A Fe[Nb]-Li center in stoichiometric LiNbO<Subscript>3</Subscript> crystals: Mechanism of formation

A new iron center in stoichiometric lithium niobate crystals has been studied by the EPR method. The angular dependences of the EPR spectrum of the center have been used to derive the parameters of its spin Hamiltonian. The data amassed on the variation in the concentrations of two iron centers in lithium niobate crystals annealed in a Li₂CO₃ powder have provided an insight into the mechanism of formation of the new center, as well as corroborated its model proposed by us earlier. According to this model, the center represents a complex of two defects aligned with the polar axis in the crystal: the iron ion at the niobium site and an interstitial lithium ion filling the nearest structural vacancy (Fe³⁺[Nb]-Li⁺[V]). The structure of other Fe³⁺ centers revealed earlier in LiNbO₃ crystals, in which the iron ion occupies the niobium site, has been discussed.     

Luminescence and scintillation properties of Y<Subscript>3</Subscript>A<Subscript>l5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

Luminescence and scintillation properties of Y₃A_l5O₁₂:Ce single crystals grown from the melt by the Czochralski and horizontal directed crystallization methods in various gas media and Y₃A_l5O₁₂:Ce single-crystal films grown by liquid-phase epitaxy from a melt solution based on a PbO-B₂O₃ flux have been comparatively analyzed. The strong dependence of scintillation properties of Y₃A_l5O₁₂:Ce single crystals on their growth conditions and concentrations of Y_Al antisite defects and vacancy defects has been established. Vacancy defects are involved in Ce³⁺ ion emission excitation as the centers of intrinsic UV luminescence and trapping centers. It has been shown that Y₃A_l5O₁₂:Ce single-crystal films are characterized by faster scintillation decay kinetics than single crystals and a lower content of slow components in Ce³⁺ ion luminescence decay during high-energy excitation due to the absence of Y_Al antisite defects in them and low concentration of vacancy defects. At the same time, the light yield of Y₃A_l5O₁₂:Ce single-crystal films is comparable to that of single crystals grown by directed crystallization due to the quenching effect of the Pb²⁺ ion impurity as a flux component and is slightly lower (∼25%) than the light yield of single crystals grown by the Czochralski method.     

The Formation of Dimeric Centers of Copper,Iron and Gadolinium in Modified Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript>

It is discovered that the electron paramagnetic resonance (EPR) spectrum of the doubly charged copper centers occurs in single crystals of Pb₅Ge₃O₁₁ doped with gadolinium or iron after annealing in an atmosphere containing chlorine and bromine. Similar annealing of the crystals doped with copper in a chlorine and fluorine atmosphere leads to redistribution of the intensities of the EPR spectra of two types of Cu²⁺ centers. The influence of annealing on the ongoing intensity of spectra of the dimeric triclinic centers Fe³⁺–A, Gd³⁺–A (where A represent Cl^?, Br^?, O^2?, F^?) was the subject of this research. Consideration is given to the mechanisms for changing the charge state and association of copper center with defects.     

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.     

Gamma-ray induced color centers in Yb:YAG crystals grown by Czochralski method

20 at.% Yb:YAG single crystals have been grown by the CZ method and gamma-ray irradiation induced color centers and valence change of Fe³⁺ and Y b³⁺ ions in Yb:YAG have been studied. One significant 255 nm absorption band was observed in as-grown crystals and was attributed to Fe³⁺ ions. Two additional absorption (AA) bands located at 255 nm and 345 nm, respectively, were produced after gamma irradiation. The changes in the AA spectra after gamma irradiation and air annealing are mainly related to the charge exchange of the Fe³⁺, Fe²⁺, oxygen vacancies and F-type color centers. Analysis shows that the broad AA band is associated with Fe²⁺ ions and F-type color centers. The transition Y b³⁺→Y b²⁺ takes place as an effect of recharging of one of the Y b³⁺ ions from a pair in the process of gamma irradiation.     

Concentration of Cr<Superscript>4+</Superscript> impurity ions and color centers as an indicator of saturation of forsterite crystals Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> with oxygen

The influence of the oxygen partial pressure P _{O 2} in the growth atmosphere on the coefficient of chromium distribution between the crystal and the melt of forsterite, the Cr³⁺ and Cr⁴⁺ ion contents in crystals, and the concentration of color centers induced by irradiation has been investigated. It has been established that the crystals grown at low oxygen partial pressures P _{O 2} (0.01–0.05 kPa) are characterized by low concentrations of Cr⁴⁺ ions and color centers. A change in the oxygen partial pressure to P _{O 2} ∼ 0.85 kPa leads to an increase in the Cr⁴⁺ center concentration by a factor of ∼10 and in the color center concentration by a factor of ∼5. A further increase in the oxygen partial pressure to P _{O 2} to 12 kPa remains the concentration of these centers almost unchanged. A model has been proposed according to which the intrinsic defects formed under conditions of a relative excess of oxygen leads to both the self-oxidation of chromium and the formation of color centers in the forsterite crystals under irradiation.     

Luminescent and photochemical processes in CdBr<Subscript>2</Subscript> : AgCl crystals

This paper reports on the results of the investigation into the optical luminescence properties of photochromic crystals CdBr₂ : AgCl grown by the Bridgman-Stockbarger method. It has been shown that, under X-ray, optical, and N₂-laser excitations of the grown crystals, there occurs emission due to Ag⁺ impurities in addition to emission from centers characteristic of CdBr₂. The photostimulated chemical reactions occurring in CdBr₂ : AgCl lead to a weakening of the luminescence and to a change in its spectral composition. Models of photosensitive centers and centers of photochemical coloring have been proposed. The mechanisms of the photochromic effect have been considered. The nature of luminescent and trapping centers has been discussed.     

Photoactive lead ions in ferroelectric Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> 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.     

Cooperative down-conversion of UV light in disordered scheelitelike Yb-doped NaGd(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> and NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

Concentration series of disordered scheelitelike Yb:NaGd(MoO₄)₂ and Yb:NaLa(MoO₄)₂ single crystals are grown by the Czochralski method. The actual concentrations of Yb³⁺ ions in the crystals are determined by optical-absorption spectroscopy. The luminescence of Yb³⁺ ions in these crystals in the region of 1 μm is studied under UV and IR excitation. In the case of UV excitation, this luminescence appears as a result of nonradiative excited state energy transfer from donor centers of unknown nature to ytterbium. The character of the concentration dependence of Yb³⁺ luminescence indicates that the energy transfer at high Yb concentrations occurs with active participation of a cooperative mechanism, according to which the excitation energy of one donor center is transferred simultaneously to two Yb³⁺ ions. In other words, the quantum yield of this transfer exceeds unity, which can be used to increase the efficiency of crystalline silicon (c-Si) solar cells.     

EPR study of impurity iron ion clusters in BaF<Subscript>2</Subscript> crystals

Tetragonal paramagnetic centers with spin S = 7/2 were detected in x-ray-irradiated BaF₂: Fe (c_Fe ≈ 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 Fe^1.5+?Fe^1.5+ 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.     

Optical and magneto-optical studies of a multiferroic GaFeO<Subscript>3</Subscript> with a high Curie temperature

Single crystals of a noncentrosymmetric orthorhombic pyroelectric ferrimagnet Ga_2?xFe_xO₃ with a Curie temperature within 260–345 K have been grown by the flux method. It has been found that the electrical properties of the single crystals varied over a broad range from 10⁵ to 10¹³ Ω cm depending on the presence of transitionmetal oxide impurities. The dispersion relations for all three principal dielectric functions of orthorhombic GaFeO₃ have been determined in the range 0.7–5.4 eV by spectroscopic ellipsometry. The spectra of the dielectric functions of the orthorhombic Ga_2?xFe_xO₃ crystals are compared with the spectra of the trigonal crystals. The Faraday effect and second-harmonic generation are studied, and the law of the transition to the paramagnetic state has been determined. The crystallographic and magnetic contributions to the second-harmonic generation are analyzed.     

Luminescence peculiarities and optical properties of MgMoO<Subscript>4</Subscript> and MgMoO<Subscript>4</Subscript>:Yb crystals

Magnesium molybdate is considered as a promising material for cryogenic scintillation bolometers. The luminescence properties of MgMoO⁴ have been investigated on single crystals grown from melts of stoichiometric and nonstoichiometric compositions and on crystals doped with Yb³⁺ ions. Their optical properties are interpreted taking into account the anisotropy of the MgMoO⁴ crystal structure.     

Growth and spectral and luminescent properties of a new Cr<Superscript>4+</Superscript>:LiAlGeO<Subscript>4</Subscript> nonlinear crystal

Single crystals of a new promising laser material, Cr⁴⁺-doped germanium eucryptite Cr⁴⁺:LiAlGeO₄, are grown from the melt for the first time. The crystals grown exhibit nonlinear optical properties. Absorption and luminescence spectra of the crystals and their luminescence decay are studied at various temperatures. The luminescence lifetime at 1.2 μ m at 300 K is 10 μ s. Active centers in the crystal are identified, and crystal field parameters are estimated.     

Magnetic properties of single crystals of ludwigites Cu<Subscript>2</Subscript><Emphasis Type="Italic">M</Emphasis>BO<Subscript>5</Subscript> (<Emphasis Type="Italic">M</Emphasis> = Fe<Superscript>3+</Superscript>, Ga<Superscript>3+</Superscript>)

High-quality single crystals of ludwigites Cu₂ MBO₅ (M = Fe³⁺, Ga³⁺) have been grown, and the magnetic, resonance, and Mössbauer studies have been performed. It is established that the Cu₂FeBO₅ and Cu₂GaBO₅ compounds are antiferromagnets with Néel temperatures of 32 and 3.4 K, respectively. A model of the magnetic structure of the compounds is proposed. It is shown that the magnetic properties of the ludwigites are substantially dependent on the degree of ion distribution over crystallographic positions.     

ESR of Fe3+ -Rhombic,Fe2+ -Vo and Fe1+ -Vo complexes in SrTiO3

Three new Fe-defect complexes have been identified by ESR in Fe doped crystals of SrTiO₃. These are an Fe³⁺ center with rhombic fine structure in the “as-grown” crystals, and the vacancy associated complexes Fe²⁺ -V_o and Fe¹⁺ -V_o in “reduced” crystals. Photo-excited charge transfer effects are observed for these centers.     

Trigonal and triclinic Fe3+ paramagnetic centers in ferroelectric Pb5Ge3O11

In the EPR spectra of iron-doped lead germanate single crystals, triclinic Fe³⁺ paramagnetic centers have been found in addition to trigonal centers. Their contribution increases upon annealing in a chlorine-containing atmosphere. The parameters of the spin Hamiltonian of three triclinic centers assigned to Fe³⁺-Cl^? dimeric complexes have been determined. The localization of iron and chlorine ions in Pb₅Ge₃O₁₁ has been discussed.     

Divalent rare-earth ions in LaF<Subscript>3</Subscript> crystals

The optical spectra and electric conductivity of LaF₃ crystals doped with 0.01, 0.1, and 0.3 mol % YbF₃, where Yb was partly or completely recharged to the divalent state, are studied. The long-wavelength absorption band of 370 nm is caused by electrons transitioning from state 4f ¹⁴ to the level of anion vacancies. The remaining bands at 300–190 nm are caused by 4f ¹⁴–5d ¹4f ¹³ transitions in Yb²⁺. The bulk electric conductivity and peaks of the dielectric losses of LaF₃–Yb²⁺ crystals are caused by Yb²⁺–anion vacancy dipoles. The activation energy of the reorientation of Yb dipoles is 0.58 eV. The optical and dielectric properties of Yb²⁺ centers are compared to those of Sm²⁺ and Eu²⁺ centers studied earlier in LaF₃ crystals.     

Paramagnetic defects in BaTiO3 and their role in light-induced charge transport — Optical studies

On the basis of a previous identification of paramagnetic defects in nominally undoped as grown BaTiO₃ single crystals, we have investigated the changes of the concentrations of these centers and their optical absorptions under illumination with light of varying wavelengths. The most pronounced charge transfers occur by hole ionization of Fe⁴⁺ and — to a lesser extent — of Cr⁵⁺ and Cr⁴⁺. At low temperatures the created holes are trapped in the form of O^–-ions next to Al³⁺ or unknown acceptor defects. Corresponding Fe⁴⁺ and O^– absorptions have been identified.     

Determination of the charge state of chromium in Cr:Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> single crystals by spectral luminescence methods

The valence state and structural localization of Cr ions in Cr:Bi₁₂GeO₂₀ single crystals grown by the Czochralski method in air and annealed in argon or UV-irradiated have been determined in the spectral luminescence study. It is established that chromium is in the state of tetrahedrally coordinated Cr⁴⁺ ions in the samples studied. Octahedrally coordinated Cr²⁺ and Cr³⁺ ions and tetrahedrally coordinated Cr⁵⁺ ions were not revealed.