Specific features of electronic and vibrational properties of I-V-V2 crystals

The ab initio density functional method with using pseudopotentials has been applied to calculate the electronic and vibrational spectra of the LiN2 and NaPN2 crystals, with optimization of their structure. The specific features of the crystal structure (highly compressed chalcopyrite) and composition of these crystals manifest themselves in the qualitative similarity of their band spectra and chemical bonding to those of chalcopyrite as well as in some substantial distinctions. The second valence band exhibits four branches instead of two branches, as in chalcopyrite crystals, and involves contributions of the p-states of the P and N atoms rather than the s states of the Group-III cations in the I-III-VI2 chalcopyrite compounds. The top of the valence band is at the side points of the Brillouin zone, the bottom of the conduction band is in the center of the Brillouin zone, and the absorption edge corresponds to the indirect band gap. The contributions of alkali metals to the vibration spectra are significant in the region of low-frequency modes; phosphorus and nitrogen make significant contributions to the modes in the high-frequency region, which is due to the prominent covalent component of bonding between these atoms.