d-Orbital-Driven Low Lattice Thermal Conductivity in TiRhBi: A Root for Potential Thermoelectric and Microelectronic Performance

Half-Heusler (HH) compounds are high-temperature thermo-electric materials with a high power factor upon appropriate doping. However, the efficiency and ZT values are still low due to their high lattice thermal conductivity, kappa l. It is essential to understand the thermal transport properties to design a potential thermoelectric material such as HH and a microelectronic device in general. At high temperatures, the kappa l is dominated by intrinsic scattering rates which arise purely from the anharmonic potential of the system. We study theoretically HH compounds, TiRhBi and TiCoBi, with the density functional theory and Boltzmann transport theory for kappa l calculation. We find that TiRhBi has a much lower kappa l (2.6 W/mK) than TiCoBi (6.4 W/mK) at 1000 K due to the weaker bond formation capability of diffused Rh 4d-electrons compared to the corresponding narrow band of 3d-electrons of Co in TiCoBi. The diffused Rh 4d-electrons near the valence band maximum participate in the nonbonding and antibonding types of overlap. This leads to an extremely weaker bond strength of TiRhBi, as evident from the COHP analysis. The weaker bond strength corresponds to an anharmonic potential which results in anharmonic effects leading to a lower kappa l. We discuss in detail several intermediate quantities such as COOP/COHP, heat capacity, phonon entropy, group velocity, Gru''neisen parameter, and anharmonic scattering rates to explain the kappa l magnitudes.