Structural and Vibrational Properties of the Ordered Y₂CaGe₄O₁₂ Germanate: A Periodic Ab Initio Study

DFT calculations with six LDA, GGA, and hybrid functionals have been performed using the CRYSTAL09 code to describe the crystal structure and vibrational spectra of Y₂CaGe₄O₁₂ cyclotetragermanate, a new optical host. Two space groups <i>P</i>4/<i>nbm</i> and <i>Cmme</i> have been considered. The former corresponds to a mixed (0.5 Ca + 0.5 Y) distribution at the octahedral sites found from the results of Rietveld refinement of room temperature powder XRD pattern; the latter refers to the model of crystallographically nonequivalent calcium and yttrium atomic setting in distorted oxygen octahedrons. The most accurate geometry description has been obtained with the WC1LYP and PBE (<i>n</i> = 6) hybrid functionals, while the B3LYP calculation provides the best agreement between the recorded infrared and Raman spectra and their computed counterparts. Assignments of most of the observed bands to vibrational modes are given. The comparison between calculated and experimental frequencies shows a general good agreement for the spectra below 600 cm^–1. The relationship between selected infrared bands and Raman lines, internal vibrations of the [Ge₄O₁₂] unit, and external modes is briefly discussed