Lattice dynamics and elastic properties of LiPN(2) and NaPN(2).

Density functional theory calculations using the ultrasoft pseudopotentials and local density approximation have been carried out to investigate the phonon spectra and elastic properties of LiPN(2) and NaPN(2) crystals with chalcopyrite structure. As follows from the results obtained, phonon spectra of LiPN(2) and NaPN(2) consist of three bands and have a high-frequency boundary at ∼1200 cm(-1). Phonon modes of the upper half of the spectrum involve mainly nitrogen atom vibrations, whereas low-frequency modes have a predominant contribution from the lithium or sodium atoms and the contribution of phosphorus is almost uniformly distributed over the entire range of allowed frequencies. The calculations performed show that the dynamical and elastic behaviour of the compounds under study is determined by the strong covalent bond between P and N atoms and the substantially weaker interaction of the alkali metal and N atoms.