Energy harvesting potential of LiHfIrZ (Z = Si, Ge) quaternary Heusler alloys: a first-principles computational study

In the present study, optoelectronic, thermoelectric (TE), and elastic characteristics of LiHfIrZ Heusler alloys (HAs) (Z = Ge and Si) are studied through first-principles calculations and semi-classical Boltzmann transport theory. The structural properties were studied using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional, while electronic and transport properties were analysed using the Tran-Blaha modified Becke-Johnson potential. The calculated elastic constants, thermodynamic parameters, and formation energy confirm the mechanical, thermodynamic, and structural stability of the HAs, as well as their anisotropic character. Moreover, the alloys exhibit semiconducting behaviour. The calculated band gaps (E_g) of LiHfIrSi and LiHfIrGe are 1.72 and 0.84 eV, respectively. The semiconducting nature of both HAs has been established by their band structure (BS) and density of states (DOS) calculations. The optical analysis reveals a high dielectric response in the low-energy region (~ 2 eV), which decreases with increasing photon energy. Our HAs exhibit strong reflectivity in the ultraviolet (UV) region, indicating significant interaction with high-energy electromagnetic radiation, while appreciable optical conductivity is observed in the visible energy range. Moreover, both HAs displayed an increasing power factor, highlighting their potential for high-temperature TE applications. Notably, LiHfIrSi achieves a maximum ZT of 0.72 at 1200 K. Given the limited studies on these HAs, this study provides a valuable basis for future theoretical and experimental investigations.