EPR spectra of Cr3+-doped LuNbO4 crystals

Ferroelastic LuNbO₄ single crystals containing 0.3 at. % Cr³⁺ ions have been grown by the floating zone technique, and their EPR spectra have been studied at a frequency of 9.8 GHz at room temperature. The lines on the spectra are due to the transitions caused by three paramagnetic centers formed as a result of the replacement of one isovalent position of a Lu₃₊ ion and two nonisovalent positions of Nb₅₊ions by Cr₃₊-ions. As a result of twinning, the line number is doubled and four principal directions arise along which the same spectra are obtained. The spectra of these centers were described by a spin Hamiltonian with S = 3/2, the D and E parameters ranging from 0.024 to 0.17 cm^?1, and the g-factors g ∥ = 1.75–2.20 and g ⊥ = 1.90–2.13.     

Growth and characterization of pure and Co2+‐doped Li2B4O7 single crystals

Pure and Co‐doped Li₂B₄O₇ (LBO) single crystals were grown by the Czochralski method. Starting concentrations of Co₂O₃ in the melt were: 0.5, 0.85 and 1 mol% relative to Li₂CO₃. Technological factors affecting the quality of both crystals were discussed. Optical absorption and EPR spectra were analyzed to define the oxidation states and lattice sites of cobalt ions. It was shown that Co²⁺ ions enter LBO crystal at octahedral Li⁺ site positions. Low‐temperature EPR measurements revealed that two types of Co²⁺ complexes can be distinguished in the Li₂B₄O₇:Co crystals. Additional absorption calculated for γ‐irradiated crystals showed V_k type defects suggesting the creation of cation vacancies during growth. The concentration of the defects decreases with an increase of intentional Co concentration. Introduction of cobalt ions to LBO crystal is limited probably by the formation of cobalt ion pairs or by the entrance of cobalt as Co⁺. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Czochralski Growth of Potassium Lithium Niobate Single Crystals with Low Li Content and Their Characterization

Single crystalline and crack free potassium lithium niobate (KLN) single crystals with low Li content were grown by the Czochralski method. The crystal composition can be written as K_2.60Li_1.17Nb_5.44O₁₅ (=K_2.95Li_1.33Nb_6.17O₁₇) which contain relatively fewer Li ions than ferroelectric K₃Li₂Nb₅O₁₅ crystals. All experimental results show that the deficiency of the Li ions in the KLN crystals strongly influences their physical properties. Especially, the as‐grown crystals do not indicate any signature for a ferroelectric phase transition in contrast to the ferroelectric K₃Li₂Nb₅O₁₅ crystals. However, due to ionic conduction, the temperature dependence of the dielectric constant of such KLN‐2 crystals show a broad anomaly near 300°C. In addition, the existence of proton defects can be revealed by infrared absorption spectroscopy near 3500 cm^‐1 in as‐grown crystals.     

Studies of EPR g factors for tetragonal and rhombic Cr5+ centers in ferroelectric PbTiO3

The anisotropic g factors (g_x, g_y, g_z) at low temperature and the motionally averaged isotropic g factor at high temperature for two Cr⁵⁺ centers, the tetragonal t‐Cr⁵⁺ center and the rhombic O‐Cr⁵⁺ center, in ferroelectric PbTiO₃ are calculated from the high‐order perturbation formulas based on a two‐mechanism model. In the model, the contributions to g factors from both the crystal‐field (CF) mechanism concerning the CF excited states and the charge‐transfer (CT) mechanism (which is neglected in CF theory) concerning the CT excited states are contained. The calculated results are in reasonable agreement with the experimental values. From the calculations, the defect models of t‐Cr⁵⁺ center (which is attributed to Cr⁵⁺ in the tetragonally‐compressed octahedron caused by Jahn‐Teller effect rather than in the tetragonally‐elongated octahedron in the host PbTiO₃) and O‐Cr⁵⁺ center (which is due to the t‐Cr⁵⁺ center perturbated by the electrical polarization perpendicular to the C₄ axis) are confirmed, and the defect structure of t‐Cr⁵⁺ center is obtained. It is found that in the precise calculations of g factors for the high valence state d¹ ions in crystals, both the contributions due to CF and CT mechanisms should be taken into account. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect structure and spectroscopic characteristics of codoped Hf: Er: LiNbO3 crystals

Codoped Hf: Er: LiNbO₃ crystals have been grown by the Czochralski technique. Defect structures of the crystals were analyzed by IR absorption spectra, and the compositions of the crystals were measured by X‐ray fluorescent spectrograph. A new OH^–‐associated vibrational peak at 3492 cm^–1 was revealed in 6 mol % Hf: 1 mol % Er: LiNbO₃ crystal. It was attributed to (Hf_Nb)^–‐OH^–‐(Er_Nb)^2– defect centers. The Er³⁺ concentrations in crystals gradually decreased with the increase of the codoped Hf⁴⁺ concentrations in the melts. The emission characteristics of the crystals were investigated by the fluorescence spectrum. It was found that the luminescent intensity in codoped 6 mol % Hf: 1 mol % Er: LiNbO₃ crystal was 3.5 times stronger than that in single doped 1 mol % Er: LiNbO₃ crystal. The luminescent enhancement effect was successfully explained on the basis of defect structure of the crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Recharging processes of Cr ions in Mg2SiO4 and Y3Al5O12 crystals under influence of annealing and γ ‐ irradiation

Recharging processes of chromium ions were investigated for Mg₂SiO₄:Mg, Cr single crystals using annealing in O₂ and in air and γ‐irradiation, as compare to YAG :Ca, Cr single crystals. The formation of tetravalent Cr ions in the Mg₂SiO₄ :Mg, Cr is related not only to the initial Cr content in the melt, oxygen partial pressure and O₂‐ vacancy existing in the crystal, but also to the external field such as γ‐irradiation. The additional absorption after γ‐irradiation shows the decrease in intensity of the absorption of Cr³⁺ and Cr⁴⁺ ions in some part of the spectrum and increase in the other giving evidence on recharging effects between Cr³⁺ and Cr⁴⁺. There arises also color centers observed between 380 nm and 570 nm that may participate in energy transfer of any excitation to Cr⁴⁺ giving rise to Cr⁴⁺ emission. Opposite to forsterite crystal, absorption spectrum of YAG:Ca, Cr crystal after γ‐irradiation reveals only increase in the absorption of the Cr bands. The observed behavior of the absorption spectrum of YAG:Ca, Cr crystal under influence of γ‐irradiation suggests that γ‐irradiation ionizes only Cr ions. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of X-ray irradiation on the optical properties of chromium-doped KliSO4 crystals

The absorption and circular-dichroism spectra of chromium-activated KLiSO₄ crystals both nonirradiated and irradiated with an X-ray beam have been studied. It was established that in nonirradiated crystals chromium ions are mainly trivalent (Cr³⁺) and have octahedral coordination. In irradiated crystals, along with the centers provided by (Cr³⁺) ions, new centers are formed associated with (^Cr4+) and (^Cr5+) ions.     

The absorption and circular dichroism spectra of langasite family crystals doped with chromium ions

The absorption and circular dichroism spectra of langasite family crystals, La₃Ga₅SiO₁₄, La₃Ga₅GeO₁₄, Ca₃Ga₂Ge₄O₁₄, Sr₃Ga₂Ge₄O₁₄ (red), Sr₃Ga₂Ge₄O₁₄ (green), La₃Ta_0.5Ga_5.5O₁₄, and La₃Nb_0.5Ga_5.5O₁₄, which were doped with chromium ions, have been investigated in the range of 240–850 nm. It is shown that chromium ions are incorporated into the structure of the investigated crystals both in the octahedrally (Cr³⁺ ion in 1a octahedron) and tetrahedrally (Cr⁴⁺ ion in 2d tetrahedron) coordinated positions. The ion ratio Cr³⁺/Cr⁴⁺ changes in a wide range in the crystals studied.     

Spectroscopy studies of pure and chromium-doped calcium gallogermanate crystals, Ca3Ga2Ge4O14

Absorption spectra, the spectra of circular dichroism and magnetic circular dichroism of pure and Cr-doped Ca₃Ga₂Ge₄O₁₄ crystals have been studied. The crystal-field parameters and the transition frequencies of Cr³⁺ ions are calculated. The location of Cr³⁺ and Cr⁴⁺ ions in oxygen octahedra and tetrahedra is confirmed. The spectra of circular dichroism show the maxima due to Cr³⁺ ions and growth-induced defects.     

Czochralski growth and constitutional studies on single crystals of potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2−xNb_5+xO₁₅ (KLN) is a very promising material for the conversion of infrared light to light in the visible region. However, growing of single crystals is known to be complicated due to the considerable anisotropy of the growth rate and the thermal expansion behaviour. The single crystals of KLN, Mg²⁺‐doped KLN, as well as the mixed crystals of potassium lithium tantalate niobate K₃Li₂(Nb_1−xTa_x)₅O₁₅ (KLTN) were grown by the Czochralski technique. The chemical analyses of the samples were performed by atomic absorption spectroscopy (AAS) and X‐ray fluorescence analysis (XRF). The element concentrations along the single crystals were measured by the electron microprobe analysis (EMPA) to clarify the segregation phenomena in the grown crystals. The elements distribution coefficients were also calculated. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

State of the chromium ion in soda silicate glasses under various oxygen pressures

Optical absorption and ESR spectra of chromium in soda silicate glasses were measured to characterize the electronic environment of the chromium ion in these glasses. Glasses produced in very strongly reducing conditions showed a broad optical absorption in the wavenumber range of 10 000–25 000 cm^?1 without any ESR absorption, which suggested the formation of Cr²⁺ ions. Glasses produced in air or moderately reducing conditions showed ESR signals suggesting that there are three different states of Cr³⁺ ions, the strongly coupled Cr³⁺ ion pairs, the weakly coupled Cr³⁺ ion pairs and the isolated Cr³⁺ ions in elongated tetragonal sites or sites with lower symmetry. The presence of Cr⁵⁺ ions in glasses produced in air was also suggested. It was indicated that the critical partial oxygen pressure of the formation of Cr²⁺ ions is in the vicinity of P_O2 = 10^?8 atm and that Cr²⁺ ions do not coexist with Cr³⁺ ions homogeneously in soda silicate glasses.     

Spectroscopy and crystal-field analysis of Cr4+-doped transparent silicate glass-ceramics

We report transparent Cr⁴⁺-doped SiO₂–Al₂O₃–ZnO–Li₂O–K₂O glass-ceramics with broadband infrared luminescence. After heart-treatment, Li₂ZnSiO₄ crystallite was precipitated in the glasses, and its average size increased with increasing heat-treatment temperature. Racah parameters of Cr⁴⁺–Li₂ZnSiO₄ glass-ceramics have been calculated, and it was confirmed from absorption spectra that the energy levels of Cr⁴⁺-doped glass-ceramics are close to the cross point ¹E and ³T states. No infrared emission was detected in the as-made glass samples, but the broadband infrared emission centered at 1210 nm with the full width at half maximum (FWHM) of more than 250 nm was observed by exciting the glass-ceramics with excitation of an 808 nm laser diode. In order to analyze the located crystal field of Cr⁴⁺ ions, the emission spectra are fitted by multi-peak Gauss fitting. It is seen that the fluorescence spectra are fitted into two Gaussian bands at around 1195 and 1263 nm with band widths of 208 and 278 nm, respectively. The two Gaussian bands at around 1195 and 1263 nm have about the same decay rate, and hence they would probably originate from the same luminescent centers. The observed infrared emission could be attributed to Cr⁴⁺ ions at low-field sites in Li₂ZnSiO₄ glass-ceramics.     

Structure and optical properties of near stoichiometric Ce:LiNbO3 crystals

The near sotichiometric Ce:LiNbO₃ (Ce:SLN) crystals were grown by the top seeded solution growth (TSSG) method by adding K₂O flux to Li₂O‐Nb₂O₅ melt. Their UV‐vis absorption spectra and IR spectra were measured and discussed to investigate their defect structure. The results showed that the grown crystals were near stoichiometric and Ce ions in the crystals located the Li site. Photorefractive properties of Ce:SLN crystals were studied by two‐wave coupling experiment. The results of the two‐wave coupling experiments of the crystals showed that as the CeO₂ doping concentrations increased, the diffraction efficiency increased, photoconductivity decreased and the writing time and erasure time increased. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of Optical Properties of YAlO3Pr3+ Single Crystals

Colour centers in YAlO₃:Pr³⁺ crystals originating under xenon lamp or y–radiation and oxidizing annealing are considered. The nature of colour centers under these treatments is similar in nominally pure and Pr³⁺ containing crystals but the intensity of induced absorption lines is higher in activated crystals which assumably is associated with valent transitions of Pr ions.     

Investigations of spin‐Hamiltonian parameters and defect structures for two tetragonal Cu2+ centers in KTaO3 crystal

Based on the defect models that the two tetragonal Cu²⁺ centers in KTaO₃ are due to Cu²⁺ ions at Ta⁵⁺ sites associated, respectively, with one and two oxygen vacancies (V₀) along C₄ axis because of charge compensation, the spin‐Hamiltonian parameters (g factors g_‖, g_⊥and hyperfine structure constants A_‖, A_⊥) of both Cu²⁺ centers in KTaO₃ are calculated from the perturbation theory method (PTM) and the complete diagonalization (of energy matrix) method (CDM). The calculated results from both theoretical methods are not only close to each other, but also in reasonable agreement with the experimental values. This suggests that both methods are effective in the explanations of spin‐Hamiltonian parameters for 3d⁹ ions in tetragonal symmetry. From the calculations, the defect models are confirmed and the defect structural data are obtained for both Cu²⁺ centers in KTaO₃. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Czochralski growth and characterization of Dy: La3Ga5.5Nb0.5O14 single crystals

Dysprosium (Dy) doped La₃Ga_5.5Nb_0.5O₁₄ single crystals were grown by the traditional Czochralski method along z‐axis. The structure of the crystal has been studied by X‐ray powder diffraction method, and the unit‐cell parameters are calculated to be a=8.22070 Å, c=5.12533 Å and V=299.965 ų. The segregation coefficient of Dy³⁺ in La₃Ga_5.5Nb_0.5O₁₄ crystal was measured by X‐ray fluorescence analysis. For 1 mol% doping level in the melt, the distribution coefficient of Dy³⁺ was determined to be 0.341 wt%. Specific heat, thermal expansion and transmission spectrum of Dy: La₃Ga_5.5Nb_0.5O₁₄ single crystals have been measured. The fluorescence spectra of Dy³⁺: La₃Ga_5.5Nb_0.5O₁₄ crystals were measured at room temperature, and there were four emission transitions occurring at 479, 576, 662 and 754 nm, respectively. The fluorescent lifetimes measurement results show 1.0% Dy: La₃Ga_5.5Nb_0.5O₁₄ possesses shorter fluorescence decay time (303.4 μs) than does 1.0%Dy:LGS (436.12 μs). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of trace metals on reactive crystallization of gypsum

The effect of Fe²⁺, Fe³⁺, and Cr³⁺ ions on crystallization of calcium sulfate dihydrate (gypsum) produced by the reaction between calcium hydroxide suspension and sulphuric acid solution was investigated at 3.5 pH and 65°C in the absence and presence of 2500 ppm citric acid concentration. Crystal size distributions, filtration rates, and morphology of gypsum were determined and discussed as a function of ion concentration. Average particle size of gypsum was not affected significantly by the presence of Fe²⁺, Fe³⁺, and Cr³⁺ ions individually. Variation of gypsum morphology depending on ion concentration affected the filtration characteristics. The presence of Fe³⁺ or Cr³⁺ ions besides 2500 ppm citric acid influenced both average particle size and filtration characteristics. The effect of citric acid on gypsum morphology was suppressed at high Fe³⁺ and Cr³⁺ ion concentrations. The change of morphology is related to the complex formation between Fe³⁺ or Cr³⁺ ions and citric acid at high ion concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cr3+ luminescence quenching in stoichiometric lithium niobate crystals

Cr³⁺-doped stoichiometric LiNbO₃ crystals were grown and detailed spectroscopic investigations were performed. The samples are characterized by extremely low frequency factor of the luminescence thermal quenching. Its numerical value 1.7 × 10⁸ s⁻¹ is about 10⁴ times lower than in crystals of lithium-aluminum fluorides. Under such conditions, radiative transitions of Cr³⁺ ions compete successfully with non-radiative ones, resulting in a relatively high quantum yield of the broadband luminescence at room temperature. The quenching of luminescence is counteracted by the effect of the dynamic removal of the radiative transition exclusion, and as a result, Cr³⁺ radiative lifetime in LiNbO₃ decreases from 8 to 4 μs when temperature grows up from 77 to 300 K. Therefore quantum yield of the broadband luminescence at room temperature is not 5–10%, as reported previously, but about 30%. The results obtained in the present study show the stoichiometric lithium niobate doped with Cr³⁺ ions to be a potential active media for tunable lasers.     

Intensities of circular dichroism and absorption bands and the states of <Emphasis Type="Italic">d</Emphasis> ions in absorbing media: II. Tetrahedrally coordinated positions of <Emphasis Type="Italic">d</Emphasis> ions

The factors affecting the band intensity in the circular dichroism (CD) and absorption spectra of tetrahedrally coordinated d ions in an absorbing medium (symmetry selection rules, structural position, and bond covalence) are analyzed. It is shown by the examples of the Cr⁴⁺ ion in Ca₃Ga₂Ge₄O₁₄ crystal and the Fe²⁺ and Fe³⁺ ions in SiO₄ crystal that the symmetry forbiddenness of the transitions from orbitally degenerate states and the covalence of the d-ion-ligand bond lead to changes in the intensity of the corresponding CD bands in a wide range, beginning from zero. It is shown by the example of Ca₃Ga₂Ge₄O₁₄, LiAlGeO₄, LiGaGeO₄, and LiGaSiO₄ crystals activated with Cr⁴⁺ ions that the preferred ion localization position corresponds to a higher effective symmetry.     

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⁺.