Electronic, optical and transport properties of α-, β- and γ-phases of spinel indium sulphide: An ab initio study

Spinel indium sulphide exists in three phases. The tetragonal β-phase transforms to the cubic α-phase at 420 °C which further transforms to the trigonal γ-phase at 754 °C. Due to wide energy bandgap, the phases of indium sulphide have possibilities of applications in photo-electrochemical solar cell devices as a replacement of toxic CdS. The electronic, optical and transport properties of the three phases have therefore been investigated using full potential linear augmented plane wave (FP-LAPW) + local orbitals (lo) scheme, in the framework of density functional theory (DFT) with generalized gradient approximation (GGA) for the purpose of exchange-correlation energy functional. We present the structure, energy bands and density of states (DOS) for α-, β- and γ-phases. The partial density of states (PDOS) of β-In2S3 is in good agreement with experiment and earlier ab initio calculations. To obtain the fundamental characteristics of these phases we have analysed their linear optical properties such as the dynamic dielectric function in the energy range of 0–15 eV. From the dynamic dielectric function it is seen that there is no directional anisotropy for α-phase since the longitudinal and transverse components are almost identical, however the β and γ-phases show birefringence. The optical absorption profiles clearly indicate that β-phase has possibility of greater multiple direct and indirect interband transitions in the visible regions compared to the other phases. To study the existence of interesting thermoelectric properties, transport properties like electrical and thermal conductivities, Seebeck and Hall coefficients etc. are also calculated. Good agreements are found with the available experimental results.