Light induced magnetic properties of spiropyrane tris(oxalato)chromate (III) single crystals

The effect of UV light on Weiss temperature and ESR spectra in 1-isopropyl-3, 3, 5′, 6′-tetramethylspiro[indolin-2,2′-[2H]pyrano[3,2-b]pyridinium] tris(oxalato)chromate (III) (Sp₃Cr(C₂O₄)₃) has been found. Additional line has been observed in the ESR spectra of irradiated samples in “strong” magnetic fields of ~15 kOe. The analysis of angular dependences of the ESR spectra allowed a contribution of Cr³⁺ ions to magnetic properties of Sp₃Cr(C₂O₄)₃ to be determined. The zero-field splitting parameters D=0.619 cm⁻¹, E=0.024 cm⁻¹ were derived from the experimental data. The parameters were typical for Cr³⁺ in the chromium oxalate. Weiss temperature changed sign from 25 to −25 K under UV irradiation. The value of Weiss temperature and its changing cannot be explained by exchange interaction, dipole-dipole interaction or the effect of crystal field. The existence of Weiss temperature is explained by the changes in amount and spin of paramagnetic particles. The change is due to thermoactivated redistribution of electrons between chromium ions and spiropyrane molecules. Light-induced transfer of electrons is also explaining the change in sign of Weiss temperature under UV irradiation.     

Solvothermal Synthesis of [Cr7S8(en)8Cl2]Cl3 ⋅ 2H2O with Magnetically Frustrated [Cr7S8]5+ Double-Cubes**

A novel transition metal chalcohalide [Cr₇S₈(en)₈Cl₂]Cl₃ ⋅ 2H₂O, with [Cr₇S₈]⁵⁺ dicubane cationic clusters, has been synthesized by a low temperature solvothermal method, using dimethyl sulfoxide (DMSO) and ethylenediamine (en) solvents. Ethylenediamine ligand exhibits bi- and monodentate coordination modes; in the latter case ethylenediamine coordinates to Cr atoms of adjacent clusters, giving rise to a 2D polymeric structure. Although magnetic susceptibility shows no magnetic ordering down to 1.8 K, a highly negative Weiss constant, θ=−224(2) K, obtained from Curie-Weiss fit of inverse susceptibility, suggests strong antiferromagnetic (AFM) interactions between S=3/2 Cr(III) centers. Due to the complexity of the system with (2S+1)⁷=16384 microstates from seven Cr³⁺ centers, a simplified model with only two exchange constants was used for simulations. Density-functional theory (DFT) calculations yielded the two exchange constants to be J₁=−21.4 cm⁻¹ and J₂=−30.2 cm⁻¹, confirming competing AFM coupling between the shared Cr³⁺ center and the peripheral Cr³⁺ ions of the dicubane cluster. The best simulation of the experimental data was obtained with J₁=−20.0 cm⁻¹ and J₂=−21.0 cm⁻¹, in agreement with the slightly stronger AFM exchange within the triangles of the peripheral Cr³⁺ ions as compared to the AFM exchange between the central and peripheral Cr³⁺ ions. This compound is proposed as a synthon towards magnetically frustrated systems assembled by linking dicubane transition metal-chalcogenide clusters into polymeric networks.     

Laser-induced luminescence of model Fe/Al2O3 and Cr/Al2O3 catalysts

The laser-induced luminescence of Cr³⁺ impurity ions in model Fe/Al₂O₃ and Cr/Al₂O₃ catalysts with different calcination temperatures was studied. It was found that an additional luminescence band at 770 nm appeared in the luminescence spectra of low-temperature samples as a result of the interaction of octahedrally coordinated Cr³⁺ ions with Fe³⁺ impurity ions. In the θ-Al₂O₃ phase with a concentration of Cr³⁺ ions higher than 0.1 wt %, the interaction of the Cr³⁺-Cr³⁺ ion pairs in the immediate surroundings resulted in the appearance of N _θ lines due to the splitting of R _θ lines. The differences of these lines from the N _α lines of α-Al₂O₃ were related to the individuality of the crystal lattice of the θ phase and the coordination of Cr³⁺ impurity ions in the immediate surroundings, which is different from that in the α phase. Based on the laser-induced luminescence spectroscopic data, it was found that regions with a local Cr³⁺ concentration higher than the average Cr³⁺ concentration in the bulk of a catalyst by one order of magnitude were formed in the α-Al₂O₃-Fe₂O₃ system with the bulk Fe and Cr concentrations of 2.5 and 0.04 wt %, respectively, which was calcined at 1220°C, as a result of the diffusion of chromium and iron ions.     

Synthesis and Characterization of Nanostructured Fe3O4 Micron‐Spheres and Their Application in Removing Toxic Cr Ions from Polluted Water

We present a simple and effective method for the synthesis of nanostructured Fe₃O₄ micron‐spheres (NFMSs) by annealing hydrothermally formed FeCO₃ spheres in argon. The phase structure, particle size, and magnetic properties of the product have been characterized by X‐ray diffraction (XRD), Fourier‐transform infrared spectroscopy (FTIR), X‐ray photoelectron spectroscopy (XPS), field‐emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and by means of a superconducting quantum interference device (SQUID). The results have shown that the as‐obtained NFMSs have a diameter of about 5 μm and are composed of nanometer‐sized porous lamellae. The NFMSs have a large specific surface area (135.9 m² g^?1), reductive Fe²⁺ incorporated into their structure, and intense magnetic properties. These properties suggest that NFMSs have potential application in removing toxic Cr⁶⁺ ions from polluted water. At 25 °C, each gram of NFMSs product can remove 43.48 mg of Cr⁶⁺ ions, as compared to just 10.2 mg for nanometer‐sized Fe₃O₄ and 1.89 mg for micron‐sized Fe₃O₄. The enhanced removal performance can be ascribed to the structural features. Moreover, the Cr⁶⁺ ion removal capacity of the NFMSs can reach up to 71.2 mg g^?1 at 50 °C. The influencing parameters in the removal of Cr⁶⁺ ions, such as contact time, pH, and temperature, have been evaluated. The Cr⁶⁺‐removal mechanism has been investigated. We have found that the NFMSs product not only serves as an effective adsorbent to remove toxic Cr⁶⁺ ions from polluted water, but also as an effective reductant in reducing the adsorbed toxic Cr⁶⁺ ions to much less toxic Cr³⁺ through the Fe²⁺ incorporated into its structure.     

Local structure distortion and spin Hamiltonian parameters for Cr4+ ions in Cr4+:α-Al2O3 crystals

The relations between the spin Hamiltonian (SH) parameters and crystal structure of Cr⁴⁺:α-Al₂O₃ crystals have been established. On the basis of this, the SH parameters including zero-field splitting parameter D and Zeeman g-factors (g_|| and g_⊥) for Cr⁴⁺ ions in Cr⁴⁺:α-Al₂O₃ crystals, taking into account the spin–spin (SS), spin-other-orbit (SOO) and orbit–orbit (OO) magnetic interactions in addition to the spin–orbit (SO) magnetic interaction, are theoretically investigated using complete diagonalization method (CDM). The theoretical results are in excellent agreement with the experimental ones when the upper three O^2? ions rotate 0.94° toward [1 1 1] axis and the lower three O^2? ions rotate 0.92° toward it. Hence, the local structure distortion effect plays an important role in explaining the spectroscopic properties of Cr⁴⁺ ions in Cr⁴⁺:α-Al₂O₃ crystals. This study shows that for Cr⁴⁺:α-Al₂O₃ the contributions arising from SS, SOO, and OO interactions to the zero-field splitting (ZFS) parameter D are appreciable, whereas those to g_|| and g_⊥ are quite small.     

Laser spectroscopy of chromium(III) in magnesium aluminate spinels and transparent glass-ceramics

Laser excitation at different wavelengths was carried out on Cr³⁺-doped transparent glass-ceramics of composition (mole%) Ac(58.7SiO₂, 16.7 Al₂O₃, 17.8 MgO, 6.7TiO₂, 0.03 Cr₂O₃) and Bc(49.1 SiO₂, 19.7 Al₂O₃, 21.9 MgO, 6.0 TiO₂, 3.2 ZrO₂, 0.03 Cr₂O₃), and in synthetic crystals of composition MgAl₂O₄ (Cr³⁺), Mg₂TiO₄ (Cr³⁺) and Mg₁₂ Ti₀₂ Al₁₆ o₄ (Cr³⁺). Analysis of the emission spectra, excitation spectra and decay curves at 4.4. K and room temperature reveals that Cr³⁺ is essentially situated on distorted single sites and in pairs exchanging Al³⁺ ions in the crystalline phase of glass-ceramics.     

Characterization and physical properties of Li2O-CaF2-P2O5 glass ceramics with Cr2O3 as a nucleating agent—Physical properties

In this paper, studies on various physical properties, viz., dielectric properties (dielectric constant, loss tan δ, a.c. conductivity σ) over a wide range of frequency and temperature, optical absorption, ESR at liquid nitrogen temperature and magnetic susceptibility at room temperature of Li₂O-CaF₂-P₂O₅: Cr₂O₃ glass ceramics, have been reported. The optical absorption, ESR and magnetic susceptibility studies indicate that the chromium ions exist in Cr⁵⁺, Cr⁴⁺ and Cr⁶⁺ states in addition to Cr³⁺ state in these samples. The dielectric constant and loss variation with the concentration of Cr₂O₃ have been explained on the basis of space charge polarization mechanism. The dielectric relaxation effects exhibited by these samples have been analysed by a graphical method and the spreading of dielectric relaxation has been established. The a.c. conductivity in the high-temperature region seems to be connected both with electronic and ionic movements.     

Caracterisation d'oxydes catalyseurs a base de cuivre et de chrome par diffraction X,spectroscopie infra-rouge et reflectance diffuse

A cubic phase close to that of γCr₂O₃ has been observed in copper chromite catalysts with atomic ratio 0,8 ⩽ Cu/Cr ⩽ 1,5. The infra-red spectroscopy shows that Cr⁶⁺ ions should be necessary for the stability of the phase.Cr³⁺ ions in tetrahedral environment have been observed in all the catalysts by diffuse reflectance spectroscopy. In reduced state, cubic structure is kept, with formation of Cu metal. The phase is now stabilised by hydroxyls and inversion decreases.     

Reactions anodiques et cathodiques des composes de Cr(VI) en milieu de nitrates alcalins fondus

The electrochemical behavior of chromate and dichromate, ions in molten NaNO₃-KNO₃ at 250°C has been studied by single-sweep voltammetry and cyclic voltammetry. The anodic dimerization of CrO₄^2- ions has been shown to proceed in three steps, the reaction: CrO₄^2-→CrO_4ad^?+e^? being rate-determining. The cathodic reduction of Cr₂O₇^2- ions includes the coupled acid-base reactions; it forms solid chromium oxide Cr₂O₃ and the soluble chromate and chromite ions.     

Preparation and characterization of silicate gel glasses containing Cr2O3

The effect of thermal treatment on the structural incorporation of Cr₂O₃ in xCr₂O₃·(100 – x)SiO₂ and 5R _n O·xCr₂O₃·(95 – x)SiO₂ (where x=0.01–1 mol.% and R=Li, Na and Ca) gel glasses was studied by optical absorption spectrophotometry, DTA-TG, XRD and electron microscopy.Samples heat treated at 60°C have green color due to the presence of Cr³⁺ in octahedral coordination. The optical transmission, and color (yellow, orange or ambar), of the samples heat treated between 200 and 700°C prove that Cr³⁺ (octahedrally coordinated) and Cr⁶⁺ (tetrahedrally coordinated) are both present. Segregation of Cr₂O₃ take place at temperatures above 800°C.In reducing conditions the gel glasses were green due to the presence of a high content of Cr³⁺ ions. Samples containing Li or Na show crystalline phases at temperatures below 850°C.     

A novel spinel coating on cobalt‐based Superalloy GH605 via an in‐situ oxidation approach

Cr‐Mn‐O spinel coating was prepared on the surface of cobalt‐based superalloy GH605 via an in‐situ oxidation method in H₂O‐H₂ environment. The composition, morphology, and chemical value state of the oxide spinel coatings were investigated by SEM, EDS, XRD, Raman spectra, and XPS. It indicated that the morphology of coating varied with oxidation temperature, and granular surface appeared when oxidation temperature increased to 1100°C. The formed Cr‐Mn‐O spinel coating was composed of Cr₂O₃ and MnCr₂O₄, and the thickness increased significantly with oxidation temperature. In the coating, Cr element existed in the state of Cr³⁺ ions and Cr⁶⁺ ions, while Mn element only existed in the form of Mn²⁺ ions.     

Effects of Sm Co-doping on Luminescent Properties of Sr4Al14O25:M (M=Mn4+, Cr3+) Phosphors

The Sr₄Al₁₄O₂₅:M and doped Sr₄Al₁₄O₂₅:M+Sm³⁺ (M=Mn⁴⁺, Cr³⁺) phosphors were syn-thesized by a solid-state reaction method and their luminescent properties were investi-gated. The results showed that the co-doping of Sm ions did not change the positions of excitation band and emission band but signi cantly improved the luminescent properties of Sr₄Al₁₄O₂₅:Cr³⁺ phosphors; whereas, the emission intensity of Sr₄Al₁₄O₂₅:Mn⁴⁺ was re-dueced remarkably when Sm ions were co-doped. In addtion, a radiative-form energy transfer from Sm³⁺ to Cr³⁺ was observed for the first time in the Cr, Sm co-doped Sr₄Al₁₄O₂₅ phos-phors. The results indicated that Sm ions could signi cantly improve the emission intensity of Sr₄Al₁₄O₂₅:Cr³⁺, making the Sm³⁺co-doped Sr₄Al₁₄O₂₅:Cr³⁺ phosphor a promising can-didate for the applications in display and solid state lightening.     

Magnetic behavior of tetrahedral Cr4+ compounds: Sr2CrO4, Ba2CrO4, and Ba3CrO5

The preparation of the compounds Sr₂CrO₄, Ba₂CrO₄, and Ba₃CrO₅ is described. The characterization of these three Cr⁴⁺ compounds by X-ray and magnetic susceptibility experiments has been conducted. The magnetic moments for Sr₂CrO₄, Ba₂CrO₄, and Ba₃CrO₅ were determined to be in good agreement with the calculated value expected for a tetrahedral Cr⁴⁺ ion. Weak antiferromagnetic ordering for all three compounds is indicated from the small paramagnetic Weiss constants determined from the susceptibility data in the temperature region 80–300 K. Distortions of the tetrahedra from ideality, as determined from the structural features, further cause a reduction in the magnetic moments from the theoretical values.     

Cr3+:SrGa12O19: A Broadband Near‐Infrared Long‐Persistent Phosphor

Cr³⁺‐doped SrGa₁₂O₁₉ is demonstrated to be a broadband near‐infrared (650–950 nm) long‐persistent phosphor whose luminescence can last for more than 2 h after ultraviolet irradiation is stopped. Detailed analysis of the photoluminescence and thermoluminescence spectra and of the persistent decay behavior of the Cr³⁺‐doped SrGa₁₂O₁₉ samples indicate that the persistent energy transfer from the SrGa₁₂O₁₉ host to the Cr³⁺ ions and the filling and release of electrons into and from the shallow and deep traps through the conduction band is responsible for the long‐persistent phosphorescence.     

Titrations with extractive end-points: The dichromate titration

A method for the titration of iron(II) with K₂Cr₂O₇ in coloured solutions is reported, using the extraction of Cr₂O₇^-2 ions as (Ph₃MeAs)₂Cr₂O₇ at the end-point. The method is applicable to titration with $\text{N}\text{10}$ K₂Cr₂O₇ in the presence of normally interfering coloured cations in molar concentrations up to 50 times that of the iron (II). The method may be used to estimate iron in ferrous alloys, usually without separation from the other metals present. A stable dichromate, (Ph₃MeAs)₂Cr₂O₇ has been isolated.     

Kinetics and Mechanism of Cr(VI) Reduction in a Water Cathode Induced by Atmospheric Pressure DC Discharge in Air

The process of reduction of Cr⁶⁺ ions (solution of potassium dichromate, K₂Cr₂O₇) in a water cathode was studied during a DC discharge in air. The concentration range of Cr⁶⁺ was (5.7–19) ×10^?5 mol/l and discharge current range was 20–80 mA. Cr⁶⁺ ions were shown to be reversibly reduced under a discharge action. The equilibrium degree of reduction increased with increasing initial concentration of the solution at fixed discharge current. At fixed initial concentration the reduction degree increased with increasing discharge current. The reduction degrees so obtained were 0.34–0.84. A kinetic scheme of the processes taking place in a solution was proposed. The calculated data obtained as a result of application of this scheme described well the experimental results on Cr⁶⁺ kinetics. The main processes of Cr⁶⁺ reduction and Cr³⁺ oxidation were revealed. HO ₂ ^· radicals and hydrogen peroxide were shown to be responsible for Cr⁶⁺ reduction whereas ^·OH radicals and O₂ molecules provide the reverse process of Cr³⁺ oxidation to Cr⁶⁺. The mechanism of action of phenol additives improving the process efficiency is discussed. The efficiency of phenol action as a radical scavenger was shown to be determined with its mass-transfer to the reaction area rather than chemical reaction rate.     

Indirect radiotracer study of the adsorption of chromate ions on γ-Al2O3 and direct study of the adsorption of sulfate ions on Cr2O3 using 35S-labelled H2SO4

The specific adsorption of sulfate ions on Cr₂O₃ was studied by a radiotracer technique using ³⁵S-labelled sulfuric acid in low concentrations (c<10^–3 mol dm^–3) in the presence of a large excess of perchlorate supporting electrolyte. The pH and concentration dependence were determined. It was found that the extent of adsorption is determined by the protonation of the surface sites, similar to other oxides studied previously. A comparison of Cr₂O₃ and Al₂O₃ in this respect shows that the protonation of the former takes place at significantly lower pH values than that of the latter. The indirect radiotracer study of the adsorption of chromate on Al₂O₃ was carried out using labelled sulfate ions as indicator species. The results obtained show that the adsorption strength of chromate species is very low in comparison to sulfate ions and a regular Langmuir-like adsorption behaviour can be observed. It is believed that the observations presented may contribute to a better understanding of the behaviour of surface layers with a mixed oxide content. Electronic Publication     

Anomalous High Temperature Structural Behavior of Potential Multifunctional Material SmCo0.5Cr0.5O3

Crystal structure parameters of the mixed cobaltite–chromite SmCo_0.5Cr_0.5O₃ in the temperature range of 298–1173 K were derived from in situ high-resolution X-ray synchrotron powder diffraction data. Similar to the parent SmCoO₃ compound, SmCo_0.5Cr_0.5O₃ reveals anomalous thermal expansion reflected in abnormal temperature dependence of the unit cell dimensions and the selected interatomic distances and angles. These anomalies are associated with temperature induced changes of spin state of Co³⁺ ions and coupled insulator-metal transition. Observed decreasing behavior of the bandwidth W points on the increasing population of the exited spin states of Co³⁺ ions in SmCo_0.5Cr_0.5O₃ with increasing temperature. First principle calculations revealed antiferromagnetic ground state of SmCo_0.5Cr_0.5O₃ as the most stable.     

Beiträge zum thermischen Verhalten wasserfreier Phosphate. IX. Darstellung und Kristallstruktur von Cr6(P2O7)4. Ein gemischtvalentes Pyrophosphat mit zwei- und dreiwertigem Chrom

Contributions on the Thermal Behaviour of Anhydrous Phosphates. IX. Synthesis and Crystal Structure of Cr₆(P₂O₇)₄. A Pyrophosphate Containing Di- and Trivalent Chromium Cr₆(P₂O₇)₄ (Cr₂²⁺Cr₄³⁺(P₂O₇)₄) can be obtained reducing CrPO₄ by phosphorus (950°C, 48 h, 100 mg iodine as mineralizer). By means of chemical transport reactions (transport agent iodine; 1050 → 950°C) the compound has been separated from its neighbour phases (Cr₂P₂O₇, CrP₃O₉) and crystallized (greenish, transparent crystals; edge length up to 0.3 mm). The crystal structure of Cr₆(P₂O₇)₄ (Spcgrp.: P-1; z = 1; a = 4.7128(8) Å, b = 12.667(3) Å, c = 7.843(2) Å, α = 89.65(2)°, β = 92.02(2)°, γ = 90.37(2) has been solved and refined from single crystal data (2713 unique reflections, 194 parameter, R = 0.035). Cr²⁺ is surrounded by six oxygen atoms which occupy the corners of an elongated octahedron (4 × d^{Cr? O} ≈? 2.04 Å; 2 × d^{Cr? O} ≈? 2.62 Å). The Cr³⁺ ions are also coordinated octahedraly (1.930 Å ≤ d_{Cr? O} ≤ 2.061 Å). The crystallographically independent pyrophosphate groups show nearly eclipsed conformation. The bridging angles (P? O? P) are 136.5° and 138.9° respectively.     

Effect of metal oxide additives on the thermal decomposition of KMnO4

The study of the thermal decomposition of potassium permanganate in air has been studied using DTA, TG and isothermal decomposition between 150 and 400°C. Rate constants were evaluated and found to be affected by the presence of foreign ions. Oxides such as Fe₂O₃ and Cr₂O₃ resulted in a lowering of the decomposition temperature. The modified catalytic mechanisms were proposed by considering the electron transfer and the oxygen-abstraction models.