Contribution to the study of some physical properties of the ternary phosphides LiAeP (Ae=Sr, Ba) and Zintl-phases Ae3AlAs3 (Ae = Sr, Ba)

Abstract

In the present work, we report the results of an ab initio study of some physical properties of two ternary phosphides: LiAeP (Ae = Sr, Ba), and two newly synthesized Zintl-phases: Ae3AlAs3 (Ae = Sr, Ba). The structural, elastic electronic, optical and thermodynamic properties of the LiBaP and LiSrP compounds were performed using the pseudopotential plane-wave method within the framework of density functional theory with the GGAPBEsol. Calculated equilibrium lattice constants are in good agreement with the available experimental data. Calculated cohesive energy confirms the thermodynamically stability of the considered materials. Single-crystal elastic constants Cij and related properties were predicted using the static finite strain technique. Polycrystalline elastic moduli and related properties were estimated from the Cij via the Voigt-Reuss-Hill approximations. Energy band dispersions along some high symmetry directions in the k-space and the density of states diagrams were computed and analysed. Obtained energy band structures show that both considered crystals are indirect band gap compounds. The band gap is approximately equal to 1.16 eV for LiSrP and 0.80 eV for LiBaP. The chemical bonding character was discussed through the electron density map plots. In order to understand the optical properties of LiSrP and LiBaP, the complex dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, electron energy-loss function and complex conductivity function were predicted for an incident radiation with an energy range up to 15 eV. The origin of the main peaks in the optical spectra was discussed in terms of the calculated electronic structure. We have predicted temperature and pressure dependence of the relative unit-cell volume, volume thermal expansion coefficient, heat capacities, Debye temperature and Grüneisen parameter via the quasi-harmonic Debye model. The structural, elastic, electronic, optical and thermoelectric properties of the Zintl-phases Ae3AlAs3 (Ae = Sr, Ba) were investigated using two complementary approaches based on density functional theory. The pseudopotential plane-wave method was used to explore the structural and elastic properties whereas the full-potential linearized augmented plane wave approach was used to study the structural, electronic, optical and thermoelectric properties. Calculated structural parameters are in good consistency with the corresponding measured ones. Single-crystal and polycrystalline elastic constants and related properties were examined in details. Electronic properties, including energy band dispersions, density of states and charge-carrier effective masses, were computed using Tran-Blaha modified Becke-Johnson functional for the exchange-correlation potential. It is found that Sr3AlAs3 has a direct band gap semiconductor while Ba3AlAs3 has an indirect band gap semiconductor. Frequencydependence of the linear optical functions were predicted for a wide photon energy range up to 15 eV. Charge carrier concentration and temperature dependences of the basic parameters of the thermoelectric properties were explored using the semi-classical Boltzmann transport model. Our calculations unveil that the studied compounds are characterised by a high thermopower for both carriers, especially the p-type conduction is more favourable.