Fluorine ions

R. Mews, Nitrogen-Sulfur-Fluorine Ions, Adv. Inorg. Chem. Radiochem., 19, 185 (1976). 

Oxygen and fluorine ions have very similar ionic radii. This steric similarity ensures relatively easy substitution of the oxygen in the compounds, by fluorine ions. Such substitution opens up huge possibilities for the synthesis of oxyfluoride compounds with desired crystal structure and properties. 

Analysis of weight loss isotherms displayed in Fig. 8 shows that the first step in the interaction between Nb02F and carbonates of other alkali metals is similar to the interaction described by Equation (11). However, compounds of the M(Nb04F form, where M = Na, K, Rb, Cs, were not found [85]. The instability of such compounds is related to the ionic radii of the alkali metals, which are greater than that of Nb5+, thus the ions are too large to occupy the octahedral cavities formed by the oxygen and fluorine ions. 

The apparently high relative stability of KF-4KNb03 can be related to the steric similarity between potassium and fluorine ions, which is hardly the case with other alkali metals. Table 10 presents a general list of compounds that can be obtained by the interaction of Nb02F with alkali metal carbonates. 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

The steric similarity of oxygen and fluorine ions enables the formation of coordination-type structures in some tantalum and niobium oxyfluoride compounds. 

Typical examples of compounds with a coordination-type structure are Nb02F and Ta02F, which crystallize in a Re03 type structure [233, 243]. Oxygen and fluorine ions are statistically distributed in the anionic sub-lattice. The compounds are characterized by X Me = 3 and can be described as MeX3 type compounds. 

Kuznetsov, Polyakov and Stangrit [317, 318] and on KC1 - NaCl - NaF -K2NbF7 by Kovalev, Ioffe and Datlina [319], all in the concentration range of up to 20-25% mass of K2NbF7. In all cases, maximum electro-conductivity was observed at concentrations of about 5-10% mass of K2NbF7. The peaking of the electro-conductivity is explained by the occurrence of two processes with opposite characters. The increase in electro-conductivity is related to the increase in the concentration of potassium ions [319] and fluorine ions [317, 318], whereas the drop in conductivity results from a decrease in free volume due to an increase in NbF72 ion concentration. 

The calculation of the ionic composition (see Fig. 66) is based on the assumption that the salts dissociate completely, yielding potassium ions, K+, fluorine ions, F, and complex fluorotantalete ions, TaF or TaF72, as described below ... 

Fluorine ions form the first coordination sphere around the tantalum ion, which is the central atom of the complex. Potassium ions form the second coordination sphere, which significantly affects the geometry and force field of the first coordination sphere. The melting of K2TaF7 leads to the dissociation of the compound into ions, as follows ... 

The equilibrium between the complexes formed according to Equation (80) depends both on the concentration of fluorine ions and on the potential of interionic interactions, namely the nature of the outer-sphere cations [358]. The influence of the concentration of fluorine ions and of the nature of the outer-sphere cations on the equilibrium in Equation (80) can be demonstrated by the spectral transformations observed at 850°C for M2TaF7 - MF systems, where M = alkali metal [358]. 

The partially reduced form of niobium accounts for the color change of samples that underwent thermal treatment in vacuum or inert atmospheres. Whereas the thermal treatment of the mixture in air leads to the simultaneous oxidation of Nb4+ by oxygen, this is actually equivalent to the replacement of fluorine ions by oxygen ions in the complex structure of oxyfluoroniobate. Extended thermal treatment of systems containing LiNbOF4 and LiF yields a mixture of LiF and LiNbOs as the final thermal decomposition product. 

Nevertheless, Ta5+ and Nb5+ interact with aqueous media containing fluorine ions, such as solutions of hydrofluoric acid. On the other hand, as was clearly shown by Majima et al. [448 - 450], the increased hydrogen ion activity can also significantly enhance the dissolution rate of oxides. The activity of hydrogen ions can be increased by the addition of mineral salts or mineral acids to the solution. 

Substitution of fluoride ions by other suitable ions in the complex acid so as to separate the fluorine ions from tantalum and niobium (liquid-liquid interaction) ... 

Floating point, 276 Fluorine, ion, electronic correlation energy, 240 London energy, 78 molecules, intramolecular London energies, 78... 

Some fluorine-ion conductors exhibit high ionic conductivities, even at room temperature [4], which are not equaled by other halide-ion conductors. However, there is a lack of known electrode materials. Further research on this topic is very worthwhile. 

The method was then applied in predicting the structure of the orthorhombic crystal topaz, AljSiC>4F. l It was assumed that each aluminum ion is surrounded by four oxygen ions and two fluorine ions at the corners 10 Linus Pauling, Proc. Nat. Acad. Sd., 14, 603 (1928). 

The crystals considered are to contain only small cations, with relatively large electric charges, that is, usually trivalent and tetravalent cations, with crystal radii not over about 0.8 A. All anions are large (over 1.35 A.) and univalent or divalent. Furthermore, they should not be too highly deformable. The most important anions satisfying this restriction are the oxygen ion and the fluorine ion, with crystal radii 1.35-1.40 A.12... 

We further expect hydrogen bonds between nitrogen and four surrounding fluorine ions. The fluorine ions should be approximately tetrahedrally arranged about the nitrogen ion, at a distance of about 2.63 A, as in NHtF. 

Oil examining the KHF2 structure we observe that each potassium ion is surrounded by eight fluorine ions at the corners of a twisted cube, no four of them forming an approximately regular tetrahedron. If, however, we make each (001) layer of HF2 ions like the one above (so that the unit of structure has c = 3.53 A rather than 7.06 A), the... 

Each nitrogen is attached to four fluorine ions at a distance of 2.76 A by hydrogen bonds, and each fluorine ion to two nitrogen and one fluorine ion, the F—H distance in the linear HF2 groups being 1.184 A. 

Fig. 1. A hydrargillite layer of octahedra. The light circles indicate oxygen ions, the heavier ones hydroxyl or fluorine ions in mica.

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