First principles study of the structural stability, lattice dynamics, optical and thermoelectric properties of NaSrX(X = As, Sb and Bi)

First principles methods are utilized in exploring the structural stability, electronic, optical, phonon, thermal properties and electronic transport coefficients of NaSrX materials. Based on the calculated cohesive and formation energies, NaSrX compounds are found to be energetically stable. NaSrX materials are also shown to be direct band gap semiconductors with strong exciton binding energies between ~0.13 and 0.20 eV. The dynamical and mechanical stabilities of NaSrX materials are further confirmed from their phonon dispersion curves and elastic constants. The calculated lattice thermal conductivities, icL, for NaSrX compounds exhibit anisotropic behaviour with the lowest value of icL found in the a-direction. Moreover, NaSrBi is shown to possess the lowest average value of icL (1.29 Wm-1K-1) as compared to NaSrAs and NaSrSb at 300 K. p-doped NaSrX compounds are shown to record large values of thermoelectric figure of merits. For instance, the highest calculated figure of merit (ZT) for p-doped NaSrAs is estimated to be 1.66 at 1200 K when p = 1020 cm-3 whereas those of NaSrSb and NaSrBi are 2.78 (at 1000 K) and 3.16 (at 900 K) respectively when p = 1019 cm-3. The large ZT values obtained in these calculations indicate that NaSrX compounds are promising candidates for thermoelectric applications.