First-Principles study of the structural, electronic, elastic, lattice dynamics and thermoelectric properties of the newly predicted half-heusler alloys NaYZ (Z = Si, Ge, Sn)

In this study, we applied Density Functional Theory (DFT) and Boltzmann Transport Theory to investigate the structural, electronic, lattice dynamic, elastic and thermometric properties of new half-Heusler compounds NaYZ (Z = Si, Ge, Sn). The structural optimization calculations revealed that the ZXY (1 /4,1 /2, 0) phase have the most stable state with the least energy among others. Our calculated lattice constants in (A) are found to be 6.861, 6.899, and 7.284 for NaYZ (Z = Si, Ge, Sn). Also, the calculated elastic constants satisfied the Born-Huang criteria and were found to be mechanically and elastically stable. The Density Functional Perturbation Theory (DFPT) was used to examined the lattice dynamics of the compounds and the results shown non-existence of negative frequencies all through dispersion band structures, thus all the compounds are dynamically stable. The Seebeck coefficient of the compounds at room temperature for the p-type (n-type) are 143.79 (-48.91) mu V/K, 132.49 (-48.86) mu V/K, 154.75 (-60.24) mu V/k for NaYZ (Z = Si, Ge, Sn). The modified Slack's approach was used to compute lattice thermal conductivity (kp) within the experimental range and the kp values obtained for NaYZ (Z = Si, Ge, Sn) at room temperature are 17.265 Wm -1 K -1, 14.671 W m- 1K-1, 10.693 W m- 1K-1, respectively. The computations of charge carriers relaxation time (tau = F(T)) as a function of temperature through deformation potential theory and the effective mass of charge carriers were used to evaluate the figure of merit (zT) for p-type and n-type of the materials. The maximum zT at 1200 K of tau = F(T) (tau = 10-13 s) are 1.76 (1.67), 1.73 (1.65), and 1.91 (1.88). These results show that tau = F(T) has improvement in determining the figure of merit as compared to the constant relaxation time (tau).