First-principles studies on the structural, electronic and thermal transport characteristics of half-Heusler compounds LiXN (X=Mg, Zn)

The electronic structures and thermal transport properties of the half-Heusler (HH) compounds LiMgN and LiZnN are calculated in detail by combining first-principles calculations with Boltzmann transport theory. Our phonon calculations showed that LiMgN and LiZnN are dynamic stability. The calculated bulk moduli and equilibrium lattice constants are in good agreement with the experimental and available theoretical data. The electronic band structures obtained by PBE (HSE06) method indicate that both LiMgN and LiZnN are direct bandgap semi-conductors with band gaps of 2.33 eV (3.23 eV) and 0.53 eV (1.42 eV), respectively. It is shown that the Li-N, Li-Mg and Li-Zn are ionic bond via ELF analysis. At room temperature, the calculated lattice thermal conduc-tivities of LiMgN and LiZnN compounds are 19.31 Wm- 1K-1 and 18.74 Wm- 1K-1, respectively; But with the external temperature increases to 1100 K, their lattice thermal conductivities are decrease rapidly to 4.96 Wm- 1K-1 and 4.32 Wm- 1K-1, respectively. Moreover, we analyzed the mean free paths, phase volume spaces (P3), Gruneisen parameters (gamma), scattering rates and group velocities in detail in order to further understand the origins of their relatively low lattice thermal conductivities and corresponding microscopic mechanism. We also investigated thermoelectric performances of LiMgN and LiZnN under different temperatures, and our results showed that with the external temperature increasing from 300 K to 1100 K, the maximum ZT values can in-crease from 0.05 (0.03) and 1.83 (1.32) for LiMgN (LiZnN), indicating that they are potential high-temperature thermoelectric materials. Our work can offer some theoretical references for improving thermoelectric perfor-mance and for future experimental studies.