Insights on the Synthesis, Crystal and Electronic Structures, and Optical and Thermoelectric Properties of Sr1-xSbxHfSe3 Orthorhombic Perovskite

Single-phase polycrystalline powders of Sr1-xSbxHfSe3 (x = 0, 0.005, 0.01), a new member of the chalcogenide perovskites, were synthesized using a combination of high temperature solid-state reaction and mechanical alloying approaches. Structural analysis using single-crystal as well as powder X-ray diffraction revealed that the synthesized materials are isostructural with SrZrSe3, crystallizing in the orthorhombic space group Pnma (#62) with lattice parameters a = 8.901(2) angstrom; b = 3.943 (1) angstrom; c = 14.480(3) angstrom; and Z = 4 for the x = 0 composition. Thermal conductivity data of SrHfSe3 revealed low values ranging from 0.9 to 1.3 W m(-1) K-1 from 300 to 700 K, which is further lowered to 0.77 W m(-1) K-1 by doping with 1 mol % Sb for Sr. Electronic property measurements indicate that the compound is quite insulating with an electrical conductivity of 2.9 S/cm at 873 K, which was improved to 6.7 S/cm by 0.5 mol % Sb doping. Thermopower data revealed that SrHfSe3 is a p-type semiconductor with thermopower values reaching a maximum of 287 mu V/K at 873 K for the 1.0 mol % Sb sample. The optical band gap of Sr1-xSbxHfSe3 samples, as determined by density functional theory calculations and the diffuse reflectance method, is similar to 1.00 eV and increases with Sb concentration to 1.15 eV. Careful analysis of the partial densities of states (PDOS) indicates that the band gap in SrHfSe3 is essentially determined by the Se-4p and Hf-Sd orbitals with little to no contribution from Sr atoms. Typically, band edges of p- and d-character are a good indication of potentially strong absorption coefficient due to the high density of states of the localized p and d orbitals. This points to potential application of SrHfSe3 as absorbing layer in photovoltaic devices.