M. M. Zaripov

Jahn-teller chromium ions in CdF2 and CaF2 crystals: An EPR spectroscopic study in the frequency range 9.3–300 GHz

The bivalent chromium impurity centers in CdF2 and CaF2 crystals are investigated using electron paramagnetic resonance (EPR) in the frequency range 9.3–300 GHz. It is found that Cr2+ ions in the lattices of these crystals occupy cation positions and form [CrF4F4]6− clusters whose magnetic properties at low temperatures are characterized by orthorhombic symmetry. The parameters of the electron Zeeman and ligand interactions of the Cr2+ ion with four fluorine ions in the nearest environment are determined. The initial splittings in the system of spin energy levels of the cluster are measured.

EPR study of impurity iron ion clusters in BaF2 crystals

Tetragonal paramagnetic centers with spin S = 7/2 were detected in x-ray-irradiated BaF2: Fe (cFe ≈ 0.002 at. %) crystals using the EPR method. Electronic transitions between the |±1/2〉 states of a Kramers doublet were observed in the X and Q ranges. In the EPR spectra of the tetragonal centers, a ligand hyperfine structure (LHFS) was observed corresponding to the interaction of the electron magnetic moment of the tetragonal center with eight equivalent ligands. The large spin moment, significant anisotropy of the magnetic properties, and the characteristic LHFS indicate that the tetragonal center is a Fe1.5+−Fe1.5+ dimer in which the two iron ions are bound via superexchange interaction. It is assumed that, before crystal irradiation, this dimer was in the Fe3+(3d5)−Fe+(3d7) state.

EPR of trivalent iron centers in a BaF2: Fe crystal

An orthorhombic paramagnetic Fe3+ ion center (concentration of iron ions is 0.1 at. %) was found using EPR in BaF2: Fe crystals irradiated by x rays. The EPR spectra recorded in the Q range at a temperature T = 77 K exhibit both the fine structure typical of a center with effective spin S = 5/2 and a superhyperfine structure (SHFS) indicating the SHFS interaction of the electronic moment of the center with the nuclear magnetic moments of its six ligands (F− ions). An analysis of the SHFS reveals that this center forms through the replacement of a Ba2+ cation by a Fe2+ cation, which transforms into a Fe3+ (6A1g) cation under x-ray irradiation and shifts into a neutral position along the C2 axis of the cubic coordination shell of the replaced host cation.

Electronic structure of bivalent copper off-center complexes in SrF2 crystals from data of EPR and ENDOR spectroscopy

The electronic structure of bivalent copper off-center complexes in SrF2 crystals is calculated from experimental data obtained earlier by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. The electronic structure parameters characterizing the unpaired-electron density in the vicinity of the nucleus of a copper impurity ion are determined, and the parameters of covalent bonds between an impurity copper ion and three groups of the fluorine ions nearest to this impurity are calculated. It is demonstrated that states of the ground electron configuration of the bivalent copper impurity complex involve an admixture of excited electron configurations due to electron transfer from the ligand to 4s and 4p unfilled shells of the copper ion.

Local structure of titanium pair centers in SrF2 crystals: EPR and ENDOR spectroscopic studies

The local structure of titanium pair centers in SrF2: Ti crystals is investigated using electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. It is found that titanium pair centers with spin moment S=2 and tetragonal symmetry of the magnetic properties are formed in SrF2: Ti cubic crystals under certain growth conditions and during annealing. The tensor components of the fine and ligand hyperfine structures in the EPR and ENDOR spectra are determined. A model of the Ti+-Ti3+ paramagnetic dimer is proposed. This model provides an adequate interpretation of both the ferromagnetic nature of the exchange interaction and the observed displacements of four ligands in the first coordination sphere of titanium impurity ions in directions perpendicular to the impurity ion-ligand bonds.

Local structure of bivalent copper centers in SrF2 crystals: An ENDOR spectroscopic study

Structural distortions of the SrF2 crystal lattice near the bivalent copper impurity Jahn-Teller center are investigated by the ENDOR method (v=9.3 GHz, T=4.2 K). The approximate directions and the magnitudes of displacements of a Cu2+ impurity ion and its surrounding F− ions are determined with respect to one of the anionic networks in the crystal matrix. The tensor components for the ligand hyperfine interaction (LHFI) with fluorine ions separated from the impurity by a distance R≤6.2 Å are obtained from the angular dependences of the location of the ENDOR resonance lines. It is found that the parameters of magnetic interactions between the impurity and these ligands contain the contributions determined by the covalence of bonds in the impurity complex and the polarization of electron shells of the ligands.

Influence of structural deformations on the magnetic properties of Jahn-Teller complexes of divalent copper in CaxSr1−xF2 mixed crystals

Crystals of variable constituency CaxSr1−xF2:Cu (0⩽x⩽0.05, x≅0.5, 1.0⩾x⩾0.95) are investigated by EPR and x-ray structural analysis. The electron-Zeeman, intrinsic hyperfine, and ligand hyperfine interaction parameters are determined; they characterize the magnetic properties of the copper paramagnetic complexes formed in the crystals. Models of the molecular structures of these complexes are discussed, along with the influence of structural deformations and vibronic interactions on their magnetic properties.

EPR of trivalent chromium centers with monoclinic symmetry in SrF2 crystals

Cr3+ centers with monoclinic symmetry in SrF2:Cr crystals are studied by EPR. Based on an analysis of the EPR data and a study of the conditions under which the observed samples were formed, a model is proposed which explains the low symmetry of the magnetic properties in terms of the simultaneous effect on the impurity ion state of interactions with the interstitial F− ion and with normal vibrations of the crystal lattice in a trigonal symmetry.

Anomalies in the magnetic properties of copper impurity clusters in barium fluoride crystals

Copper impurity trimers with antiferromagnetic coupling between Cu2+ ions are synthesized in BaF2 crystals. In some of the BaF2: Cu crystals thus prepared, the concentration of copper stable trimers turns out to be higher than that of copper dimers and copper single centers. The BaF2: Cu crystals are studied using electron paramagnetic resonance (EPR) spectroscopy at frequencies of 9.3, 23.0, and 37.0 GHz in the temperature range 4.2–77 K. It is found that the components of the effective tensor g for copper trimers depend strongly on the external magnetic field and temperature. A static model of the molecular structure of the copper trimer is proposed on the basis of the experimental data obtained.

Trimers of bivalent copper impurity ions in CaF2 crystals: The structure and mechanism of formation

Crystals of CaF2: Cu (with a copper impurity content higher than 0.1 at. %) grown by the Czochralski method from a melt in a mixed helium-fluorine atmosphere are investigated using electron paramagnetic resonance (EPR) spectroscopy. It is found that the crystals contain paramagnetic centers whose magnetic properties at low temperatures are identical to those of [CuF4F4]6− (S=1/2) single centers. The magnetic properties of the centers exhibit a qualitative change in the temperature range 77–300 K. These changes are described within a model according to which the center is treated as a cluster composed of three [CuF4F4]6− impurity complexes involved in exchange interactions and interactions occurring in the field of Jahn-Teller lattice distortions.