Electron paramagnetic resonance of Ce<Superscript>3+</Superscript> and Nd<Superscript>3+</Superscript> impurity ions in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.13</Subscript>

The electron paramagnetic resonance (EPR) spectra of Ce³⁺ and Nd³⁺ impurity ions in unoriented powders of the YBa₂Cu₃O_6.13 compound are observed and interpreted for the first time. It is demonstrated that, upon long-term storage of the samples at room temperature, the EPR signals of these ions are masked by the spectral line (with the g factor of approximately 2) associated with the intrinsic magnetic centers due to the significant increase in its intensity.     

Enhanced superhyperfine structure of the EPR spectra of a U<Superscript>3+</Superscript> ion introduced into the Van Vleck paramagnet LiTmF<Subscript>4</Subscript>

The observation of the superhyperfine structure (SHFS) in EPR spectra due to enhanced nuclear magnetism is reported. The X-band spectrum of a U³⁺ ion introduced into the Van Vleck paramagnet LiTmF₄ is measured in the temperature range of 5–20 K and compared with the spectra of LiLuF₄:U³⁺ and LiYF₄:U³⁺ single crystals. The spectra reveal well-resolved and strikingly different SHFS. The SHFS of Li(Lu, Y)F₄:U³⁺ is due to the fluorine ions forming the nearest surroundings of the U³⁺ ion. The main contribution to the SHFS of the U³⁺ spectrum in LiTmF₄ comes from the Tm³⁺ ions with a highly enhanced nuclear gyromagnetic tensor. The text was submitted by the authors in English.     

EPR of trivalent iron centers in SrF<Subscript>2</Subscript>: Fe crystals

Paramagnetic centers of three types are found in SrF₂: Fe(0.2 at. %) crystals. Two types are observed in the untreated crystals, and the third type appears only in the crystals irradiated by x-rays. The EPR spectra of one type of centers in a nonirradiated crystal and of the centers that appear after irradiation are described by the orthorhombic Hamiltonians with an effective spin S _eff = 5/2. In both cases, the centers are observed at 4.2 and 77 K. The principal axes of the spin Hamiltonians for them are along the 〈001〉, 〈1 \(\overline 1 \) 0〉, and 〈110〉 axes. However, the fine-structure parameters of their EPR spectra differ significantly. An analysis of the superhyperfine structure (SHFS) of the EPR spectra shows that the radiation center forms through substitution of a Fe²⁺ ion for a Sr²⁺ cation. Under x-ray irradiation, the Fe²⁺ ion transforms into the Fe³⁺(⁶ A _1g ) state and is displaced to an off-center position along the C ₂ axis of its coordination cube. The absence of a SHFS in the EPR spectra of the orthorhombic centers in a nonirradiated crystal makes it impossible to determine their molecular structure unambiguously. The most probable model is proposed for this structure. The EPR spectra of centers of the third type were observed only at 4.2 K, and the structure of these centers was not studied.     

Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

EPR and optical spectroscopy of Yb<Superscript>3+</Superscript> ions in CaF<Subscript>2</Subscript>: an analysis of the structure of tetragonal centers

CaF₂ crystals doped with Yb³⁺ ions have been studied by electron paramagnetic resonance (EPR) and optical spectroscopy. EPR spectra of paramagnetic centers (PCs) for cubic (T_c) and tetragonal (T_tet) symmetries were identified. Empirical energy level diagrams were established and crystal field parameters were determined. Information on the CaF₂∶Yb³⁺ phonon spectra was obtained from the electron-vibrational structure of the optical spectra. The crystal field parameters were used to analyze the crystal lattice distortions in the vicinity of the Yb³⁺ ion. Within the framework of a superposition model, it is established that four F⁻ ions located symmetrically with respect to the fourfold axis from the side of the ion-compensator approach the impurity ion and deviate from the PC axis. The second set of four fluorine ions also approaches the Yb³⁺ ion and the PC axis. The ion-compensator F⁻ also approaches considerably the impurity ion.     

Spectral properties of Nd<Superscript>3+</Superscript> ions in samples of transparent Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramics

The spectral and luminescent characteristics of samples of Y₂O₃:Nd³⁺ ceramics obtained from different precursors under different preparation conditions (the concentration of an HfO₂ compacting additive, the temperature and time of synthesis) are studied at 300 and 77 K. It is shown that the spectral positions of absorption and luminescence lines of ceramics correspond to those of a Y₂O₃:Nd³⁺ single crystal. At the same time, the absorption and luminescence spectra show an inhomogeneous broadening, characteristic of disordered crystals and glass. The energies of the ⁴ I _9/2 and ⁴ F _3/2 Stark states of the Nd³⁺ ion are calculated. The calculation results nearly coincide with the data from the literature for the Y₂O₃:Nd³⁺ single crystal and transparent ceramics. Samples containing the compacting additive show additional lines, whose intensities correlate with its concentration and the method of preparation of Y₂O₃:Nd³⁺ ultradispersed powders. It is assumed that these lines are related to the fact that either Nd³⁺ ions enter the composition of the HfO₂ compacting additive or Hf⁴⁺ ions are present in the nearest environment of Nd³⁺ ions at the boundaries of granules enriched with HfO₂.     

High-frequency EPR of Tb<Superscript>3+</Superscript>-doped KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb₂Cl₅:Tb³⁺ crystal have been investigated. Three types of spectra were observed in the frequency range of 74–200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the¹⁵⁹Tb³⁺ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb²⁺ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values ofg-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb³⁺ lines was discussed.     

Multiphonon relaxation of optical excitations in CsCdBr<Subscript>3</Subscript>:Pr<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Nd<Superscript>3+</Superscript> crystals

The technique of calculation of the n-phonon transition rates between electronic sublevels of impurity rare earth ions in dielectric crystals is developed in the case when n>2. The n-phonon transition probabilities are calculated according to the 1st and 2nd orders of perturbation theory. The Hamiltonian of the electron-phonon interaction is constructed in the framework of the exchange charge model and developed as series in relative displacements of the rare earth ion and ligands. The contribution of the lattice anharmonicity on the probabilities of n-phonon transitions is taken into account. On the basis of the developed technique, the nonradiative relaxation rates of ⁴ G _7/2 multiplet of Nd³⁺ ions in LiYF₄:Nd³⁺crystal and ³P₁ multiplet of Pr³⁺ ions in CsCdBr₃:Pr³⁺ crystal were computed. The results of our calculations show that the 2nd order terms in the expressions for the probabilities studied here are comparable with, and in some cases prevail over the 1st order terms. An account of lattice anharmonicity in case of LiYF₄:Nd³⁺ crystal substantially modifies the corresponding multiphonon relaxation rates. The calculated nonradiative relaxation rates for both crystals agree well with the experimental data.     

Study of ESR spectra of Ce<Superscript>3+</Superscript> ions in polycrystalline Sr<Subscript>2</Subscript>B<Subscript>5</Subscript>O<Subscript>9</Subscript>Br

ESR spectra of Ce³⁺ ions in polycrystalline Sr₂B₅O₉Br were studied, and the two crystallographic positions of the Ce³⁺ ion in this compound were identified on the basis of the data obtained. The ESR spectrum of Ce³⁺ ions with local charge compensation contains a broad line indicating the existence of several types of charge compensation. ESR spectra of Ce³⁺ ions in samples activated additionally by K⁺ ions are similar to those of the regular Ce³⁺ centers, which indicates that the effect of the univalent cation on Ce³⁺ is negligible.     

Spectral and generation properties of a new laser crystal (Cr<Superscript>3+</Superscript>,Li): Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

Based on the results of a comparative analysis of luminescence spectroscopy and EPR spectroscopy data, it was found for the first time that the wide-band luminescence of Cr³⁺ ions in a forsterite crystal is due to the Cr³⁺-V_Mg center or, in a crystal additionally doped with lithium, to a Cr³⁺-Li⁺ center. For the first time, tunable laser action was obtained with Cr³⁺-Li⁺ centers responsible for the wide-band luminescence.     

EPR of Yb<Superscript>3+</Superscript> ions in a monoclinic KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystal

The electron paramagnetic resonance (EPR) of Yb³⁺ ions in a KY(WO₄)₂ single crystal was investigated at T=4.2 K and fixed frequency of 9.38 GHz. The resonance absorption observed on the lowest Kramers doublet represents the complex superposition of three spectra, corresponding to the ytterbium isotopes with different nuclear moments. The EPR spectrum is characterized by a strong anisotropy of the g-factors. The temperature dependence of the g-factors is shown to be caused by the strong spin-orbital and orbital-lattice coupling. The resonance lines broaden with increasing temperature due to the short spin-lattice relaxation times.     

Superhyperfine structure in the EPR spectra and optical spectra of impurity f ions in dielectric crystals: A review

The results of observation and simulation of the superhyperfine (ligand hyperfine) structure (SHFS) of the electron paramagnetic resonance (EPR) spectra of rare-earth and uranium impurity ions in dielectric crystals have been systematized. The resolved SHFS of the EPR spectra of doped cubic crystals (with the fluorite and perovskite structures) has been observed for orientations of a constant magnetic field along the crystallographic axes. Most attention has been paid to tetragonal double fluorides LiRF₄ (R = Y, Lu, Tm), in which the SHFS of the EPR spectra has also been found for intermediate orientations of the magnetic field. For the LiYF₄: Nd³⁺ single crystal, the splitting of optical spectral lines due to the interaction of Nd³⁺ ions with nuclear magnetic moments of the nearest neighbor fluorine ions has been observed for the first time. The Van Vleck paramagnet LiTmF₄: U³⁺ is characterized by the SHFS with clearly distinguishable components due to the interaction of uranium ions both with nuclei of the fluorine ions and with enhanced magnetic moments of the thulium nuclei. The SHFS envelopes of the EPR spectra of Yb³⁺, Ce³⁺, Nd³⁺, and U³⁺ ions in LiYF₄ and LiLuF₄ crystals are well reproduced by numerical calculations based on the microscopic model using only three fitting parameters: the width of transitions between the electron-nuclear sublevels of the complex containing the paramagnetic ion and nuclei of the ligands and two constants of covalent bonding of the f electrons with 2s and 2p electrons of the nearest neighbor fluorine ions.     

Multi-range high-frequency EPR spectroscopy of LiYF<Subscript>4</Subscript> and LiLuF<Subscript>4</Subscript> crystals doped by rare-earth ions

EPR spectra of isostructural LiYF₄ and LiLuF₄ crystals doped by Dy³⁺, Er³⁺, and Ho³⁺ ions are measured at 4.2 K in the frequency range 40–800 GHz. The effects caused by isotopic disorder in the lithium sublattice, the random crystal field, and the interaction between paramagnetic impurity ions are detected and studied. The results of the measurements are used to determine the spectral characteristics of the compounds and the crystal field parameters. It is demonstrated that the formation of the isotope structure of the EPR signal is dominated by local deformations of the crystal lattice induced by mass defects.     

Electron paramagnetic resonance study of Fe<Superscript>3+</Superscript> ions at octahedral and tetrahedral mirror symmetry sites in the LiScGeO<Subscript>4</Subscript> crystal

An electron paramagnetic resonance (EPR) study of a synthetic single crystal of LiScGeO₄ doped with Cr ions carried out earlier at the X- and Q-bands at 300, K has indicated additional weak lines. A detailed analysis of these EPR lines, which were tentatively attributed to the Fe³⁺ ions at two different mirror symmetry sites, is presented in this paper. The angular dependences in the three crystallographic planes were resolved by fitting the two distinct spectra denoted Fe³⁺(I) and Fe³⁺(II) with a spin Hamiltonian (S=5/2) of monoclinic symmetry. The rank-4 crystal field tensors at tetrahedral and octahedral sites were calculated with the point-charge model to determine the principal axis orientations of their cubic, tetragonal and trigonal components. A comparative analysis of the zero-field splitting tensors and the crystal field ones indicates that Fe³⁺(I) ions substitute for Sc³⁺ at octahedral sites and Fe³⁺(II) ions substitute for Ge⁴⁺ at tetrahedral sites with no significant distorition of the coordination polyhedra in the structure of LiScGeO₄.     

Spectral properties of neodymium in POCl<Subscript>3</Subscript>-BCl<Subscript>3</Subscript>-Nd<Superscript>3+</Superscript>

We measured the absorption and luminescence spectra of Nd³⁺ ions in an inorganic solvent POCl₃-BCl₃. The spectra were analyzed in terms of the Judd-Ofelt theory. We calculated the Judd-Ofelt parameters, oscillator strengths, spontaneous emission probabilities, luminescence quantum yield, and the stimulated emission cross section for the laser transition ⁴ F _3/2 → ⁴ I _11/2 of the neodymium ion in a POCl₃-BCl₃-Nd³⁺ solution.     

The influence of the specific surface of grains on the luminescence properties of Nd<Superscript>3+</Superscript>-doped Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> nanopowders

Nd³⁺:Y₃Al₅O₁₂ (Nd:YAG) powders were prepared by the Pechini method in the temperature range of 800 to 1400 °C. The pure garnet phase of the obtained materials was confirmed by XRD studies. The size of the grains was controlled by the annealing temperature of the samples. Their morphologies were investigated by TEM and porosity measurements (BET). The effect of annealing temperature on the morphology and luminescence properties of Nd:YAG nanocrystallites was studied, and the results were compared to the properties of a Nd:YAG single crystal. A significant enhancement of the ⁴F_3/2→⁴I_9/2/⁴F_3/2→⁴I_11/2 intensity ratio with decreasing grain size was observed. It was found that the decay times of the Nd³⁺ luminescence depends on the specific surface and is significantly longer for well crystallized nanocrystalline grains than for single crystals having the same concentration of Nd³⁺ ions. The role of crystallinity and specific surface on the radiative processes is analyzed. PACS 78.55.-m; 78.20.Ci; 78.67.Bf; 78.68.+m     

Electron paramagnetic resonance and photoluminescence of NaBi(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals doped with gadolinium ions

The results of electron paramagnetic resonance (EPR) and photoluminescence studies of large NaBi(MoO₄)₂ crystals grown by the low-gradient Czochralski method and doped with gadolinium ions (0.1 wt %) have been presented. It has been found from the analysis of the angular dependence of EPR spectra that the gadolinium ions enter into the crystal structure in the state Gd³⁺ and occupy the bismuth position. The parameters of the EPR spectra of the gadolinium ions have been calculated and the analogy has been drawn based on these data between the specific features of the incorporation of gadolinium ions into the structures of double tungstates and molybdates. The observed shift of the maximum of the photoluminescence band of the NaBi(MoO₄)₂ crystals doped with Gd³⁺ ions with respect to the spectrum of the undoped crystal suggests the influence of gadolinium ions on the formation of the bottom of the conduction band caused by the states of the (MoO₄)^2?.     

EPR and optical absorption studies of Cu<Superscript>2+</Superscript> ions in [ZnPd(CN)<Subscript>4</Subscript>(C<Subscript>4</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>O<Subscript>2</Subscript>)] single crystals

A Cu²⁺-doped single crystal of catena-trans-bis(N-(2-hydroxyethyl)-ethylenediamine) zinc(II)-tetra-m-cyanopaladate(II) [ZnPd(CN)₄(C₄H₁₂N₂O₂)] complex has been investigated by electron paramagnetic resonance (EPR) technique at room temperature. EPR spectra indicate that Cu²⁺ ions substitute for magnetically equivalent Zn²⁺ ions and form octahedral complexes in [ZnPd(CN)₄(C₄H₁₂N₂O₂)] hosts. The crystal field affecting the Cu²⁺ ion is nearly axial. The optical absorption studies show two bands at 322 nm (30864 cm⁻¹) and 634 nm (15337 cm⁻¹) which confirm the axial symmetry. The spin Hamiltonian parameters and the relevant wave function are determined.     

Investigations of the electron paramagnetic resonance spectra of VO<Superscript>2+</Superscript> in CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> system

The EPR spectra for VO²⁺ in CaO-Al₂O₃-SiO₂ system are calculated using complete diagonalization method (CDM) and perturbation theory method (PTM). The calculated results are in good agreement with the observed values. By comparing the calculated results by CDM and PTM in a wide range of crystal field parameters, the validity of the PTM formulas has been studied. It is shown that, the PTM formulas are reasonable and reliable in a wide range of CF parameters.     

Structural and Near-Infrared Properties of Nd<Superscript>3+</Superscript> Activated Lu<Subscript>3</Subscript>NbO<Subscript>7</Subscript> Phosphor

Undoped and Nd³⁺ doped lutetium niobate phases have been prepared by a conventional solid state reaction method using lutetium acetate and niobium oxide at 1250 °C for 6 h. X-ray diffraction patterns of the 6 mol% Lu₃NbO₇ sample exhibited a cubic fluorite single phase. Phase structure exhibited interesting crystallization behaviour depending on increasing Nd³⁺ concentration which led to a Lu₃NbO₇ single phase formation during the heat treatment process. SEM investigations were also in agreement with the XRD results. Morphologies of Nd³⁺ doped lutetium niobate powders exhibited oval like shapes and grain sizes varied between 0.3 and 5 μm. Near-infrared luminescence properties of Nd³⁺ doped Lu₃NbO₇ were also studied. 1.06 μm laser transition characteristics of Nd³⁺ doped lutetium niobate have been observed. Concentration quenching phenomenon was not detected depending on increasing Nd³⁺ doping concentrations at room temperature.