Remarkable high thermoelectric efficiency of the half-Heusler compounds BXGa (X = Be, Mg, and Ca).

The thermoelectric materials with a high value of the dimensionless figure of merit (ZT) are one of the most important new energy resources. Too much attention has been paid to the search of high ZT thermoelectric materials and the one with ZT = 5 is reported recently. Here a remarkable high ZT = 7.38 is predicted for the n-type half-Heusler compound of BCaGa at 700 K. To understand the high ZT behavior, we perform electronic properties of BXGa (X = Be, Mg, and Ca) with first-principles calculations based on density-functional theory. The stabilities of the structure of BXGa (X=Be, Mg, and Ca) are confirmed by phonon dispersion. The transport properties are determined by the semi-classical Boltzmann transport theory. We evaluate the relaxation time by using the deformation potential theory and the lattice thermal conductivity based on the elastic coefficients. The results demonstrate that such a high efficiency of BCaGa arises from the intrinsic coordination of the ultralow lattice and electronic thermal conductivity and the larger power factor at certain carrier concentration and temperature. The high n-type power factor originates from the large relaxation time which is resulted in a light, twofold degenerate conduction-band pocket at the Γ point. In contrast, the power factors of BBeGa and BMgGa are smaller because of their flat-and-dispersive valence band. It is expected the remarkable results for BXGa could encourage more experimental and theoretical investigations to develop efficient thermoelectric materials with BXGa.