Cathodic deposition of molybdenum and vanadium mixed oxyhydroxide films from V-substituted polymolybdophosphate

We report a new electrochemical route for fabricating molybdenum and vanadium mixed oxyhydroxide films on Au electrode from Keggin-type vanadium-substituted polymolybdophosphate. The process involves a potentiodynamic reduction of aqueous 9-molybdo(VI)-3-vanadophosphosphate(V) ([PMo₉V₃O₄₀]⁶⁻) in the potential region between 0 and −0.7 V versus Ag/AgCl. The resulting MoV oxyhydroxide film electrode gave a stable redox behavior in Na₂SO₄ electrolyte of pH 3. X-ray photoelectron spectroscopy revealed that this results from oxidation/reduction of Mo⁵⁺/Mo⁶⁺ in the film which accompanies extraction/insertion of protons for charge compensation. The deposited V ions remained in the film upon repetitive potential cycling without affecting their oxidation state. Voltammetric data in the presence of sodium nitrite showed electrocatalytic activity of the MoV oxyhydroxide toward the electroreduction of nitrite.     

TPR and XPS study of cobalt–copper mixed oxide catalysts: evidence of a strong Co–Cu interaction

In this work the results of a TPR and XPS investigation of Co_xO_y–CuO mixed oxides in the range of composition Co : Cu=100:0–8:92 are reported and compared. The final catalysts were obtained by thermal decomposition in air and N₂ at 723 K for 24 h of singlephase cobalt–copper hydroxycarbonates prepared by coprecipitation at constant pH. The Co : Cu=100 : 0 specimen calcined in air formed the Co²⁺[Co³]₂O₄ (Co₃O₄) spinel phase. The coppercontaining catalysts (Co : Cu=85 : 15–8 : 92) showed mainly two phases: (i) spinels, like Co²⁺[Co³⁺]₂O₄, Co _1-x ²⁺ Cu _x ²⁺ [Co³⁺]₂O₄ and (ii) pure CuO, the relative amount of each phase depending on the Co : Cu atomic ratio. The results of the XPS study are consistent with the bulk findings and revealed the presence of Co²⁺, Co³⁺ and Cu²⁺ species at the catalyst surface. Moreover, the surface quantitative analysis evidenced a cobalt enrichment, in particular for the most diluted cobalt samples. The TPR study showed that the catalyst reduction is affected by a strong mutual influence between cobalt and copper. The reducibility of the mixed oxide catalysts was always promoted with respect to that of the pure Co₃O₄ and CuO phases and the reduction of cobalt was markedly enhanced by the presence of copper. Cobalt and copper were both reduced to metals regardless of the catalyst composition. On the other hand, the Co : Cu=100 : 0 specimen calcined in N₂ formed, as expected, CoO. The initial addition of copper resulted in the formation of the Cu⁺Co³⁺O₂ compound, besides CoO, up to a Co/Cu=1 atomic ratio at which the CuCoO₂ phase was the main component. A further addition of copper led to the formation of CuCoO₂ and CuO phases. The XPS results were in good agreement with these findings and the surface quantitative analysis revealed a less enrichment of cobalt with respect to the catalysts calcined in air. The TPR analysis confirmed that the reduction of the N₂calcined catalysts was also remarkably promoted by the presence of copper. Also in this case cobalt and copper metal were the final products of reduction.     

Ambient Temperature Hydrolysis of Carbonyl Sulfide Using γ-Alumina Catalysts: Effect of Calcination Temperature and Alkali Doping

The hydrolysis of COS using -alumina as catalyst at 20 °C is described and discussed. In particular, the effect of calcination on the catalyst activity is investigated. Catalysts calcined at 100 and 500 °C are found to give the highest catalyst activities, in terms of both specific (mol COS converted/g catalyst/h) and intrinsic (mol COS converted/m² catalyst/h) activity. Calcination at other temperatures leads to diminished catalyst activity. The effects are discussed in terms of the known surface chemistry of -alumina involving physisorbed water, surface dehydroxylation and defect formation. The addition of alkali additives (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Ba²⁺, Si²⁺) is also reported. Only K⁺ and Cs⁺ give a sustained enhancement in catalyst activity, whereas all the other additives act as catalyst poisons for the steady-state performance measured following 5 h time-on-stream. Interestingly, addition of Na⁺ and Mg²⁺ leads to a very high initial activity (>95% COS conversion) but the effect is very short-lived and, after 5 h time-on-stream, a much lower steady-state activity (15-30% COS conversion) is observed.     

Decisive role of mixed‐valence structure in colossal dielectric constant of LaFeO3

The role of mixed‐valence structure in colossal dielectric constant (CDC) behavior has been investigated in LaFeO₃ ceramics by tuning the ratio of Fe²⁺/Fe³⁺ through substituting Al for Fe. The ratio of Fe²⁺/Fe³⁺ is decreased gradually from 1.0 to 0.0 by increasing the concentration of Al³⁺. Two clear‐cut correlations have been found: (i) the relationship between the CDC behavior and the ratio of Fe²⁺/Fe³⁺ follows an exponential function and (ii) the activation energy of the polaron relaxation is proportional to , where is the intrinsic dielectric constant. These findings underscore the role of the mixed‐valence structure in CDC behavior and suggest that adjusting the mixed‐valence structure through doping/alloying can be a promising strategy to achieve superior CDC behavior in transition‐metal oxides.     

On methanethiol synthesis from H2S‐containing syngas over K2MoS4/SiO2 catalysts promoted with transition metal oxides

The catalysts K₂MoS₄/SiO₂ promoted with transition metal oxides Fe₂O₃, CoO, NiO and MnO₂ were prepared and used to catalyze the synthesis of methanethiol from H₂Scontaining syngas. The results of activity assay show that the catalysts promoted with Fe₂O₃, CoO and NiO can remarkably increase the hour space yield of methanethiol. Nevertheless, MnO₂ was found to have a disadvantageous effect on the selectivity of methanethiol. The results of XRD and XPS characterization indicate that the addition of the transition metal oxides promoters is in favor of the formation of a Mo–S–K active phase and also retards the decomposition of K₂MoS₄ to MoS₂, thereby suppressing both the deep reduction of Mo species and the formation of (S–S)² species, which are reflected by the increment of the concentration ratios of both Mo⁶⁺/Mo⁴⁺ and S²/(S–S)².     

Comparison of multi-valent manganese oxides (Mn4+, Mn3+, and Mn2+) doping in BiFeO3-BaTiO3 piezoelectric ceramics

Different manganese oxides-doping effects were compared in piezoceramic BiFeO₃-BaTiO₃ system. 0.67Bi_1.05(Fe_0.99Mn^x_0.01)O₃-0.33BaTiO₃ (valence state x = 4+, 3+, and 2+) ceramics were prepared via a solid-state reaction process followed by furnace-cooling (FC) or water-quenching (WQ) process. For the FC ceramics, the direct piezoelectric sensor coefficient (d₃₃) was almost independent of valence state of doped Mn, while d₃₃ depended on the fraction of Fe³⁺/Fe²⁺ in WQ ceramics. The d₃₃ value was highest for the donor Mn⁴⁺-doped ceramic, among the FC ceramics, with 175 pC/N. However, acceptor-doping with Mn²⁺ prevented the transition of Fe ion valence state from 3+ to 2+ in the WQ ceramics, the Mn²⁺-doped WQ ceramic showed the largest d₃₃ of 313 pC/N and converse piezoelectric actuator coefficient, d₃₃^* of 352 pm/V, with high Curie phase transition temperature (482 °C).     

Identification of Highly Active Iron Sites in N2O-Activated Fe/MFI

Reduction in an H₂ flow at 600 °C of Fe/MFI prepared by chemical vapor deposition, followed by its exposure to N₂O at 250 °C, produces a highly active state characterized by an unusual TPR spike at 200 °C. In situ X-ray absorption near-edge structure, X-ray absorption fine structure data and literature data on DFT calculations suggest that in this state some Fe will be present in the oxidation state of Fe⁴⁺.     

Some Factors Influencing the Morphology of α-Zirconium Phosphate and its Intercalation Reactions with a Bifunctional Thioether Amine

Crystalline samples of -zirconium phosphate, (-ZrP, -Zr(HPO₄)₂·H₂O) have been prepared by decomposition of zirconium fluoride complexes in the presence of phosphoric acid under a variety of conditions. The crystallinity and morphology have been shown to depend on a number of factors including the F^–/Zr⁴⁺ ratio, the concentration of Zr⁴⁺ ions, the material of the reaction vessel and the reaction temperature. Under conditions of rapid precipitation small plate-like crystals of -ZrP are produced whereas under conditions of slow crystallisation larger crystals with a lower aspect ratio are formed. The relative intensities of the d ₀₀₂, d ₁₁₀ and d ₁₁₂ reflections observed by X-ray powder diffraction show a correlation with the crystal morphology as determined by SEM.The intercalation reaction of 4-(methylmercapto)aniline with different samples of -ZrP under a variety of conditions has been studied. Incomplete intercalation is observed in each case, with the extent of intercalation depending on both the morphology of the -ZrP and the reaction conditions. The intercalated amine has been shown to exist as a mixture of protonated cations and neutral molecules.     

Nitrous oxide decomposition and reduction over copper catalysts supported on various types of carbonaceous materials

Copper supported on three different allotropic forms of carbon materials have been prepared and evaluated as catalysts for the N₂O decomposition and reduction reactions. It was found that all the catalysts underwent severe deactivation during the N₂O decomposition reaction due to the gasification of carbon substrates. This behavior was particularly evident when activated carbon was used as the support medium. The chemical identity of the active entity involved in the carbon gasification process is believed to consist of a mixture of Cu⁺ and Cu²⁺ species and, according to the well established mechanism, the reaction proceeds in such a manner so that the surface of the catalyst undergoes a redox cycle at the gas/solid carbon interface. The introduction of CO into the system was shown to result not only in an enhancement in the activity of the desired N₂O decomposition reaction, but also served to inhibit the deleterious carbon gasification process. In addition, this procedure stabilized the copper particles in the metallic state, which is the active species responsible for the dissociation of N₂O. Copper dispersed on a diamond substrate appeared to attain the highest activity for the N₂O reduction reaction, a feature that is associated with the ability of the metal to undergo a wetting and spreading action on the support surface, possibly resulting in an epitaxial relationship between the two components.     

紫外-可见吸收光谱法表征氟磷酸钒钠系列化合物中钒的表观价态

采用紫外?可见吸收光谱表征Na3(VO1?xPO4)2F1＋2x(0≤x≤1)中钒的表观价态,用X射线衍射精修所得晶胞参数对紫外?可见光谱法的表征结果进行验证. 结果表明,Na3(VOPO4)2F中钒的表观价态为+3.993, Na3(VO0.5PO4)2F2和Na3(VOPO4)2F3中钒的价态分别为+3.097和+3.603,紫外?可见吸收光谱法得到的结果是可信的.     

Deformation Dynamics of a Reactive Medium During Gasless Combustion

Deformation during gasless combustion of 5Ti + 3Si and Ti + C samples was studied experimentally. The dynamics of motion of the material in a gasless combustion wave was studied using highspeed video recording (500 frames/sec) with a spatial resolution of the order of 10 m. It was shown that behind the combustion front, the medium was first expanded and then compressed. The dimensions of the expansion and compression zones were determined.     

Effect of Mn-doping on the structure and electrical properties of (Pb0.325Sr0.675)TiO3 ceramics

Pb_0.325Sr_0.675Ti_1-xMn_xO₃ ceramics (x?=?0, 0.001, 0.005, 0.01, and 0.05) were successfully prepared by traditional solid-state reaction method. It was found that the lattice constant calculated through Rietveld refinement initially increased and then decreased with increasing Mn content, which was attributed to the variation in valence state of Mn and Ti ions. The microstructure gradually varied from the coexistence of large grains and fine grains for x?=?0 to the uniform grain for x?=?0.05 by increasing the doping Mn ions. With increasing Mn content from x?=?0 to x?=?0.05, the Curie temperature (Tc) dramatically decreased from 25?°C to ??40?°C and dielectric maximum decreased from 27,100 to 13,200. Pb_0.325Sr_0.675Ti_1-xMn_xO₃ ceramics with x?=?0.001 showed the lowest dielectric loss of 0.006 with a relatively high dielectric peak value of ~ 21,000. The grain boundaries resistance obtained from the complex impedance decreased with the increase of Mn content. The decrease in resistance was ascribed to oxygen vacancies and electronics produced by the change of ionic valence state. X-ray photoemission spectroscopy revealed that Ti ions were Ti⁴⁺ and the valences of Mn ions were deduced to be mainly in the form of Mn²⁺ and/or Mn³⁺ for ceramics with low content of Mn, while the Ti ions were in the form of Ti³⁺ and Ti⁴⁺ and Mn ions were diverse valence states with the coexistence of Mn²⁺, Mn³⁺, and Mn⁴⁺ for ceramics with x?=?0.01 and 0.05.     

Mechanisms of Iron Activation on Fe-Containing Zeolites and the Charge of α-Oxygen

According to previous Mössbauer data [1] -sites formation at the activation of Fe-containing zeolites is accompanied by irreversible self-reduction of the iron, proceeding without participation of an external reducing agent. Reduced Fe²⁺ ions are inert to O₂ but are reversibly oxidized to Fe³⁺ by N₂O, generating the -oxygen species, O, which provide selective oxidation of hydrocarbons.In this work, the mechanism of -sites formation was studied via quantitative measurement of the dioxygen amount desorbed into the gas phase at the step of self-reduction. A prominent role of the zeolite matrix chemical composition has been revealed. For example, with zeolites of Al–Si composition (FeZSM-5 and Fe-), heating to 900 °C in a closed vacuum space leads to irreversible evolution of O₂, which is accompanied by the immediate formation of -sites. Similar heating of B–Si and Ti–Si zeolites also leads to dioxygen evolution; however, this evolution is reversible and is not accompanied by formation of -sites. Activation of these zeolites occurs only in the presence of water vapor. Stoichiometric measurements showed that in terms of charge one regular O^2- ion, removed at the activation, is equivalent to two -oxygen atoms. So, -oxygen is identified as an ion-radical species O ^-., whose unique oxidation properties still distinguish it from the generally observed O^-. radicals.The mechanism of -sites formation is proposed, in which the process of strong chemical stabilization of reduced Fe²⁺ atoms in the zeolite structure is a key step, making impossible the reoxidation of the iron with O₂.     

RuO2 doping and its influence on phase structure,cations state,and electrical properties of Mn1·6Co0·4CuO4 ceramics

RuO₂ is a typical metal conductor with low resistivity and is available in a wide range of electronic components to decrease resistance and temperature resistance coefficient. Mn–Cu–Cu–O compounds with different RuO₂ dopping contents were prepared with RuO₂ and other oxide powders via a solid-state reaction method. The X-ray diffraction patterns of the compounds revealed a primarily spinel-structure, moreover, as the Ru content increased, diffraction spectrum for other impurities appeared. Images of Mn–Cu–Cu–Ru (MCCR) ceramics showed significant differences in grain size. The energy-dispersive X-ray spectroscopy mapping results revealed a non-homogenous element distribution. The valence states of metal elements were investigated using X-ray photon spectroscopy, which demonstrated conversion of the valence state in Cu (Cu²⁺ to Cu⁺) and Mn (Mn²⁺ to Mn³⁺ and Mn⁴⁺) cations, whereas Co may have a single valence state, Co²⁺. With increasing RuO₂ content from 0 to 0.3, the resistivity of the material decreased to 7.43 and 0.13 Ω cm, whereas the activation energy were 0.205, 0.147, 0.132, and 0.115 eV, while the thermal sensitivity (B) constant of the materials were 2378, 170, 153, and 1329 K, respectively. These results can be used to design a negative temperature coefficient (NTC) thermistor with low resistance and B value.     

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.     

Photoluminescence investigation of novel reddish-orange phosphor Li2NaBP2O8:Sm3+ with high CP and low CCT

The exploration of appropriate inorganic phosphors with high color purity (CP) and low correlated color temperature (CCT) has always been a hot issue for solid state light applications. In this work, we have developed series of Sm³⁺ doped Li₂NaBP₂O₈ (abbreviated as: LNBP) phosphors by means of the solid state synthesis route. The crystalline phase compositions, micromorphology, valence state of elements as well as photoluminescence properties were systematically illustrated. Upon the excitation wavelength at 400?nm, emission peaks are located at 561，597，643 and 699?nm, corresponding to the ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2, ⁶H_9/2 and ⁶H_11/2 transitions of Sm³⁺ in the same order. The optimal doping amount of Sm³⁺ ions is 2?mol% for the reddish-orange photoluminescence of the LNBP:Sm³⁺ phosphor system. The critical energy transfer distance, mechanism of concentration quenching, CIE chromaticity coordinate, CP, CCT and internal quantum efficiency were extensively investigated. The title product might be considered as a promising candidate of phosphor in near UV-based warm white LEDs.     

Spectral management and energy‐transfer mechanism of Eu3+‐doped β‐NaGdF4:Yb3+,Er3+ microcrystals

β‐NaGdF₄:Yb³⁺,Er³⁺ upconversion (UC) microcrystals were prepared by a facile hydrothermal process with the assistance of ethylene diamine tertraacetic acid (EDTA). The β‐NaGdF₄ UC microcrystal morphology was controlled by changing the doses of EDTA and NaF. Uniform hexagonal structure can be obtained at the 2 mmol EDTA and 9‐10 mmol NaF. The UC emissions of β‐NaGdF₄:Yb³⁺,Er³⁺ microcrystals were tuned by the variation of Eu³⁺ doping level (0%‐5%), where the red/green intensity ratio decreased with the Eu³⁺ concentration increase. It was found on the base of rate equations that with the Eu³⁺ doping, the energy back transfer process ²H_11/2/⁴S_3/2 (Er³⁺) → ⁴I_13/2 (Er³⁺) decreased. In addition, an energy‐transfer process from ⁴F_7/2 (Er³⁺) to ⁵D₁ (Eu³⁺) and a cross relaxation process of ⁷H_9/2 (Er³⁺) + ⁵D₀ (Eu³⁺) → ⁴F_7/2 (Er³⁺) + ⁵D₂ (Eu³⁺) were proposed and verified by rate equations, which dominated the energy‐transfer mechanism between Er³⁺ and Eu³⁺, resulted in the spectra tuning of β‐NaGdF₄:Yb³⁺,Er³⁺. The results suggested that the color tuning of β‐NaGdF₄:Yb³⁺,Er³⁺,Eu³⁺ UC microcrystals would have potential applications in such fields as flat‐panel displays, solid‐state lasers, and photovoltaics.     

Transport of phosphorus forms and of nitrate through a clay soil under grass and cereal production

Many nutrients are lost from soil to water viatile drains. However, there are very few reliable studies of such phosphorus(P)losses under Swedish agriculture practices, especially in connection tofertiliser and slurry applications and related to nutrient balances. Tile drainlosses were measured from nine experimental plots in south-west Sweden; fourplots were measured for 7 years and five plots for 2–3 years. Cereals,mainly spring barley (Hordeum vulgare, L.) and oats(Avena sativa, L.), were grown in six plots, while oneplotwas cultivated with grass, timothy (Phleum pratense L.)andmeadow fescue (Festuca pratensis, L.), one with lucerne(Medicago sativa, L.) and meadow fescue (F.pratensis, L.), and the last one was a set-aside withgrass (Lolium perenne, L. and Trifoliumrepens, L.) that was neither fertilised nor used for crop removal.Attention was paid to the forms in which P was transported to water since thishas important ecological implications. Average losses of particulate P (PP)fromthe set-aside land was significantly lower than from the cerealproduction, but the average losses of soluble reactive P (SRP) and dissolvednonreactive P (DUP) were the same. Average loss of PP from the grass ley during 3years (0.09 kg per ha^–1 y^–1) wassignificantly less than the losses from cereal production. Total P loss of 0.3kg per ha^–1 y^–1 was equal to the averagelossfrom the Swedish monitoring network of observation fields. Compared to theaverage nitrate-nitrogen (NO₃-N) leaching, which was 7 kg perha^–1 y^–1 from cereals, the NO₃-Nleaching was very low from the plots with lucerne-grass and set-aside.After applying and directly ploughing in 80 kg ha^–1 ofsuperphosphate for cereal production in November 1998, 0.6–1.8 kgSRP ha^–1 y^–1 was lost through the drain tiles.Surface application of cattle slurry in spring 1999 to the grass ley alsoresulted in a very pronounced increased base level of SRP. This paper documentsthe importance of applying slurry during dry conditions and of placingfertiliser into the soil according to crop requirements in the time perspectiveof one year.     

Nutrient transformation in soil due to addition of organic manure and growing crops

Maintaining organic pools of nitrogen (N) in soil is important for providing a steady flux of N in soil solution. Bioslurry, which is the product obtained from anaerobically digested (methanised) farm yard manure (FYM), is an efficient source of organic manure with capability to supply nutrients, particularly N to crops. A study was conducted to see the equilibrium relationship between the inorganic and organic N fractions as affected by application of bioslurry and fertilizer N in a maize (Zea mays L.) — mustard (Brassica campestris) crop sequence. Results obtained revealed that 75.7 percent of the total soil N was in the hydrolyzable N fraction. Among the hydrolyzable fractions, aminoacid N, unidentified N and hydrolyzable NH ₄ ⁺ constituted 25.8, 25.7 and 18.6 percent of the total N, respectively. Ammonium fixed in clay lattice constituted 19.1 percent of the total N. Application of bioslurry @ 13.32 t ha^–1 under N-unfertilized conditions increased NO₃-N, fixed NH ₄ ⁺ , aminoacid N, hexosamine N and hydrolyzable NH ₄ ⁺ . The magnitude of increase in total hydrolyzable and inorganic N fractions was 31.4 and 15.2 percent, respectively. Growing crops decreased N in the inorganic fractions. Transformation reaction of organic N to inorganic N was evident after second crop in the sequence. Fertilizer N application encouraged build-up of N in organic fractions, particularly in aminoacid, hydrolyzable NH ₄ ⁺ and unidentified N fractions. Application of bioslurry maintained higher status of N in both organic and inorganic N fractions. Linear regression relationship between N content in different fractions and bioslurry applied both under fertilized and unfertilized conditions assisted in developing prediction models on the rate of bioslurry to be applied to arrive at the desired N content in different fractions. Significant intercorrelation coefficients (r²) between different fractions indicated free mobility between the N fractions under limited N conditions suggesting a dynamic equilibrium between them. Path coefficient analysis showed that exchangeable NH ₄ ⁺ and NO₃-N had substantial direct positive effect on N uptake by mustard with bioslurry application. Under untreated conditions exchangeable NH ₄ ⁺ , hexosamine and hydrolyzable NH ₄ ⁺ fractions had higher direct contribution to meet mustard N requirement. Most of the hydrolyzable N fractions contributed to N uptake by mustard by first transforming to exchangeable NH ₄ ⁺ and NO₃—N and thus setting an equilibrium condition for maintaining the steady flux of N to plants.Part of Ph.D. Thesis of the senior author     

Effect of thermal history on high‐valence chromium ion dissolution in merwinite (3CaO·MgO·2SiO2)

Solubility and local structure of transmission elements in calcium silicate compounds has not been well understood. We investigate the local structure of chromium ions dissolved in merwinite (3CaO·MgO·2SiO₂) of a monoclinic crystal structure. The acceptance of doping elements into merwinite has not been reported before. We found that chromium ions are soluble in merwinite in air and that chemical valence of the dissolved Cr ions varies with annealing temperature. The absorption edge in the x‐ray absorption near edge structure (XANES) of Cr‐doped merwinite indicated that octahedrally coordinated Cr³⁺ ions were mainly formed when annealed at 1673 K in air. A pre‐edge peak was also detected, indicating the existence of tetrahedrally coordinated high‐valence Cr ions. Conversely, through annealing of merwinite at 1123 K in air, tetrahedrally coordinated Cr⁶⁺ ions were found to be the main form of chromium. XANES spectra simulated by first‐principle calculations were used to explain the structural features in the observed spectra. We propose the coexistence of Cr³⁺ ions in octahedral Mg²⁺ sites and high‐valence Cr ions in tetrahedral Si⁴⁺ sites. In addition, a change in the chromium ion oxidation state in tetrahedral coordination sites was suggested by XANES spectroscopy of Cr‐doped merwinite synthesized at 1673 K and reannealed at 1123 K.