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In yttrium aluminum garnet (YAG) crystals containing terbium and cerium impurities, along with the main EPR signals of Tb3+ and Се3+ ions located in the dodecahedral sites of the YAG lattice in a regular environment, EPR lines with a lower (several percent) intensity were observed. They also belong to the paramagnetic centers of terbium and cerium, but are characterized by slightly altered parameters—the initial level splitting for non-Kramers Tb3+ ions and g-factors for Се3+ ions. It is shown that the nature of such centers and their number can be explained by the presence of YAl antisite defects, i.e. yttrium ions in the octahedral aluminum positions, in the environment of Tb3+ and Се3+.

Investigation of the EPR spectrum of Y3Al5O12:V crystals have made it possible to determine the parameters of the fine and hyperfine structures of trigonal V2+ centers. The existence of V2+ triclinic centers that arise as a result of association of V2+ sites with defects that lower the symmetry is established.

Electron paramagnetic resonance (EPR) spectra of Ce3+, Yb3+, Cr3+, and Gd3+ impurity ions in yttrium aluminum garnet Y3Al5O12 (YAG) ceramics were detected and identified at frequency of 94 GHz. The advantage of measuring the EPR spectra in the high-frequency range compared to the standard EPR technique is shown, which makes it possible to separate the EPR spectra characterized by different anisotropic g-factors and also to isolate the EPR signals due to the splitting of the fine structure for centers with high-spin states. In ceramics with a high content of magnetic Gd3+ ions, EPR and electron spin echo (ESE) spectra of multi-ionic gadolinium complexes were observed, and EPR spectra of complexes with the maximum number of exchange-coupled gadolinium ions are seen at low temperatures. The temperature dependences of the EPR spectra indicates ferromagnetic ordering of exchange-coupled complexes of gadolinium.

In crystals of yttrium orthoaluminate doped with the 151Eu isotope, Eu2+, Cr3+, Gd3+, and Мо3+ paramagnetic centers are detected. The fine structure parameters of the spin Hamiltonian are determined or refined for these centers. The orientation behavior of previously unobserved forbidden transitions is considered for the Cr3+ centers. It is shown that two Mo3+ signals that are detected in magnetic fields directed parallel to the crystallographic axes do not belong to different molybdenum centers, but are two intradoublet transitions of the same center.

The crystals of yttrium–aluminum garnet with an admixture of iron and cerium have been studied by the method of wideband EPR spectroscopy. Resonance absorption of microwave radiation in the frequency range 220–300 GHz was detected. Based on the experimental data set, it is concluded that the signals belong to the Mo3+ ion, which is included in the crystals as an uncontrolled impurity. A straightforward method was used to measure the splitting in a zero field between the Kramers doublets |±1/2〉 and |±3/2〉 (261.6 ± 0.5 GHz). The calculated angular and frequency-field dependences of the EPR spectra are in good agreement with the experiment.

The ESR spectrum of Gd3+ and Eu2+ monoclinic centers, which substitute for yttrium ions in YAlO3 crystals doped with europium and zirconium, has been investigated. Parameters of the fine structure of these centers are determined. The hyperfine-interaction parameters are determined for the centers with 151Eu isotope, and the hyperfine structure of the centers with 153Eu isotope is discussed.

Electron paramagnetic resonance spectra of Eu2+ and Gd3+ centers in Lu3Al5O12 single crystals have been studied and the parameters of their rhombic spin Hamiltonian have been determined.

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.

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.

Luminescence of yttrium-aluminum garnet Y3Al5O12: Eu,Si samples is studied. It is found that luminescence is associated with Eu2+ ions, but does not correspond to intracenter 4f65d1–4f7 transitions. Measurements of the excited state lifetime point to the luminescence mechanism involving charge-transfer states.

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 4f 05d 1-4f 1 (2 F 7/2) band of the Ce3+ ion luminescence indicates the existence, at least, two types of Ce3+ ion centers in PbGa2S4.

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.

The YAlO3: Tm3+ single crystal has been studied on a wide-band EPR spectrometer. The EPR spectra of Tm3+ ions in the frequency range of 90–160 GHz have been detected for the first time. It has been confirmed that thulium ions substitute the position of Y3+ in the crystal lattice. The detected spectra have been described with the use of a spin Hamiltonian with the effective spin S = 1/2. A comparative analysis of the orientation of the magnetic axes of the Tm3+ paramagnetic center with earlier data on other rare-earth ions has been performed.

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. © 2013 Pleiades Publishing, Ltd.

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.

An analysis of the electron spin resonance saturation effects for the molecular impurity ion MnO42− in proper ferroelastic K3Na(CrO4)2 has shown that two different types of active paramagnetic centers coexist near the local structural transition. A new model of local structural transition is proposed based on the temperature-dependent Jahn-Teller effect (JTE); this model makes it possible to substantiate the occurrence of two types of such centers and explain their nature. It is also shown that two types of the local structural Jahn-Teller transition are implemented within the theory developed.

The results of investigation of electron paramagnetic resonance of Er3+ ions in the thermally populated first excited state in (Y1 − xLux)3Al5O12 (YLuAG) mixed yttrium-lutetium garnet single crystals (0 ≤ x ≤ 1) are considered. In composition-disordered YLuAG, a number of new (as compared to Y3Al5O12 (YAG)) Er3+ paramagnetic centers are detected; these centers appear due to a change in the crystal field symmetry and magnitude upon isomorphic substitution of Lu3+ for Y3+ in the yttrium sublattice of garnets. The origin of new paramagnetic centers is established and their formation probability is calculated.

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.

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.

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.

Electron paramagnetic resonance (EPR) was detected in the ground state of Ce3+ ions in YAG:Ce single crystals by monitoring the intensity of photoluminescence under nonresonant excitation. The photoluminescence intensity was found to increase strongly in magnetic field at liquid helium temperature. The optically detected magnetic resonance (ODMR) effect was of the order of percents and corresponded to resonance decrease in the luminescence intensity. ODMR correlated with the conventional EPR spectra of Ce3+ ions, which occupy the dodecahedral Y3+ sites in the YAG crystal lattice. The increase of the PL intensity in magnetic field was concluded to be caused by Boltzmann distribution between spin sublevels of the ground state of Ce3+ ions. The ODMR tend to equalize the populations of levels in resonance and results to a decrease of PL intensity. Different types of Ce3+ centres characterized by the presence of nearby antisite defects were observed, which should be taken into account in applications of single ion spectroscopy based applications of YAG:Ce.