Ab initio study of MnScNbX (X = Al, Ga) quaternary Heusler alloys as promising thermoelectric materials

Quaternary Heusler alloys are emerging as promising candidates for thermoelectric applications due to their favorable electrical and thermal transport properties. In this study, we investigate the thermal and electronic transport properties of MnScNbX (X = Al, Ga) quaternary Heusler alloys, using density functional theory in combination with semi-classical Boltzmann transport theory under the constant relaxation-time approximation. The calculated mechanical properties and phonon dispersion curves confirm the structural stability of both alloys. They exhibit non-magnetic semiconducting behavior and follow the Z−18 Slater–Pauling rule. A low lattice thermal conductivity, κL∼4.0Wm−1K−1 at 300 K, is obtained, primarily due to reduced phonon group velocities, shorter phonon lifetimes, inhomogeneity of chemical bonding among the constituent atoms, and high Grüneisen parameter. Furthermore, the calculated thermoelectric figure of merit attains values up to ∼1.6 at T=650K for p-type and at T = 900 K for n-type doping, arising from the combined effect of low κL and favorable electronic transport properties. These results suggest that p-type MnScNbX (X = Al, Ga) alloys are suitable for mid-temperature, while n-type alloys are promising for high-temperature thermoelectric applications.