S. Yu. Shashkin

Local structure of Gd3+ impurity center at cubic sites in fluorites

A computer controlled ENDOR spectrometer and special software are used to determine the coordinates of19F nuclei in four anionic spheres of cubic MeF2:Gd3+ (Me = Ca, Sr, Ba, Pb) centers. The computer simulation of Gd3+ cubic impurity center in crystals MeF2 (Me = Ca, Sr, Ba) has been also performed. The analysis of lattice relaxation near the impurity center carried out on the base of ENDOR experiments data has allowed us to obtain potential of interaction Gd3+-F−. For the first time not only radial displacements but also angular displacements of F− ions of the third sphere have been taken into account and it has been shown that it influences the determination of lattice relaxation around the impurity center essentially. The influence of hydrostatic pressure (up to 30 kbar) on the local structure of the impurity center has also been investigated using computer simulation. The comparison of the experimental and calculated displacements of distant ions gives a reliable test of the validity of theoretical lattice relaxation model and accuracy of calculations of impurity-directed shifts of ligands.

The influence of the Jahn–Teller effect on phonons in LaMnO3

Abstract A model of the low-temperature orthorombic phase (O′) of LaMnO 3 was constructed using pair potentials with explicitly allowing for the many-body Jahn–Teller contribution to the crystal energy. The analysis of the structure, elastic constants and phonon frequencies is based on the comparison of the structural phases that obtained with including and “switching-off” Jahn–Teller contribution to the energy and dynamical matrix of the crystal.

The influence of pressure on crystal and magnetic structures of K2CuF4

Abstract The effect of hydrostatic pressure on structural and magnetic characteristics of the two-dimensional ferromagnetic K 2 CuF 4 is studied using a pair interaction model for the crystal energy. Multi-body Jahn-Teller (JT) contributions to the energy are also taken into account. It is predicted that application of hydrostatic pressure induces a first order structural transition which gives rise to antiferromagnetic ordering of the copper spins within the c -plane.

Modeling of a structural phase transition in La2−xSrxCuO4

The structural, vibrational, and elastic properties of La2CuO4 are calculated using a model for calculating the energy of the crystal based on interionic potentials with the multiparticle Jahn-Teller contribution included explicitly. The microscopic reasons for the structural instability of the La2CuO4 lattice relative to rotations of the oxygen octahedra are investigated. A structural phase transition from the orthorhombic phase (space group D2h18) into the tetragonal phase (space group D4h17) under hydrostatic compression of an La2−xSrxCuO4 crystal is modeled. The (P,x) structural phase diagram for La2−xSrxCuO4 is constructed.

Local lattice distortions and ligand hyperfine interactions in Eu2+-and Gd3+-doped fluorites

A computer simulation of lattice distortions around an impurity ion Eu2+ in MeF2 fluorites (Me=Ca, Sr, Ba) is reported. ENDOR data on displacements of F− ions distant from an impurity center were used to determine the parameters of the Eu2+-F− short-range interaction potential. A theoretical study of the effect of hydrostatic pressure on the impurity-center local structure has been made. A comparison with experimental data permits a conclusion that the calculated ligand displacements are reliable. An experimental ENDOR investigation of the ligand hyperfine interaction (LHFI) in MeF2:Gd3+ crystals (Me=Ca, Sr, Pb, Ba) has been performed. The results obtained in the simulation are used to describe the LHFI of impurity ions with the nearest-neighbor fluorine environment. The contributions to LHFI associated with ligand polarization are shown to constitute 10–50% of the experimental LHFI constants. The inclusion of such contributions results in practically linear dependences of the remaining short-range part of the LHFI on distance.

Semiempirical model for calculating the lattice dynamics of the Jahn-Teller oxide La2CuO4

Calculations of the properties of NiO, CaO, SrO, and La2NiO4 crystals are used as a basis for determining empirical pairwise potentials which are employed in calculating the structure, lattice dynamics, and certain properties of La2CuO4. The model developed here differs from those proposed before. First, the many-particle Jahn-Teller contributions to the energy and dynamic matrix of the crystal are taken into account. Second, the analytic expressions for the pairwise potentials contain an additional short-range electrostatic screening term. The results are in satisfactory agreement with experimental data.

Elastic properties of La2CuO4 under pressure

Within the framework of the pair potential approximation and the shell model including the Jahn-Teller contribution to the crystal energy we have studied the elastic properties of La2 CuO4 . The strong anisotropic correlation between the lattice deformations exx and ezz and the internal displacements of the accompanying sublattices within the La2 O2 layers are shown to be responsible for the giant anisotropy of the elastic moduli in the xz -plane. We have predicted the softening of several elastic moduli in the orthorhombic phase region near the pressure-induced structural phase transition. But we have not observed the softening of c 11 , c 12 , and c 66 in the tetragonal phase region that was measured near the temperature-induced structural phase transition.