V. P. Petrov

Lattice dynamics of rare-earth titanates with the structure of pyrochlore R2Ti2O7 (R = Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu): Ab initio calculation

The ab initio calculation has been performed for the crystal structure and the phonon spectrum of titanates with the structure of pyrochlore R2Ti2O7 (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 Gd2Ti2O7 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 structural, dynamic, and elastic properties of the R2Ti2O7 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.

Structure and lattice dynamics of rare-earth ferroborate crystals: Ab initio calculation

The ab initio calculation of the crystal structure and the phonon spectrum of crystals RFe3(BO3)4 (R = Pr, Nd, Sm) has been performed in the framework of the density functional theory. The ion coordinates in the unit cell, the lattice parameters, the frequencies and the types of fundamental vibrations, and also the intensities of lines in the Raman spectrum and infrared reflection spectra have been found. The elastic constants of the crystals have been calculated. For low-frequency A2 mode in PrFe3(BO3)4, a “seed” vibration frequency that strongly interacts with the electronic excitation on a praseodymium ion was found. The calculation results satisfactory agree with the experimental data.

Lattice dynamics of Cs2NaYbF6 and Cs2NaYF6 elpasolites: Ab initio calculation

The ab initio calculations of the crystal structure and the phonon spectrum of Cs2NaYbF6 and Cs2NaYF6 crystals with the elpasolite structure have been performed. The frequencies and types of fundamental vibrations have been determined. The calculations have been performed in the framework of the density functional theory using the molecular orbital method with hybrid functionals in the CRYSTAL09 program developed for the simulation of periodic structures. The outer 5s and 5p shells of the rare-earth ion have been described in Gaussian-type basis sets. The influence of inner shells, including 4f electron shells, on the outer shells has been described using the pseudopotential. It has been shown that this approach allows the description of the phonon spectrum with the inclusion of the splitting of the longitudinal and transverse optical modes.

Phonon spectra of YTiO3 and Y2Ti2O7: Ab initio calculations

Using the Hartree-Fock and density-functional methods, we have calculated the structure and the lattice dynamics of YTiO3. Frequencies of Raman and IR phonons of this crystal have been determined. The crystal structure and phonon spectrum of Y2Ti2O7 have been calculated.

Structure and lattice dynamics of PrFe3(BO3)4: Ab initio calculation

The crystal structure and phonon spectrum of PrFe3(BO3)4 are ab initio calculated in the context of the density functional theory. The ion coordinates in the unit cell of a crystal and the lattice parameters are evaluated from the calculations. The types and frequencies of the fundamental vibrations, as well as the line intensities of the IR spectrum, are determined. The elastic constants of the crystal are calculated. A “seed” frequency of the vibration strongly interacting with the electron excitation on the praseodymium ion is obtained for low-frequency A2 mode. The calculated results are in agreement with the known experimental data.

Effect of pressure on the lattice structure and dynamics of elpasolites Cs2NaRF6 (R = Y, Yb): ab initio calculation

The effect of hydrostatic compression on the lattice structure and dynamics of elpasolites Cs2NaYbF6 and Cs2NaYF6 (sp. gr. 225) has been investigated ab initio. The frequencies and types of fundamental oscillations are determined, and elastic constants are calculated. The computation is performed within the molecular orbitals-linear combinations of atomic orbitals (MO LCAO) approach using the density functional theory (DFT) method with hybrid functionals B3LYP and PBE0 in the CRYSTAL09 program. The rare-earth ion was described by representing the inner (in particular, 4f) orbitals in the form of a pseudopotential. The outer 5s and 5p orbitals, which determine chemical bonding, were described using valence basis sets.