Effect of γ‐irradiation on spectral properties of undoped Y2SiO5 crystals

The absorption spectra of undoped Y₂SiO₅ crystals were studied before and after γ‐irradiation. After γ‐irradiation, the additional absorption peaks at 260‐270 and 320nm were observed in as‐grown and H₂‐annealed Y₂SiO₅ crystal, but it did not occur in air‐annealed Y₂SiO₅ crystal. These absorption peaks were attributed to F color centers and O^– hole centers, respectively. Owing to more oxygen vacancies and color centers in H₂‐annealed Y₂SiO₅ crystal than that in as‐grown Y₂SiO₅ crystal after γ‐irradiation, the additional absorption peaks were more intense in the former than that in the latter. With the irradiation dose increasing from 20 to 220kGy, the intensity of additional absorption peaks increased. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Cr3+‐doped LiNbO3 crystals grown by the Bridgman method

The growth of LiNbO₃ crystals doped with Cr³⁺ ions in 0.1, 0.2, and 0.5 mol % concentrations by Bridgman method were reported. The Cr³⁺ ion concentrations in crystals were measured by inductively coupled plasma spectrometry. Electron paramagnetic resonance had been used to investigate the sites occupied by the Cr³⁺ ions. Two Cr³⁺ ion centers located at Li⁺ and Nb⁵⁺ sites (Cr_Li³⁺ and Cr_Nb³⁺ centers, respectively) were observed. Optical absorption and temperature‐dependence emission spectra of the Cr³⁺ ions were reported. The crystal‐field parameters and Racah parameters of the Cr³⁺ ion defect sites were reported and compared with those grown by Czochralski technique. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of Cr ion in lithium metasilicate Li2O · SiO2 transparent glass-ceramics

The optical properties of Cr³⁺ ions in lithium metasilicate (Li₂O · SiO₂) transparent glass-ceramics were investigated. The main crystalline phase precipitated was the lithium metasilicate (Li₂O · SiO₂) crystal. The percent crystallinity and crystalline size were ranging 65-75% and 20-35 nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are found to be derived from Cr³⁺ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr³⁺ ions substitute for three Li⁺ ions and occupy the distorted octahedral site between single [SiO₄]_n chains of lithium metasilicate crystal. The ligand field parameters can be estimated: 10Dq = 13 088 cm⁻¹, B = 453 cm⁻¹, Dq/B = 2.89 and C = 2036 cm⁻¹. The near-infrared luminescence centered at 1250 nm was not detected in the deep pink glass-ceramics unlike emerald green glass.     

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.     

Distribution of oxidation states of Cr ions in Ca or Ca/Mg co-doped Cr:Y3Al5O12 single-crystal fibers with nitrogen or oxygen annealing environments

The valence states of Cr ions in Ca or Ca/Mg co-doped Cr:Y₃Al₅O₁₂ (YAG) single-crystal fibers are studied. The fibers were grown using the laser-heated pedestal growth method, followed by annealing treatments up to 1500 °C. The concentrations of the Cr³⁺ and Cr⁴⁺ ions in octahedral and tetrahedral sites in oxygen or nitrogen environments were characterized. Above 700 °C, migration of Cr⁴⁺ between octahedral and tetrahedral sites takes place; its relative stabilization energy was estimated. For Ca,Cr:YAG annealed in an oxygen or nitrogen environment, it was 0.25 and 0.3 eV, respectively. For Mg,Ca,Cr:YAG annealed in oxygen or nitrogen, it was 0.47 and 0.49 eV, respectively. For the Ca,Cr:YAG crystal fiber (Ca/Cr=113.1%) with oxygen annealing, about 35% and 2.5% of Ca ions took part in charge compensation for Cr⁴⁺ in the octahedral and tetrahedral sites, respectively. The density of oxygen vacancies depends on the concentration of Ca ions. The estimated ratios of the unreacted oxygen vacancies to total oxygen vacancies were about 63% and 88% for oxygen and nitrogen annealing, respectively. The main limitation on the concentration of Cr⁴⁺ in the tetrahedral site of YAG is the presence of unreacted oxygen vacancies.     

Investigation on the blue photorefractive properties varied with MgO codoping in Ru:Fe:LiNbO3 crystals

Mg:Ru:Fe:LiNbO₃ crystals with various doping concentration of MgO have been grown by Czochralski method. The type of charge carriers and photorefractive properties in Mg:Ru:Fe:LiNbO₃ crystals were measured by two‐wave coupling method using Kr⁺ laser (476 nm) and He‐Ne laser (633 nm) as light sources. We found that holes were the dominant charge carriers under blue light irradiation while electrons were the dominant charge carriers under red light irradiation. Mg²⁺ ions behaved no longer as damage resistant, but promoter to the photorefractive properties at 476 nm wavelength. The photorefractive properties under blue light improved with the increase concentration of Mg²⁺ ions. The enhancement mechanisms of the blue photorefractive were suggested. Experimental results definitely showed that Mg‐doped two‐centre Ru:Fe:LiNbO₃ was a promising blue photorefraction material for holographic volume storage.     

Investigation of the crystallization features,atomic structure,and microstructure of chromium-doped monticellite

A series of Cr⁴⁺:CaMgSiO₄ single crystals is grown using floating zone melting, and their microstructure, composition, and crystal structure are investigated. It is shown that regions with inclusions of second phases, such as forsterite, akermanite, MgO, and Ca₄Mg₂Si₃O₁₂, can form over the length of the sample. The composition of the single-phase regions of the single crystals varies from the stoichiometric monticellite CaMgSiO₄ to the solid solution Ca_{(1 ? x)}Mg_{(1 + x)}SiO₄(x = 0.22). The Cr:(Ca_0.88Mg_0.12)MgSiO₄ crystal is studied using X-ray diffraction. It is revealed that, in this case, the olivine-like orthorhombic crystal lattice is distorted to the monoclinic lattice with the parameters a = 6.3574(5) Å, b = 4.8164(4) Å, c = 11.0387(8) Å, β = 90.30(1)^o, Z = 4, V = 337.98 ų, and space group P2₁/c. In the monoclinic lattice, the M(1) position of the initial olivine structure is split into two nonequivalent positions with the center of symmetry, which are occupied only by Mg²⁺ cations with the average length of the Mg-O bond R _av = 2.128 Å. The overstoichiometric Mg²⁺ cations partially replace Ca²⁺ cations (in the M(2) position of the orthorhombic prastructure) with the average bond length of 2.347 Å in the [(Ca,Mg)-O₆] octahedron. The average distance in SiO₄ distorted tetrahedra is 1.541 Å.     

Effect of annealing and gamma irradiation on undoped and Eu3+-doped Y2SiO5 single crystals

The absorption spectra of the undoped Y₂SiO₅ and Eu³⁺-doped Y₂SiO₅ crystals grown by the Czochralski technique were compared before and after annealing and, similarly, the unannealed and annealed crystals after γ-ray irradiation. The absorption bands of Eu²⁺ ions with peaks at 300 and 390 nm were observed in the as-grown Y₂SiO₅:Eu³⁺ crystal. These peaks were more intense in H₂-annealed and irradiated Y₂SiO₅:Eu³⁺ crystals. The additional absorption peaks at 260 and 320-330 nm which were attributed to F color centers and O⁻ hole centers were observed in irradiated undoped Y₂SiO₅ and Y₂SiO₅:Eu³⁺ crystals, respectively.     

Structure analysis of MgSiO3 glass

The structure of MgSiO₃ glass was investigated by a combination of radial distribution function (RDF) analysis with an intensity comparison method on the basis of X-ray diffraction data. There appeared five peaks at 1.63, 2.08, 2.58, 3.25 and 4.2 Å in the RDF curve. The short range structure was found to be similar to that observed in pyroxenes: single chains composed of SiO₄ tetrahedra are linked by Mg²⁺ ions laterally. The Mg²⁺ ion is surrounded by six oxygen atoms on average, four of which form MgO pairs with an average distance of 2.08 Å and the remainder Mg O pairs of 2.5 Å which contribute to the formation of the third peak at 2.58 Å in the RDF curve together with the OO pairs (2.66 Å) in SiO₄ tetrahedra. The calculated intensity curve based on this model was in good agreement with the observed one.     

Growth Habit and Habit Variation of γ‐CuI Crystallites under Hydrothermal Conditions

The octahedra were observed in the γ CuI crystallites synthesized by hydrothermal method using 1.6g (C₂H₅OO)₂Cu and 2.66g KI as precursors at 200 °C for 12 hours in the de‐ionized water. The effect of additives on the morphology of γ‐CuI crystallites was investigated. Results show that the morphology of γ‐CuI crystallites prepared in the mixed solution of de‐ionized water and alcohol at 200 °C for 12 hours is the tetrahedron. In order to disclose the effect of the additive on the growth habit of γ‐CuI crystallites, the microcosmic growth mechanism of γ‐CuI crystal is investigated from the complex of I^‐ and Cu⁺ ions to each other. It is concluded that the effect of alcohol on the morphology of γ‐CuI crystallites is carried out through changing the relative rate of complex of anion and cation to each other at the interface. Based on the above analysis, the growth habit of γ‐CuI crystallites and the habit variation under hydrothermal conditions are explained reasonably.     

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 development of SiO2 resistant Cr-doped glass ceramics for high Cr4+ emission

The influence of SiO₂ inter-diffusion into Cr-doped CaO–Al₂O₃ glass, Ca₂GeO₄, and Mg₂SiO₄ glass ceramics at high temperature is so severe that almost none of Cr⁴⁺ emission can remain. Although the Cr⁴⁺ emission can be recovered up to 13% from its pristine value after re-heat treatment in the initial composition of Mg₂SiO₄ glass ceramics, the new composition based upon SiO₂ pre-deficiency technique proposed in this paper can retrieve up to about 4 times better, which indicates much less vulnerability to SiO₂ incorporation. Further improvement is possible with finer composition optimization. The results indicate the potential of pre-deficiency technique for making high Cr⁴⁺ emission in silica-based optical fibers under the fact that the Cr⁴⁺ emission is seriously degraded upon SiO₂ incorporation.     

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)     

Fabrication of Cr4+,Nd3+:YAG transparent ceramics for self-Q-switched laser

Transparent 0.1 at.%Cr,1.0 at.%Nd:YAG (Y₃Al₅O₁₂) ceramics were fabricated by a solid-state reaction and vacuum sintering with CaO as a charge compensator and tetraethyl orthosilicate (TEOS) as a sintering aid using high-purity powders of Al₂O₃, Y₂O₃, Nd₂O₃ and Cr₂O₃. The mixed powder compacts were sintered at 1800 °C for 5 h and 30 h under vacuum. The optical transmittance of the Cr,Nd:YAG ceramics sintered at 1800 °C for 5 h and 30 h is ∼63% and ∼78% in the infrared wavelengths, respectively. The two samples exhibit pore-free structures and the average grain size is about 10 and 20 μm. For the sample sintered at 1800 °C for 5 h, the dominant fracture mechanism is the transgranular fracture. With increase of holding time up to 30 h, the ratio of intergranular fracture surfaces increase and more Cr³⁺ ions in the Cr,Nd:YAG ceramic transform to Cr⁴⁺. High-quality Cr⁴⁺,Nd³⁺:YAG transparent ceramics may be a potential self-Q-switched laser material.     

Mild hydrothermal synthesis and structures of mixed‐valence iron phosphates: SrFe3(PO4)3 and the interesting Mg2+‐doped AFe3 (PO4)3 (A = Ba,Pb) in Fe2+ site

Series of mixed valence monophosphates AFe_3‐xMg_x(PO₄)₃ [A = Sr(x = 0), Ba(x = 0.6), Pb(x = 0.6)] were synthesized by mild hydrothermal treatment at 210 °C. Refinements of single crystal X‐ray diffraction datas show all these compounds are isostructural. The attempts to make AFe₃(PO₄)₃ (A = Ba, Pb) hydrothermally in the experiment were unsuccessful. However, the Mg‐doped homologues AFe_2.4Mg_0.6(PO₄)₃ (A = Ba, Pb) were synthesized with the addition of MgCO₃ in the reactants as mineralizer. EDS and single crystal X‐ray data refinement indicated that the Mg²⁺ cations were doped in the Fe²⁺ sites of AFe_2.4Mg_0.6(PO₄)₃ (A = Ba, Pb). The influence of the Mg‐doping on the structure and the reason why the Mg doped in the Fe(II) site instead of A site was discussed from the point of view of the bond valence model.     

Spectroscopic properties of Cr3+ ions in nanocrystalline glass–ceramic composites

Luminescence properties of Cr³⁺ ions in a silica-based precursor glass, and in fabricated optically transparent glass-based nanocrystalline composites, have been investigated. The luminescence spectra of the precursor glass revealed a wide range of crystal fields and showed the ⁴T₂–⁴A₂ broadband emission of Cr³⁺ ions in a weak crystal field, combined with ²E–⁴A₂ emission characteristic for Cr³⁺ ions in a strong crystal field. Glass–ceramic nanocomposites, with gallium oxide nanocrystals nucleated in a host glass matrix, indicated the prevailing contribution of the crystal-like ²E–⁴A₂ emissions (R-lines) of Cr³⁺ ions in a strong crystal field. The low-temperature studies demonstrated that the fluorescence of Cr³⁺ ions could be altered from sharp R-lines of the ²E–⁴A₂ transition, below 70 K, to a combination of R-lines and their sidebands, above 70 K. Our results indicate that, in the developed glass–ceramic nanocomposites, most of the Cr³⁺ ions have migrated from the host glass matrix to the nucleated gallium oxide nanocrystalline phase.     

Optical and Luminescence Properties of YAlO3 ‐Tm Crystals

The results on optical absorption, photoluminescence and thermoluminescence of YAP‐Tm crystals of 1 at.% and 4 at.% of Tm are presented as well as influence of UV‐, γ‐radiation and thermal treatments on optical and luminescence properties of the crystals. The blue‐green luminescence of defect centers was observed in YAP‐Tm crystals at Ar⁺‐laser excitation. The possibility of energy transfer processes between defect centers and Tm³⁺ ions was shown.     

Transmission spectra of crystals at elevated temperatures for the calculation of internal radiant heat transport during crystal growth – Part 2: Spectra of YAG:Cr,YVO4:Nd and the bandgap variation of various materials

The transmission spectra in the range from 350 to 3500 nm of the two most complex materials with regard to radiant heat transport – YAG:Cr⁺⁴ and YVO₄:Nd – are reported for temperatures between 25 and 1310 °C. The extreme shift of the short wavelength cut‐off (band gap) of YVO4:Nd with temperature is shown in comparison to that of some other materials. In YAG:Cr⁴⁺ we found a light‐induced absorption band near 2000 nm besides a strong reduction of the transmission in the visible and the near IR with increasing temperature. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects induced by chemical non‐stoichiometry and γ‐irradiation on the habit and unit cell parameters of ammonium tetrachlorozincate

The effect of chemical non‐stoichiometry and γ‐irradiation on the unit cell parameters of ammonium tetrachlorozincate (NH₄)₂ZnCl₄ (A₂ZC₄) has been studied. The unit cell parameters of crystal grown from solution with NH₄Cl/ZnCl₂ molar ratio 1:1, apparently non‐stoichiometrric, are nearly the same as those given for ammonium tetrachlorozincate in the literature. The 2:1 ratio is actually ‘pseudo‐stoichiometric’ due to the hygroscopic nature of ZnCl₂. The unit cell parameters of crystal grown from solution with molar ratio 2:1 match those of the structure (NH₄)₃ZnCl₅ (A₃ZC₅). The habit of the crystal grown in the former case, from solution with excess ZnCl₂, was different from that of the crystal grown in the later case, from solution with excess (NH₄)Cl. Between these two limits, a set of four samples were prepared from solutions with an excess of ZnCl₂ of 20, 30, 60 and 80 wt% in order to detect exact stoichiometric composition to grow A₂ZC₄. Analysis by X‐ray diffraction shows that the first two crystals out of this set are mixed from A₂ZC₄ and A₃ZC₅ The third and fourth crystals still contain traces of A₃ZC₅. Analysis of the X‐ray diffraction was then confirmed by DTA study. Irradiating A₂ZC₄ with γ‐dose of 250 kGy slightly increased the unit cell volume due to imperfections created by irradiation. Two computer programs were used to calculate the lattice constants and the results were compared. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Absorption,emission and absorption saturation of Cr4+ ions in calcium aluminate glass

Absorption, luminescence and absorption saturation of Cr ions in a calcium aluminate glass are studied. In the absorption spectrum, the absorption bands of Cr³⁺, Cr⁴⁺ and Cr⁶⁺ ions are revealed. The emission spectrum presents luminescence of Cr³⁺ ions centered at 0.82 μm and that of Cr⁴⁺ ions at 1.3 μm. The luminescence signal demonstrates short decay times of 120 ± 10 ns and 300 ± 20 ns for Cr³⁺ and Cr⁴⁺ ions, respectively. Absorption saturation measurements allowed an estimate of the ground-state absorption cross-sections for Cr⁴⁺ ions at 1.06 μm of 0.7 × 10⁻¹⁸ cm² and at 0.69 μm of 1.5 × 10⁻¹⁸ cm².