G. R. Asatryan

Electron paramagnetic resonance and optical spectroscopy of K3Na(CrO4)2 ferroelastics activated by MnO 4 2− molecular impurity ions

Crystals of a proper ferroelastic K3Na(CrO4)2 containing molecular impurity ions MnO42− are studied using electron paramagnetic resonance (EPR) and optical spectroscopy. The EPR spectrum of the Mn6+ ion contained in the molecular impurity ion MnO42− is identified at low temperatures (T ≤ 20 K). The intensity of this spectrum decreases unusually fast as the temperature increases. A broad IR luminescence band with a vibronic structure well resolved at a temperature of 8 K is revealed. Theoretical treatment of the Mn6+ ion involved in the molecular impurity ions MnO42− of the K3Na(CrO4)2 ferroelastic crystal suggests that an important role in this case is played by the pseudo-Jahn-Teller. The pseudo-Jahn-Teller effect offers an explanation for the appearance of a fine structure in the vibronic replicas in the luminescence spectrum, on the one hand, and accounts for the fast decrease in the intensity of the EPR spectrum of K3Na(CrO4)2: MnO42− with increasing temperature, on the other.

EPR of Er3+, Nd3+, and Ce3+ ions in YAlO3 single crystals

EPR spectra of the Er3+, Nd3+, and Ce3+ ions substituting for the Y3+ ion in the YAlO3 yttrium orthoaluminate lattice are studied. The EPR spectra of these rare-earth ions are described by a spin Hamiltonian of rhombic symmetry with an effective spin S=1/2. The principal values of the g tensors were determined from an analysis of the angular dependences of the EPR spectra. The orientation of the local magnetic axes of paramagnetic centers relative to the YAlO3 crystallographic directions are shown to depend on the actual rare-earth species. The EPR spectra exhibit a hyperfine structure due to the 167Er, 143Nd, and 145Nd odd isotopes, which permitted unambiguous identification of these spectra. The hyperfine coupling constants for the odd erbium and neodymium isotopes are determined.

High-spin europium and gadolinium centers in yttrium–aluminum garnet

Electron-spin resonance spectra of Eu2+ and Gd3+ centers substituting Y3+ ions in single-crystal yttrium–aluminum garnet have been studied and the parameters of their rhombic spin Hamiltonian have been determined. The fine-structure parameters of the above ions have been calculated in the superposition model disregarding changes in the angular coordinates of the ligand environment of the impurity defect thus demonstrating the necessity of taking these changes into account.

Family of paramagnetic centers of Ce3+ ions in yttrium aluminum garnet

The electron paramagnetic resonance (EPR) spectra of Ce3+ ions in single crystals of yttrium aluminum garnet have been investigated. It has been found that, in addition to the usually observed EPR signals of Ce3+ ions located in the regular environment at dodecahedral sites of the crystal lattice, the spectra contain a group of less intense anisotropic lines with g-factors close to the corresponding parameters of Ce3+ ions in the regular environment. It has been concluded that the observed satellite lines belong to the family of Ce3+ ions in the immediate vicinity of which there are permutation defects that lead to a change in the strength and symmetry of the crystal field in the vicinity of the paramagnetic center.

Electron paramagnetic resonance of Dy3+ ions in lead thiogallate single crystals

The results of electron paramagnetic resonance (EPR) studies of Dy3+ ions in lead thiogallate PbGa2S4 single crystals have been presented. It has been shown that the ground state of these ions corresponds to the lowest Stark sublevel Γ6 of the term 6H15/2. The spectra are well described by the axially symmetric spin Hamiltonian with the effective spin S = 1/2 with the factors g‖ = 15.06 and g⊥ = 2.47. The Dy3+ ions substitute Pb2+ ions in the crystal lattice of PbGa2S4. The observed hyperfine structure has allowed to unambiguously interpret the EPR spectra. The hyperfine interaction constants of two odd isotopes of dysprosium in lead thiogallate single crystals have been found to be A‖ = 675 × 10−4 cm−1 and A⊥ = 111 × 10−4 cm−1 for 163Dy and A‖ = 472 × 10−4 cm−1 and A⊥ = 77 × 10−4 cm−1 for 161Dy.

Paramagnetic resonance of yttrium aluminum garnet doped with 151Eu2+ ions

The 151Eu2+ orthorhombic centers in yttrium aluminum garnet crystals have been investigated. The initial splitting and hyperfine interaction parameters have been obtained taking into account the positions of the hyperfine structure components. The relative signs of the fine and hyperfine structure parameters have been determined from the analysis of the type of the hyperfine structure formed by the allowed and forbidden electron–nuclear transitions.

Origin of the structural local transition in the molecular impurity ion MnO42− in the K3Na(CrO4)2 ferroelastic

A new model is proposed for a local transition in a Jahn-Teller impurity center in a crystal with a ferroelastic (ferroelectric) phase transition. This model is based on direct interaction of the order parameter of the phase transition in the matrix with the Jahn-Teller impurity degrees of freedom. It is shown that, under these conditions, the order parameter field can induce lifting of degeneracy of the electronic states active in the Jahn-Teller effect, which is accompanied by a transition from the Jahn-Teller effect to the pseudo-Jahn-Teller effect with its subsequent suppression. As a result, a decrease in temperature gives rise to a structural local transition in the region of the low-symmetry ferroelastic (ferroelectric) matrix phase from the many-well local adiabatic to a single-well potential. The model proposed allows interpretation of experimental data obtained in an EPR study of the molecular impurity ion MnO42− in the K3Na(CrO4)2 ferroelastic.

Luminescence of lead thiogallate single crystals doped with cerium ions

The luminescence of single crystals of lead thiogallate PbGa2S4: Ce3+ has been studied. The observed nonmonotonic temperature dependence of the integrated luminescence intensity of PbGa2S4: Ce3+ has been explained by the energy transfer from the electronic excitations of the matrix to impurity ions and by the temperature quenching of the Ce3+ ion luminescence. The observed structure of the 4f05d1-4f1 (2F7/2) band of the Ce3+ ion luminescence indicates the existence, at least, two types of Ce3+ ion centers in PbGa2S4.

Paramagnetic defects in manganese-doped lead tungstate

In manganese-doped PbWO4 crystals, low-intensity signals of triclinic clusters Mn4+-VO and Fe3+-VPb have been revealed in addition to signals of Mn2+ tetragonal centers. The Mn4+-VO cluster is formed by a Mn4+ ion in the W6+ position, which is associated with a vacancy of the nearest neighbor O2−ion, and the Fe3+-VPb cluster consists of a Fe3+ ion substituting for Pb2+ with a local compensation of by a lead vacancy. It has been shown that, in PbWO4: Mn, there is also a small amount of Mn4+ tetragonal centers located in the Pb2+ position with a nonlocal compensation of an excess charge.

Photosensitive bismuth ions in lead tungstate

Electron paramagnetic resonance (EPR) signals of Bi2+ ions have been detected in the EPR spectrum of manganese-, bismuth-, and tin-doped PbWO4 single-crystals irradiated by xenon and mercury lamps at 100 K. The parameters of the Zeeman, hyperfine, and superhyperfine interactions and the localization of Bi2+ ions have been determined.