Elastic properties and lattice dynamics of alkali chalcogenide compounds Na2S,Na2Se and Na2Te

We report on first principles study of the elastic, vibrational and dielectric properties of the alkali chalcogenide compounds Na₂S, Na₂Se and Na₂Te using the pseudopotential method within the local density approximation and linear response theory. The calculated lattice constants for the studied compounds are in good agreement with available experimental data as well as with other theoretical results. The phonon dispersion curves and phonon density of states are calculated by using density functional perturbation theory. For Na₂S the experimental features of lattice dynamics data are well reproduced by our calculations. The width of the acoustic frequency range decreases with increasing chalcogen atomic number. The phonon densities of states show that Na⁺ ions are involved in the lower frequency modes in Na₂S. However, the inverse occurs in the case of Na₂Se and Na₂Te. The mean-square ionic displacements show that Na⁺ ions perform large thermal vibrations even at temperatures well below the melting point. The Born effective charge increases, while the IR oscillator strength decreases with anion atomic number.