Peculiar temperature-dependent fluorescence of EuCl3 in LiCl molten salt as a result of phase changes

Fluorescence spectra were obtained in situ as a function of temperature (ranging from room temperature to 956 K) from a melt of EuCl₃ and LiCl. Three different characteristic Eu²⁺ fluorescence bands, associated with phase changes, were observed. The critical fluorescence dependence on temperature appearing in the blue fluorescence were resulted from the radiative relaxation from 4f⁶5d¹ excited state to 4f⁷ (⁸S_7/2) ground state of Eu²⁺, which was reduced from Eu³⁺ of EuCl₃ at high temperature. The fluorescence studies could provide information regarding the phase changes estimated as the stable dihalide, aggregation, and precipitation states of Eu²⁺ in alkali halide not only crystallic but also fluidic melting matrix.     

Irradiation-induced change of valence of Cr3+ ions doped in alkali sulphates

It is shown by means of investigation of both optical absorption spectra and Roentgen K-lines of chromium doped in LiKSO₄, LiNaSO₄, and Li₂SO₄ · H₂O crystals, that X-irradiation results in change of the impurity charge in a following way: Cr³⁺ + h → Cr⁴, Cr⁴⁺ + h → Cr⁵⁺.     

Reduction and luminescence of europium ions in glass ceramics containing SrF2 nanocrystals

Reduction of Eu³⁺ → Eu²⁺ and luminescence of europium (Eu) ions in glass ceramics containing SrF₂ nanocrystals have been investigated. The formation of SrF₂ nanocrystals in glass ceramics was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Blue luminescence of the Eu²⁺ ions was observed in the Eu doped glass ceramics which were prepared by the heat treatment of the glass in air atmosphere. The double-exponential decay curves of ⁵D₀ state of Eu³⁺ in the Eu doped glass ceramics indicated that there were two different surroundings of the Eu ions in the glass ceramics.     

Luminescence characteristics of undoped and Eu‐doped GdCa4O(BO3)3 single crystals and nanopowders

Single crystals of GdCa₄O(BO₃)₃ (GdCOB) pure and doped with Eu concentration of 1 and 4 at% were grown by the Czochralski and micropulling‐down methods. The distribution of Eu ions in GdCOB crystals was uniform. The substitutions of Eu³⁺ in Gd, Ca(1) and Ca(2) cation sites and eventually formation Eu²⁺ have been investigated. The spectroscopic properties of crystals are compared with the properties of nanopowders obtained by sol‐gel method. Radioluminescence spectra of undoped GdCOB crystal show the characteristic emission of Gd³⁺ at about 312 nm, whereas this emission dramatically decreases in Eu‐doped crystals upon X‐ray excitation, as well as in Eu‐doped nanopowders excited in vacuum ultraviolet (VUV) region. The VUV excitation in the range 125‐333 nm for Eu‐doped samples leads to strong emission in red coming from the ⁵D₀ multiplet of Eu³⁺, only. In the photoluminescence decay kinetics of 312 nm emissions substantial shortening and departure for single exponential decay in Eu‐doped samples is clearly observed. Higher Eu doping results in further acceleration of the decay. In undoped GdCOB crystal, the lifetime of the Gd^{3+ 6}P_7/2 multiplet is 2.79 ms. The Eu^{3+ 5}D₀ decay kinetics monitored at 613 nm are rather constant. Numerical fitting of fully exponential curves, reveals lifetimes 2.7 ms for nanopowder and 2.5 ms for single crystal. The results suggest that this material may be used as a red phosphor in plasma display panels in nanopowder form because of strong excitation band of Eu³⁺ luminescence in the 160‐200 nm regions. Contrary to nanopowder sample, such an excitation band, attributed to the Gd³⁺–O^2– charge transfer was not observed in crystal obtained by the micropulling‐down method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Luminescence properties of Y3Al5O12:Eu3+-coated submicron SiO2 particles

Silica submicron spherical particles coated with an yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) layer doped with Eu³⁺ were prepared by the sol–gel method. The structure and morphology of samples determined by the X-ray powder diffraction measurements and transmission electron microscope images, respectively, indicated that well-crystallized garnet nanocrystallites were formed with successive coating cycles. Similar trends were deduced from the evolution of the luminescence spectra. The ratio of integrated intensities of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions was used to analyze the structural variations in the surroundings of the Eu³⁺ ion. The effect of coating was analyzed by comparing the luminescence properties of the Y₃Al₅O₁₂:Eu³⁺ nanocrystalline powders and composite Y₃Al₅O₁₂:Eu³⁺/SiO₂ materials.     

Characterization of Eu3+-doped fluorophosphate glasses for red emission

Fluorescence spectra and decay curves of the ⁵D₀ level for different concentrations of Eu³⁺ (4f⁶) ions in K–Ba–Al fluorophosphate glasses have been measured at room temperature and are analyzed. The Judd–Ofelt intensity parameters Ω₂ and Ω₄ have been determined from the intensity ratios of emission peaks corresponding to ⁵D₀ → ⁷F_J (J = 2 and 4) to ⁵D₀ → ⁷F₁ transitions for 1.0 mol% glass. The intensity parameters thus obtained are in turn used to calculate the radiative properties of the fluorescent levels of Eu³⁺ ions. Second and fourth rank crystal-field parameters have been evaluated by assuming a C_2V site symmetry for the local environment of Eu³⁺ ions to estimate the crystal-field strength experienced by Eu³⁺ ions in the present host. The decay profiles of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions in the present glasses are found to be single exponential for all the studied Eu³⁺ ion concentrations. A marginal increase in lifetime of the ⁵D₀ level has been noticed with Eu³⁺ ion concentration up to 2.0 mol% and then the lifetime marginally decreases for higher Eu³⁺ ion concentrations.     

Energy transfer from Bi3+ sensitizing the luminescence of Eu3+ in clusters embedded into sol–gel silica glasses

Y³⁺(La³⁺), Eu³⁺ and Bi³⁺ ions co-doped sol–gel silica glasses were synthesized. Photoluminescence spectra show that there is energy transfer from Bi³⁺ ions to the emission band of Eu³⁺ ions. The co-dopants Y³⁺ or La³⁺ have strong effects on the local structure and luminescence of Eu³⁺ ions. For 0.5 mol% Eu³⁺ ions doped glasses, the co-doping of 1 mol% Bi³⁺ and 1 mol% Y³⁺ is the most appropriate for the sensitization from Bi³⁺ to Eu³⁺. The sensitization effectiveness from Bi³⁺ ions to Eu³⁺ ions was studied by changing the amount of Bi³⁺ and Y³⁺, and clusters containing rare earth ions and Bi³⁺ ions dominate the energy transfer processes. The comparison of luminescent R-values (the intensity ratio of ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ in Eu³⁺ ions) between glasses containing La³⁺ and containing Y³⁺ verifies the formation of clusters in sol–gel glasses. As a favorable configuration for energy transfer, the accurate design and synthesis of clusters-contained glasses may provide a new kind of luminescent materials.     

Characterization,luminescence and EPR investigations of Eu2+ activated strontium aluminate phosphor

Strong blue-green light emitting Eu doped SrAl₂O₄ phosphor was synthesized by a low-temperature initiated, self-propagating and gas producing combustion process in a very short time (<5 min). The prepared powder was characterized by X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscopy. The excitation spectrum shows a peak at 397 nm. Upon excitation at 397 nm, the emission spectrum exhibits a well defined broad band with maximum at 493 nm corresponding to 4f⁶5d → 4f⁷ transition. Electron paramagnetic resonance (EPR) measurements at X-band showed low field signals due to Eu²⁺ ions in SrAl₂O₄:Eu.     

Time-resolved fluorescence measurements on Eu3+- and Eu2+-doped glasses

Various fluoride, phosphate and borosilicate glasses with known properties and global structure have been doped with Eu³⁺ (4f⁶) at doping concentrations between 10¹⁸ and 10²¹ cm^?3. By applying reductive melting conditions Eu³⁺ could partially be transformed to Eu²⁺ (4f⁷). Four fluoroaluminate glasses with varying phosphate content between 0 and 20 mol% (FPx), a pure phosphate glass (P100) and two borosilicate glasses with low (DURAN^®-like) and high optical basicity (NBS1) have been used for these investigations. The time-resolved fluorescence in the visible range has been studied for both ions. Although static and dynamic fluorescence of Eu³⁺ and Eu²⁺ are dramatically different (f–f-transitions for Eu³⁺; d–f-transitions for Eu²⁺), glass structure changes have a similar influence on the dynamic fluorescence behavior of both ions. A strongly ionic surrounding due to fluorine ligands as in fluoroaluminate glass samples provides the longest fluorescence lifetime (about 7 ms for Eu³⁺; about 1.3 μs for Eu²⁺). Increasing phosphate content decreases the fluorescence lifetime due to more oxygen ligands. Interesting differences have been found for the two borosilicate glasses due to the difference in their optical basicity (Na₂O/B₂O₃ ratio). Measurements indicate a homogeneous distribution of europium ions in most FP samples. NBS1 measurements suggest that two different local europium sites are formed. For Duran-like samples only one specific europium site was found, although these samples show phase separation at high doping concentrations into a SiO₂-rich phase and borate- and europium-rich droplets. Fluorescence quenching due to energy transfer from Eu²⁺ to Eu³⁺ could be found for co-doped samples; Eu³⁺-doped samples show no fluorescence quenching.     

Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses

Europium-doped lead germanate and lead fluorogermanate glasses are studied by using differential thermal analysis, X-ray diffraction, photoluminescence and fluorescence lifetimes measurements of the ⁵D_j, j = 0, 1, 2 levels. PbF₂ addition increases the thermal stability of the lead germanate glass, while Eu³⁺ ions promote the crystallization of β-PbF₂:Eu³⁺ nano-crystals embedded in a glassy matrix. In the lead fluorogermanate glasses, Eu³⁺ ions exhibit a strong affinity for F⁻ ions although oxygen ions are much more numerous. It appears that luminescence concentration quenching is not important, while cross relaxation is very efficient in the glasses. The results allow to propose for these glasses a molecular model in which small fluorine rich island, incorporating the Eu³⁺ ions in low symmetry sites, are separated from each other by chains of germanate (GeO₄)⁴⁻ ions linked together.     

Structural chemistry of peroxo compounds of group VI transition metals: I. Peroxo complexes of chromium (a review)

The specific features revealed in the structure of the d ³ Cr(III), d ² Cr(IV), d ¹Cr(V), and d ⁰ Cr(VI) peroxo complexes with the ratios M:O₂ = 1:1, 1:2, and 1:4 are considered. It is noted that, in eleven compounds of the general formula Cr(O₂)_nO_m A _p (n = 1, 2, 4; m = 0, 1; p = 0–4), the metal atoms can be in four oxidations states: +3 (d ³), +4 (d ⁴), +5 (d ¹), and +6 (d ⁰). This property distinguishes chromium peroxo compounds from molybdenum and tungsten dioxygen complexes, which, with one exception, are represented by the d ⁰ M(VI) compounds.     

Effect of impurities on the optical properties of PbWO<Subscript>4</Subscript> crystals

The effect of excess W and La³⁺, Y³⁺, and Mo⁶⁺ impurities on the luminescence spectra and the thermally stimulated luminescence of PbWO₄ crystals is studied. A high tungsten content (up to 1 at %) in the charge mixture results in a high luminescence intensity in the green spectral region. Lanthanum impurity decreases the transmission in the short-wavelength (k > 29 000 cm^?1) spectral region. Lanthanum and yttrium impurities decrease the luminescence intensity. At low (10² ppm) concentrations, Mo impurity can somewhat increase the light yield. However, the presence of Mo in PbWO₄ shifts the absorption-band edge to longer wavelengths, while an increase in the Mo concentration above 10² ppm decreases the luminescence intensity.     

Luminescent nano-composites generated from a spray

The spray-pyrolysis (SP) synthesis technique has been employed to obtain SiO₂:Eu³⁺ and γ-AlOOH:Eu³⁺. It was possible to obtain sub-micrometric spherical particles of SiO₂ with luminescent Eu³⁺ ions bonded to the silica surface or embedded in amorphous silica beads, by controlling the synthesis and annealing process. Boehmite γ-AlOOH doped with Eu³⁺ nanoparticles were synthesized by SP at moderate temperature (200 °C) with Eu³⁺ ions bonded to the surface hydroxyls of the boehmite nanocrystals. Luminescent nano-composites were obtained by controlled reaction of γ-AlOOH:Eu³⁺ nanocrystals with ASN (asparagine). In these nano-composites, the Eu³⁺ are held at the surface of the boehmite nanocrystals and partially shielded from interactions with additional luminescence quenchers (hydroxyl groups, water molecules).     

X-ray diffraction study of defects in zinc-diffusion-doped silicon

Samples of CZ n-Si〈Zn〉(111) are prepared by high-temperature zinc-diffusion annealing followed by quenching and are studied by X-ray diffraction. The silicon contains an initial phosphorus impurity and zinc-compensating admixture at concentrations N _P = 1.5 × 10¹⁴ cm^?3 and N _Zn = 1 × 10¹⁴ cm^?3; i.e., the relation N _P/2 < N _Zn < N _P is fulfilled. Microdefects are studied by double- and triple-crystal X-ray diffraction in the dispersion free modes (n, ?n) and (n, ?n, +n). The samples are found to contain microdefects with two characteristic sizes (average sizes of about 1 μm and 70 nm). The interplanar distance in the near-surface layer with a thickness of 0.1 μm is smaller than this parameter in the remaining matrix, the difference being equal to d ₀ Δd/d ₀ ≈ 2 × 10^?5. This layer contains mainly vacancy-type microdefects. The angle between the reflecting planes and the local surface relief is Δψ = (7 ± 1) arcmin.     

Spectral analysis of a novel Eu2+ activated SiO2–BaO glass ceramics phosphor under UV-LED excitation

A novel Eu²⁺ activated 60SiO₂–40BaO (mol%) glass ceramics phosphor was prepared and the optical properties were investigated. X-ray diffraction (XRD) and Raman spectra confirmed the formation of Ba₂Si₃O₈ nano-crystals in the glass matrix. The Eu²⁺ activated glass ceramics exhibited broad emission band centered at 518 nm due to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. Compared with the glass, the emission intensity of Eu²⁺ activated glass ceramics was much stronger, and the peak wavelength shifted toward shorter wavelength. The photoluminescence excitation (PLE) spectra of the glass ceramics showed an overlap with the main emission region of an ultraviolet light-emitting diode (UV-LED). According to the photoluminescence (PL) spectra, the CIE chromaticity coordinates of the Eu²⁺ activated glass and glass ceramics were calculated. The results indicated that the Eu²⁺ activated glass ceramics containing Ba₂Si₃O₈ nano-crystals can be used as a potential green emitting phosphor under UV-LED excitation.     

Chemical co-precipitation of hybrid precursors to synthesize Eu3+/Dy3+ activated YNbxP1−xO4 and YNbxV1−xO4 microcrystalline phosphors

A modified wet-chemical synthesis technology was put forward to fabricate Eu³⁺/Dy³⁺ doped with YNb_xP_1−xO₄ and YNb_xV_1−xO₄ phosphors with varying x from the assembly of multicomponent hybrid precursors. The morphologies have been found to be 1–2 μm crystalline spheres using XRD and SEM. The red photoluminescence intensity reaches the strongest in YNb_0.8P_0.2O₄:Eu³⁺ and YNb_0.1V_0.9O₄:Eu³⁺ phosphors and the red to orange intensity ratio (RO) values decrease with the content of P(V) increasing. Besides this, the optimum concentration for Dy³⁺ luminescence in YNb_0.5P_0.5O₄ is 1 mol%, while that in YNb_0.5V_0.5O₄ is 0.5 mol% or lower than 0.5 mol%. The yellow to blue intensity ratio (YB) value of Dy³⁺ increases when the Dy³⁺ concentration increases from 0.5 to 8 mol% in YNb_0.5P_0.5O₄; however, that of Dy³⁺ does not vary much in YNb_0.5V_0.5O₄ matrix.     

Impurity-trapped excitons: Experimental evidence and theoretical concept

The paper summarizes experimental evidence of anomalous luminescence in RE-doped dielectric materials. Special attention is paid to Pr³⁺-doped LiNbO₃ and LiTaO₃, and Eu²⁺-doped XF_2, where X = Ba, Sr. Luminescence, luminescence excitation spectra and luminescence kinetics results are discussed obtained at ambient and high hydrostatic pressure at various temperatures. Hydrostatic pressure has been shown to cause a red shift of anomalous luminescence. The experimental data show the existence of temperature- and pressure-induced spectral transformation where anomalous luminescence is replaced with normal d–f emission in Eu²⁺ centers. The model predicts strong electron-lattice coupling of bound excitons and relaxation in the system’s excited states, as well as the pressure effect of spectral transformation from anomalous to normal emission.     

Photoluminescence from the 5D4 level of Tb3+ ions in K–Ba–Al fluorophosphate glass under pressure

Fluorescence spectra of Tb³⁺ ions in 55.5P₂O₅–17K₂O–14.5BaO–8Al₂O₃–4AlF₃–1.0Tb₄O₇ (PKBAFTb) glass have been measured as a function of high pressures up to 35 GPa at room temperature. A strong red shift of ⁵D₄ → ⁷F_J (J = 3, 4 and 5) transitions with different rates is observed under pressure, which is attributed to the increasing spin–orbit coupling constant, ζ_4f. There is a considerable pressure effect on the magnitude of the crystal-field splittings observed for the ⁵D₄ → ⁷F₄ and ⁷F₅ transitions. The increase in Stark splittings observed under pressure is due to pressure induced electrostatic interaction between the 4f electrons and their ligands. Decay times of the emission from the ⁵D₄ level fit to a single exponential function for the entire pressure range studied. Analyses indicate that the luminescence properties under pressure are more or less reversible, which shows the absence of considerable hysteresis in the present studied glass.     

The validity of the double layer model on the amorphous semiconductor surface including the bulk localized state within the gap

Under the two assumptions that the origin of surface states may be different from that of bulk localized states within the gap and the density of surface states is sufficiently high, the validity of the double layer model on the amorphous semiconductor surface is investigated in comparison with the case of a crystal. It is suggested that the criteria concerning the double layer should be determined by the relative value of the surface states to that of bulk localized states. The existence of the double layer can be confirmed when the bulk localized state density n(E_f) is smaller than 10¹⁹ cm^?3 eV^?1. When n(E_f) is high at about 10²⁰ cm^?3 eV^?1, surface states cannot be distinguished from the localized states within the gap. This double layer model is strongly supported by the results of previous experiments by others who have measured the dependence of the Schottky barrier height on the work function of metal and the dependence of the surface potential on the preparation conditions of a-SiH.     

Novel luminescent rare earth hybrids covalently trapped through phthalic anhydride linkage

In the present work, a novel functional molecular bridge (PASi) is achieved through the modification of ortho phthalic anhydride (PA) by N-2-aminoethyl-3-aminopropyl-methyl-dimethoxylsiliane (abbreviated as AEAPMMS). The PASi plays the role of a functional bridge to link Eu³⁺ (Tb³⁺) through the oxygen atom to a Si–O network formed from tetraethoxysilane. Subsequently, a novel chemically bonded hybrid material with double chemical bonds (RE–O coordination bond and Si–O covalent bond) has been fabricated. The characteristic red luminescence of Eu³⁺ and green luminescence of Tb³⁺ indicate that an efficient energy transfer process exists within the hybrid systems.