Ab Initio Calculations of the Vibrational Spectra of AgInSe2 and AgInTe2

The phonon spectra and densities of states of AgInSe2 and AgInTe2 semiconductor crystals with a chalcopyrite structure have been calculated from first principles by the linear response method. The frequencies calculated at the center of the Brillouin zone are in agreement with the experimental data obtained using IR and Raman spectroscopy. According to the atomic contributions to the vibrational modes, the spectra of the AgInSe2 and AgInTe2 crystals exhibit three groups of bands: the vibrations in the low- and medium-frequency ranges are mixed in character with approximately identical contributions of all sublattices, and the bands at higher frequencies are associated with the contributions of Ag, C-VI and In, C-VI (C-VI = Se, Te) atoms. The position of these bands allows us to make the inference that, in the crystals under investigation, the In. CVI bonding is stronger than the Ag-C-VI bonding.