Synthesis,Structure, and Properties of Nonlinear Optical Crystal Na2SiF6

Nonlinear optical crystals of fluosilicate Na₂SiF₆ are synthesized via hydrothermal method and its structure is determined by single‐crystal X‐ray diffraction (XRD). The space group of Na₂SiF₆ is P321 with cell parameters a = 8.8715(3) Å, c = 5.0484(5) Å, Z = 3, V = 344.09(4) ų. The properties of the crystal are measured by powder XRD, infrared (IR) spectroscopy, ultraviolet/visible (UV/Vis) near‐infrared (NIR) diffuse reflectance spectroscopy, thermogravimetric (TG), and differential scanning calorimetry (DSC) analysis. The bandgap calculated using CASTEP is 7.41 eV, indicating that the cut‐off edge of the Na₂SiF₆ crystal can be down to deep‐UV energy region. The first‐principles studies were performed to elucidate the structure/property relationship of Na₂SiF₆.     

Phase Analysis of the Binary System of Cryolite (Na3AlF6) and Sodium Metasilicate (Na2SiO3)

The phase analysis of cryolite (Na₃AlF₆) and sodium metasilicate (Na₂SiO₃) was performed by thermal analysis. The eutectic system with a region of two immiscible substances at a concentration of Na₂SiO₃ between 42.8 and 46.3 mol‐% was identified and the eutectic temperature determined to (886±2) °C. Based on the results of mass‐loss measurements, it was assumed that the introduced Na₂SiO₃ reacts with Na₃AlF₆ due to the formation of some nonvolatile stable compounds. The stable reaction products were identified by X‐ray diffraction analysis and IR spectroscopy of the spontaneously cooled samples, which established the formation of NaF and stable amorphous aluminosilicate compounds.     

Mechanism and Parameters Controlling the Decomposition Kinetics of Na2SiF6 Powder to SiF4

Sodium hexafluorosilicate (Na₂SiF₆) powder has been used as a silicon source for formation of Si₃N₄ coatings by the hybrid precursor system‐chemical vapor deposition (HYSY‐CVD) route. The quantitative effect of processing time, temperature, gas flow rate, and process atmosphere (N₂ and N₂:5% NH₃) upon the fractional weight loss during the decomposition of Na₂SiF₆ was studied using a standard L₉ Taguchi experimental design and analysis of variance. The decomposition kinetics of Na₂SiF₆(s) was studied theoretically and experimentally in the temperature range of 550–650ºC by applying the shrinking core model. It was found that regardless of atmosphere type, the reaction order is n ≈ 0.12 and that a two‐stage mixed mechanism consisting of chemical reaction and boundary layer gas transfer controls the decomposition rate. The determined fractional weight loss during Na₂SiF₆ decomposition in nitrogen atmosphere is about 1.05–1.5 orders of magnitude greater than that in N₂:NH₃. The gas flow rate affects the dissociation activation energy, being of 121, 109, and 94 kJ/mol in N₂ and of 140, 120, and 115 kJ/mol in N₂:NH₃, for the flow rates of 20, 60, and 100 cm³/min, respectively, in both atmosphere types. A good agreement is observed by comparing experimental weight loss data with model predictions.     

Phase relations and thermodynamic properties in the ternary reciprocal system LiFNaFNa3AlF6Li3AlF6

The ternary reciprocal sytem LiFNaFNa₃AlF₆Li₃AlF₆ has been investigated by thermal analysis, differential thermal analysis, quenching, X-ray diffraction, microscopy, and calorimetry. The phase diagrams of the following systems are given: LiFNaF (revised), LiFAlF₃, Na₃AlF₆LiF, and LiFNaFNa₃AlF₆Li₃AlF₆. Some values of heat of mixing and heat content in the system have been measured.It is shown that molten mixtures in this system can be treated as consisting of the following species: Li⁺, Na⁺, AlF^3-₆, AlF₃ and F^-. At high contents of alkali fluoride the dissociation of the AlF^3-₆ ion to AlF₃ and F^- will, however, be negligible.On the basis of the calorimetric data, heats of mixing and dissociation, together with the degree of dissociation of AlF^3-₆, in the systems LiFAlF₃ and LiFNa₃AlF₆ have been calculated. The partial Gibbs free energy, enthalpy and entropy of Na₃AlF₆ in the system LiFNa₃AlF₆ have also been calculated. Finally the activity of Na₃AlF₆ in the latter system has been calculated by treating it as a part of the ternary reciprocal system 3LiF+Na₃AlF₆→Li₃AlF₆+3NaFA satisfactory agreement between the Flood, Førland and Grjotheim theory and the experimental values is obtained at small Na₃AlF₆ concentrations.     

Preparation of High-Purity Silicon Tetrafluoride by Thermal Dissociation of Na2SiF6

The possibility of preparing high-purity silicon tetrafluoride by the thermal dissociation of pure grade Na₂SiF₆ was studied. The impurity composition of the product was studied by IR and atomic emission spectroscopy and by mass spectrometry.     

ChemInform Abstract: First‐Principles Molecular Dynamics Investigation on Na3AlF6 Molten Salt.

The partial radial distribution function, coordination numbers, bond angles, F atoms type, self‐diffusion coefficient, viscosity, and ionic conduction of molten Na₃AlF₆ are investigated by first‐principles molecular dynamics simulation.     

Na5AlF2(PO4)2: Darstellung,Kristallstruktur und Ionenleitfähigkeit

Na₅AlF₂(PO₄)₂: Synthesis, Crystal Structure and Ionic Conductivity Two different procedures (precipitation from aqueous solution and solid state reaction) for the synthesis of hitherto unknown Na₅AlF₂(PO₄)₂ were optimized. The crystal structure was determined using diffractometer data (P3 , a = b = 10.483(1), c = 6.607(1) Å, MoKα, 1080 independent reflections, R_w = 0.025). PO₄-tetrahedra and AlO₄F₂-“octahedra” are connected via common vertices forming a twodimensionally extended heteropolyanion. Sodium is located in interconnected spacings of the [AlF₂(PO₄)₂]-part of the structure. Ionic conductivity as expected because of these structural features was affirmed experimentally.     

Thermal Analysis of the System Na3AlF6–NaVO3

Summary. The phase diagram of the system Na₃AlF₆–NaVO₃ was determined by means of thermal analysis. The system is a simple binary eutectic one. The eutectic point was estimated at x(NaVO₃) = 0.975 and t _eut = 617°C. The XRD patterns of samples after thermal analysis revealed the presence of cryolite and NaVO₃ only supporting the above assumption of a simple eutectic binary system.     

Solid state reactions of hexafluorosilicates

At beginning thermal decomposition K₂[SiF₆] loses SiF₄-planes from [SiF₆]^2?-octahedrons, which has been proved by x-ray-diffraction [1], [2]. Analogous disorder structures are supposed to be present with all solids having complex ions including carbonates, sulfates and others. The result is a high reactivity at this spots. Another reactive form in hexefluorosilicates is represented by mobile SiF-species, perhaps SiF₃⁺. The reactivity is shown by heterogenous reactions with CHCl₃ and by solid-solid reactions for instance with halides, oxides etc. As an example corundum (α-Al₂O₃) reacts at 600°C giving K₃ AlF₆ and KAlSiO₄ [3].     

Intrinsic kinetics of the oxidation of Na2TiF6 and Na3TiF6

The intrinsic kinetics, unaffected by diffusional and mass transfer effects, of the air oxidation of Na₂TiF₆ and Na₃TiF₆ were determined by using a nonisothermal technique. The oxidation of these sodium fluorotitanates proceeds through two-step reactions involving the formation of oxyfluorotitanate, i.e. Na₃TiOF₅, as the intermediate. The oxidation rate shows a first-order dependence on the amount of the unreacted solids for each of the two-step reactions for both fluorotitanates. The activation energy for the further oxidation of Na₃TiOF₅ to a mixture of NaF + TiO₂ was determined to be 52.4 kJ/mol and 55.3 kJ/raol for Na₂TiF₆ and Na₃TiF₆ as reactants, respectively.     

Zum Modifikationswechsel von SiO2 in Gegenwart von Alkali-Fluoriden und -Silicaten

On the Modification Change of SiO₂ in Presence of Alkali Fluorides and Silicates The influence of NaF and Na₂SiF₆ on SiO₂ and sodium silicates was investigated. The SiF₄, developed in this process, influences the reaction as shown by a comparison of experiments in open and closed systems. With low Pressures of SiF₄ the reactions of NaF or Na₂SiF₆ with certain SiO₂ modifications lead to sodium silicates, in presence of a perceptible pressure of SiF₄ to cristobalite and tridymite and also to low-quartz. Sodium silicates react with NaF mainly to Na₂SiO₃. Na₆Si₈O₁₉ could be obtained from Na₂SiO₃ only when NaF was replaced by Na₂SiF₆. In this case additional low-quartz was formed. The formation of tridymite, only possible in the presence of certain foreign ions, is attainable at high temperatures (1150°C) if HF is added together with an alkali fluoride (A = Na, K, Rb, Cs). The amount of fluoride added was decisive for the size of the crystals of tridymite, which were prepared by chemical transport. The impurities of the used low-quartz (Furka) are not sufficient for the formation of tridymite.     

Zur Kenntnis des Aluminiumtrifluorides und seiner Hydrate

Zusammenfassung Es wird die Herstellung konzentrierter Aluminiumfluoridlösungen durch Umsetzung von Al(OH)₃ mit H₂SiF₆ und die Abscheidung des Aluminiumfluorides in Form verschiedener Hydrate aus dieser Lösung beschrieben. Die Hydrate, die je Mol AlF₃ 9 Mol H₂O und 3,5 Mol H₂O enthalten, sowie ein neu identifiziertes Hydrat mit 5,5 Molen H₂O bilden eine zusammenhängende Reihe von aquo-Komplexen. Ein Hydrat mit 3 H₂O entsteht durch eine innere, irreversible Umlagerung der anderen Hydrate. Bei der Entwässerung des AlF₃·3,5 H₂O entsteht als Zwischenstufe eine Verbindung AlF₃·H₂O, bei der Entwässerung des AlF₃·3 H₂O die Verbindung AlF₃·0,5 H₂O. Sowohl das AlF₃·0,5 H₂O als auch das AlF₃·H₂O haben dieselbe Struktur wie das wasserfreie AlF₃, das Wasser ist hiebei in den Hohlräumen des AlF₃-Gitters eingebaut.

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The preparation of concentrated solutions of aluminum fluoride from Al(OH)₃ with H₂SiF₆ is described. Aluminum fluoride precipitates from these solutions as a hydrate. The hydrates AlF₃·9 H₂O, and AlF₃·3.5 H₂O as well as the newly identified hydrate AlF₃·5 H₂O give a continuous series of aquo-complexes. The hydrate AlF₃·3 H₂O is formed in an internal, irreversible rearrangement of the other hydrates. Dehydration of AlF₃·3.5 H₂O yields intermediately AlF₃·H₂O, while AlF₃·3 H₂O gives AlF₃·0.5 H₂O. AlF₃·0.5 H₂O as well as AlF₃·H₂O have the same structure as anhydrous AlF₃. The H₂O is built-in in the cavities of the AlF₃ lattice.

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Mit 10 Abbildungen     

Über Hexafluorovanadate(III). Cs2MVF6 und Rb2MVF6 (MTl,K und Na); mit einer Bemerkung über Na3VF6

On Hexafluorovanadates(III). Cs₂MVF₆ and Rb₂MVF₆ (M?Tl, K. and Na); with a Remark on Na₃VF₆ By heating the binary fluorides in a closed system we obtained Cs₂TlVF₆ (a = 9.23₄ Å), Cs₂KVF₆ (a = 9.04₇ Å), Rb₂KVF₆ (a = 8.85₅ Å) and Rb₂NaVF₆ (a = 8.46₈ Å), all cubic Elpasolithes of soft green colour as well as Cs₂NaVF₆ (hexagonal a = 6.24 Å, c = 30.58 Å, isotypic with Cs₂NaCrF₆) and Na₃VF₆ (monoclinic a = 5.51₃ Å, b = 5.72₁ Å, c = 7.96₃ Å, β = 90.4₇°, isotypic with Na₃AlF₆). VF₃ (3.0–296.2°K), Cs₂TlVF₆, Cs₂KVF₆ and Rb₂KVF₆ (all from 70–299°K) have been measured magnetically. The spectra of reflection in the range of 9 000 to 33 000 cm^?1 of VF₃ and the new quaternary fluorides are measured and discussed. The Madelung Part of Lattice Energy (MAPLE) is calculated and discussed.     

Effects of Mn2+ on the chrome‐free colored Ti/Zr‐based conversion coating on 6063 aluminum alloy

The conversion coating with golden color and improved corrosion resistance had been prepared by adding Mn²⁺ in the Ti/Zr conversion coating solution. Comparing with that of conversion coating without Mn²⁺, the optimal treatment time of this conversion coating was much shorter and the corrosion resistance was obviously improved. The effect of Mn²⁺ on the formation of golden Ti/Zr conversion coating was thoroughly investigated by means of energy dispersive X‐ray spectroscopy, SEM, XPS, and Raman and electrochemical workstation. The results showed that the conversion coating had a double‐layer structure: the outer layer consisted of the metal‐organic complex and the inner layer was mainly made up of Na₃AlF₆. Mn²⁺ was oxidized into MnOOH in solution and precipitated on the substrate surface which provided the nucleus to Na₃AlF₆ crystal and accelerated Na₃AlF₆ crystal formation and also made the microstructure of conversion coating change to the cubic. The mechanism of the formation of the conversion coating can be deemed as nucleation, growth of Na₃AlF₆ crystal, and formation of metal‐organic complex. Copyright © 2015 John Wiley & Sons, Ltd.     

The reaction of aluminum fluoride solution with cryolite

The title reaction was investigated kinetically and the products obtained were analyzed and examined by physicochemical methods. The reaction was found to result in the formation of NaAlF₄.H₂O and its solid solutions with aluminum fluoride with compositions down to Na_0.5AlF_3.5.1.3H₂O, except in the presence of chiolite seed crystals, which cause the reaction to give it as the final product.It was suggested that the solid solution are brought about by coprecipitation of approximately monohydrated hexagonal ° -AlF₃ direct from solution, both compounds being presumably isostructural each other.Differences in the infrared spectra of Na₂AlF₆, Na₅Al₃F₁₄ and NaAlF₄.H₂O were indicated.Homogenous mixtures of Na₃AlF₆ and NaAlF₄.H₂O are found to react exothermally at 350°C to give Na₅Al₃F₁₄, while NaAlF₄.H₂O alone decomposes into Na₅Al₃F₁₄ and AlF₃.     

High-temperature powder synchrotron diffraction studies of synthetic cryolite Na3AlF6

A high-resolution synchrotron diffraction study of the structures of a synthetic sample of cryolite Na₃AlF₆ from room temperature to 800°C is reported. At room temperature Na₃AlF₆ is monoclinic and the structure is described in space group P2₁/n. Heating the sample to 560°C results in only minor changes to the structure. A first-order transition from this monoclinic structure to a high-temperature cubic structure is observed near 567°C. The cubic structure is characterized by disorder of the fluoride atoms.     

Darstellung,Struktur und magnetische Eigenschaften der Natriumeisenchalkogenide Na6FeS4 und Na6FeSe4

Synthesis, Structure, and Magnetic Properties of the Sodium Iron Chalcogenides Na₆FeS₄ and Na₆FeSe₄ The compounds Na₆FeS₄ and Na₆FeSe₄ have been synthesized by fusion reactions of sodium carbonate with iron and chalcogen in a stream of hydrogen. Structural investigations on single crystals show that both compounds crystallize in an atomic arrangement isotypic with Na₆ZnO₄ (space group P6₃mc). The structure is characterized by isolated [FeX₄]-tetrahedra. The magnetic susceptibilities show Curie-Weiss behaviour. The deviations at low temperatures are obviously caused by antiferromagnetic interactions.     

Kristallstrukturen von Na3TiCl6 und K3TiCl6

Ternary Halides of the Type A₃MX₆. IX Crystal Structures of Na₃TiCl₆ and K₃TiCl₆ Light yellow single crystals of Na₃TiCl₆ and K₃TiCl₆ are obtained from the binary components, TiCl₃ and NaCl and KCl, respectively, in 1 : 3 molar ratios. Na₃TiCl₆ crystallizes with the cryolite type of structure (monoclinic, P2₁/n, Z = 2, a = 668,02(8), b = 709,13(6), c = 981,38(12) pm, β = 90,31(2)°) while K₃TiCl₆ belongs to the K₃MoCl₆ type of structure (monoclinic, P2₁/c, Z = 4, a = 1261,6(2), b = 751,36(8), c = 1210,7(2) pm, β = 108,30(2)°).     

Die Kryolithstruktur von Na3ScF6 und die Oktaederkippung in isostrukturellen Natriumhexafluorometallaten Na3MF6

The Cryolite Structure of Na₃ScF₆ and the Tilting of Octahedra in Isostructural Sodium Hexafluorometallates Na₃MF₆ X-ray studies at single crystals of Na₃ScF₆ confirmed the monoclinic cryolite type structure of this compound: a = 559.5, b = 580.2, c = 811.6 pm, β = 90.72°, Z = 2, space group P2₁/n; R1 = 0.021 for 512 symmetry independent reflections. The octahedra of [ScF₆] (average Sc? F = 200.7 pm), as well as those of [NaF₆] (Na1? F = 229.1 pm) linked to them, are titled by about 20° with respect to the axes of the perovskite-like pseudocell. This tilting of octahedra is discussed in comparison with other cryolites and with orthorhombic perovskites NaMF₃; there results a correlation between tilt angle and tolerance factor t ? 0.88 of these compounds, the [NaF₈] coordination of which invariably exhibits a constant mean value of Na2? F = 231.5 ± 1 pm for the four shortest distances.