K3TaF8 from laboratory X‐ray powder data

The crystal structure of tripotassium octafluoridotantalate, K₃TaF₈, determined from laboratory powder diffraction data by the simulated annealing method and refined by total energy minimization in the solid state, is built from discrete potassium cations, fluoride anions and monocapped trigonal–prismatic [TaF₇]²⁻ ions. All six atoms in the asymmetric unit are in special positions of the P6₃mc space group: the Ta and one F atom in the 2b (3m) sites, the K and two F atoms in the 6c (m) sites, and one F atom in the 2a (3m) site. The structure consists of face‐sharing K₆ octahedra with a fluoride anion at the center of each octahedron, forming chains of composition [FK₃]²⁺ running along [001] with isolated [TaF₇]²⁻ trigonal prisms in between. The structure of the title compound is different from the reported structure of Na₃TaF₈ and represents a new structure type.     

Über die Phasenbildungen in den Reaktionssystemen Alkalihexafluorosilicat und Zirconiumdioxid

On the Formation of Phases in the Systems Alkali Hexafluorosilicate and Zirconium Dioxide The reactions of alkali hexafluorosilicates with ZrO₂ are described resulting watersoluble fluorozirconates as intermediate products already at lower temperatures (650–800°C). It is a solid state reaction in cooperation with reactive Si? F compounds. Complexes with monomeric anions are formed at first if the reaction product does not represent a double fluoride. This has been proved by reactions with variable oxides. From aqueous solutions can be obtained either K₃ZrF₇ or K₂ZrF₆ as well as Na₃ZrF₇ or Na₅Zr₂F₁₃, respectively, according to the conditions of crystallization. The results of longtime-reactions with formation of silicates (K₂ZrSi₃O₉, K₂ZrSi₂O₇) as well as the thermolysis of the fluoro-zirconates are also discussed.     

Röntgenographische Einkristallstrukturbestimmungen an den Kalium-Kupfer(II)-Fluoriden K2CuF4 und K3Cu2F7

X-Ray Single Crystal Structure Determinations of the Potassium Copper(II) Fluorides K₂CuF₄ and K₃Cu₂F₇ With single crystals of the tetragonal compounds K₂CuF₄ (a = 414.7(2), c = 1273(3) pm) and K₃Cu₂F₇ (a = 415.6(3), c = 2052(3) pm), showing no superstructure reflections, crystal structure determinations were based on space group I4/mmm of the K₂NiF₄ and Sr₃Ti₂O₇ type, resp. The shape of F₀-Fourier maxima at the positions of linking fluorine atoms within the layers of octahedra suggested disorder of bridging ligands caused by multidomain structure, which could be refined assuming half occupation of higherfold positions. The results confirmed the Jahn-Teller-distortions of octahedra being elongated with a significant orthorhombic component: Cu? F = 190.9(7)/193.9(2)/223.8(7) pm in K₂CuF₄ (R_W = 0.020) and 190.0(7)/194.7/225.6(7) pm in K₃Cu₂F₇ (R_W = 0.023). In the acentric octahedra of the latter compound the intermediate distance is averaged from the splitted lengths 192.7(4)/196.8(1) pm of axes along the c direction.     

ChemInform Abstract: First‐Principles Calculations of the Structural,Electronic, Optical and Elastic Properties of the New Phosphors,Na2ZrF6 and K2ZrF6.

Na₂ZrF₆ and K₂ZrF₆ are characterized using the plane‐wave based first‐principles calculations.     

The structure of cubic YbZrF7

The structure of primitive-cubic YbZrF₇ has been determined using X-ray and neutron diffraction techniques. A unit cell (a = 4.07 Å, space group Pm3m) contains one formula unit of Yb_0.5Zr_0.5F_3.5, with no ordering of cations, in materials prepared by rapid quenching from 1000°C. Metal and fluorine displacements from ideal sites are in accord with results previously obtained on Zr_0.8Yb_0.2F_3.2O_0.3. The separation between FF pairs bridging neighboring metal ions is similar to those observed in other complex zirconium fluorides. The metal displacements, metal-fluorine distances and fluorine-fluorine distances are discussed with respect to the formation and stability of disordered fluorine-excess ReO₃-type phases. These materials are intermediate in character between phases such as monoclinic YbZrF₇, with perfect order on both metal and nonmetal sublattices, and ZrF₄-based glasses where there is disorder on the metal as well as on the fluorine sublattice. No ordering effects are observed on heating to near 200°C, but near 400°C there is a slow transformation to the monoclinic YbZrF₇ structure.     

Electrical conductivity of the molten KF—K2TaF7 system

The specific conductivity of the molten KF—K₂TaF₇ system was measured within the temperature range from 1155 K to 1255 K using a two-molybdenum electrode cell. It was found that the addition of up to 10 mass % K₂TaF₇ caused a decrease of the system specific conductivity.     

A density‐functional theory approach to the separation of K2ZrF6 and K2HfF6 via fractional crystallization

Because of the low absorption cross‐section for thermal neutrons of zirconium (Zr) as opposed to hafnium (Hf), Zr‐metal must essentially be Hf‐free (<100 ppm Hf) to be suitable for use in nuclear reactors. However, Zr and Hf always occur together in nature, and due to very similar chemical and physical properties, their separation is particularly difficult. Separation can be achieved by traditional liquid–liquid extraction or extractive distillation processes, using Zr(Hf)Cl₄ as feedstock. However, the production of K₂Zr(Hf)F₆ via the plasma dissociation route, developed by the South African Nuclear Energy Corporation Limited (Necsa), could facilitate the development of an alternative separation process. In this theoretical study, the results of density‐functional theory (DFT) simulations of K₂Zr_(1‐z)Hf_zF₆ solid solutions [using Cambridge Serial Total Energy Package (CASTEP)] are presented, for which the supercell approach was applied in an attempt to determine whether solid solution formation during crystallization from aqueous solutions (fractional crystallization) is thermodynamically possible, which would hinder the separation efficiency of this method. Consequently, the calculated thermodynamic properties of mixing were used to evaluate the separation efficiency of Zr and Hf by fractional crystallization using a thermodynamic model to calculate the relative distribution coefficients. The small mixing enthalpies that were calculated from the DFT results, indicates that lattice substitution of Zr(IV) by Hf(IV) in K₂ZrF₆ could occur with relative ease. This is not surprising, considering the close similarities between Zr and Hf, and it was therefore concluded that K₂Zr_(1‐z)Hf_zF₆ solid solution formation might well restrict the separation efficiency of Zr and Hf by fractional crystallization of K₂Zr(Hf)F₆. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Interaction of metallic zirconium and its alloys Zry-2 and E110 with molten eutectic salt of LiF—NaF—KF containing zirconium fluoride components

The interactions of selected zirconium alloys used as special construction materials (Zr-pure, Zry-2, E-110) with the molten system of (LiF—NaF—KF)_(eut.) with additions of K₂ZrF₆ or Na₇Zr₆F₃₁ were studied. Corrosion losses of pure zirconium decrease sharply with 5 mole% addition of K₂ZrF₆ to (LiF—NaF—KF)_(eut.). The presence of alloying additives (Sn, Nb) has a positive influence on corrosion resistance in the eutectic mixture — up to 60% increase in corrosion resistance in comparison with pure zirconium. The mass losses of pure zirconium indicate an increasing corrosion attack with increasing Na₇Zr₆F₃₁ content in (LiF—NaF—KF)_(eut.) mixtures.     

Ion Mobility and Electrophysical Properties of Potassium Hexafluorozirconate K2ZrF6

In our ¹⁹F NMR and impedance spectroscopy study, we investigated the mobility of fluoride ions (270-550 K) in polycrystalline K₂ZrF₆. A structural phase transition was found for K₂ZrF₆ in the temperature range of 513 to 518 K. The high-temperature -K₂ZrF₆ modification is characterized by translational diffusion of the fluoride ions and uniaxial anisotropy of the ¹⁹F nuclear magnetic screening tensor. The electrophysical characteristics of K₂ZrF₆ were investigated in the temperature range of 300 to 560 K. It is established that the -K₂ZrF₆ phase is characterized by high ionic (superionic) conductivity above 540 K ( 3.2× 10^-4 S/cm, T=560 K).     

Derivate des Flußspat-Typs: [Fe(NH3)6][TaF6]2 und [Ni(NH3)6][TaF6]2

Derivatives of the Fluorite Type: [Fe(NH₃)₆][TaF₆]₂ and [Ni(NH₃)₆][TaF₆]₂ Light blue single crystals of [Fe(NH₃)₆][TaF₆]₂ and [Ni(NH₃)₆][TaF₆]₂ are obtained from 36 : 1 : 6 molar mixtures of (NH₄)F, iron/nickel and tantalum powders, respectively, in sealed Monel metal ampoules at 400 °C. They both crystallize isotypic with [Co(NH₃)₆][PF₆]₂ (cubic, Fm-3m, Z = 4, a = 1259.0(2)/1260.4(2) pm) in a structure that can be derived from the basic fluorite-type of structure according to [Ca][F]₂≡[Fe(NH₃)₆][TaF₆]₂, for example.     

Synthese und Struktur von Ag7M6F31 (M = Zr,Hf, Ce)

Synthesis and Structure of Ag₇M₆F₃₁ (M = Zr, Hf, Ce) Colorless single crystals of Ag₇Zr₆F₃₁ have been obtainend by heating up a mixture of AgF and ZrF₄ in a closed goldtube (T = 450 °C, t ∼ 2 d). The compound crystallizes trigonal, space group R3‐C (No. 148) with a = 1400,9(3) pm, c = 979,0(2) pm, Z = 3. Also prepared were the isotypic compounds Ag₇Hf₆F₃₁ with a = 1393,8(2) pm, c = 978,7(2) pm, and Ag₇Ce₆F₃₁ with a = 1469,8(1) pm, c = 998,5(1) pm.     

Redetermination of Na3TaF8

The crystal structure of trisodium octafluoridotantalate, Na₃TaF₈, has been redetermined using diffractometer data collected at 153 K, resulting in more accurate bond distances and angles than obtained from a previous structure determination based on film data. The structure is built from layers running along [101], which are formed by distorted [TaF₈] antiprisms and [NaF₆] rectangular bipyramids sharing edges and corners. The individual layers are separated by eight‐coordinated Na ions. Two atoms in the asymmetric unit are in special positions: the Ta atom is on a twofold axis in Wyckoff position 4e and one of the Na ions lies on an inversion centre in Wyckoff site 4d.     

Neubestimmung der Kristallstrukturen der Hexahydroxometallate Na2Sn(OH)6, K2Sn(OH)6 und K2Pb(OH)6

Redetermination of the Crystal Structures of the Hexahydroxometallates Na₂Sn(OH)₆, K₂Sn(OH)₆, and K₂Pb(OH)₆ Slow cooling down of hot saturated hydroxo stannate‐ resp. ‐plumbate solutions gives crystals of Na₂Sn(OH)₆, K₂Sn(OH)₆, and K₂Pb(OH)₆ well suited for an X‐ray structure determination. With these crystals the so far known crystal data were verified, determined more precisely and H‐positions found for the first time. The compounds crystallize rhombohedral in the space group R 3. The hexagonal unit cells contain three formula units with Na₂Sn(OH)₆: a = 5.951(1) Å, c = 14.191(2) Å, c/a = 2.384 K₂Sn(OH)₆: a = 6.541(1) Å, c = 12.813(4) Å, c/a = 1.959 K₂Pb(OH)₆: a = 6.625(1) Å, c = 12.998(2) Å, c/a = 1.962 The compounds are not isotypic whereas the atoms occupy in all three cases the same Wyckoff positions. Na₂Sn(OH)₆ has with an hcp packing of O a CdI₂ like superstructure with Na and Sn in octahedral interstices. Hydrogen bonds O–H…O–H play a role in solid K₂Sn(OH)₆ and K₂Pb(OH)₆. In these compounds the potassium ions are shifted from an octahedral coordination in an hcp packing of O. They have nine nearest O‐neighbours. The hydrogen bonds are investigated by Raman spectroscopy.     

Magnesium Hexafluoridozirconates MgZrF6·5H2O,MgZrF6·2H2O,and MgZrF6: Structures,Phase Transitions,and Internal Mobility of Water Molecules

The MgZrF₆ · n H₂O (n = 5, 2 and 0) compounds were studied by the methods of X‐ray diffraction and ¹⁹F, MAS ¹⁹F, and ¹H NMR spectroscopy. At room temperature, the compound MgZrF₆ · 5H₂O has a monoclinic C‐centered unit cell and is composed of isolated chains of edge‐sharing ZrF₈ dodecahedra reinforced with MgF₂(H₂O)₄ octahedra and uncoordinated H₂O molecules and characterized by a disordered system of hydrogen bonds. In the temperature range 259 to 255 K, a reversible monoclinic ? two‐domain triclinic phase transition is observed. The phase transition is accompanied with ordering of hydrogen atoms positions and the system of hydrogen bonds. The structure of MgZrF₆ · 2H₂O comprises a three‐dimensional framework consisting of chains of edge‐sharing ZrF₈ dodecahedra linked to each other through MgF₄(H₂O)₂ octahedra. The compound MgZrF₆ belongs to the NaSbF₆ type and is built from regular ZrF₆ and MgF₆ octahedra linked into a three‐dimensional framework through linear Zr–F–Mg bridges. The peaks in ¹⁹F MAS spectra were attributed to the fluorine structural positions. The motions of structural water molecules were studied by variable‐temperature ¹H NMR spectroscopy.     

Barium,Potassium, and Tris(ethane‐1,2‐diamine)nickel(II) Fluoroalkanedisulfonates and the X‐ray Crystal Structures of K2(O3S)2CHF,K2(O3S)2CF2, K2(O3S)2(CF2)3 · H2O,and [Nien3][(O3S)2(CF2)n] (n = 1, 3)

The barium perfluoroalkanedisulfonates Ba(O₃S)₂(CF₂)_n (n = 1, 3–5) and the new potassium fluoroalkanedisulfonates K₂(O₃S)₂CHF, K₂(O₃S)₂CF₂, and K₂(O₃S)₂(CF₂)₅ have been prepared by reaction of (CF₂)_n(SO₂F)₂ (n = 1, 3–5) or CHF(SO₂F)₂ with CaO (or Ca(OH)₂) and M(OH)_x (M = Ba, x = 2; M = K, x = 1) or with Ba(OH)₂ alone (n = 1) in water. In each of the crystal structures of K₂(O₃S)₂CHF and K₂(O₃S)₂CF₂, there is an eight‐coordinate and a six‐coordinate potassium ion, whilst in K₂(O₃S)₂(CF₂)₃H₂O, two different eight‐coordinate potassium ions are linked by a bridging water molecule. One potassium has additionally six sulfonate oxygen and one fluorine donor atoms, and the other, five sulfonate oxygens and two fluorine donor atoms. The preparation of highly crystalline [Nien₃][(O₃S)(CF₂)_n] (en = ethane‐1,2‐diamine; n = 1, 3–5) and the X‐ray crystal structures for n = 1 or 3 provide evidence for the value of perfluoroalkanedisulfonate ions as counter ions for the crystallization of cationic complexes.     

Phase analysis of the solidified KF–(LiF–NaF–UF4)–ZrF4 molten electrolytes for the electrowinning of uranium

The X-ray diffraction (XRD) phase analysis of different solidified uranium-based fluoride systems ((LiF–NaF)_eut–UF₄; (KF–LiF–NaF)_eut–UF₄; (LiF–NaF)_eut–UF₄–ZrF₄ and (KF–LiF–NaF)_eut–UF₄–ZrF₄) were examined in order to provide the basis for pyro-electrochemical extraction of uranium in molten fluorides. Several uranium-based species (Na₂UF₆, Na₃UF₇, K₂UF₆, K₃UF₇, UO₂, K₃UO₂F₅) were identified in the solidified melts. The role of oxygen in argon atmosphere was found to be critical in the formation of uranium species during the melting and solidification. In order to reduce the accumulated level of free oxygen traces in our experiments, zirconium (in the form of ZrF₄) was used inside the melt as an oxygen buffer. It was found that ZrF₄ can really stabilize the uranium species by complexation and protects them against the oxygenation. The results of this work highlight the importance of oxygen removal for obtaining pure deposit in the electrorefinning of uranium.     

Thermal stability and X-ray-luminescent properties of fluorozirconates and fluorosulfatozirconates

The thermolysis of fluorozirconates (M₂ZrF₆, M₅Zr₄F₂₁ · 3H₂O, MZrF₅ · H₂O, Rb₂Zr₃OF₁₂, and Cs₂Zr₃F₁₄ · 1.5H₂O) and fluorosulfatozirconates (M₂ZrF₄SO₄, Rb₃Zr₂F₉SO₄ · 2H₂O, and Cs₈Zr₄F₂(SO₄)₁₁ · 16H₂O) with M = K, Rb, or Cs in undried air was studied by thermal analysis in tandem with X-ray powder diffraction. The X-ray luminescence (XRL) intensity was determined for these compounds and their thermolysis products. A mixture of Rb₂Zr₃OF₁₂ and Rb₂ZrF₆ luminescent phases was detected in the thermolysis products of Rb₅Zr₄F₂₁ · 3H₂O and RbZrF₅ · H₂O for the first time. After heat treatment, a considerable quantum yield was observed for ZnZrF₆ · 5H₂O, ZnZrF₆ · 6H₂O, and ZnZr₂F₁₀ · 7H₂O. The XRL luminescence was affected by the composition of the phase and the density of excited states (F* and O*).     

Determination of dissociation degrees of K3NbF8 and K3TaF8 by thermodynamic analysis of subsystems of the KF-K2NbF7 and KF-K2TaF7 systems

A special form of the LeChatelier-Shreder equation describing the equilibrium between the crystalline phase and the melt in system A-AB in which the substance AB partially dissociates upon melting was applied to systems KF-K₃NbF₈, K₂NbF₇-K₃NbF₈ and to KF-K₃TaF₈, K₂TaF₇-K₃TaF₈ subsystems of the binary systems KF-K₂NbF₇ and KF-K₂TaF₇ in which the additive compounds K₃NbF₈ and K₃TaF₈ are formed. Using the phase diagram of the system KF-K₂NbF₇ determined by McCawley and Barclay (1971) and the values of the fusion enthalpy of K₃NbF₈ taken from literature, the intervals of the dissociation degree values of K₃NbF₈ for both branches of the liquidus curve of K₃NbF₈ were calculated. The calculated values of the dissociation degree depend on the coordinates of the liquidus curve of K₃NbF₈ of the pertinent phase diagram, on its used branch and section, and on the value of the fusion enthalpy of K₃NbF₈. For the measured fusion enthalpy of K₃NbF₈ (57 kJ mol⁻¹), a common interval of the dissociation degree values of K₃NbF₈ for both branches of the liquidus curve of K₃NbF₈ is 0.71–0.72. Similarly, intervals of the dissociation degree values of K₃TaF₈ for both branches of the liquidus curve of K₃TaF₈ were calculated using the phase diagram of the system KF-K₂TaF₇ determined by Boča et al. (2007) and the measured fusion enthalpy of K₃TaF₈ ((52 ± 2) kJ mol⁻¹). The error of the determination of the fusion enthalpy of K₃TaF₈, the common interval of the dissociation degree values of K₃TaF₈ for both branches of the liquidus curve of K₃TaF₈ is 0.68–0.69.     

Kryometrische Untersuchungen. IV. Lösungen von oxidischen Mo6+- und Cr6+-Verbindungen in geschmolzenem K2Cr2O7 und KNO3

The depression of freezing point of molten K₂Cr₂O₇ and KNO₃ as solvents was measured after addition of small concentrations of the following compounds: to K₂Cr₂O₇: MoO₃, CrO₃, (NH₄)₂CrO₄, K₂MoO₄, Na₂MoO₄, Li₂MoO₄, and Na₂Mo₂O₇, respectively; to KNO₃: CrO₃, (NH₄)₂Cr₂O₇ K₂Cr₂O₇, K₂CrO₄ and MoO₃, (NH₄)₆(Mo₇O₂₄) · 4 H₂O, K₂Mo₂O₇, K₂MoO₄, Na₂MoO₄ and Li₂MoO₄, respectively. It could be concluded from the measured values of the freezing point depression if a reaction between solvent and solute took place.