Ultra-low lattice thermal conductivity in 18 VEC quaternary Heusler alloys—Efficient thermoelectric materials

Quaternary Heusler alloys are known for their distinctive electrical and thermal properties, present fascinating opportunities for improving thermoelectric performance. By leveraging the unique characteristics of these alloys and employing advanced design strategies, we can drive significant improvements in waste heat treatment. In the present study, we have identified novel stable quaternary Heusler alloys guided by the 18-valence electron rule through ab initio calculations. The ground state properties of presented alloys have studied using the generalized gradient approximation, whereas the accurate bandgap values have been determined using mBJ and GGA + U. The Boltzmann transport equation is used to investigate the thermoelectric properties of the alloys. The alloys are stable at room temperature and even at a high temperature of 900 K. The investigated quaternary Heusler alloys, MgZrFeSn and MgHfFeSn have indirect bandgaps of 0.69 eV and 0.77 eV, respectively, under mBJ approximation. The presence of flatbands and degenerated valley bands near the edges of the conduction and valence bands significantly enhance both the Seebeck coefficient and electrical conductivity of the alloys. Strong interactions between acoustic and optical phonon modes result in low lattice thermal conductivity of 0.58 and 1.49 W/m K at 900 K. This leads to high figure of merit of 1.28 and 1.55 for MgZrFeSn and MgHfFeSn, respectively, indicating their potential for advanced thermoelectric materials.