Nanocrystalline nature of high-symmetry Ce<Superscript>4+</Superscript>-Eu<Superscript>3+</Superscript> centers in silica gel glasses

The spectral-luminescent properties of silica gel glasses coactivated by Ce⁴⁺ and Eu³⁺ ions are investigated. The structure of the glasses is studied using x-ray powder diffraction and small-angle neutron scattering. The inference is drawn that Ce⁴⁺-Eu³⁺ centers with high-symmetry Eu(III) oxo complexes, which are formed in the glasses, have nanocrystalline nature. These centers are characterized by a weak vibronic interaction of Eu³⁺ ions with the matrix. The size of nanocrystallites formed under the synthesis conditions and at coactivator concentrations used is approximately equal to 10 nm.     

ESR spectra of Eu centers in defect Ga2S3 single crystals

In this paper, the author presents the results of measurements of the low-temperature and angular dependences of the ESR spectra of Eu²⁺ centers in defect Ga₂S₃ single crystals in the temperature range 8–29 K and for 0–180° orientations of the static magnetic field. The electron structure of impurity ¹⁵¹Eu atoms in Ga₂S₃:Eu single crystals has been studied by using the ESR method at different doping proportions of Eu atoms. Ga₂S₃ single crystals were grown from the melt using the Bridgman method. The Eu concentration was determined by atomic absorption analysis and X–ray fluorescence analysis (XRFA). By investigation on the ESR spectra, the author has first determined the values of charge states for Eu, which have turned out to be a Eu²⁺(4f⁷) ion with spin S=7/2, g=4.18±0.02 and concentration of the states of Eu N=6.3×10¹⁴ cm⁻³.     

Stabilization of Eu2+ in KLuS2 crystalline host: an EPR and optical study

Electron paramagnetic resonance and luminescence spectroscopies were applied to study the incorporation and charge stability of Eu²⁺ luminescent ions in single crystals of KLuS₂:Eu found in an earlier optical study [Jary et al., Chem. Phys. Lett. 574 , 61 (2013)]. The location of Eu²⁺ in the structure was unambiguously determined and three different centers were identified and described. Two of these centers correspond to substitution of Eu²⁺ for K⁺ and Lu³⁺ ions providing thus effective mechanism for Eu²⁺ incorporation due to the charge self‐compensation in the lattice. The observed luminescence spectra are consistent with the results of electron paramagnetic resonance experiment and can be decomposed accordingly. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Superhyperfine Structure of the EPR Spectra of Nd<Superscript>3+</Superscript> Impurity Ions in Fluorite CaF<Subscript>2</Subscript>

EPR spectra of a CaF₂ single crystal that was grown from melt containing a small addition of NdF₃ were studied. Signals corresponding to tetragonal centers of Nd³⁺ ions and cubic centers of Er3+ and Yb3+ ions were found. Superhyperfine structure (SHFS) in the spectra of the Nd³⁺ ions was observed for the first time in this crystal; parameters of the superhyperfine interaction of the Nd³⁺ ions with the nearest nine fluorine ions were determined. The dependence of the resolution of the Nd³⁺ EPR spectrum SHFS on the incident microwave power at the temperature of T ≈ 6 K was studied. Obtained results are discussed and compared with the literature data.     

Effect of isoelectronic impurities K<Superscript>+</Superscript> and I<Superscript>−</Superscript> on luminescence of CsBr:Eu<Superscript>2+</Superscript> crystals

We have investigated the photoluminescence (PL) and photostimulated luminescence (PSL) spectra at 300 K to study the effect of isoelectronic impurities K⁺ and I⁻ on the formation and energy structure of Eu²⁺-V_Cs isolated dipole centers and aggregate centers in the form of single crystals of CsEuBr₃ in CsBr:Eu²⁺ single crystals. We have shown that K⁺ and I⁻ impurities in a concentration of 5 mol% do not have a substantial effect on the energy spectrum of isolated dipole centers in CsBr:Eu²⁺ single crystals and the processes for the formation of such centers during growth of CsBr:Eu single crystals from the melt by the Bridgman method. We have established that in Cs_0.95K^0.05Br:Eu²⁺, more favorable conditions are realized for the formation of aggregate centers than in CsBr:Eu²⁺ and CsBr^0.95K^0.05Br:Eu²⁺ single crystals. So in order to improve the storage properties of phosphors based on CsBr:Eu²⁺, in particular for increasing the efficiency of PSL excitation, it is expedient to dope them with K⁺ impurity in a concentration up to 5 mol%. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 627–630, September–October, 2007.     

Luminescence properties of phosphors based on Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> (TbAG) terbium-aluminum garnet

The processes of excitation energy transfer in phosphors based on single-crystal Tb₃Al₅O₁₂:Ce (TbAG:Ce) and Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb³⁺ cations) to Ce³⁺ and Eu³⁺ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions and from Ce³⁺ ions to Eu³⁺ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb³⁺ cation sublattice.     

Luminescence of CsBr:Eu films grown by liquid-phase epitaxy

By liquid-phase epitaxy from an aqueous alcoholic solution, we have obtained films of the well-known storage phospor CsBr:Eu, and we have studied their cathodoluminescence and photoluminescence (PL) spectra compared with the undoped CsBr films. We have established that the structure of the photoluminescence centers of the CsBr:Eu films when excited by laser radiation in the absorption band of the Eu²⁺ ions (λ = 337 nm) includes Eu²⁺-V_Cs isolated dipole centers and CsEuBr₃ aggregate centers, and also luminescence centers based on inclusions of hydroxyl group OH⁻ with the corresponding emission bands in the 440 nm, 520 nm, and 600 nm regions. We have studied the dependence of the spectra and the intensity of the photoluminescence for CsBr:Eu films on annealing temperature in air at 423–483 K, compared with analogous dependences for CsBr:Eu single crystals obtained from the melt. We have shown that annealing the films at T = 423–463 K leads to rapid formation of CsEuBr3 aggregate luminescence centers, while for T > 473 K thermal degradation of these centers occurs. We conclude that the observed differences between the photoluminescence spectra of CsBr:Eu films and CsBr:Eu single crystals may be due to additional doping of the films with OH⁻ ions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 191–194, March–April, 2006.     

Ionoluminescence of Eu<Superscript>2+</Superscript>–Eu<Superscript>3+</Superscript> clusters in NaF: Eu single crystals

The radiation-impurity modification of NaF: Eu crystals results in the formation of optically active planar heterostructures with a complex set of luminescence centers, including, in particular, clusters of the Eu²⁺–Eu³⁺ type. The luminescence spectra of Eu²⁺–Eu³⁺ centers exhibit bands at wavelengths of 409 and 442 nm, which are associated with Eu²⁺ ions in nonequivalent crystallographic positions, and a band at a wavelength of 610 nm, which is attributed to Eu³⁺ ions. The luminescence spectra of irradiated NaF: Eu samples contain a broad band with a maximum at 506 nm due to the presence of F₂ + F ₊ ³ color centers in the crystal.     

Molecular self-organization of Ho<Superscript>3+</Superscript> impurity ions in synthetic forsterite

The structure of paramagnetic centers formed by impurity Ho³⁺ ions in synthetic forsterite is studied by submillimeter EPR spectroscopy in the frequency range 65–200 GHz. It is found that Ho³⁺ enters into the Mg²⁺ sublattice in the form of single ions and dimer centers. The concentration of dimer centers considerably exceeds the concentration of single ions, which points to the molecular self-organization of Ho³⁺ impurity ions into dimers during the growing of the crystals from melt. Possible structures of the dimer center are discussed. The parameters of the effective spin Hamiltonian describing the behavior of the electron-nuclear sublevels of the two lowest electronic levels of the Ho³⁺⁵I₈ ground multiplet are determined for a single ion and a dimer center.     

Thermally stimulated recombination processes and luminescence in Li<Subscript>6</Subscript>(Y,Gd,Eu)(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li₆(Y,Gd,Eu)(BO₃)₃ have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃: Eu, and Li₆Gd(BO₃)₃: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the ⁶ P _J → ⁸ S _7/2 transitions in the Gd³⁺ ion and the group of lines at 580–700 nm due to the ⁵ D ₀ → ⁷ F _J transitions (J = 0–4) in the Eu³⁺ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.     

Spectral separation of Cr<Superscript>3+</Superscript> optical centers in stoichiometric magnesium-doped lithium niobate crystals

The broadband luminescence of chromium optical centers with strongly overlapping spectral lines and similar emission probabilities from excited ⁴ T ₂ states of red and green Cr³⁺ centers in stoichiometric magnesium-doped lithium niobate crystals has been separated for the first time. The spectral-luminescence characteristics and parameters of intracenter interaction between red and green optical Cr³⁺ centers in stoichiometric lithium niobate have been calculated. The luminescence quantum efficiencies of red and green chromium centers are determined.     

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.     

Fluorinated Eu‐Doped SnO2 Nanostructures with Simultaneous Phase and Shape Control and Improved Photoluminescence

Fluorinated Eu‐doped SnO₂ nanostructures with tunable morphology (shuttle‐like and ring‐like) are prepared by a hydrothermal method, using NaF as the morphology controlling agent. X‐ray diffraction, field‐emission scanning electron microscopy, high‐resolution transmission electron microscopy, X‐ray photoelectron spectroscopy, and energy dispersive spectroscopy are used to characterize their phase, shape, lattice structure, composition, and element distribution. The data suggest that Eu³⁺ ions are uniformly embedded into SnO₂ nanocrystallites either through substitution of Sn⁴⁺ ions or through formation of Eu‐F bonds, allowing for high‐level Eu³⁺ doping. Photoluminescence features such as transition intensity ratios and Stark splitting indicate diverse localization of Eu³⁺ ions in the SnO₂ nanoparticles, either in the crystalline lattice or in the grain boundaries. Due to formation of Eu‐F and Sn‐F bonds, the fluorinated surface of SnO₂ nanocrystallites efficiently inhibits the hydroxyl quenching effect, which accounts for their improved photoluminescence intensity.     

Silica gel glasses with a high efficiency of luminescence sensitization in the Ce<Superscript>3+</Superscript>-Tb<Superscript>3+</Superscript> system

Terbium-and (Ce, Tb)-containing glasses prepared using the direct sol-gel-glass transition are studied. It is shown that glasses doped with one activator contain two main types of optical centers, namely, isolated and complex centers, which are characterized by weak and strong cross-relaxation quenching of luminescence from the ⁵D₃ state of Tb³⁺ ions, respectively. The Ce⁴⁺-Tb³⁺ (Tb⁴⁺) complex centers are formed during sintering of coactivated xerogels in oxygen and can be transformed into Ce³⁺-Tb³⁺ centers through saturation of the samples with hydrogen. The Ce³⁺-Tb³⁺ centers exhibit efficient luminescence from the ⁵D₄ state upon excitation into the absorption bands of Ce³⁺ ions.     

Photoluminescence of colloidal YVO4:Eu/SiO2 core/shell nanocrystals

YVO₄:Eu, and YVO₄:Eu/SiO₂ nanocrystals (NCs) were prepared by hydrothermal method with citrate as capping ligands. Their morphologies, structures, components, and photoluminescence properties were investigated and presented in this paper. A remarkable fluorescence enhancement up to 2.17 times was observed in colloidal YVO₄:Eu/SiO₂ NCs, compared to that of colloidal YVO₄:Eu NCs. This is mainly attributed to the formation of the outer protecting layers of biocompatible SiO₂ shells; which shield the Eu³⁺ ions effectively from water and thus reduces the deleterious effects of water on the luminescence. Meanwhile, on the basis of laser selective excitation, two kinds of luminescent centers were confirmed in the NCs, namely, inner Eu³⁺ ions and surface Eu³⁺ ions. The surface modifications for YVO₄:Eu NCs effectively reduced the surface defects and accordingly enhanced the luminescence. The core/shell NCs exhibited long fluorescence lifetime and high photostability under ultraviolet radiation.     

Local structure of Tm<Superscript>2+</Superscript> and Yb<Superscript>3+</Superscript> impurity centers in <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Ca,Sr, Ba) fluoride crystals

The local structure of Tm²⁺ and Yb³⁺ cubic impurity centers in MeF₂: Tm²⁺ and MeF₂: Yb³⁺ (Me = Ca, Sr, Ba) fluoride crystals, as well as Yb³⁺ trigonal and tetragonal impurity centers in MeF₂: Yb ³⁺ crystals, is calculated within the shell model in the pair potential approximation.     

Electron paramagnetic resonance of Cr<Superscript>3+</Superscript>-Li<Superscript>+</Superscript> centers in (Cr,Li): Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> synthetic forsterite

Synthetic single crystals of chromium-and lithium-doped forsterite, namely, (Cr,Li): Mg₂SiO₄, are studied using electron paramagnetic resonance spectroscopy. It is revealed that, apart from the known centers Cr³⁺(M1) and Cr³⁺(M2) (with local symmetries C_i and C_s, respectively), these crystals involve two new types of centers with C₁ symmetry, namely, Cr³⁺(M1)′ and Cr³⁺(M2)′ centers. The standard parameters D and E in a zero magnetic field [zero-field splitting (ZFS) parameters expressed in GHz] and principal components of the g tensor are determined as follows: D=31.35, E=8.28, and g=(1.9797, 1.9801, 1.9759) for Cr³⁺(M1)′ centers and D=15.171, E=2.283, and g=(1.9747, 1.9769, 1.9710) for Cr³⁺(M2)′ centers. It is found that the lowsymmetric effect of misalignment of the principal axes of the ZFS and g tensors most clearly manifests itself (i.e., its magnitude reaches 19°) in the case of Cr³⁺(M2)′ centers. The structural models Cr³⁺(M1)-Li⁺(M2) and Cr³⁺(M2)-Li⁺(M1) are proposed for the Cr³⁺(M1)′ and Cr³⁺(M2)′ centers, respectively. The concentrations of both centers are determined. It is demonstrated that, upon the formation of Cr³⁺-Li⁺ ion pairs, the M1 position for chromium appears to be two times more preferable than the M2 position. Reasoning from the results obtained, the R₁ line (the ²E → ⁴A₂ transition) observed in the luminescence spectra of (Cr,Li): Mg₂SiO₄ crystals in the vicinity of 699.6 nm is assigned to the Cr³⁺(M1)′ center.     

Luminescence VUV spectroscopy of cerium-and europium-doped lithium borate crystals

The paper reports on a study of the luminescence of lithium borate crystals (Li₆Gd(BO₃)₃ doped by Eu³⁺ and Ce³⁺ ions, Li_5.7Mg_0.15Gd(BO₃)₃: Eu, and Li₆Eu(BO₃)₃) initiated by selective excitation by synchrotron radiation at excitation energies of 3.7–27 eV at 10 and 290 K. Efficient energy transfer between the rare-earth ions Gd³⁺ → Ce³⁺ and Gd³⁺ → Eu³⁺ was found to proceed by the resonance mechanism, as well as by electron-hole recombination. Fast decay kinetics of luminescence of the Ce³⁺ activator centers was studied under intracenter photoexcitation and excitation in the interband transition region. The mechanisms involved in luminescence excitation and radiative relaxation of electronic states of rare-earth ions are analyzed, and the energy transfer processes operating in these crystals are discussed.     

Synthesis,crystal structure,and luminescence properties of CaY2Ge3O10:Ln3+, Ln = Eu,Tb

The crystal structure and luminescence properties of CaY₂Ge₃O₁₀:Ln³⁺ (Ln = Eu, Tb) germanates synthesized via a conventional solid-state reaction and an ethylenediaminetetraacetic acid complexing process are studied. The CaY_{2 ? x} Ln _x Ge₃O₁₀ (Ln = Eu, Tb; x = 0–1.0, 2.0; Δx = 0.1) solid solutions have a monoclinic structure (space group P2₁/c, Z = 4), in which dopant ions occupy three nonequivalent noncentrosymmetric sites with different Ca²⁺/Ln³⁺ ratios. The effect of the synthesis methods, dopant concentrations, and excitation wavelengths on the luminescence properties of the compounds obtained is determined.     

Ligand electron-nuclear double resonance of <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> Trigonal Gd<Superscript>3+</Superscript> centers in CaF<Subscript>2</Subscript> with a mixed oxygen-fluorine environment

This paper reports on the results of ligand electron-nuclear double resonance (ENDOR) investigations of T₁ trigonal ¹⁵⁷Gd³⁺ centers in the CaF₂ compound. It is experimentally found that the nearest environment of an impurity center contains only one ¹⁹F ion. Anions in the other coordination shells are identical to those in the pure CaF₂ crystal. However, ¹⁹F ions in these shells are displaced from their ideal positions in the lattice. The parameters of the ligand hyperfine interaction (LHFI) for ¹⁹F nuclei and their coordinates and displacements with respect to the positions in the lattice of the pure CaF₂ crystal are determined. It is demonstrated that the unusual isotropic LHFI constant A _s >0 for Gd³⁺ ions in the lattice with a mixed oxygen-fluorine nearest environment can be associated with the strong polarization of impurity centers in accordance with the empirical model proposed in [1], provided the structural model of the nearest environment of impurities in the T₁ centers [2] is correct. This structural model is confirmed by the analysis of the isotropic hyperfine constant A(s) for ¹⁵⁷Gd³⁺ centers.