Ab initio Insights into Novel FeZrCh (Ch = S, Se) Half Heusler Alloys as Promising Thermoelectric and Solar Absorber Materials

In this work, a comprehensive ab initio investigation of the structural stability, mechanical behaviour, optoelectronic response, and thermoelectric performance of FeZrCh (Ch = S, Se) half-Heusler (HH) alloys in cubic MgAgAs-type structure is carried out by the full-potential linearized augmented plane wave (FP-LAPW) method. The optimized lattice constants are found to be 5.69Å and 5.86Å for FeZrS and FeZrSe respectively. Both compounds are thermodynamically stable as confirmed by the negative formation energy. Mechanical analysis shows large values of Young's and bulk modulus, indicating strong structural stiffness and good resistance to thermal stress. Electronic structure analysis reveals that FeZrCh (Ch = S, Se) compounds exhibit semiconducting nature with indirect band gap of 0.890 eV and 0.630 eV for FeZrS and FeZrSe respectively. DOS results disclose strong Fe-d and chalcogen-p hybridization near the valence band, while Zr-d states dominate the conduction band, indicating favourable electronic transport characteristics. Optical analysis reveals pronounced absorption in the visible and near-ultraviolet regions, with absorption coefficients reaching the order of 10 4 cm -1 , indicating strong UV absorption capability in both compounds. The high static dielectric constants of 20.1 for FeZrS and 20.2 for FeZrSe, together with strong interband transition peaks around 2-3 eV and superior reflectivity in the higher-energy region, further highlight their suitability for optoelectronic and UV-absorbing applications. Thermoelectric analysis shows positive Seebeck coefficients of ∼232 μV/K for FeZrS and ∼248 μV/K for FeZrSe at room temperature, indicating dominant p-type carrier transport while the figure of merit (ZT) increases with temperature and reaches maximum values of 0.71 for FeZrS and 0.60 for FeZrSe at 1200 K, highlighting FeZrCh (Ch = S, Se) half-Heusler alloys as promising candidates for high-temperature thermoelectric and optoelectronic applications.