthorhombic (SnSe)1−x(AgBiSe2)x (0.00 ≤ x < 0.28) samples retained 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.

Powder X-ray diffraction. Room temperature powder X-ray diffraction for all the samples were recorded using a Cu Kα (λ = 1.5406 Å) radiation on a Bruker D8 Diffractometer. Rietveld refinement of the PXRD pattern was performed using FULLPROF software.
Field emission scanning electron microscopy (FESEM) in back-scattered electron (BSE) mode. FESEM-BSE images were taken using ZEISS Gemini SEM -Field Emission Scanning Electron Microscope.
Electrical transport. Electrical conductivity and Seebeck coefficients were measured simultaneously under helium atmosphere from room temperature to 850 K on a ULVAC-RIKO ZEM-3 instrument system. The SPS processed sample were cut and polished in a rectangular shape with the dimensions of ~ 2 × 2 × 8 mm 3 to carry out the measurements. Electrical and thermal transport were measured in same direction.
Hall measurement. For determining the carrier concentrations, Hall measurements were carried out on the same rectangular specimens used for electrical transport measurement in four-contact geometry up to a magnetic field of 0.57 T at room-temperature using custom-built equipment developed by Excel Instruments.

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Thermal transport. Temperature dependent thermal diffusivity (D) was evaluated using a laser flash diffusivity technique in a Netzsch LFA-457 instrument. In addition, temperature dependent heat capacity was also measured in the same instrument by using a standard pyroceram (Fig. S12). Next, the total thermal conductivity (κ) was derived using the formula, κ = D.Cp.ρ, where ρ is density of the sample and the experimentally determined density was found to be ~97% of the theoretical density. Further, the electrical thermal conductivity, κele were derived using Wiedemann-Franz Law, κele= LσT, where L denotes the Lorenz number which was estimated by fitting the temperature dependent Seebeck data 1-3 and provided in Fig.   S13.
Computational details. Our first-principles calculations within density functional theory (DFT) were performed with QUANTUM ESPRESSO Package (QE) and projector augmented wave (PAW) potentials. 4 Electronic exchange and correlation energy was treated within a generalized gradient approximated (GGA) 5 functional with Perdew, Burke, and Ernzerhof (PBE) parametrization. 6 Electronic wave functions and charge density were represented using plane wave basis sets truncated at cut-off energies of 45 Ry and 360 Ry respectively. The discontinuity in occupation numbers of electronic states was smeared with broadening temperature of kBT = 0.003 Ry in a Fermi-Dirac distribution function. We determined electronic structure of (SnSe)1-