A. D. Gorlov

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 of paramagnetic defects in cadmium fluoride doped with yttrium and gadolinium

Trigonal and monoclinic Gd3+ centers with close initial splittings that have no analogs in crystals without yttrium are detected in the electron spin resonance spectrum of Y0.03Cd0.97F2.03: Gd3+ single crystals. The centers observed are attributed to gadolinium ions localized in tetrahedral yttrium and yttrium-cadmium clusters. The values of the parameters of the second-rank spin Hamiltonian are explained (in order of magnitude) in terms of a superposition model that uses the tetrahedral-cluster structure obtained by minimizing the energy of a lattice with a rare-earth cluster.

Paramagnetic resonance and off-center location of Cu2+ ions in ferroelectric lead germanate

This paper reports on the results of investigations into the orientation behavior of the splitting of the electron paramagnetic resonance spectrum due to the location of paramagnetic ions in different domains of Cu2+-containing lead germanate. The off-diagonal components of the tensor g for Cu2+ quasi-monoclinic centers are determined. A model for off-central displacement of copper ions is proposed with due regard for the revealed symmetry of paramagnetic centers, their superhyperfine structure, and the orientation of the magnetic axes.

Local structure of Tm2+ and Yb3+ impurity centers in MeF2 (Me = Ca, Sr, Ba) fluoride crystals

The local structure of Tm2+ and Yb3+ cubic impurity centers in MeF2: Tm2+ and MeF2: Yb3+ (Me = Ca, Sr, Ba) fluoride crystals, as well as Yb3+ trigonal and tetragonal impurity centers in MeF2: Yb 3+ crystals, is calculated within the shell model in the pair potential approximation.

Magnetic and electrical properties of intercalated phases in the (Cr, Cu)-HfSe2 system

It has been shown that intercalation of the HfSe2 compound with chromium atoms results in an increase in the magnetic susceptibility and a decrease in the resistivity while retaining the semiconductor conductivity type. The CrxHfSe2 compounds exhibit a paramagnetic behavior at temperatures above 2 K in the entire concentration range 0 ≤ x ≤ 0.25. It has been revealed that an increase in the electron density due to additional introduction of copper can cause the appearance of a cluster-spin-glass-type state in CrxCuyHfSe2 compounds. The data obtained indicate a large role of the indirect exchange interaction via conduction electrons in the formation of the magnetic state in layered intercalated compounds based on transition metal dichalcogenides.

Ligand hyperfine interaction in Gd3+ tetragonal centers in CaF2 and SrF2 and the structure of the impurity nearest surroundings

ENDOR experimental spectra of Gd3+ tetragonal impurity centers in CaF2 and SrF2 crystals were used to determine the superhyperfine interaction (SHFI) constants of the impurity with 19F nuclear spins of its first coordination sphere and the compensator ion. The distances in the Cd3+F9 complex were estimated within the model of isotropic SHFI constants suggested in [1]. An analysis of the data on the SHFI and spin-Hamiltonian constants [2] in terms of the superposition model indicates significant changes in the contributions (due to the Gd3+ mixed states) to these parameters for the tetragonal centers in comparison with the corresponding contributions for the cubic and trigonal centers in the same crystals.

Hyperfine and quadrupole interactions of trigonal 157Gd3+ centers in SrF2 and BaF2. Analysis of distortions in the nearest atomic environment

Trigonal 157Gd3+ impurity centers in SrF2 and BaF2 were experimentally studied by EPR and double electron-nuclear resonance (DENR) techniques. Parameters of the hyperfine and quadrupole interactions between these centers were determined. Possible distortions of the nearest atomic environment of the impurity centers are estimated within the framework of a superposition model using the EPR and DENR data for the centers of cubic and trigonal symmetry in the crystals studied.

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.

Paramagnetic Resonance of Gd[sup 3+] Ions in Nonstoichiometric Fluorite R[sub x]M[sub 1 – x]F[sub 2 + x] (R = Y, Gd; M = Ca, Cd)

EPR studies of Y0.03Ca0.97F2.03: Gd3+ and Y0.03Cd0.97F2.03: Gd3+ single crystals revealed the presence of Gd3+ ions embedded in yttrium clusters. The symmetry of the paramagnetic centers was determined, and the fine-structure parameters were estimated both experimentally and theoretically.

Ligand electron-nuclear double resonance of T1 Trigonal Gd3+ centers in CaF2 with a mixed oxygen-fluorine environment

This paper reports on the results of ligand electron-nuclear double resonance (ENDOR) investigations of T1 trigonal 157Gd3+ centers in the CaF2 compound. It is experimentally found that the nearest environment of an impurity center contains only one 19F ion. Anions in the other coordination shells are identical to those in the pure CaF2 crystal. However, 19F ions in these shells are displaced from their ideal positions in the lattice. The parameters of the ligand hyperfine interaction (LHFI) for 19F nuclei and their coordinates and displacements with respect to the positions in the lattice of the pure CaF2 crystal are determined. It is demonstrated that the unusual isotropic LHFI constant As>0 for Gd3+ ions in the lattice with a mixed oxygen-fluorine nearest environment can be associated with the strong polarization of impurity centers in accordance with the empirical model proposed in [1], provided the structural model of the nearest environment of impurities in the T1 centers [2] is correct. This structural model is confirmed by the analysis of the isotropic hyperfine constant A(s) for 157Gd3+ centers.