On the reduction of Cr3+ during the thermal decomposition of fluorochromates(III) with nitrogen containing cations

The thermal decomposition of [CO(NH₂)₂H]CrF₆·H₂O, (C₃N₆H₈)CrF₅·H₂O and the solid state reaction of CrF₃ and melamine are investigated under non-reciprocal quasi-static conditions and compared with the thermal behaviour of other fluorochromates(III) ([Cr(NH₃)₆]CrF₆, (NH₄)₃CrF and [C(NH₂)₃]₃CrF₆). The comparison of the results shows that the amount of chromium(II) in the final product is determined by the thermal stability and consequently by the decomposition temperature of the intermediates. Neither bonding properties in the starting materials nor the absolute amount of generated NH₃ influence the composition of the final product.     

Chromium(III) Hydroxide Solubility in the Aqueous K+-H+-OH?-CO2-HCO 3? -CO 32? -H2O System: A?Thermodynamic Model

Chromium(III)-carbonate reactions are expected to be important in managing high-level radioactive wastes. Extensive studies on the solubility of amorphous Cr(III) hydroxide solid in a wide range of pH (3–13) at two different fixed partial pressures of CO₂(g) (0.003 or 0.03 atm.), and as functions of K₂CO₃ concentrations (0.01 to 5.8 mol⋅kg⁻¹) in the presence of 0.01 mol⋅dm⁻³ KOH and KHCO₃ concentrations (0.001 to 0.826 mol⋅kg⁻¹) at room temperature (22±2 °C) were carried out to obtain reliable thermodynamic data for important Cr(III)-carbonate reactions. A combination of techniques (XRD, XANES, EXAFS, UV-Vis-NIR spectroscopy, thermodynamic analyses of solubility data, and quantum mechanical calculations) was used to characterize the solid and aqueous species. The Pitzer ion-interaction approach was used to interpret the solubility data. Only two aqueous species [Cr(OH)(CO₃)₂²⁻ and Cr(OH)₄CO₃³⁻] are required to explain Cr(III)-carbonate reactions in a wide range of pH, CO₂(g) partial pressures, and bicarbonate and carbonate concentrations. Calculations based on density functional theory support the existence of these species. The log ₁₀ K° values of reactions involving these species [{Cr(OH)₃(am) + 2CO₂(g)⇌Cr(OH)(CO₃)₂²⁻+2H⁺} and {Cr(OH)₃(am) + OH⁻+CO₃²⁻ ⇌Cr(OH)₄CO₃³⁻}] were found to be −(19.07±0.41) and −(4.19±0.19), respectively. No other data on any Cr(III)-carbonato complexes are available for comparisons.     

H2O−D2O solvent isotope effect in the volume behavior of some high-charge high-size electrolytes

The apparent molar volumes V of KCl, BaCl₂, K₂SO₄, LaCl₃, Co(en)₃Cl₃ [Tris(ethylenediamine)cobalt(III) chloride], K₃Co(CN)₆, K₃Fe(CN)₆, K₄Fe(CN)₆, and Ba₃[Co(CN)₆]₂ have been determined at 25°C in both light and heavy water. The V values in D₂O are systematically lower and increase more rapidly with salt concentration than the V in H₂O. The volume of transfer from H₂O to D₂O as well as the partial molar volume at infinite dilution in both solvents have also been calculated. These results together with literature values for other electrolytes were used to estimate both of these quantities for D₂O solutions of individual ions. The predictions of ion hydration models and ion-ion interactions are compared with experimental observations.     

[Cl3Cu(I)-As(III)O3]: Ein kristallchemisch neues Bauelement in der Struktur des Pb6Cu(AsO3)2Cl7

The synthetic compound Pb₆Cu(AsO₃)₂Cl₇ crystallizes in space group R witha ₀=9.8614(4),c ₀=17.089(2)Å,Z=3. The crystal structure, determined by single crystal X-ray work, is a typical layer structure. Complex Pb₆(AsO₃)₂Cl₆ layers are combined via monovalent Cu and Cl atoms. A novel element within the structure is a [Cl₃Cu(I)-As(III)O₃] group with the interatomic distances (Å): Cu-Cl=2.44 (3×), As-O=1.76 (3×), Cu-As=2.34 (1×).

     

The crystal chemistry of Bi6TP2O15+x, T=Fe, Ni, Zn: Isomorphism and polymorphism, structural relationship to Bi6TiP2O16

The crystal structures of compounds with nominal compositions Bi₆FeP₂O_15+x (I), Bi₆NiP₂O_15+x (II) and Bi₆ZnP₂O_15+x (III) were determined from single-crystal X-ray diffraction data. They are monoclinic, space group I2, Z=2. The lattice parameters for (I) are a=11.2644(7), b=5.4380(3), c=11.1440(5) Å, β=96.154(4)°; for (II) a=11.259(7), b=5.461(4), c=11.109(7) Å, β=96.65(1)°; for (III) a=19.7271(5), b=5.4376(2), c=16.9730(6) Å, β=131.932(1)°. Least squares refinements on F² converged for (I) to R1=0.0554, wR₂=0.1408; for (II) R₁=0.0647, wR₂=0.1697; for (III) R₁=0.0385, wR₂=0.1023. The crystals are complexly twinned by 2-fold rotation about , by inversion and by mirror reflection. The structures consist of edge-sharing articulations of OBi₄ tetrahedra forming layers in the a-c plane that then continue by edge-sharing parallel to the b-axis. The three-dimensional networks are bridged by Fe and Ni octahedra in (I) and (II) and by Zn trigonal bipyramids in (III) as well as by oxygen atoms of the PO₄ moieties. Bi also randomly occupies the octahedral sites. Oxygen vacancies exist in the structures of the three compounds due to required charge balances and they occur in the octahedral coordination polyhedron of the transition metal. In compound (III), no positional disorder in atomic sites is present. The Bi-O coordination polyhedra are trigonal prisms with one, two or three faces capped. Magnetic susceptibility data for compound (I) were obtained between 4.2 and 350 K. Between 4.2 and 250 K it is paramagnetic, μ_eff=6.1 μ_B; a magnetic transition occurs above 250 K.     

Calorimetric study and thermal analysis of [Gd4/3Y2/3(Gly)6(H2O)4](ClO4)6·5H2O and [ErY(Gly)6(H2O)4](ClO4)6·5H2O (Gly: glycin)

Two solid-state coordination compounds of rare earth metals with glycin, [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O and [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O were synthesized. The low-temperature heat capacities of the two coordination compounds were measured with an adiabatic calorimeter over the temperature range from 78 to 376 K. [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O melted at 342.90 K, while [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O melted at 328.79 K. The molar enthalpy and entropy of fusion for the two coordination compounds were determined to be 18.48 kJ mol⁻¹ and 53.9 J K⁻¹ mol⁻¹ for [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O, 1.82 kJ mol⁻¹ and 5.5 J K⁻¹ mol⁻¹ for [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O, respectively. Thermal decompositions of the two coordination compounds were studied through the thermogravimetry (TG). Possible mechanisms of the decompositions are discussed.     

Low-temperature heat capacities and thermodynamic properties of rare-earth triisothiocyanate hydrates (III) —Sm(NCS)3 · 6H2O, Gd(NCS)3 · 6H20, Yb(NCS)3 · 6H20 and Y(NCS)3 · 6H20

The heat capacities of four RE isothiocyanate hydrates, Sm(NCS)₃, · 6H₂0, Gd(NCS)₃ · 6H₂0, Yb(NCS)₃, · 6H₂O and Y(NCS)₃, · 6H₂0, have been measured from 13 to 300 K with a fully-automated adiabatic calorimeter. No obvious thermal anomaly was observed for the above-mentioned compounds in the experimental temperature ranges. The polynomial equations for calculating the heat capacities of the four compounds in the range of 13–300 K were obtained by the least-squares fitting based on the experimentalC _P, data. TheC _P, values below 13 K were estimated by using the Debye-Einstein heat capacity functions. The standard molar thermodynamic functions were calculated from 0 to 300 K. Gibbs energies of formation were also calculated. Project supported by the National Natural Science Foundation of China.     

DSC study of the phase transitions in [Ni(D2O)6](ClO4)2 and [Ni(H2O)6](ClO4)2

DSC measurements were carried out for [Ni(H₂O)₆](ClO₄)₂ (sampleH) and [Ni(D₂O)₆](ClO₄)₂ (sampleD) in the temperature range 300–380 K. For both compounds two anomalies on the DSC curves were detected. The results for sampleH are compared to those previously obtained using adiabatic calorimetry method. For both compounds studied in this work the high-temperature transition appears at the same temperature while the low-temperature one is shifted towards higher temperatures in sampleD. Disorder connected with H₂O or D₂O groups is suggested in the intermediate phase between the low- and high-temperature transitions.     

Hydration behavior of mixed ligand chromium complexes. Part I: partial molar volumes and partial molar adiabatic compressibilities of oxalato and mixed oxalato — Ethylxanthato chromate(III) complexes

Densities and ultrasonic velocities in aqueous solutions of cis-K[Cr(ox)₂(H₂O)₂], K₃[Cr(ox)₃] and KSSCOC₂H₅ and a mixed ligand complex K₂[Cr(ox)₂SSCOC₂H₅], where SSCOC₂H₅ is the ethylxanthate ligand, have been measured at 25, 35 and 45°C. Limiting partial molar volumes, partial molar adiabatic compressibilities and hydration numbers of the complex salts as well as of the ligand have been calculated. The implication of the findings are discussed.     

Reactivity of Binary Mixtures of La(III) Oxide and Cu(II) Oxalate or Nitrate and Synthesis of La(III) Cuprate

Reactivity of mixtures of La(III) oxide and Cu(II) oxalate/nitrate in hydrated as well as anhydrous state was studied using TG, DTA and XRD. Cu(II) oxide formed in the endothermic decomposition of mixture containing hydrated Cu(II) nitrate and La(III) oxide could not form La₂CuO₄ while Cu(II) oxide formed in the exothermic decomposition of mixture containing hydrated/anhydrous Cu(II) oxalate and La(III) oxide reacts with La(III) oxide and develops the phases CuLaO₃ and La₂CuO₄. The maximum reactivity with respect to the formation of La₂CuO₄phase was observed in mixture containing anhydrous Cu(II) oxalate. This revised version was published online in July 2006 with corrections to the Cover Date.     

New mixed-valence chromium structure type: NH4Cr(CrO4)2

Synthesis and crystal structure of a new structure type of mixed Cr(III)/Cr(VI) chromates is reported. NH₄Cr(CrO₄)₂ was prepared from CrO₃ in the presence of (NH₄)₂Ce(NO₃)₆. Since this is the first preparation of mixed valence ternary chromium oxides from aqueous solution, a reaction pathway for this synthesis is suggested. The crystal structure of NH₄Cr(CrO₄)₂ has been determined from three-dimensional X-ray data collected at low temperature, 173 K. The structure belongs to the orthorhombic space group Pnma, with a=14.5206(10), b=5.4826(4), and Z=4. The title compound consists of corner-sharing chromium(III) octahedra and chromium(VI) tetrahedra forming a three-dimensional network with the composition [Cr(CrO₄)₂]_n^n-, containing channels in which zigzag rows of ammonium ions balance the net charge.     

Apparent molar volumes of alkaline earth hexacyanocobaltates (III) in aqueous solution at 25°C

The apparent molar volumes of Mg, Ca, Sr, and Ba hexacyanocobaltates (III) have been determined from 1×10⁴ to 0.2M (mol-dm^–3), using both vibrating tube densimeter and dilatometric methods. The semiempirical Pitzer equation has been used to reproduce the experimental data. Positive deviations from the Debye-Hueckel limiting law (DHLL) have been observed at C<0.01M and are compared with the predictions of two other electrostatic approaches, the DHLL+B₂ approximation of the Mayer theory and the numerical integration of the exponential Poisson-Boltzmann equation. A least squares procedure has been used to obtain the best fit parameters, including the apparent molar volume at infinite dilution.     

Thermal and calorimetric studies of M(IO3)2·6H2O and M(IO3)2·6D2O forM 2+=Ca2+ and Sr2+

Thermal and calorimetric studies were carried out on M(IO₃)₂·6H₂O and M(IO₃)₂·6D₂O forM ²⁺=Ca²⁺ and Sr²⁺, using DTA and DSC methods. The thermal behaviour of the ordinary and deuterated hydrates is outlined and the differences observed between them are discussed. The enthalpies of the phase transitions were determined. The H _f ^o for Ca(IO₃)₂·6H₂O, Ca(IO₃)₂·6H₂O(D₂O) and Sr(IO₃)₂·6H₂O(D₂O) were calculated from the H _deh data and comments are made on the isotope effect observed.

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Zusammenfassung Mittels DTA- und DSC-Methoden wurden Me(IO₃)₂·6H₂O und Me(IO₃)₂·6D₂O (mitMe ²⁺=Ca²⁺ und Sr²⁺) thermisch und kalorimetrisch untersucht. Es wird ein Überblick über das thermische Verhalten ordentlicher und deuterierter Hydrate gegeben, in dem auch die Unterschiede zwischen beiden diskutiert werden. Die Enthalpien der untersuchten Phasenumwandlungen wurden bestimmt. Aus den Daten für H_deh wurde H_f von Ca(IO₃)₂·6H₂O, Ca(IO₃)₂H₂O(D₂O) und Sr(IO₃)₂·6H₂O(D₂O) berechnet und Bemerkungen zum beobachteten Isotopeneffektes gemacht.

     

Synthesis and properties of the double perovskites La2NiVO6, La2CoVO6, and La2CoTiO6

The double perovskites La₂CoVO₆, La₂CoTiO₆, and La₂NiVO₆, are described. Rietveld fitting of neutron and powder X-ray diffraction data show La₂NiVO₆ and La₂CoVO₆ to have a disordered arrangement of B-cations whereas La₂CoTiO₆ shows ordering of the B-cations (with ∼5% Co/Ti inversion). Curie-Weiss fits to the linear region of the 1/χ plots reveal Weiss temperatures of −107, −34.8, and 16.3 K for La₂CoVO₆, La₂CoTiO₆, and La₂NiVO₆, respectively, and magnetic transitions are observed. La₂CoTiO₆ prepared by our method differs from material prepared by lower-temperature routes. A simple antiferromagnetic spin model is consistent with the data for La₂CoTiO₆. These compounds are semiconductors with bandgaps of 0.41 (La₂CoVO₆), 1.02 (La₂CoTiO₆) and 0.45 eV (La₂NiVO₆).     

Thermal Bevaviour of the Coordination Compound [Mn(urea)6](NO3)2·2H2O

Thermoanalytical, mass spectrometric and X-ray diffraction data were associated with a reliable assignation of the thermal transformations of the coordination compound [Mn(urea)₆](NO₃)₂·2H₂O. This revised version was published online in August 2006 with corrections to the Cover Date.     

Crystal growth, structure and magnetic properties of the double perovskites Ln2MgIrO6 (Ln=Pr, Nd, Sm-Gd)

Single crystals of double-perovskite type lanthanide magnesium iridium oxides, Ln₂MgIrO₆ (Ln=Pr, Nd, Sm-Gd) have been grown in a molten potassium hydroxide flux. The compounds crystallize in a distorted 1:1 rock salt lattice, space group P2₁/n, consisting of corner shared MO₆ (M=Mg²⁺ and Ir⁴⁺) octahedra, where the rare earth cations occupy the eight-fold coordination sites formed by the corner shared octahedra. Pr₂MgIrO₆, Nd₂MgIrO₆, Sm₂MgIrO₆, and Eu₂MgIrO₆ order antiferromagnetically around 10-15 K.     

Thermodynamic Model for the Solubility of Cr(OH)3(am) in Concentrated NaOH and NaOH–NaNO3 Solutions

The main objective of this study was to develop a thermodynamic model for predicting Cr(III) behavior in concentrated NaOH and in mixed NaOH–NaNO₃ solutions for application to developing effective caustic leaching strategies for high-level nuclear waste sludges. To meet this objective, the solubility of Cr(OH)₃(am) was measured in 0.003 to 10.5 m NaOH, 3.0 m NaOH with NaNO₃ varying from 0.1 to 7.5 m, and 4.6 m NaNO₃ with NaOH varying from 0.1 to 3.5 m at room temperature (22 ± 2°C). A combination of techniques, X-ray absorption spectroscopy (XAS) and absorptive stripping voltammetry analyses, were used to determine the oxidation state and nature of aqueous Cr. A thermodynamic model, based on the Pitzer equations, was developed from the solubility measurements to account for dramatic increases in aqueous Cr with increases in NaOH concentration. The model includes only two aqueous Cr species, Cr(OH) ₄ ^– and Cr₂O₂(OH) ₄ ^– (although the possible presence of a small percentage of higher oligomers at >5.0 m NaOH cannot be discounted) and their ion–interaction parameters with Na⁺. The logarithms of the equilibrium constants for the reactions involving Cr(OH) ₄ ^– [Cr(OH)₃(am) + OH^– Cr(OH) ₄ ^– ] and Cr₂O₂(OH) ₄ ^2– [2Cr(OH)₃(am) + 2OH^– Cr₂O₂(OH) ₄ ^2– + 2H₂O] were determined to be –4.36 ± 0.24 and –5.24 ± 0.24, respectively. This model was further tested and provided close agreement between the observed Cr concentrations in equilibrium with Cr(OH)₃(am) in mixed NaOH–NaNO₃ solutions and with high-level tank sludges leached with and primarily containing NaOH as the major electrolyte.     

Experimental and theoretical study of the IR spectrum of [(CH2OH)3CNH3]2SiF6 crystal

[(CH₂OH)₃CNH₃]₂SiF₆, (tris(hydroxymethyl)aminomethane)₂SiF₆ crystal, abbreviated as (TRIS)₂SiF₆ crystal, exhibits a solid-solid phase transition (PT) at 182 K. The phase transition is connected with reorientational motion of SiF₆²⁻ and -CH₂OH groups. The vibrational infrared spectra of powdered (TRIS)₂SiF₆ crystal in Nujol and Fluorolube mulls were studied in the wide range of temperatures, from 320 K to 133 K. A wide region of internal vibrations of the TRIS⁺ and SiF₆²⁻ ions was investigated. Temperature changes of wavenumber, width, centre of gravity, and intensity of bands were analyzed to clarify the molecular mechanism of the phase transitions. Theoretical calculations were made based on density functional theory (DFT). The calculated normal vibrational modes of the molecules, their frequencies and intensities were compared with those obtained from experimental data.     

Kinetics of the thermal decomposition of [Co(NH3)6]2(C2O4)3·4H2O

The thermal decomposition of [Co(NH₃)₆]₂(C₂O₄)₃·4H₂O was studied under isothermal conditions in flowing air and argon. Dissociation of the above complex occurs in three stages. The kinetics of the particular stages thermal decomposition have been evaluated. The R_N and/or A_M models were selected as those best fitting the experimental TG curves. The activation energies,E, and lnA were calculated with a conventional procedure and by a new method suggested by Kogaet al. [10, 11]. Comparison of the results have showed that the Arrhenius parameters values estimated by the use of both methods are very close. The calculated activation energies were in air: 96 kJ mol^–1 (R_1.575, stage I); 101 kJ mol^–1 (Ain1.725 stage II); 185 kJ mol^–1 (A _2.9, stage III) and in argon: 66 kJ mol^–1 (A _1.25, stage I); 87 kJ mol^–1 (A _1.825, stage II); 133 kJ mol^–1 (A _2.525, stage III).