First-principles studies on structural, elastic, electronic, optical, thermodynamic and thermoelectric properties of RbScC and CsScC half-Heusler alloys

The semi-classical Boltzmann transport theory under the constant relaxation time approximation, combined with density functional theory (DFT) calculations performed using the WIEN2k code are utilized to investigate the elastic, structural, optoelectronic, and thermoelectric properties of XScC (X = Rb, Cs) half-Heusler alloys. Our study reveals that RbScC and CsScC half-Heusler alloys are both thermodynamically stable, as shown by their negative formation energies—an encouraging sign for their possible experimental realization. To gain deeper insight into their mechanical behavior, we have explored how their elastic constants respond to applied pressure from 0 to 25 GPa. The results show smooth, positive trend, indicating strong interatomic forces and robust mechanical stability under compression. Both RbScC and CsScC half-Heusler alloys are classified as semiconductors. RbScC has a direct X → X bandgap and CsScC displays an indirect Γ → X bandgap. Moreover, these systems exhibit semiconducting properties marked by a flat band next to the Fermi energy (EF), rendering them viable candidates for thermoelectric applications. The RbScC and CsScC alloys have minimal reflectivity and absorptivity in the ultraviolet spectrum and significant absorption in IR. Investigations are achieved on the Seebeck coefficient, figure of merit (ZT), power factor, electrical conductivity, and electronic thermal conductivity in relation to the temperature and chemical potential. RbScC and CsScC have a high power factor for p-type doping. At ambient temperature, it is found that high Seebeck coefficients are 245.36 µV/K for RbScC and 244.06 µV/K for CsScC, with a figure of merit nearing unity. Our results show that these alloys have the potential to be thermoelectric materials. Furthermore, the thermodynamic parameters of XScC (X = Rb, Cs) half-Heusler alloys at various pressures, 0–25 GPa and temperatures 0–900 K are determined by utilizing the quasi-harmonic Debye model. Graphical abstract