The phase diagram for the binary system K2CrO4CaCrO4

The phase diagram for the binary system K₂CrO₄CaCrO₄ has been determined for CaCrO₄ concentrations up to 60 mole%, using the techniques of differential thermal analysis, X-ray diffraction, and drop calorimetry. Essential features of the phase diagram are: the solid-solid phase transition for pure K₂CrO₄ at 670°C, β-K₂CrO₄ ? α-K₂CrO₄; a eutectoid reaction at 14 mole% CaCrO₄ and 548°C, β-K₂CrO₄ ? α-K₂CrO₄ + K₂CrO₄ · CaCrO₄; a peritectoid event at 50 mole% CaCrO₄ and 640°C, β-K₂CrO₄ + CaCrO₄ ? K₂CrO₄ · CaCrO₄; and a eutectic reaction at 51 mole% CaCrO₄ and 678°C, L ? β-K₂CrO₄ + CaCrO₄. X-ray diffraction studies lead to the determination of the unit cell dimensions for the K₂CrO₄ · CaCrO₄ double salt, a C-centered monoclinic form with a₀ = 7.615(6) Å, b₀ = 22.797(15) Å, c₀ = 9.777(9) Å, β = 115.45(5)°.     

Phase equilibria in the stable tetrahedron NaF-KF-KBr-K2CrO4 of the quaternary mutual system Na,K‖F,Br,CrO4

The stable triangle NaF-KBr-K₂CrO₄ and the united stable tetrahedron NaF-KF-KBr-K₂CrO₄ of the quaternary mutual system Na,K‖F,Br,CrO₄ were studied by differential thermal analysis, and the characteristics of eutectics were determined.     

Li2CrO4 · 2H2O: Ungewöhnliche Wasserstoffbrückenbindung und Koordination der O-Liganden des Anions CrO42−

Li₂CrO₄ · 2H₂O: Unusual Hydrogen Bridge Bonding and Coordination for Oxygen of the Anions CrO₄^2? The crystal structure of Li₂CrO₄ · 2H₂O was solved including the positions of hydrogen by X-ray methods. Li₂CrO₄ · 2H₂O: P2₁2₁2₁, Z = 4, a = 5.503(1) Å, b = 7.733(2) Å, c = 11.987(2) Å, Z(F_o) with (F_o)² ? 3σ(F_o)² = 2284, Z (parameter) = 99, R/R_w = 0.025/0.029 LiCrO₄ · 2H₂O contains a locally bordered hydrogen bridge bonding system between water molecules as donors and two O of CrO₄^2? as acceptors. This system connects anions in the direction [010]. It is noticeable that oxygen ligands of the anion CrO₄^2? have strongly differing coordination.     

A new chromate of tetravalent cerium: Ce2(CrO4)4·2H2O

Dicerium(IV) tetrachromate(VI) dihydrate, Ce(CrO₄)₄·2H₂O, has been prepared from an acidic aqueous solution at room temperature. Its novel crystal structure, which was solved from single‐crystal X‐ray diffraction data, is built from isolated CrO₄ tetrahedra and isolated Ce(O,H₂O)_n (n = 8 and 9) polyhedra. All atoms are in general positions. The mean Ce—O and Cr—O bond lengths are 2.358 and 1.651 Å, respectively. Comparisons are drawn with the structure of Ce^IV(CrO₄)₂·2H₂O.     

Reactivity in The Solid State Between Ag2S and Ag2CrO4

Reactivity, in the solid state between Ag₂S and Ag₂CrO₄, was investigated by DTA, XRD and IR methods. It was found that, according to a composition of an initial Ag₂S/Ag₂CrO₄ mixture, the products of a reaction of Ag₂S with Ag₂ CrO₄ can be: solid solution with Ag₂CrO₄ structure (Ag₂Cr_1–xS_xO₄) and AgCrO₂; or solid solution Ag₂Cr_1–xS_xO₄, Ag₂SO₄, AgCrO₂ and metallic silver; or Ag₂S, β-Ag₈S₄O₄, Ag, AgCrO₂, Ag₂SO₄ and Ag₂Cr_1–xS_xO₄ solid solution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal decomposition of silver chromate complex with 1, 10 phenanthroline,Ag2(phen)2CrO4

The decomposition of silver chromate complex with 1, 10 phenanthroline has been studied by TG/DTG/DTA method and X-ray diffraction analysis. Intermediate decomposition products have been isolated and characterized. The thermolysis of Ag₂(phen)₂CrO₄ occurs in the two major stages: Ag₂(phen)_xCrO₄ as a product of the first stage and various mixtures of silver, silver chromate and silver metachromite as the final residue. The comparison of the decomposition process of the complex with that of parent Ag₂CrO₄ indicates that the presence of ligand changes thermal behaviour of the latter.

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Zusammenfassung Mittels TG/DTG/DTA und Röntgendiffraktionsanalyse wurde die Zersetzung des Silberchromat/1,10-Phenanthrolinkomplexes untersucht. Zwischenprodukte dieser Zersetzung wurden isoliert und beschrieben. Die Thermolyse von Ag₂(phen)₂CrO₄ verläuft in zwei Hauptschritten: im ersten Schritt entsteht Ag₂(phen)_xCrO₄, als Endprodukt entstehen verschiedene Gemische aus Silber, Silberchromat und Silbermetachromit. Ein Vergleich des Zersetzungsprozesses des Komplexes mit dem der Stammverbindung Ag₂CrO₄ ergab, daß die Gegenwart von Liganden das thermische Verhalten von Ag₂CrO₄ verändert.

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The authors thank Dr W. Mielcarek for making X-ray analysis.     

Contribution à l'étude du système quaternaire K+—NH4+—CrO4−−—SO4−−—H2O: I. les isothermes de 25° des systèmes ternaires limites

The four ternary limiting isotherms of the system K⁺− NH₄⁺ CrO₄⁻⁻ SO₄⁻⁻ H₂O are given for 25° C. In the systems K₂SO₄ (NH₄)₂SO₄ H₂O and K₂SO₄ K₂CrO₄ H₂O only one solid phase has been encountered; both systems belong to the type I of the ROOZEBOOM classification. A miscibility gap is present in the systems (NH₄)₂CrO₄ K₂CrO₄ H₂O and (NH₄)₂CrO₄ (NH₄)₂SO₄ H₂O; they belong to ROOZEBOOM'S type V.     

Synthesis,structure and magnetic properties of ultra-high purity CrO2 prepared under high O2-gas pressure

Ultra-high purity CrO₂ was prepared by decomposing CrO₃ in O₂ with gas pressures up to 40 MPa, which were maintained throughout the decomposition process of CrO₃ to prevent the formation of any other phases of chromium oxides. Our method is different from the traditional methods that start from or under ambient pressures. The high oxygen pressure makes the meta-stable CrO₂ stable from the initial stage of preparation. As a result, the purity of the as-prepared CrO₂ is improved, and this has been further proved by the highest magnetization of the samples. The as-prepared CrO₂ particles show very large grains with flat surfaces, octagonal cross-section, and straight edges, owing to the high mobility of Cr ions in CrO₂ at temperatures above its melting point. The lattice parameters of CrO₂ are a = 4.4176 Å and c = 2.9144 Å. The maximum value of the magnetic entropy change of the high purity CrO₂ particles is ∼2.83 J/kg·K for an applied field of 1.5 T. The preparation of pure CrO₂ is important for studying its intrinsic properties and for applications in spintronic devices.     

Iranite,CuPb10(CrO4)6(SiO4)2(OH)2, isomorphous with hemihedrite

This study presents the first structural report of iranite, ideally CuPb₁₀(CrO₄)₆(SiO₄)₂(OH)₂ [copper decalead hexachromate bis(orthosilicate) dihydroxide], based on single‐crystal X‐ray diffraction data. Iranite is isomorphous with hemihedrite, with substitution of Cu for Zn and OH for F. The Cu atom is situated at the special position with site symmetry . The CrO₄ and SiO₄ tetrahedra and CuO₄(OH)₂ octahedra form layers that are parallel to (120) and are linked together by five symmetrically independent Pb²⁺ cations displaying a rather wide range of bond distances. The CuO₄(OH)₂ octahedra are corner‐linked to two CrO₄ and two SiO₄ groups, while two additional CrO₄ groups are isolated. The mean Cr—O distances for the three nonequivalent CrO₄ tetrahedra are all slightly shorter than the corresponding distances in hemihedrite, whereas the CuO₄(OH)₂ octahedron is more distorted than the ZnO₄F₂ octahedron in hemihedrite in terms of octahedral quadratic elongation.     

Phase equilibria in the ternary systems NaCl–NaI–Na<Subscript>2</Subscript>CrO<Subscript>4</Subscript> and KCl–KI–K<Subscript>2</Subscript>CrO<Subscript>4</Subscript>

The ternary systems NaCl–NaI–Na₂CrO₄ and KCl–KI–K₂CrO₄ were studied by differential thermal analysis. In the systems, the melting points and compositions of alloys at ternary eutectic points were determined. The compositions of crystallizing phases in the eutectics were confirmed by X-ray powder diffraction analysis.     

Zur Kenntnis von Na4[CrO4]

On Na₄(CrO₄) Dark-green single crystals of Na₄[CrO₄] were obtained for the first time. (Na₂O/Cr₂O₃; Na:Cr = 4:1 not exactly closed Ni-tube; 1000°C; 30 d). It is isostructural to Na₄(CoO₄); space group P1 , a = 859.7 pm, b = 569.8 pm, c = 640 pm, α = 124°C, β = 98.4°, γ = 98.9°, Z = 2. The Madelung Part of Lattice Energy, MAPLE, Effective Coordination Numbers, ECoN, these via Mean Effective Ionic Radii, MEFIR, are calculated.     

Instant capture of recoil51Cr(III) from neutron activated K2CrO4

We have recently developed a new method of measuring the initial⁵¹Cr(III) produced from nuclear recoil of K₂CrO₄. In our method, K₂CrO₄ was mixed with MgO in the presence of a small amount of water, and the mixture was irradiated in a nuclear reactor. After irradiation, the mixture was dissolved in water, and MgO precipitate was separated from the solution. The yield of recoil⁵¹Cr(III) could be calculated from the⁵¹Cr activity in the precipitate measured. On the other hand, the yield of retention of⁵¹Cr as chromate could be calcualted from the activity found in the supernatant. The⁵¹Cr(III) yield thus obtained is almost a factor of 2 higher than observed in pure K₂CrO₄ without mixing with MgO, irradiated under the same condition. Another important observation is that the⁵¹Cr(III) yield is independent of irradiation time in the presence of MgO. Without MgO, the observed⁵¹Cr(III) yield decreases with increasing irradiation time, suggesting possible oxidation of Cr(III) to chromate during irradiation. This variation is not observed in the system of K₂CrO₄ containing MgO, indicating that the initial Cr(III) is adsorbed immediately after nuclear recoil by MgO and is protected from oxidation by gamma radiation.     

Investigations of CsCl,K2SO4, and K2CrO4 as high temperature calibrants for differential scanning calorimetry

The selection of reference materials having calibration quality data sets is essential to meaningful measurements with DSC. In the present work the results of critical evaluations of the data sets for such materials are reported. New studies for three salt systems, CsCl, K₂SO₄, and K₂CrO₄ as calibrants for high temperature DSC, were undertaken, and are reported. The studies were extended to include Bi and KNO₃, two prospective candidate materials for measurements at lower temperatures, and these results are also reported herewith.     

Combination of Ag2CrO4 and AgI semiconductors with g-C3N4: Novel nanocomposites with substantially improved photocatalytic performance under visible light

Novel g-C₃N₄/Ag₂CrO₄/AgI nanocomposites with improved photocatalytic performance under visible light were synthesized by consecutive deposition of Ag₂CrO₄ and AgI semiconductors over g-C₃N₄ sheets by refluxing method. The synthesized g-C₃N₄/Ag₂CrO₄/AgI photocatalysts were fully characterized by XRD, EDX, SEM, TEM, UV–vis DRS, TGA, FT-IR, and PL instruments. Photocatalytic performance of g-C₃N₄/Ag₂CrO₄/AgI (30%) nanocomposite for degradation of RhB was 27.9, 4.0, and 3.1 folds greater than those of the g-C₃N₄, g-C₃N₄/Ag₂CrO₄ (20%), and g-C₃N₄/AgI (30%) photocatalysts, respectively. The substantially increased photocatalytic performance was related to efficient retardation of the charge carriers from recombination and more absorbing of visible light, due to the synergistic effects of Ag₂CrO₄ and AgI on g-C₃N₄. The photocatalytic performance of the ternary nanocomposite did not considerably change after several cycles, indicating that the ternary nanocomposite is stable and it could be reused in successive runs.     

Dilead(II) trimercury(II) tetraoxide chromate(VI), Pb2(Hg3O4)(CrO4)

Pb₂(Hg₃O₄)(CrO₄) consists of [CrO₄]²⁻ tetra­hedra, linear O—Hg—O dumbbells and divalent Pb atoms in [3+5]‐coordination. The HgO₂ dumbbells are condensed into [Hg₃O₄]²⁻ units and can be regarded as a section of the HgO structure. The [Hg₃O₄]²⁻ complex anions are connected by inter­stitial Pb²⁺ ions, while the [CrO₄]²⁻ tetra­hedra are isolated.     

Notiz zum Magnetischen Verhalten von Li3CrO4

Notice on the Magnetic Behaviour of Li₃CrO₄ Li₃CrO₄, a smaragd green powder, is due to powder photographs (Guinier-Simon-technique) isotypic with orthorhombic HT? Li₃PO₄, a = 6.30₉, b = 10.85₁, c = 4.95₂ Å, Z = 4. Between 298 and 10K the Curie-Weiss-Law is obeyed with μ=1.60 B.M. and Θ=+10 K. Below 5 K ferromagnetism is observed. ESR measurements at 4.2 K and more pronounced at 1.8 K show anisotropy of the ligand field.     

The First Sodium Uranyl Chromate,Na4[(UO2)(CrO4)3]: Synthesis and Crystal Structure Determination

The first sodium uranyl chromate, Na₄[(UO₂)(CrO₄)₃], has been obtained by high‐temperature solid‐state reaction. The structure (triclinic, P1¯, Z = 2, a = 7.1548(3), b = 8.4420(3), c = 11.5102(5)Å, α = 80.203(1)°, β = 79.310(1)°, γ = 70.415(1)° V = 639.24(4)ų ) has been solved by direct methods and refined on the basis of F² for all unique reflections to R1 = 0.024 [calculated on the basis of 4374 unique observed reflections (‖F_o‖ 4σF)]. The structure is based on chains of composition [(UO₂)(CrO₄)₃] that are parallel to [1¯01]. The chains contain UrO₅ pentagonal bipyramids (Ur = Uranyl) that share all equatorial corners with CrO₄ tetrahedra. Cr(1)O₄ and Cr(3)O₄ tetrahedra bridge between two adjacent UrO₅ bipyramids, whereas Cr(2)O₄ tetrahedra share one corner with one UrO₅ bipyramid each. The [(UO₂)(CrO₄)₃] chains are planar and oriented parallel to (313). The Na⁺ cations provide linkage of the chains in the structure.     

NaIn(CrO4)2·2H2O,the first indium(III) member of the kröhnkite family

Sodium indium(III) chromate(VI) dihydrate, NaIn(CrO₄)₂·2H₂O, synthesized from an aqueous solution at room temperature, is the first indium(III) member of the large family of compounds with kröhnkite [Na₂Cu^II(S^VIO₄)₂·2H₂O]‐type chains. The crystal structure is based on infinite octa­hedral–tetra­hedral [In(CrO₄)₂(H₂O)₂]⁻ chains along [010], linked via charge‐balancing Na⁺ cations. The slightly distorted InO₄(H₂O)₂ octa­hedra are characterized by a mean In—O distance of 2.125 Å. The CrO₄ tetra­hedra are strongly distorted (mean Cr—O = 1.641 Å). The Na atom shows an octa­hedral coordination, unprecedented among compounds with kröhnkite‐type chains. The NaO₆ octa­hedra share opposite edges with the InO₄(H₂O)₂ octa­hedra to form infinite [001] chains. The hydrogen bonds are of medium strength. NaIn(CrO₄)₂·2H₂O belongs to the structural type F2 in the classification of Fleck, Kolitsch & Hertweck [Z. Kristallogr. (2002), 217 , 435–443], and is isotypic with KAl(CrO₄)₂·2H₂O and MFe(CrO₄)₂·2H₂O (M = K, Tl or NH₄). All atoms are in special positions except one O atom.     

Formation of Cr(II) Species in the H2/CrO3 System. Parameter control

The thermal behaviour of CrO₃ on heating up to 600°C in dynamic atmospheres of air, N₂ and H₂ was examined by thermogravimetry (TG), differential thermal analysis (DTA), IR spectroscopy and diffuse reflectance spectroscopy (DRS). The results revealed three major thermal events, depending to different extents on the surrounding atmosphere: (i) melting of CrO₃ near 215°C (independent of the atmosphere), (ii) decomposition into Cr₂(CrO₄)₃ at 340–360°C (insignificantly dependent), and (iii) decomposition of the chromate into Cr₂O₃ at 415–490°C (significantly dependent). The decomposition CrO₃ → Cr₂(CrO₄)₃ is largely thermal and involves exothermic deoxygenation and polymerization reactions, whereas the decomposition Cr₂(CrO₄)₃ → Cr₂O₃ involves endothermic reductive deoxygenation reactions in air (or N₂) which are greatly accelerated and rendered exothermic in the presence of H₂. TG measurements as a function of heating rate (2–50°C min⁻¹) demonstrated the acceleratory role of H₂, which extended to the formation of Cr(II) species. This could sustain a mechanism whereby H₂ molecules are considered to chemisorb dissociatively, and then spillover to induce the reduction. DTA measurements as a function of the heating rate (2–50°C min⁻¹) helped in the derivation of non-isothermal kinetic parameters strongly supportive of the mechanism envisaged. This revised version was published online in August 2006 with corrections to the Cover Date.     

Infrared spectral investigation of orthovanadates and CrO6 groups in TlMIICr2(VO4)3 compounds

The i.r. spectra of triple orthovanadates TlM it Cr₂(VO₄)₃ have been obtained in the region 4000-200 cm⁻¹ and are discussed with the aid of site group analysis. The complexity of the spectra is attributed to the possibility of VO³⁻₄ groups being located on two sites of different symmetries as well as to appearance of vibrational modes corresponding to condensed CrO₆ lattice.