Elucidating the Role of Hierarchical Bonding, Electronegativity, and Chemical Pressure on the Thermoelectric Properties of Double Half Heuslers

Double half-Heusler (DHH) alloys (XY0.5Y '(0.5)Z) stabilized by mixing two unstable HHs (XYZ and XY ' Z) have been the subject of extensive research as an alternative to HHs for high-temperature thermoelectric applications because of the former's low lattice thermal conductivity. In this work, using a combination of density functional theory (DFT)-based calculations and semiclassical Boltzmann transport theory, we elucidate the role of hierarchical bonding, reduction of electronegativity of X, and chemical pressure induced by variation in its atomic size on the electronic properties, transport, and thermoelectric properties, of a family of DHH compounds, namely, XFe0.5Ni0.5Sb (where X = Ti, Zr, and Hf). Compared to the parent compounds, we observe a larger variation in the nature of the bonds in the DHH lattice that aids in the reduction of their lattice thermal conductivity. Our calculations show that electronegativity in the X element and chemical pressure influence the band convergence observed in the conduction band of these materials in a reverse way. While reduction of electronegativity favors band convergence, tensile strain induced in the lattice due to the larger size of X is detrimental for the same. However, electronegativity has a much stronger effect. We observe that HfFe0.5Ni0.5Sb, which shows the largest band convergence, has the highest value of zT for n-type charge carriers among the three materials considered in our work. Moreover, hole-doped (p-type) HfFe0.5Ni0.5Sb also exhibits zT > 1. Therefore, we envisage that HfFe0.5Ni0.5Sb can be a good candidate for both the n and p legs of a thermoelectric device.