Effect of M elements (M  =  Ti, Zr, and Hf) on thermoelectric performance of the half-Heusler compounds MCoBi

Half-Heusler compounds are a promising kind of candidate for thermoelectric applications, due to the excellent transmission properties of electricity and heat. In this paper, we present a theoretical investigation on the geometrical structure, phonon dispersion, electronic properties, relaxation time, carrier mobility and thermoelectric properties of the half-Heusler compounds of MCoBi (M  =  Ti, Zr, and Hf) by using the first-principles density functional theory combined with the deformation potential theory, the semi-classical Boltzmann transport theory with relaxation time approximation, and Slack's model. The transport coefficients including thermal conductivity, Seebeck coefficient, electrical conductivity, and the dimensionless figure of merit (ZT) are obtained and used to evaluate the impact of the metal M on the thermoelectric properties of MCoBi. The results show that MCoBi is dynamically stable and the M elements obviously affect the lattice thermal conductivities (κ _l), electronic transport properties and ZTs of MCoBi. Moreover, the p -type half-Heusler compounds of MCoBi exhibit more excellent thermoelectric performance than those of the n-type ones, and the maximum ZTs of the p -type TiCoBi, ZrCoBi, and HfCoBi can reach 1.4, 1.7, and 1.5, respectively. These findings provide a valuable reference for developing effective thermoelectric materials with MCoBi compounds. (paper)