DFT calculations with six LDA, GGA, and hybrid functionals have been performed using the CRYS-TAL09 code to describe the crystal structure and vibrational spectra of Y 2 CaGe 4 O 12 cyclotetragermanate, a new optical host. Two space groups P4/nbm and Cmme 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 (n = 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 [Ge 4 O 12 ] unit, and external modes is briefly discussed.

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The ab initio calculation has been performed for the crystal structure and the phonon spectrum of titanates with the structure of pyrochlore R 2 Ti 2 O 7 (R = Gd-Lu). The frequencies and types of fundamental vibrations have been found. For R = Tb, Tm, and Yb, this calculation has been carried out for the first time; furthermore, there is no available information on experimental studies of the phonon spectrum for Tm and Yb. The influence of hydrostatic pressure to 35 GPa on the structure, dynamics, and elastic properties of the Gd 2 Ti 2 O 7 lattice has been investigated. The dependence of the phonon frequencies on the pressure has been obtained. The calculations have predicted that the relative change in the pyrochlore structure volume during compression at pressures to 35 GPa is well described by the third order Birch-Murnaghan equation of states. The results of the calculations agree with the available experimental data. It has been shown that the struc tural, dynamic, and elastic properties of the R 2 Ti 2 O 7 crystal lattice can be adequately described in the case where the inner shells of the RE ion up to 4f are replaced by the pseudopotential.