A. E. Nikiforov

Interplay between orbital, charge and magnetic orderings in R1−xAxMnO3 (x=0, 0.5)

Abstract This work is devoted to the description of the orbital and magnetic structures of pure and charge-ordered manganites, caused by crystal and charge structures and the Jahn–Teller effect. The model under consideration is based on the orbital dependency of the magnetic interactions in Jahn–Teller crystal. The orbital structure is considered dependent upon the Jahn–Teller distortions of the coordination of the Mn 3+ ions and upon the crystal field. It is shown that exchange interactions forming the magnetic structure of the charge-ordered manganite could be described in the superexchange approximation because of localization of the charge carriers. For pure manganite and for charge-ordered phase, the exchange parameters dependencies and the single-ion anisotropy dependency, upon the angle of orbital states mixing, are drawn. Therefore, it is shown that the orbital structure of the charge-ordered phase is formed mainly by cooperative Jahn–Teller distortions and, taking into account the charge superstructure, causes the non-trivial magnetic structure.

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.

Structure and the electronic and magnetic properties of LaTiO3

A model is proposed for the LaTiO3 compound, which allows for the strong coupling of the lattice, orbital, and magnetic subsystems of this crystal. The interaction of the lattice with the electronic structure of the LaTiO3 compound is described by the vibronic Hamiltonian with the constants calculated using the ab initio Hartree-Fock-Roothaan method with the inclusion of the electron correlations. It is demonstrated that a considerable contribution to the formation of the orbital structure of the LaTiO3 compound is made by the next-nearest neighbors of the Ti3+ ion. The orbital-dependent superexchange interactions in LaTiO3 are described within the Hubbard model and agree well with experiment.

Crystal Fields of Hexameric Rare-Earth Clusters in Fluorites

In solid solutions of alkaline-and rare-earth fluorides with a fluorite structure, ions of most elements of the rare-earth (RE) row form hexameric clusters that assimilate the minor component of the solid solutions (fluorine) and build it into the cubic fluorite lattice without changing its shape. An analysis of the EPR spectra of paramagnetic RE ions (Er3+, Tm3+, Yb3+) in clusters of diamagnetic ions (Lu3+, Y3+) confirms their hexagonal structure, which was established when studying the superstructures of the compounds under study. In such a cluster, a RE ion is in a nearly tetragonal crystal field, with the parameters of this field differing radically from those of single cubic and tetragonal RE centers in crystals with a fluorite structure. In particular, this field causes high (close to limiting) values of the g∥ factors of the ground states of the paramagnetic RE ions. Computer simulation is used to determine the atomic structure of a hexameric cluster in MF2 crystals (M = Ca, Sr, Ba). The crystal field and energy spectrum of Er3+, Tm3+, and Yb3+ ions in such clusters are calculated, and the spectroscopic parameters of the ground states of these ions are determined. The calculations confirm the earlier assumption that the unusual EPR spectra of nonstoichiometric fluorite phases are related to RE ions in hexameric clusters.

Effect of orbital ordering on the magnetic-structure formation in the LaMnO3 Jahn-Teller magnet

A study is reported on the relation between orbital ordering and the magnetic structure of an LaMnO3 crystal. The dependence of the exchange parameters on the orbital-structure angle Φ has been determined. Inclusion into the spin Hamiltonian of isotropic exchange interaction and single-ion anisotropy, which depends on the angle Φ and rotational distortions, results in a four-sublattice structure (AX, FY, GZ), with the sublattice magnetic moments oriented close to the long axis of the orthorhombic cell in the basal plane of the crystal . The effect of the rare-earth-ion size in RMnO3 manganites on the orbital and magnetic structures is considered.

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.

Theory of magnetic resonance as an orbital state probe

It is demonstrated that magnetic resonance in a magnetically ordered state is a sensitive indirect method for the investigation of the orbital ground state. This idea is illustrated for two perovskite titanates:

Antiferromagnetic resonance spectrum in LaMnO3: Interrelation of the orbital structure and the magnetic properties

{\text{LaTiO}}_{3}

The influence of pressure on crystal and magnetic structures of K2CuF4

and

The structure of mixed fluorides Ca1−xSrxF2 and Sr1−xBaxF2 and the luminescence of Eu2+ in these crystals

{\text{YTiO}}_{3}