Calculation of the vibrational properties of LiMgAs

Abstract

We have studied the vibrational properties of the filled tetrahedral semiconductor LiMgAs and its binary analog AlAs by using the plane-wave pseudopotential method within density functional theory. The calculated lattice constants for the studied compounds are in good agreement with previous theoretical and experimental results. The phonon dispersion curves and phonon density of states are calculated by using density functional perturbation theory. The sound speeds in different directions are quantitatively similar in LiMgAs and AlAs. The assignment of the zone center modes to the relative motion of the atoms shows that the lower optic modes are due to the Mg-As pair vibrations, while for the upper ones the Li-Mg pair dominates, which is attributed to the smaller Mg atom mass. The longitudinal interatomic force constant of Mg-As is about 66% higher than that of Li-As, showing the relatively high covalency of the former bond.

8 –NRule and Chemical Bonding in Main-Group MgAgAs-Type Compounds

“…Thereby, cation and polyanion realize a closed-shell electronic configuration. The view of the polyanionic network is considered to be consistent with the phonon spectra, the analysis of the density of states (DOS) and crystal orbital overlap populations (COHP), the distribution of valence charge density, and the analysis of the valence part of the electron localization function (valence ELF) . The model is formally applicable to all 8- and 18-electron MgAgAs-type compounds.…”