Half-Heusler (HH) compounds are high-temperature thermoelectric 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, κl. 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 κl 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 κl calculation. We find that TiRhBi has a much lower κl (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 κl. We discuss in detail several intermediate quantities such as COOP/COHP, heat capacity, phonon entropy, group velocity, Grüneisen parameter, and anharmonic scattering rates to explain the κl magnitudes.

We study the thermoelectric properties of monolayer indium triphosphide (InP3) under uniaxial compressive and tensile strains using density functional theory in conjunction with Boltzmann transport formalism. InP3 is a recently predicted two-dimensional (2D) material with a host of interesting multi-functional properties. Though InP3 is a low lattice thermal conductivity material, its thermoelectric figure of merit, ZT is found to be low. We thoroughly examined how its thermoelectric transport properties evolve under external strain. We find that the tensile (t) and compressive (c) strains have contrasting effects on the transport coefficients, both leading to the same effect of enhancing the ZT value strongly. While t-strain enhances the power factor dramatically, c-strain gives rise to an ultra-low lattice thermal conductivity. Both these effects lead to an enhancement of ZT value at high temperatures by an order of magnitude compared to the corresponding value for free InP3. The maximum ZT value of InP3 at 800 K is found to be ∼0.4 under t-strain and ∼0.32 under c-strain, values which are comparable to those observed for some of the leading 2D thermoelectric materials. Another finding relevant to optoelectronic properties is that under c-strain the material shows a transition from an indirect to a direct band gap semiconductor with an accompanying increase in the valley degeneracy. The structural, electronic, and thermal properties of the material are thoroughly analyzed and discussed.

The lattice heat transport properties of the thermoelectric (TE) material SnTe and the doped Sn7SbTe8 and Sn7BiTe8 are examined using Boltzmann transport theory supplemented with first-principle calculations. We illustrate the microscopic origin of the lattice thermal conductivity, κ l of the materials by calculating the mode Grüneisen parameters, phase space volume for three-phonon processes, the anharmonic scattering rates (SR), and the phonon group velocities. SnTe is found to be a low κ l material with a value of ∼3 W mK−1 at room temperature in agreement with experiments. The phonon scatterings in pristine SnTe mainly originates in the strong anharmonicity of the material, as evidenced by the large values of its mode Grüneisen parameters. Doping with Sb or Bi reduces the anharmonic strength. For Sb doped Sn7SbTe8, it results in a drop in the SR and hence a higher κ l value. However in the Bi doped Sn7BiTe8, the number of allowed three-phonon processes gets greatly enhanced which compensates for the reduction in anharmonicity. This coupled with lower phonon group velocities lowers the κ l value for the Bi doped system below that of pristine SnTe. In nanowire structures, κ l values for the doped systems get drastically reduced yielding an ultra-low value of 0.84 W mK−1 at 705 K for the Bi doped material for a nanowire of 10 nm diameter.

The electronic structure and thermoelectric properties of ZrRuTe-based half-Heusler compounds are studied using density functional theory and Boltzmann transport formalism. Based on rigorous computations of electron relaxation time τ considering electron-phonon interactions and lattice thermal conductivity κ l considering phonon-phonon interactions, we find ZrRuTe to be an intrinsically good thermoelectric material. It has a high power factor of ∼ 2 × 10 −3 W m −1 K −2 and low κ l ∼ 10 W m −1 K −1 at 800 K. The thermoelectric figure of merit ZT ∼ 0.13 at 800 K is higher than similar other compounds. We have also studied the properties of the material as a function of doping and find the thermoelectric properties to be substantially enhanced for p-doped ZrRuTe with the ZT value raised to ∼ 0.2 at this temperature. The electronic, thermodynamic, and transport properties of the material are thoroughly studied and discussed.

The electronic structure and thermoelectric properties including lattice thermal conductivity of ZrRuTe, p-type ZrTcxRu1−xTe and n-type ZrRu1−xRhxTe (x=0.125) are studied using density functional theory (DFT) and Boltzmann transport formalism. The electron relaxation time for the undoped compound is estimated rigorously from electron-phonon interactions computed using Wannier wavefunctions. We find the undoped compound to have a high power factor of 1.12 × 10 −3 Wm −1 K −2 s −1 and a low lattice thermal conductivity of ∼ 10 Wm −1 K −1 at 800 K which are comparable or even better than some of the known good thermoelectric materials. Our calculations show ZrRuTe to be a promising thermoelectric material with a high ZT value of 0.08 at 800 K for the undoped compound. The thermodynamic, electronic, and transport properties of the material are thoroughly studied and discussed.