New aspects of the luminescence of magnesiumtitanate part I: Activation with chromium

Mg₂TiO₄:Cr³⁺ annealed at 560°C shows luminescence properties which differ considerably from those of the unannealed phosphor used so far. In both cases the regular Cr³⁺ ions only absorb the irradiated energy transferring it radiationless to the real luminescence centres, but the annealed phosphor reveals a new spectrum consisting of a few sharp lines. The radiance of the single lines can be influenced by powdering, which in connection with the annealing effect shows that the centres of both types of phosphors consist of clusters of lattice defects and octahedrally coordinated Cr³⁺ ions. These clusters or associates form themselves by thermal diffusion during the annealing process. Furthermore criteria have been found showing that the R lines do not come from only one but from two Cr³⁺ ions situated in non-equivalent lattice sites.     

Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO₃, congruent Cr:LiNbO₃ and congruent Cr,Mg:LiNbO₃ were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr³⁺ impurity ions that preferentially occupy Li⁺ sites (Cr_Li) in the Cr:LiNbO₃ crystals. Low-field centers related to Cr³⁺ substitution of Nb⁵⁺ (Cr_Nb) occur in addition to Cr_Li in co-doped Cr,Mg:LiNbO₃ samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr³⁺ centers from low- to high-field type due to strong pressure-induced blue shift of the ⁴ T ₂ state, resulting in its crossing with the ² E state of Cr³⁺. This level-crossing effect was observed for the dominant Cr³⁺ _Li and Cr³⁺ _Nb centers at pressures that correlate well with estimations based on the ⁴ T ₂-² Eenergy gap (230 cm^-1 and 1160 cm^-1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^-1/kbar, respectively). We also studied inhomogeneous broadeningof the ² E?⁴ A ₂transitions at ambient pressure for the minor high-field “defect” Cr³⁺ _Li centers in congruent LiNbO₃. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr³⁺ ions in the LiNbO₃ lattice. Received: 25 June 2001 / Published online: 2 November 2001     

Cr<Superscript>3+</Superscript> spectroscopy study in ZnO-codoped and Zn-in-diffused congruent LiNbO<Subscript>3</Subscript>:Cr crystals

This paper reports on the spectroscopy properties, absorption and luminescence, of Cr³⁺ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO₃:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr³⁺ ions located in Li⁺ and Nb⁵⁺ sites; [Cr]_Li and [Cr]_Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr³⁺ ions of the two defect centres. The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO₃:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr]_Nb centres in the diffusion zone. The location of the diffused Zn²⁺ ions is considered to be in Li⁺ site, displacing the Cr³⁺ ions from the Li⁺ sites, [Cr]_Li, to the Nb⁵⁺ positions, [Cr]_Nb.     

Electron spin resonance of chromia-yttria solid solutions

Electron spin resonance spectra of chromia-yttria solid solutions have been studied at room temperature for Cr concentrations between 0.20 and 2.00 mol%. Isolated Cr³⁺ ions in sites with two different symmetries were observed, as well as well as Cr³⁺ ions coupled by the exchange interaction. The relative concentration of isolated to coupled Cr³⁺ ions decreases with increasing chromium concentration. The results are consistent with the assumption that the chromium ions occupy preferentially the C₂ symmetry sites. A theoretical calculation based on this model yields an effective range of the exchange interaction between Cr³⁺ ions of 0.64 nm, of the same order as that of Cr³⁺ ions in MgO.     

The temperature dependence of the Raman spectra of chromium‐doped titanite (CaTiOSiO4)

The temperature dependence of the Raman bands of Cr⁴⁺ modes which show enhanced intensities due to pre‐resonance effects is reported from 293 to 673 K in chromium‐doped titanite (CaTiOSiO₄). Some aspects of the temperature dependence of Raman bands in pure, synthetic titanite which have not been previously published are also included in this study. Two Raman‐active components of A_g and B_g symmetry, respectively, of the symmetric Si–O mode in titanite are predicted under P2₁/a symmetry and also been identified in this phase for the first time. The one component of B_g symmetry disappears just above the antiferroelectric–paraelectric transition at ~500 K in accordance with the predictions under A2/a symmetry for the high‐temperature phase. Two resonance‐enhanced components of the Cr⁴⁺–O stretch are also evident in the P2₁/a phase and only one could be identified in the A2/a phase, again in accordance with group‐theoretical predictions. These observations can be used to characterize the P2₁/a and A2/a phases of pure synthetic and chromium‐doped titanite. The temperature dependence of the Cr⁴⁺–O modes can be approximated by two‐dimensional Ising behavior with the critical exponent β ≈ 1/8 below 450 K. Between 450 and 498 K, anomalous behavior is observed and this could be due to the appearance of mobile anti‐phase boundaries (APBs). Anomalous behavior also persists to temperatures above 500 K. The half‐width of the Ti–O stretching mode reflects the influence of the order parameter (Ti–O displacements) as well as mobile anti‐phase boundaries. No evidence could be found of the existence of other ions such as Cr⁴⁺‐ions in Ti‐sites and/or Cr³⁺‐ions also in Ti‐positions in Cr‐doped titanite in the Raman spectra using different laser lines to excite the spectra. Copyright © 2013 John Wiley & Sons, Ltd.     

Eu2+激活的Ca3SiO5绿色荧光粉的制备和发光特性研究

研究了Eu²⁺激活的绿色发光材料Ca₃SiO₅的制备条件和发光性质. Eu²⁺中心形成主峰值为501 nm和次峰值为570 nm的特征宽带，两峰值叠加形成发射峰值为502nm的绿色发射光谱带. 利用这些光谱结果和Van Uitert 经验公式，确认Ca₃SiO₅:Eu²⁺中存在两种性质有差异的Eu²⁺发光中心，它们分别占据基质中八配位的Ca²⁺(Ⅰ)格位和四配位的Ca²⁺(Ⅱ)格位. 其激发光谱分布在250—450 nm的波长范围，峰值位于375 nm处，可以被InGaN管芯产生的350—410 nm辐射有效激发. 关键词： 发光 荧光粉 绿色荧光粉 3SiO₅')" href="#">Ca₃SiO₅ 2+')" href="#">Eu²⁺     

Dominant Cr3+ centers in LiNbO3 under hydrostatic pressure

Low-temperature luminescence spectra of stoichiometric Cr: LiNbO₃ and of congruent Cr, Mg: LiNbO₃ were studied. Cr³⁺ impurity ions preferentially occupy Li⁺ sites (Cr_Li) in the LiNbO₃ crystal lattice, while Cr³⁺ ions substituting for Nb⁵⁺ ions (Cr_Nb) occur in addition to Cr_Li centers in codoped Cr, Mg: LiNbO₃ crystals. Application of a high hydrostatic pressure leads to a transformation of (dominant in concentration) Cr³⁺ centers from low-to high-crystal-field centers. Due to a strong pressure-induced blue shift of the ⁴ T ₂ state resulting in crossing with the ² E state, the replacement of the broad band ⁴ T ₂ → ⁴ A ₂ emission by a narrow R-line emission ² E → ⁴ A ₂ occurs in the luminescence spectra of the samples. This effect of level crossing was observed for the dominant Cr _Li ³⁺ and Cr _Nb ³⁺ centers at pressures which correlated well with estimations based on the ⁴ T ₂-² E energy gap (230 and 1160 cm^?1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^?1/kbar, respectively).     

Spectroscopic characteristics of LiGaSiO4:Cr nano-glass-ceramics and glassy precursors

We have performed a complex of spectroscopic investigations (extinction, luminescence, and luminescence excitation spectra and luminescence decay kinetics) of LiGaSiO₄:Cr nano-glass-ceramics and Cr-Li-Ga-Si-O glassy precursors. It has been shown that 94–96% of chromium in precursors synthesized in air are in the trivalent form, while remaining chromium is in the form of Cr⁶⁺ and Cr⁴⁺. The luminescence of the latter form in precursors at 300 K is strongly quenched. In precursors synthesized in an inert atmosphere, only trivalent chromium occurs. We have calculated the absorption cross sections of Cr³⁺ and Cr⁶⁺ ions in Cr-Li-Ga-Si-O glassy precursors. It has been shown that, in the case of synthesis of these precursors, a considerable amount of chromium is lost as a result of evaporation and fails to enter the specimens. Upon partial controlled crystallization of the precursors (formation of LiGaSiO₄:Cr nano-glass-ceramics), the oxidation state of chromium that passed to crystallites from the glass phase becomes tetravalent. In this case, two types of luminescence centers arise, which correspond to Cr⁴⁺ ions localized in two different polymorphic modifications of LiGaSiO₄. Spectral characteristics of these two centers (bandshape, peak position, and spectral range) are almost identical, whereas luminescence lifetimes are radically different.     

Luminescence of impurity 3d and 4f metal ions in different crystalline forms of Al2O3

The ² E-⁴ A ₂ luminescence spectra of Cr³⁺ ions in Al₂O₃ are investigated in the course of transitions between the structural forms γ-δ-θ-α. The spectral lines observed are assigned to Cr³⁺ ions in these structural forms, which are identified by an X-ray powder diffraction analysis. The lifetimes of the Cr³⁺ excited states in transient forms of Al₂O₃ are measured. Investigations of the luminescence spectra of Al₂O₃: Eu³⁺ demonstrate that the Eu³⁺ ions can form regular centers only in α-Al₂O₃ and, unlike the Cr³⁺ ions, give no rise to similar centers in moderately ordered θ-Al₂O₃.     

Spin Hamiltonian and structural disorder analysis for two high temperature Cr3+ defect centers in α-LiIO3 crystals—low symmetry effects

Complete diagonalization method (CDM) is developed for 3d³ ions in the trigonal type I crystal-fields (CFs). The CDM enables microscopic spin-Hamiltonian (MSH) calculations of the zero-field splitting (ZFS) parameter D and the Zeeman g-factors: g_∥ and g_⊥. CDM/MSH program is used for CF and MSH calculations for the high temperature (HT) Cr³⁺ defect centers (A) and (B) in α-LiIO₃. The contributions to the SH parameters from the single Li⁺ vacancy and the lattice distortions are considered. The calculated SH parameters of the two HT Cr³⁺ centers agree well with the experimental ones. Structural distortions are theoretically predicted with the bond angles α and β for the Cr–O₆ complex in Cr³⁺:α-LiIO₃ greater than those in the host crystal. By incorporating the MSH approach, the comprehensive CF analysis (CFA) package for 3d^N ions at arbitrary symmetry sites is at present extended as a CFA/MSH package to study SH parameters for 3d³ ions at trigonal type I CFs as well as the additional contributions to D, g_∥, and g_⊥ arising from the non-zero distortion angle ϕ at trigonal type II (e.g. C₃) CFs. The new finding for Cr³⁺:α-LiIO₃ is that g_⊥ depends strongly on ϕ, whereas g_∥ and D are rather insensitive to ϕ.     

Study of divalent and trivalent chromium in forsterite by high-frequency EPR spectroscopy

Divalent and trivalent chromium ions Cr²⁺ and Cr³⁺ replacing magnesium ions at octahedral positions in Mg₂SiO₄: Cr and Mg₂SiO₄: Cr: Li crystals are investigated by submillimeter EPR spectroscopy in the frequency range 65–230 GHz. The crystals are grown from the melt by the Czochralski method. The content of mixed-valence chromium species in forsterite is analyzed. It is demonstrated that, in crystals grown in argon (the oxygen partial pressure is \(P_{O_2 } \) = 0.01 kPa), approximately half of the chromium ions are in the divalent form. The Cr²⁺ ions are distributed over the M1 and M2 positions in a ratio of approximately 2: 1. A change in the oxygen partial pressure \(P_{O_2 } \) and the chromium concentration, as well as an additional doping with lithium, does not lead to substantial changes in the distribution of divalent chromium ions over the positions. It is shown that an increase in the oxygen partial pressure \(P_{O_2 } \) from 0.01 to 2.00 kPa results in a decrease in the coefficient of divalent chromium distribution between the crystal and the melt. Doping with lithium also decreases the concentration of Cr²⁺ centers. In crystals grown without lithium, approximately half of the trivalent chromium ions are associated with magnesium vacancies. The addition of lithium leads to the destruction of these associates, an increase in the concentration of individual Cr³⁺ centers, and the formation of lithium associates with trivalent chromium ions. The conditions for the formation of associates of trivalent chromium ions with lithium ions are optimum when the crystal contains approximately identical amounts of Cr³⁺ and Li⁺ ions. Doping with lithium increases the concentration of Cr³⁺ ions and, thus, decreases the fraction of Cr²⁺ and Cr⁴⁺ ions in the total content of chromium centers.     

High-frequency tunable EPR spectroscopy of Cr3+ in synthetic forsterite

The electron paramagnetic resonance spectra of isolated and dimer impurity centers of trivalent chromium ions in the octahedral Ml sites in synthetic forsterite are studied in the frequency range of 65–90 GHz. The measurements are performed at 4.2 K in magnetic field from ?0.04 to 0.3 T. The zero-field splitting between spin doublets of the isolated Cr³⁺ ion Δ_s = 66.7 GHz and between spin sublevels of the Cr³⁺-Cr³⁺ dimer Δ_d1 = 71.5 GHz and Δ_d2 = 73.0 GHz is measured directly at zero field. The analysis of the spin Hamiltonian parameters shows that the dimer center consists of a pair of Cr³⁺ ions with an Mg²⁺ vacancy between them replacing three Mg²⁺ ions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the exchange interaction in the dimer is ferromagnetic with parameters J_z = 0.47 GHz and J_t = 0.79 GHz.     

A semi-empirical analysis of ZFS parameters for transition metal (TM) ions (Cr3+ and Mn2+) centers in K2MgF4

The local structure around the transition metal (TM) ions (Cr³⁺ and Mn²⁺) centers in K₂MgF₄, namely tetragonal (TE) center I, monoclinic (MO) center II and orthorhombic (OR) centers III, have been explored by semi-empirical calculations in the frame of superposition model (SPM). We proposed the most appropriate structural model for each center by matching the theoretically predicted zero-field splitting parameters (ZFSPs) with the experimental ones obtained by EPR spectroscopy. It is shown that the tetragonality of MgF₆ octahedra increases with the substitution of both Cr³⁺ and Mn²⁺ for Mg²⁺ sites as well as with a rather higher value for the latter one. A tilt angle of 12.51° is found for MO Cr³⁺ center, which is comparatively small, as compared to the other A₂BF₄ crystals such as K₂ZnF₄ and Tl₂ZnF₄. Almost three times larger relaxation of intervening F-ligand than that of other equatorial F-ligands is also indicated for OR Cr³⁺ center.     

Spectroscopy of radiation-induced and structural defects in garnet crystals with V and Cr impurity ions

The optical absorption spectra of crystals with garnet structure (A₃B₂C₃O₁₂), containing V or Cr impurity ions, before and after oxidation annealing, γ irradiation, and coactivation with Ca and Mg ions have been experimentally and theoretically investigated. It is shown that V³⁺ and Cr⁴⁺ ions are incorporated into both octahedral and tetrahedral sites of the garnet lattice. The absorption spectra and energy level diagrams of impurity ions are identified.     

EPR study of charge compensation of chromium centers in the strontium titanate crystal

Chromium centers and their charge compensation in two single strontium titanate crystals, i.e., SrTiO₃: Cr (0.05 at %) and Sr_0.9995TiO₃: Cr_0.0005 grown with strontium deficiency, have been studied by the electron paramagnetic resonance method. The crystals have been investigated both immediately after growth and after oxidation and reduction procedures. Oxidation and reduction are performed by crystal annealing in a corresponding gas atmosphere at high temperature. Chromium centers associated with oxygen vacancy (Cr³⁺-V _O) are detected in the reduced crystals. It is shown that strontium vacancies are formed in the crystal grown with strontium deficiency, which leads to a lowering of the tetragonal symmetry of (Cr³⁺-V _O) and Cr⁵⁺ centers to the orthorhombic symmetry. Possible compensation mechanisms for charges of various chromium centers are considered.     

Magnetic behaviour of the mixed spinel ferrite GaxFe1-xNiCrO4

Mössbauer, X-ray diffraction, dc magnetization and ac susceptibility measurements have been carried out in the spinel ferrite Ga_xFe_1-xNiCrO₄ (0⩽x⩽1) in different temperature ranges between 1.4 and 700 K. These measurements show A site occupation of Fe³⁺ ions and a coexistence of ferrimagnetic and frustrated (entropic) spins in this system for all x. The frustration in this system arises due to the random distribution of Fe³⁺ and diamagnetic Ga³⁺ ions on the A sites, and those of Ni²⁺ and Cr³⁺ ions on B sites.     

Effect of non-stoichiometry on the occurrence of Er3+ luminescence sites in erbium-implanted zinc selenide

Erbium-implanted ZnSe cubic crystals with a high V_Zn concentration gave luminescence due to Er³⁺ ions at Zn_sub sites (Centre C) but no luminescence due to Er³⁺ ions at Zn_int sites (Centre A) or Se_sub sites (Centre B). On the other hand, Centres A and B were detected in samples with low V_Zn concentrations but Centre C was absent. To detect luminescence due to all three erbium centres moderate V_Zn concentrations were necessary. In such samples post-implantation annealing at 250–325°C caused Er³⁺ ions to occupy Se_sub and Zn_int sites but annealing at 400°C caused Er³⁺ ions to migrate from the Se_sub sites and be detected at Zn_sub sites.     

Neutron diffraction and Mössbauer study on FeGa x Cr2−x S4

Ga doped sulphur spinel FeGa _x Cr_2?x S₄ (x = 0.1 and 0.3) have been studied with X-ray, neutron diffraction, and Mössbauer spectroscopy. Rietveld refinement of X-ray, neutron diffraction, and Mössbauer spectroscopy lead to the conclusion that the samples are in inverse spinel type, where most Ga ions are present at octahedral site (B). The neutron diffractions on FeGa _x Cr_2?x S₄ (x = 0.1) above 10 K show long range interaction behaviors and reveal a ferrimagnetic ordering, with the magnetic moment of Fe²⁺(?3.45 μ_B) aligned antiparallel to Cr³⁺ (+2.89 μ_B) at 10 K. Fe ions migrate from the tetrahedral (A) site to the octahedral (B) site with an increase in Ga substitutions. The electric quadrupole splittings of the A and B sites in Mössbauer spectra give direct evidence that Ga ions stimulate an asymmetric charge distribution of Fe ions in the A site.     

Interactions hyperfines magnetiques et mecanisme de compensation de la charge pour les ions d'impurete 119Sn4+ dans Cr2O3

Mössbauer spectra of Cr₂O₃ sesquioxide doped with ¹¹⁹Sn⁴⁺ ions lead to the hypothesis of three non-equivalent magnetic sites for Sn⁴⁺. The identification of these sites has allowed to estimate the relative importance of exchange interactions between Cr³⁺ ions along [111] axis and in (111) plane.     

Spectroscopic investigations on ZnF2-MO-TeO2 (MO=ZnO, CdO and PbO) glasses doped with chromium ions

Differential scanning calorimetric studies, spectroscopic studies (viz., optical absorption, ESR, infrared spectra) and thermoluminescence studies of ZnF₂-MO-TeO₂ (MO=ZnO, CdO and PbO) glasses doped with different concentrations of chromium ions have been investigated. Results have been analyzed in the light of different oxidation states of chromium ion. The analysis indicates chromium ions mostly exist in Cr³⁺ state in ZT, CT and PT glasses when Cr₂O₃ is present upto 0.1%, 0.2% and 0.3%, respectively; these ions seems to be present in Cr⁶⁺ state that take part network forming positions with CrO₄²⁻ structural groups in all the three glasses when the concentration of Cr₂O₃ is greater than the above quantities. Quenching of thermoluminescence light output of the glasses has been observed with increase in Cr₂O₃ concentration upto 0.1%, 0.2% and 0.3%, respectively in ZT, CT and PT glasses. This has been identified due to the killing effect of Cr³⁺ ions. Further it has also been concluded that ZnF₂-PbO-TeO₂ is more favourable glass network for the presence of large concentration of laser-emitting Cr³⁺ ions, among the three glass series studied.