Formation and Local Structure Analysis of High‐Valence Chromium Ion in Dicalcium Silicate

Dicalcium silicate (2CaO·SiO₂) is a typical silicate compound that exists even at elevated temperatures. Different types of elements can be dispersed in dicalcium silicate to form a solid solution phase. However, the dispersion behavior of transition elements that can display different ionic valences is not well understood. Herein, we investigate the local structure of chromium ions dispersed in dicalcium silicate of different phase states under inert and air atmospheres using Cr K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy and first‐principle calculations. The measured XANES spectra indicated that Cr ions exist as high‐valence states when dispersed in dicalcium silicate in air atmosphere. Specifically, Cr⁶⁺ ions were detected in γ‐dicalcium silicate, which was prepared by annealing a mixture of 2CaO·SiO₂ and Cr₂O₃ at 973 K in air atmosphere. In addition, the XANES spectra obtained via first‐principle calculations revealed that Cr ions, with high‐valence states between Cr⁴⁺ and Cr⁶⁺, in dicalcium silicate, occupy the tetrahedral Si⁴⁺ sites.     

Assessment of conversion efficiency of Cr4+ ions by aliovalent cation additives in Cr:YAG ceramic for edge cladding

0.25at.% Cr:YAG ceramics were successfully fabricated as the edge cladding of Yb:YAG transparent ceramic slabs through vacuum sintering of co‐precipitated powders, using oxide additives to introduce different cations. The effects of various cation additives (Si⁴⁺, Ca²⁺, and Si⁴⁺ + Ca²⁺) on the conversion efficiency of Cr⁴⁺ ions and optical characteristics of the Cr:YAG edge cladding were investigated. Measurements of the absorption spectra of the Cr:YAG ceramics without any additives revealed 2 absorption bands centered at 430 nm and 600 nm, which imparted the sample with a green color. The introduction of only Si⁴⁺‐bearing additive did not promote the transition of Cr ions from the 3+ to 4+ state. Theoretical analysis and experimentation revealed that the addition of CaO not only enhanced the microstructure and improved the transmittance of the Cr:YAG ceramic, but also introduced vacancies that assisted in the formation of Cr⁴⁺ ions. It was determined that CaO has the same effect on the conversion efficiency of Cr⁴⁺ ions whether it is added as a single additive or in combination with SiO₂. The underlying mechanisms by which these aliovalent cation additives influence the formation of Cr⁴⁺ ions and affect optical properties are discussed in detail. High quality composite ceramics with Yb:YAG transparent ceramic slabs and dark brown‐colored Cr⁴⁺: YAG ceramic edge cladding were achieved through the addition of 0.05 wt.% CaO to the edge cladding, with no interfacial effects between the 2 regions being observed.     

Effect of Ca2+ and Mg2+ ions on the sintering and spectroscopic properties of cr-doped yttrium aluminum garnet ceramics

In this work, we investigated the effects of Ca²⁺ and Mg²⁺ ions and annealing temperature on the spectroscopic parameters of chromium-doped yttrium aluminum garnet ceramics (Cr:YAG). Samples were obtained with either a separate or a simultaneous addition of calcium and magnesium oxides. To achieve this, aqueous suspensions were prepared using Y₂O₃, Al₂O₃, Cr₂O₃, MgO, and CaO high-purity powders as raw materials. The obtained suspensions were freeze-granulated, pressed into pellets, debinded, and subjected to reactive sintering in vacuum at 1715°C for 6 h. Each material was annealed in air with temperatures between 1300 and 1700°C. Samples were also compared to Cr:YAG ceramics with the addition of silica as a sintering aid. All the materials obtained were then exposed to 445 nm excitation, and emission spectra in the visible and infrared wavelengths were recorded. The results showed that the emission spectra of Cr:YAG ceramics varied according to the annealing conditions: as-sintered samples exhibited strong emissions of around 680 nm and, after air annealing, of around 1400 nm. This phenomenon is attributed to the Cr³⁺→Cr⁴⁺ transition. Samples doped solely with MgO exhibited the highest emission intensity in the infrared region. Thus, Mg²⁺ ions provided the best conversion efficiency of chromium ions.     

Conversion of Valence State and Coordination State of Fe in Transparent Glass‐Ceramics Containing Li2ZnSiO4 Nanocrystals

Transparent glass‐ceramics containing Li₂ZnSiO₄:Fe nanocrystals were prepared by melt‐quenching method followed by post‐heat‐treatment process. The as‐prepared glasses and glass‐ceramics showed red to near‐infrared photoluminescence centered at 730 nm, ascribed to Fe³⁺ ions in tetrahedral coordination. The intensity of the photoluminescence was enhanced by two technologically simple techniques—the valence state of irons was controlled from Fe²⁺ to Fe³⁺ ions using oxidizing agents, whereas the coordination state was compulsorily converted from octahedral to tetrahedral symmetry by performing a ceramization process. The presence of Fe²⁺ ions was considered a major origin for Fe³⁺ photoluminescence quenching. Oxidation of Fe²⁺ and conversion of Fe²⁺ ions into tetrahedral symmetry contribute to suppression of energy transfer from Fe³⁺ emitters to Fe²⁺ quenching centers.     

Chromium in lead metasilicate glass: Solubility,valence, and local environment via multiple spectroscopy

In this article the doping of chromium in a lead metasilicate glass is explored using Raman, UV–Vis, and Cr K-edge XANES spectroscopy. Our results indicate that the Cr ions are predominantly present as CrO₄^2? complexes, with a minor amounts of Cr(III). The solubility of Cr is limited to ～1 mol % Cr₂O₃. This boundary was inferred from the intensity changes to the CrO₄^2? stretching vibrations and confirmed by the inability to produce a homogeneous glass containing 2 mol % Cr₂O₃. The establishment of the solubility limit of Cr in lead glasses is important to design more effective glass compositions depending on the desired application. Finally, this low solubility indicates that Cr₂O₃ could be used as a potential nucleating agent for lead-silica based glass-ceramics.     

Local Ordering Around Tetrahedral Co2+ in Silicate Glasses

Alkali lime silicate glasses containing 5 wt% of CoO were investigated by Co K‐edge XANES and EXAFS and optical absorption spectroscopy. Our results reveal the presence of tetrahedral Co²⁺ connected with the glass network, with a ^IVCo–O–Si angle of 134°. Changing the alkali from K⁺ to Na⁺ induces an increase of the local disorder around Co²⁺, as shown by a decrease of the contribution from the second neighbors in the EXAFS signal. We propose two models for interpreting the structure of the second shell of neighbors. Our results provide a structural basis for rationalizing the optical properties of Co²⁺ species in glasses.     

Multicomponent ion exchange isotherms in NaX zeolite: evaluation of Cr/Ca/Mg,Cr/Ca/K and Cr/Mg/K systems

BACKGROUND: One of the most important heavy metals is Cr³⁺, which is commonly found together with Ca²⁺, Mg²⁺ and K⁺ ions in tannery wastewaters. Such metals can be removed through ion exchange process using zeolite NaX. Unfortunately, with single exchange mechanisms it is impossible to predict muti‐component and competitive interactions. This work is aimed at overcoming this problem. Ion exchange isotherms of Cr³⁺, Mg²⁺, Ca²⁺ and K⁺ in ternary mixtures (Cr/Mg/Ca, Cr/Ca/K, Cr/Mg/K) using NaX zeolite at 30 °C, 45 °C and 60 °C are studied and reported. RESULTS: Ion exchange is dependent on temperature, and on the nature and interaction of ingoing cations. Sequential ion exchange occurs where one cation is able to displace another already located in the site. Values of the equilibrium constant of all cations investigated are close to each other, which indicates a weak preference for Cr⁺³. The affinity order is as follows: for Cr/Ca/Mg–NaX isotherms, Cr³⁺ > Ca²⁺ > Mg²⁺ at 30 °C, Ca²⁺ > Cr³⁺ > Mg²⁺ at 45 °C and Ca²⁺ > Mg²⁺ > Cr³⁺ at 60 °C. For Cr/Ca/K–NaX isotherms the sequence is Cr³⁺ > Ca²⁺ > K⁺ for the three temperatures and for Cr/Mg/K–NaX isotherms, Cr³⁺ > Mg²⁺ > K⁺ at 30 and 45 °C and Mg²⁺ > K⁺ > Cr³⁺ at 60 °C. CONCLUSION: It can be concluded that ions with a high charge such as Cr³⁺ are not always preferentially removed from multi‐component solution. Therefore, exchanges in NaX zeolite are useful when there is no need to have high chromium removal when compared to the competitive cations investigated here. Copyright © 2008 Society of Chemical Industry     

Broadband near-infrared emission enhancement in K2Ga2Sn6O16:Cr3+ phosphor by electron-lattice coupling regulation

Cr³⁺-doped phosphors have recently gained attention for their application in broadband near-infrared phosphor-converted light-emitting diodes (pc-LEDs), but generally exhibit low efficiency. In this work, K₂Ga₂Sn₆O₁₆:Cr³⁺ (KGSO:Cr) phosphor was designed and synthesized. The experimental results show that the Cr³⁺-doped phosphor exhibited broadband emissivity in the range 650-1300 nm, with a full width at half maximum (FWHM) of approximately 220-230 nm excited by a wavelength of 450 nm. With the co-doping of Gd³⁺ ions, the internal quantum efficiency (IQE) of the KGSO:Cr phosphor increased from 34% to 48%. The Gd³⁺ ions acted neither as activators nor sensitizers, but to justify the crystal field environment for efficient Cr³⁺ ions broad emission. The Huang-Rhys factor decreased as the co-doping of Gd³⁺ ions increased, demonstrating that the nonradiative transitions were suppressed. An efficient strategy for enhancing the luminescence properties of Cr³⁺ ions is proposed for the first time. The Gd³⁺–co-doped KGSO:Cr phosphor is a promising candidate for broadband NIR pc-LEDs.     

Molten salt synthesis of orthorhombic CrB and Cr2AlB2 ceramics

High-purity and fine CrB powders are firstly synthesized from Cr and B powders in (Na, K)Cl molten salt at 850 °C for 2 h. Then the as-achieved CrB powders are used as precursors to synthesize homogeneous and size-controlled Cr₂AlB₂ powders in (Na, K)Cl molten salt at 900 °C for 2 h. Additionally, the formation mechanisms of CrB and Cr₂AlB₂ in (Na, K)Cl molten salt are also investigated. Results show that the Cr atom, formed by disproportionation of Cr²⁺ ions in molten salt, reacts in-situ with B powders to form CrB. For the formation of Cr₂AlB₂, Al and CrB migrate mutually and assemble firstly in the molten salt, and then Al intercalates into CrB to generate Cr₂AlB₂.     

Synthesis of Y3Al5O12:Eu2+ Phosphor by a Facile Hydrogen Iodide‐AssistedSol–Gel Method

Pure yttrium aluminum garnet (YAG) phosphor doped with Eu²⁺ has been successfully synthesized by a facile sol–gel method. The use of hydrogen iodide aimed to get Eu²⁺ ions, confirmed by X‐ray absorption near‐edge structure (XANES) analysis. Nearly spherical and well dispersed particles were synthesized. The produced YAG:Eu²⁺ phosphor powder had a broad emission band in the range of 400–600 nm with a peak at 480 nm, attributed to the allowed 4f⁷–4f⁶5d¹ transition of electrons in Eu²⁺ ions. The proposed method could also be expanded to prepare many other Eu²⁺‐doped phosphors with a solution method.     

Influence of Ti ions valence states on the optical and magnetic properties of GdCr1-xTixO3 (0≤x≤0.05)

The vacancy defects, optical and magnetic properties of GdCr_1-xTi_xO₃ (0≤x ≤ 0.05) polycrystalline samples were investigated in this research. The crystal structural analyses show that the orthorhombic perovskite structure exists in all the samples. The positron annihilation spectra reveal that the vacancy defects and local electron density vary with the change of Ti ions valence states. The magnetic measurements show that the antiferromagnetic transition is shifted to lower temperature with the replacement of Ti ions in Cr sites, and the value of spin reorientation caused by the exchange interaction between Gd³⁺ ion and Cr³⁺ ion can be influenced by the change of Ti ions valence states, which rises from x = 0 to 0.04 and then decreases at x = 0.05 sample. Simultaneously, the optical band gap decreases from 2.7 eV to 0.8 eV with the introduction of Ti ions into the lattice, the result can provide a reference for improving optical activity of rare-earth chromium oxides.     

Oxidative Dehydrogenation of Propane with CO2 Over Cr/H[B]MFI Catalysts

Abstract  

Cr/silicalite-1 and Cr/H[B]MFI catalysts were prepared by the impregnation method, and Cr/H[B]MFI were further treated by steaming. The catalysts were employed for the oxidative dehydrogenation of propane to propylene with CO₂ as the oxidant. Cr/H[B]MFI showed significantly higher catalytic activity than Cr/silicalite-1, and steamed Cr/H[B]MFI was superior in the reaction stability to Cr/H[B]MFI. The nature of the supported chromium species have been characterized by a number of physicochemical techniques, such as Raman, UV–vis and NMR. It is concluded that the steaming led to the auto-reduction of some Cr⁶⁺ to Cr³⁺, and resultant Cr³⁺ species might be located near the boron center in the borosilicate framework to counterbalance the negative charge of the framework. The transformation of Cr⁶⁺ species to Cr³⁺ species, facilitated by the steaming process and the presence of boron in the catalyst, is responsible for the enhanced stability of oxidative dehydrogenation of propane to propylene with carbon dioxide as the oxidant.     

Effects of Sintering Aids on the Transparency and Conversion Efficiency of Cr4+ Ions in Cr: YAG Transparent Ceramics

Tetravalent chromium‐doped Y₃Al₅O₁₂ ceramics were fabricated by solid‐state reactive sintering method using high‐purity Y₂O₃, α‐ Al₂O₃, and Cr₂O₃ powders as the starting materials. CaO and MgO were co‐doped as the sintering aids. The effects of TEOS and divalent dopants (CaO and MgO) on the optical qualities, the conversion efficiency of Cr⁴⁺ ions, and the microstructure evolutions of 0.1 at.% Cr⁴⁺: YAG ceramics were investigated. Fully dense, dark brown colored Cr⁴⁺: YAG ceramics with an average grain size of 3.1 μm were achieved. The in‐line transmittance of the as‐prepared ceramic at 2000 nm was 85.3% (4 mm thick), and the absorption coefficient at 1030 nm (the characteristic absorption peak of Cr⁴⁺ ions) was as high as 3.7 cm^?1, which was higher than that of corresponding single crystals fabricated by Czochralski method.     

Cr3+‐activated Li5Zn8Al5Ge9O36: A near‐infrared long‐afterglow phosphor

Near‐infrared long‐afterglow (LAG) materials have attracted considerable attention owing to their high potential for in vivo imaging applications. Here, we present a series of near‐infrared LAG phosphors Li₅Zn₈Al_5?xGe₉O₃₆:xCr³⁺ (LZAG:Cr³⁺), which were synthesized using a solid‐state reaction method. The pure LZAG host exhibits blue photoluminescence and LAG emission. We investigated the effect of the zinc vacancy contents on the photoluminescence and LAG performance by adjusting the zinc content and introducing Ga³⁺ ions to substitute the Zn²⁺ sites in LZAG host. When Cr³⁺ ions were introduced into the LZAG host, LZAG:Cr³⁺ produced a strong, broad blue emission band centered at 456 nm and a near‐infrared emission band at 700 nm caused by the ²E → ⁴A₂ transition of Cr³⁺. The energy transfer processes from the LZAG host to Cr³⁺ were identified in the photoluminescence and LAG process. After irradiation at 258 nm for 10 minutes, the LAG emission of LZAG:0.008Cr³⁺ can last nearly 2.5 hours. Moreover, the LAG intensity and duration of LZAG: 0.008Cr³⁺ were significantly improved by introducing a small dose of Ga³⁺ ions. Finally, the traps and mechanism of LAG in LZAG, LZAG:Ga³⁺, and LZAG:Cr³⁺ were discussed in detail.     

Synthesis and properties of novel inorganic red chromophore based on Cr4+ in tetrahedral coordination in Bi24Al2-xCrxO39

The novel orange to red color Bi₂₄Al_2-xCr_xO₃₉ (0≤ x ≤ 0.5) compounds were successfully synthesized by the solid state method. With the increase of Cr content, the colors of compounds are tuned from orange red to dark red. The X-ray diffraction patterns of all samples are indexed with cubic Bi₂₄Al₂O₃₉ structure with the space group of I23. The oxidation state of Cr in the oxide is confirmed to be +4 valence by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). Based on the analysis of Cr oxidation state and UV–Visible spectrum, the red hue is due to the d-d transition of Cr⁴⁺ in tetrahedral sites. The as-synthesized red color compounds also show excellent stability in acid and base environments.     

Thermodynamic Properties of LaCrO4, La2CrO6, and La2Cr3O12, and Subsolidus Phase Relations in the System Lanthanum–Chromium–Oxygen

In the system La–Cr–O, there are three ternary oxides (LaCrO₄, La₂Cr₃O₁₂, and La₂CrO₆) that contain Cr in higher valence states (V or VI). On heating, LaCrO₄ decomposes to LaCrO₃, La₂Cr₃O₁₂ to a mixture of LaCrO₄ and Cr₂O₃, and La₂CrO₆ to LaCrO₃ and La₂O₃ with loss of oxygen. The oxygen potentials corresponding to these decomposition reactions are determined as a function of temperature using solid‐state cells incorporating yttria‐stabilized zirconia as the electrolyte. Measurements are made from 840 K to the decomposition temperature of the ternary oxides in pure oxygen. The standard Gibbs energies of formation of the three ternary oxides are derived from the reversible electromotive force (EMF) of the three cells. The standard enthalpy of formation and standard entropy of the three ternary oxides at 298.15 K are estimated. Subsolidus phase relations in the system La–Cr–O are computed from thermodynamic data and displayed as isothermal sections at several temperature intervals. The decomposition temperatures in air are 880 (±3) K for La₂Cr₃O₁₂, 936 (±3) K for LaCrO₄, and 1056 (±4) K for La₂CrO₆.     

Soft magnetic properties of Ni–Cr and Co–Cr alloy thin films electrodeposited from aqueous solutions containing trivalent chromium ions and glycine

Ferromagnetic Ni–Cr and Co–Cr alloy thin films were electrodeposited from aqueous solution containing trivalent chromium (Cr³⁺) ions and glycine. According to the Tafel slopes obtained from the cathode polarization curves for Ni–Cr and Co–Cr alloy deposition, it was estimated that Cr³⁺ ions inhibited Ni²⁺ and Co²⁺ ions from electrodepositing. Ni and Co preferentially electrodeposited rather than Cr and the electrodeposition process of Ni–Cr and Co–Cr was categorized to “normal co-deposition type.” At the cathode potential of −1.8 V versus Ag/AgCl/KCl sat., Ni—9.5 %Cr and Co—8.4 %Cr alloy deposits were obtained. X-ray diffraction patterns of the electrodeposits revealed that pure Ni and pure Co consist of large crystal grains, while Ni—9.5 %Cr and Co—8.4 %Cr alloys were composed of a solid solution phase with fine crystal grains. Magnetization of Ni—9.5 %Cr and Co—8.4 %Cr alloy thin films with fine crystalline phase reached to saturation at ca. 2.5 kOe in perpendicular direction to the film plane, while pure Ni and pure Co thin film with large crystal grains were hardly magnetized in the perpendicular direction. Soft magnetic properties were improved with increasing Cr content in the deposits.     

Grain-size effect on Cr3+ and F-centres photoluminescence in nanophase MgAl2O4 ceramics

Photoluminescence (PL) spectroscopy of transparent MgAl₂O₄ spinel ceramics with grain size between 100 and 300 nm was studied at 7 K temperature in the near-IR-VUV range of spectrum with synchrotron radiation excitation. The PL spectra were composed of optical transitions from spatially different regions of the ceramics, which analysis evidenced grain size effect on the emission line-shapes and intensities. In particular, emission of impurity Cr³⁺ ions, being structured in the crystalline bulk, became broad-band in the grain boundary regions, which was associated with respectively strong and weak local crystalline fields. It was observed that (i) excitons and F centres transfer energy to Cr³⁺ and (ii) Cr(²E_g)/Cr(⁴T_2g) and F-centres/Cr³⁺ PL intensity ratios underwent a linear dependence on the grain size.     

Investigation of a transparent chromate (III) passive film on electroless Ni–P coating by XPS and electrochemical methods

A transparent passive film on electroless nickel phosphorus coating (ENPC) was obtained in a chromate (Cr³⁺) bath. An ENPC sample passivated in a Cr⁶⁺-containing bath was used for comparison. The corrosion properties were tested by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). SEM, XPS, and EDX were employed to analyze the chemical composition and surface morphology of the films. SEM results indicate that the passive film is too thin to be observed by SEM. The potentiodynamic polarization tests show that the corrosion currents of the two passivated samples are only about 1/25 of the un-passivated coating. The XPS analysis illustrate that the Cr⁶⁺-treated film comprises Cr, Ni and O, while the Cr³⁺-treated film is made up of Cr, C, O, Ni, P and N. High-resolution XPS analysis show that, both in Cr⁶⁺-treated and Cr³⁺-treated films, element Cr is only in the form of trivalent compounds, no hexavalent chromium existing. By XPSpeak analyzing, trivalent chromium compounds on the two treated surfaces were fitted as Cr(OH)₃ and Cr₂O₃. However, for the Cr³⁺ and Cr⁶⁺ passivation routes, the Cr(OH)₃ and Cr₂O₃ contents in passive films are widely divergent.     

Wavelength Tailorability of Broadband Near‐Infrared Luminescence in Cr4+‐Activated Transparent Glass‐Ceramics

Four Cr⁴⁺‐activated transparent glass‐ceramics containing different species of silicate nano‐crystals (Zn₂SiO₄, Mg₂SiO₄, Li₂ZnSiO₄, and Li₂MgSiO₄) were successfully prepared. Absorption spectra, photoluminescence spectra, lifetime decay curves, and quantum yield of these transparent glass‐ceramics were measured. According to the crystal field strength of Cr⁴⁺‐incorporated tetrahedral sites, the broadband near‐infrared (NIR) luminescence of Cr⁴⁺ can be tailored from 1130 to 1350 nm and the lifetime of Cr⁴⁺ luminescence can be prolonged from 6 to 100 μs. Quantum yield in the transparent glass‐ceramics containing Li₂ZnSiO₄ nano‐crystals reached at 17%, which is the highest value of NIR luminescence in transition‐metal‐activated glass materials.