V. A. Vazhenin

Paramagnetic resonance of Mn4+ and Mn2+ centers in lanthanum gallate single crystals

An increase in the manganese concentration in lanthanum gallate in the range 0.5–5.0% has been found to result in a complete replacement of individual Mn4+ ions by Mn2+ ions. The relative concentrations and binding energies of individual Mn4+, Mn3+, and Mn2+ ions have been determined. The spin Hamiltonians of the Mn2+ and Mn4+ centers in the rhombohedral and orthorhombic phases, respectively, have been constructed and the orientation of the principal axes of the fine-structure tensor of Mn4+ at room temperature has been found. The possibility of using electron paramagnetic resonance for determining the rotation angles of oxygen octahedra of lanthanum gallate with respect to the perovskite structure has been discussed.

Fine structure of the electron paramagnetic resonance spectrum of Fe3+ centres in LiTaO3

The electron paramagnetic resonance spectrum of trigonal Fe3+ centres has been investigated and parameters of the spin Hamiltonian obtained for nominally pure congruent LiTaO3 crystals annealed at ~1500 K under Li2O vapour pressure corresponding to the pressure over the stoichiometric lithium tantalate. The possibility of calculating the zero-field splitting of the ground state of the impurity ion on the basis of the superposition model is discussed.

Photoactive lead ions in ferroelectric Pb5Ge3O11 and paramagnetic resonance

The recharging of lead matrix ions upon exposure to light in pure, doped, and nonstoichiometric lead germanate crystals has been studied using electron paramagnetic resonance. It has been shown that the maximum concentration of metastable Pb3+ ions is achieved in crystals doped with chlorine, fluorine, titanium, and in samples with excess lead oxide. The annealing activation energy and the parameters of the superhyperfine interaction of Pb3+ paramagnetic centers have been determined.

Paramagnetic centers in two phases of manganese-doped lanthanum gallate

An EPR study of two phases of manganese-doped lanthanum gallate (with a first-order structural transition occurring at 430 K) has revealed Gd3+, Fe3+, and Mn4+ centers at room temperature and 438 K. The parameters of spin Hamiltonians are determined for the Gd3+, Fe3+, and Mn4+ rhombohedral centers in the high-temperature phase (with no other centers found here) and for the monoclinic center Gd3+ in the low-temperature phase. Both in the orthorhombic and in the rhombohedral phase, crystallographic twins (or ferroelastic domains) are observed.

Structural transition in lanthanum gallate and transformation of the fine structure of the EPR spectrum of a Gd3+ impurity center

Abrupt changes in resonance positions, hysteretic temperature behavior, and coexistence of phases, which indicate a first-order phase transition, have been revealed from measurements of temperature dependences of the EPR spectra of Gd3+ and Mn4+ centers in the vicinity of the structural transition of lanthanum gallate. The transformation of monoclinic Gd3+ centers into trigonal Gd3+ centers upon the phase transition has been used to estimate the adequacy of two approximations of the superposition model for parameters of the zero-field splitting of the ground state.

Gadolinium ion localization in RbPb2Cl5 crystals

The spectrum of the electron paramagnetic resonance of high-spin triclinic Gd3+ centers in lead-rubidium chloride single crystals has been studied. The spin Hamiltonian parameters have been determined. It has been concluded that the rare-earth ion is localized at the lead site whose environment is a tricapped trigonal prism.

Gd3+-F− dimer centers in the ferroelectric lead germanate

The Gd3+ centers formed in lead germanate doped with fluorine have been studied using electron paramagnetic resonance. The parameters of the triclinic spin Hamiltonian are found at room temperature. The center is shown to be a gadolinium ion having the nearest environment in which the fluorine ion is substituted for the oxygen ion. The Gd-F dimer binding energy relative to the energy of a single gadolinium ion is determined. The interaction of the dimer center with the spontaneous polarization is lower than the measurement error.

Order parameters in lanthanum gallate lightly doped with manganese and paramagnetic resonance

The Cr3+ centers have been revealed, transitions at room temperature have been identified, and spin Hamiltonian parameters have been determined for the Cr3+ and Fe3+ triclinic centers in lanthanum gallate lightly doped with manganese. The principal axes of the fourth-rank fine-structure tensor for the Fe3+ triclinic centers have been established and used to determine the order parameters, i.e., the angles of rotation of oxygen octahedra of lanthanum gallate with respect to the perovskite structure. The order parameter in the rhombohedral phase has been estimated.

Gd3+ EPR in solid solutions based on ferroelectric lead germanate

The splitting of the ERP spectrum of a single Gd3+ ion into four intense components has been observed in crystals of Pb5(Ge1 − xSix)3O11 solid solutions, which is caused by the appearance of silicon ions in three nearest germanium spheres. The atomic structure of the observed centers has been determined. The conclusion has been made that silicon mainly substitutes for germanium in oxygen tetrahedra and for one of germanuim atoms in oxygen bitetrahedra, which is based on comparison of the measured ratio of component intensities with the results of the calculation of the concentrations of Gd3+ associations with silicon ions.

Models of tetrahedral rare-earth clusters in cadmium fluoride crystals and paramagnetic resonance

The parameters of the zero-field level splitting of Gd3+ ions localized in three kinds of tetrahedral clusters are estimated within the superposition approximation. The structure of a cluster is determined by minimizing the energy of the crystal with the embedded cluster. As a result, the strong monoclinic EPR spectrum with b20 = −345 MHz is attributed to the [CdY2CdF26] cluster and a weaker spectrum with b20 ≈ −600 MHz is attributed to [Cd2YGdF26]. The difference between the absolute values of the calculated and experimental parameters b20 is explained by the fact that the experimental spectrum is due to gadolinium ions located in clusters that are part of associations.