Titanium atoms dimerization phenomenon and magnetic properties of titanium-antisite (TiO) and chromium doped rutile TiO2, ab-initio calculation

The ab-initio calculations based on the Korringa Kohn Rostoker approximation approach combined with coherent potential approximation (KKR-CPA), were used to study the magnetic properties of the titanium anti-site (Ti_O) and chromium (Cr) doped TiO₂. In the considered systems, we used different concentrations for Ti_O defect and Cr doping. In TiO_2(0.98)(Ti_O)_0.02, the obtained results indicate that Ti_O is a donor having half-metal behavior. Ti_O[3d] band is located at the Fermi level, although isn’t 100% polarized, the ferromagnetic (FM) state is verified as being more stable than disordered local moment (DLM) state. For Ti_0.98Cr_0.02O₂ the Cr doping introduced new states which give the material half-metallic feature. The majority spin of Cr impurities are located at the Fermi level and the conduction electrons around the Fermi level are 100% spin polarized. This indicates the stability of (FM) state. Moreover, in Ti_0.98Cr_0.02O_2(0.98)(Ti_O)_0.02, the top of the valence band is shifted to lower energy compared to pure TiO₂, and the n-type of TiO₂ is verified. The majority spin of Cr[3d] are located at 0.025 Ry close to the Fermi level. The predicted Curie temperatures (Tc) were calculated using the mean field approximation (MFA) and we predicted that Ti_O defect in Cr doped TiO₂ makes Tc higher. This kind of defect makes the material useful for spinotronics's applications and devices.     

Direct chromium speciation using X‐ray spectrometry and chemometrics

Chromium is a chemical element that occurs in two principal distinct forms: trivalent (Cr(III)) and hexavalent (Cr(VI)). As chromium toxicity depends on its oxidation state, the concentrations of both the species should necessarily be determined, and not only that of total chromium. Using X‐ray spectrometry allied to chemometrics, this work presents a new method for chromium speciation, with no prior separation and/or preconcentration of Cr(III) and Cr(VI) species. Spectra of several solutions containing different concentrations of Cr(III) and Cr(VI) were obtained. The data were treated through principal component analysis and partial least squares regression. The results obtained show that direct speciation using a conventional X‐ray spectrometer is possible, with calculated limits of detection and quantification for Cr(III) and Cr(VI) being lower than 17 and 50 µg g^?1, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Properties of erbium silicate doped with chromium in porous silicon

The possible formation of chromium-doped erbium silicate Er₂SiO₅: Cr in thin layers of porous silicon is demonstrated. This paper reports on studies of the photoluminescence, electron paramagnetic resonance, and transverse current transport in porous silicon layers (with different chromium and erbium contents) grown on n-and p-silicon single crystals heavily doped with shallow impurities. The Er₂SiO₅: Cr phase with the photoluminescence maxima at approximately 1.3 and 1.5 μm manifests itself after high-temperature annealing at 1000°C. The introduction of erbium and annealing at 700°C increase the intensity of the red photoluminescence of porous silicon by several factors. The decrease in the electrical conductivity of porous silicon suggests the onset of the formation of erbium silicate. The current-voltage characteristics exhibit a nonlinear behavior with an exponential dependence of the current on the voltage due to the discrete electron tunneling. An electron paramagnetic resonance spectrum of P _b centers in p-type heavily doped silicon is observed for the first time.     

Optical properties of LiNbO3:Cr crystals co-doped with germanium oxide

Lithium niobate (LiNbO₃) crystals doped with chromium ions show a clear green colouring reflecting the absorption profile of the dominating [Cr]_Li defect centres. A significant change in its colouration takes place when it is co-doped with other valency impurities such as Mg²⁺, Sc³⁺ and W⁶⁺, above a certain threshold concentration. This concentration singularity has been attributed to the formation of [Cr]_Nb centres coexisting with the [Cr]_Li centres.In this work, we extended the investigation on the effect of co-dopant ions in Cr:LiNbO₃ to tetravalent cation such as GeO₂. A singularity in the relative intensity of the ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ absorption band was observed for a concentration of ～1.5 mol%, compared with 4.5 mol% for Mg²⁺. The photoluminescence emission spectra also reveal a new emission band, at a lower energy than the [Cr]_Li centre, corresponding to this threshold concentration. A charge compensation model is proposed to explain the role of cation impurities and results are compared with those of other valence impurities.     

Optical absorption spectrum of chromium doped rubidium aluminium sulphate dodecahydrate

Optical absorption spectrum of chromium doped rubidium aluminium sulphate dodecahydrate single crystal is investigated. The electronic spectrum in the UV-VIS region is characteristic of Cr ³⁺ in trigonal symmetry and vibrational spectrum in the i.r. region is ascribed to the SO ₄²⁻ and H ₂O groups. The following crystal field parameters are evaluated for Cr ³⁺ in trigonal symmetry.Dq = 1840cm ⁻¹, Dσ = 140cm ⁻¹, Dτ = 140cm ⁻¹, B = 700cm ⁻¹ C = 3100cm ⁻¹.     

In situ Raman spectroscopy studies on chromium oxide catalyst in an anhydrous hydrogen fluoride atmosphere

Changes of different chromium oxide species [Cr(VI), Cr(V) and Cr(III)] were investigated using in situ Raman spectroscopy in an anhydrous hydrogen fluoride (AHF) atmosphere. It was found that the Cr(VI) (CrO₃) and Cr(V) (YCrO₄) species were easy to fluorinate, and generated CrO₂F₂ or/and CrOF₃, while the Cr(III) (Cr₂O₃) species was difficult to fluorinate. Moreover, the CrO₂F₂ and CrOF₃ species were stable in the AHF atmosphere, but they easily transform into CrO₃ in N₂ atmosphere and further decomposed to Cr₂O₃ at 400 °C. Moreover, Cl/F exchange reaction of CF₃CH₂Cl to CF₃CH₂F over these Cr species indicated that the high‐oxidation‐state CrO_xF_y (CrO₂F₂ and CrOF₃) species exhibited much higher reactivities than Cr₂O₃, suggesting that these species (CrO₂F₂ and CrOF₃) may be the active phases of the reaction. Copyright © 2010 John Wiley & Sons, Ltd.     

EPR measurements on chromium doped GaAs,GaP and InP

EPR measurements have been made on chromium doped GaAs samples at 4.2 K. An n-type sample doped with chromium and silicon was irradiated with 2 MeV electrons to lower the Fermi level. No resonance from substitutional Cr⁺ (3d⁵) was detected, although the Cr_s²⁺ spectrum was observed. The generally accepted assignment of a spectrum to Cr_s⁺ for photoexcited samples must therefore be revised. An isotropic resonance with g = 2 is observed in p-type chromium doped GaAs, GaP and InP but it is still not clear whether this is due to Cr_s⁴⁺ or interstitial Cr_i⁺ (3d⁵).     

掺杂PVK光电特性研究

将磷光材料三-（2-苯基吡啶）-铱[Ir（ppy）3]掺杂在聚乙烯基咔唑（PVK）中作为发光层,制作了多层有机电致发光器件。采用常规的光电测量方法,研究其光致发光及电致发光特性,得到了激子形成截面随电压的变换关系。     

Chromium isotopes tracking the resurgence of hexavalent chromium contamination in a past-contaminated area in the Friuli Venezia Giulia Region,northern Italy

The origin of a resurgent hexavalent chromium contamination in groundwater from a phreatic aquifer in the Friuli Venezia Giulia Region plain was investigated by chromium isotopic systematics. The area underwent a severe Cr(VI) contamination by industrial effluents in 1997, when Cr(VI) concentration in groundwater reached 4500?µg/L. In subsequent years the contamination naturally attenuated, totally disappearing in 2003. A renewal of water contamination was observed in 2008, Cr(VI) reaching 1560?µg/L. The δ⁵³Cr value in groundwater and extracts from sediments was measured in 2009–2011, and it ranges between ?3.21 and +0.21‰ and between ?4.71 and +1.26‰, respectively. Due to the lack of geogenic Cr-sources, these data are interpreted as evidence of the subsequent oxidation through Mn-oxides of the Cr(III) hosted in the aquifer and originated by the reduction of the original industrial chromates. Cr(III) is characterized by negative δ⁵³Cr, starting from the δ⁵³Cr value around zero of Cr(VI) in industrial effluents. Oxidation liberates soluble Cr(VI) which is transported by groundwater and permeated soils. The complex Cr-isotopic vs. concentration distribution reflects both the new Cr(VI) reduction and dilution processes in the aquifer system. From an environmental point of view, the data raise concerns regarding the potential impact of past Cr(VI)-contamination.     

Characterization of Cr/SiO2 catalysts and ethylene polymerization by XPS

Cr/SiO₂ catalysts with 1 or 3 wt.% Cr loadings and different chromium precursors were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A method to determine chromium species in the sample was developed through the decomposition of the Cr 2p XPS spectrum in Cr⁶⁺ and Cr³⁺ standard spectra. The results of the binding energy from the Cr 2p region and of the distribution of chromium species allowed to evaluate the dynamic photo-reduction of the surface chromium species during XPS analysis. Photo-reduction of surface Cr⁶⁺ to Cr³⁺ species was verified for all samples supported in silica, depending on the precursor and chromium content. Bulk CrO₃ and Cr₂O₃ standards did not reveal variation in the binding energy of Cr 2p_3/2, but a physical mixture of CrO₃ with SiO₂ presented photo-reduction. The behavior of this mixture resembled to the catalysts and suggests the participation of the surface hydroxyls of silica in the photo-reduction process. XPS intensity measurements for assessing dispersion of chromium oxide were used to compare the calcined and reduced catalysts to different chromium precursors. Polyethylene chains were detected by in situ XPS, while oligomerization products were not observed.     

Ultrasonic-assisted synthesis of graphene oxide – fungal hyphae: An efficient and reclaimable adsorbent for chromium(VI) removal from aqueous solution

In this study, a hybrid film bio-nanocomposite material was developed based on the graphene oxide/fungal hyphae (GO-FH) interaction. The developed GO-FH bio-nanocomposite material was used for the removal of hexavalent chromium from aqueous solution. The GO-FH bio-nanocomposite material was prepared by ultrasonic irradiation technique. The synthesized GO-FH bio-nanocomposite material was characterized by XRD, FT-IR, SEM, TEM and TGA. The adsorption experiments were carried out in batch mode to optimize parameters such as pH, adsorbent dosage, initial Cr(VI) ion concentration, contact time and shaking speed. The results indicated that the adsorption of Cr(VI) onto GO-FH bio-nanocomposite material was pH dependant, with the maximum adsorption capacity of 212.76 mg/g occurred at pH 2.0. The adsorption studies followed, Langmuir isotherm and pseudo second order kinetic model. Findings demonstrates that GO-FH bio-nanocomposite material exhibited excellent regeneration performance.     

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.     

Photoluminescence and EPR of porous silicon formed on <Emphasis Type="Italic">n</Emphasis><Superscript>+</Superscript> and <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript> single crystals implanted with boron and phosphorus ions

The effect of combined doping by shallow donor and acceptor impurities on boosting the quantum yield of porous-silicon photoluminescence (PL) in the visible and near IR range was studied using phosphorus and boron ion implantation. Nonuniform doping of samples and subsequent oxidizing annealing were performed before and after porous silicon was formed on silicon single crystals strongly doped by arsenic or boron up to ≈10¹⁹ cm^?3. The concentration of known P_b centers of nonradiative recombination was controlled by electron paramagnetic resonance. It is shown that there is an optimal joined content of shallow donors and acceptors that provides a maximum PL intensity in the vicinity of the red part of the visible spectrum. According to estimates, the PL quantum yield in the transitional n ⁺⁺-p ⁺ or p ⁺⁺-n ⁺ layer of porous silicon increases by two orders of magnitude as compared to that in porous silicon formed on silicon not subjected to ion irradiation.     

Unit-cell vibrational spectra of chromium trichoride and chromium tribromide

Raman spectra (at temperatures down to 4.2°K) and infrared spectra (at room temperature) of chromium trichloride and chromium tribromide have been obtained. Based on a factor group analysis and on the insensitivity of the CrCl₃ Raman spectrum to the monoclinic ā rhombohedral structural transformation, it is concluded that the spectra are characteristic of the unit cell of the diperiodic halogen-metal-halogen “sandwich” and not of the unit cell of the true triperiodic crystallographic space group.     

Optical absorption and reflectivity spectra of highly chromium doped CdTe alloy and layer

In the present work, a CdTe alloy doped with a relatively high concentration of chromium (1%), and a CdTe:Cr layer, have been studied. Absorption and reflectivity spectra were measured at room temperature. They indicate the presence of chromium in the divalent state, both in the alloy and in the layer.      

Effect of high-energy electron irradiation on forsterite laser crystals

The effect of 21-MeV electron irradiation on the optical absorption characteristics of Czochralski-grown forsterite (Mg₂SiO₄) single crystals (both undoped and chromium-doped) has been investigated. The irradiation is found to induce additional optical absorption (AOA) in the crystals in the range of 225–1200 nm due to the formation of color centers based on intrinsic host point defects and the change in the oxidation state of chromium ions. The AOA spectra have been decomposed into elementary bands. The influence of the chromium concentration in crystals, the oxygen content in the growth atmosphere, and additional doping with lithium on the behavior of these bands has been analyzed. A possible structure of the color centers responsible for the AOA is discussed. It is shown that the electron irradiation somewhat decreases the intensity of the characteristic absorption bands of tri- and tetravalent chromium ions and gives rise to a new absorption band in Mg₂SiO₄:Cr and Mg₂SiO₄:Cr,Li crystals heavily doped with chromium.     

Chromium speciation in mildly heated Cr(VI)‐doped latosol soil

Cr(VI) chemical reduction in natural organic matter (NOM)‐bearing latosol soil was investigated under various heating conditions at ≤378 K. An enhanced Cr(VI) reduction rate has been observed for the reaction at 353–378 K. The effect of Fe(II) naturally occurring in the latosol soil on Cr(VI) chemical reduction is negligible compared with the effect of NOM. Cr(OH)₃ was quantitatively specified by X‐ray absorption spectroscopy to be the key chromium species (～80%) after ～90% of Cr(VI) was chemically reduced by NOM at 353–378 K. This study indicates a potential strategy for using the heat extracted from industrial flue gas with a heat exchanger to chemically reduce Cr(VI) in NOM‐bearing or organics‐amended soils that contain Cr(VI).     

Flow injection thermal lens spectrometric detection of hexavalent chromium

A new highly sensitive flow-injection analysis (FIA) method with thermal lens spectrometric detection (TLS) was developed for determination of hexavalent chromium (Cr(VI)) in water. TLS measurements of Cr(VI) with diphenylcarbazide (DPC) method in a batch mode are subject to lower sensitivity and are less reproducible due to photodegradation of Cr-DPC complex during the measurement. In a FIA mode the decomposition of Cr-DPC induced by an intensive excitation laser source (140 mW at 514.5 nm) can be avoided or significantly reduced. For this purpose a FIA-TLS system was assembled and Cr(VI) determined by on-line generation of Cr(VI)-DPC. Under optimal conditions, a linear calibration graph was obtained for the range of Cr(VI) concentrations 1–20 μg/L. A LOD (limits of detection) value of 0.07 μg/L was achieved for Cr(VI) in water.     

Green Emitting Cerium Doped CaS Whiskers Grown by Solid State Diffusion Method

Undoped and cerium doped Calcium sulfide (CaS) phosphors were synthesized using solid state diffusion method. The X-ray diffraction pattern revealed that both undoped and doped CaS crystallites have cubic structure with average crystallite size varying from 20 to 30 nm. Scanning electron micrographs indicated that Ce doped CaS phosphors were composed of whiskers with different dimensions and orientations. The optical properties of undoped and Ce doped particles were characterized using Photoluminescence (PL) and UV-Vis absorption spectroscopy. The PL emission spectrum of cerium doped CaS phosphors for an excitation wavelength 465 nm showed a main peak at 500 nm and a shoulder peak at 556 nm due to 5d?→?4f transition in Ce³⁺ ions. The variation of PL intensity with cerium concentration was investigated and the maximum PL intensity was obtained for a doping concentration of 3 wt.%. The optical band gap of the samples was estimated from the diffuse reflectance spectrum and was found to increase with increase in cerium concentration. The enhanced optical properties of these phosphors can be exploited in various optoelectronic devices including displays and bioimaging techniques.     

掺Er凝胶玻璃中Er离子发光性质的研究

用溶胶－凝胶方法合成了掺铒的二氧化硅玻璃。在室温下可产生１．５４μｍ波长的红外荧光。实验结果表明：荧光强度随掺杂浓度的不同而改变，并在０．５Ｗ％的掺杂浓度下出现最大值。当温度从４Ｋ升至３００Ｋ时，荧光强度下降了７４％。