V. A. Chernyshev

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.

Phonon spectrum of lead oxychloride Pb3O2Cl2: Ab initio calculation and experiment

IR and Raman spectra of Pb3O2Cl2 in the range of 50–600 cm–1 have been detected for the first time. Ab initio calculations of the crystal structure and the phonon spectrum of Pb3O2Cl2 in the framework of LCAO approach have been performed by the Hartree–Fock method and in the framework of the density functional theory with the use of hybrid functionals. The results of calculations have made it possible to interpret the experimental vibration spectra and reveal silent modes, which do not manifest themselves in these spectra but influence the optical properties of the crystal.

Structure, lattice dynamics, and exchange interaction in Lu2V2O7, Y2V2O7: an ab initio approach

Ab initio calculations of the crystal structure and fundamental vibrations of vanadium pyrochlores Lu2V2O7 and Y2V2O7 are performed. The calculations are performed in the framework of the density functional theory (DFT) with the use of hybrid functionals. The ions involved in the vibrations are determined by the isotope substitution method. Values of the isotropic exchange interaction constant were calculated. Theoretical results for the crystal structure parameters, the vibrational frequencies, and the isotropic exchange interaction parameter are compared with the experimental data.

Vibrational spectroscopy of GdCr3(BO3)4: quantitative separation of crystalline phases

This work is devoted to the investigation of GdCr3(BO3)4 crystals by the method of infrared spectroscopy. Incongruently melting borate GdCr3(BO3)4 was obtained as a result of spontaneous crystallization. Crystal structures were identified by the method of infrared spectroscopy. Ab initio calculations in the frame of density functional theory enabled us to separate modes belonging to the R32 and C2/c phases and to estimate the ratio of these phases in GdCr3(BO3)4 crystals. We have found that the content of the rhombohedral R32 (non- centrosymmetric) modification is about 85%.

Ab initio calculation of the structure and optical properties of lead oxyhalides Pb 3 O 2 X 2 ( X = Cl, Br, I)

Using ab initio methods, we obtained information on the crystal structure, calculated the phonon spectra, and determined the optical properties of oxyhalides Pb3O2Br2 and Pb3O2I2. A compound of Pb3O2Br2 is synthesized, and its phonon spectra are recorded. The experimentally observed vibrational bands are assigned. A comparative analysis of the crystal lattice parameters, the phonon spectra, and the anisotropy parameters of lead oxyhalides Pb3O2X2 (X = Cl, Br, I) is performed.

Relaxation of the environment of Gd3+ and Eu2+ impurity ions in the Y3Al5O12 garnet

The second-rank spin Hamiltonian parameters of Gd3+ and Eu2+ orthorhombic centers in crystals of the yttrium aluminum garnet Y3Al5O12 have been analyzed within the framework of the superposition model for the zero-field splitting of the ground state. It has been shown that the description of the experimental data in this model is possible only under the assumption of relaxation of the ligand environment of the paramagnetic impurity.

Exchange interaction in pyrochlore vanadates Lu2V2O7 and Y2V2O7: Ab initio approach

The exchange interaction in vanadates with the pyrochlore structure, namely, Lu2V2O7 and Y2V2O7, has been investigated using the first-principles approach. The isotropic exchange coupling constants have been determined. The calculations have been performed within the unrestricted Hartree–Fock (UHF) approximation, as well as in the framework of the density functional theory (DFT), using hybrid functionals. It has been shown that, in the description of the exchange interaction in the compounds under investigation, the nonlocal Hartree–Fock exchange should be taken into account. The splitting patterns of the 3d1 level of the V4+ ion in the crystal field have been obtained within the model approach. The calculation has been carried out in the approximation of point charges taking into account the spin–orbit interaction. It has been revealed that the “orbital liquid” state cannot be observed in the compounds under investigation, because the exchange interaction energy is significantly less than the energy spacing between the ground state and the first excited state. The orbital ordering has been analyzed, and the spin density maps have been constructed.