Fluoride complex

Fluorine is the most electronegative element and thus can oxidize many other elements to their highest oxidation state. The small size of the fluorine atom facihtates the arrangement of a large number of fluorines around an atom of another element. These properties of high oxidation potential and small size allow the formation of many simple and complex fluorides in which the other elements are at their highest oxidation states. 

The most popular device for fluoride analysis is the ion-selective electrode (see Electro analytical techniques). Analysis usiag the electrode is rapid and this is especially useful for dilute solutions and water analysis. Because the electrode responds only to free fluoride ion, care must be taken to convert complexed fluoride ions to free fluoride to obtain the total fluoride value (8). The fluoride electrode also can be used as an end poiat detector ia titration of fluoride usiag lanthanum nitrate [10099-59-9]. Often volumetric analysis by titration with thorium nitrate [13823-29-5] or lanthanum nitrate is the method of choice. The fluoride is preferably steam distilled from perchloric or sulfuric acid to prevent iaterference (9,10). Fusion with a sodium carbonate—sodium hydroxide mixture or sodium maybe required if the samples are covalent or iasoluble. 

Both the binary and complex fluorides of aluminum have played a significant role in the aluminum industry. Aluminum trifluoride [7784-18-17, A1F., and its trihydrate [15098-87-0] 3 3H2O, have thus far remained to be the only binary fluorides of industrial interest. The nonahydrate [15098-89-2] 3 9H2O, and the monohydrate [12252-28-7, 15621 -55-3], AIF 20, are of only academic curiosity. The monofluoride [13595-82-9], AIF, and the difluoride [13569-23-8], AIF2, have been observed as transient species at high temperatures. 

Chromium Chromate. Chromium chromate treatment baths are acidic and made up from sources of hexavalent chromium and complex fluoride, fluorosiHcate, fluorozirconate, fluorotitanate, and siHcofluorides. Optional additional components added to accelerate coating rate are free fluoride, ferricyanide, and other metal salts such as barium nitrate. Conversion coating on aluminum precedes by the following reactions (2,3,17) ... 

Chromic Acid Electrolysis. Alternatively, as shown in Figure 1, chromium metal may be produced electrolyticaUy or pyrometaUurgicaUy from chromic acid, CrO, obtained from sodium dichromate by any of several processes. Small amounts of an ionic catalyst, specifically sulfate, chloride, or fluoride, are essential to the electrolytic production of chromium. Fluoride and complex fluoride catalyzed baths have become especially important in recent years. The cell conditions for the chromic acid process are given in Table 7. 

Better yields are attributed to intimate association of the basic nitrile group at the surface of the mtrosomum salt causing nitrosative decomposition of the azide to occur in close proximity to the weakly nucleophilic complex fluoride anion Fluorination yields can be further enhanced to 59-81% by lengthening the azido nitrile chain, but the reaction is accompanied by pronounced secondary fluoronitnle formation arising from rearrangement [100, 101] (Table 8)... 

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

The discoveiy of the process for the separation of tantalum and niobium using fluorination marked, in fact, the beginning of the development of the chemistry and technology of tantalum and niobium in general, and initiated the development of complex fluoride compound chemistry in particular. 

The second method of tantalum and niobium production is related historically to Marignac s process of tantalum and niobium separation, in the form of complex fluoride compounds, and is based on the fluorination of raw material. The modem production process consists of slightly different steps, as described below. 

An increase in the Me F ratio leads to an increase in the acidity of the initial solution, whereas the acidity of alkali metals increases according to their molecular weight, from Li to Cs. Therefore the additives of fluorides of alkali metals having higher atomic weight provide formation of complex fluorides with lower coordination number of tantalum or niobium. 

Predominant formation of either complex fluoride or complex oxyfluoride depends on the interaction rates ratio of processes (25) and (26). The relatively high interaction rates of (27) and (28) lead to the synthesis of simple fluorides or oxyfluorides, respectively. With the availability of two or more cations in the system, the ammonium complex fluorometalates interact forming stable binary fluorides or oxyfluorides or mixtures thereof. 

Increasing the temperature to 260-300°C leads to the decomposition of the ammonium-tantalum complex fluoride ... 

The hydrofluoride method can be used successfully both for the preparation of complex fluoride compounds and of complex oxides. The main advantage is that the synthesis is performed at relatively lower temperatures. In addition, the complex oxide material is formed through its respective fluoride compound and the product obtained is therefore more consistent. For instance, Co4Nb209 can be prepared using the hydrofluoride method at 900-1100°C, whereas the regular synthesis, based on the interaction of simple oxides, requires extended treatment at about 1400°C. 

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. 

Among the different families of tantalum and niobium complex fluorides and oxyfluorides, the family of compounds with an X Me ratio equal to 6 is the largest. Table 22 presents the main structural characteristics of hexafluoroniobates and hexafluorotantalates. All known cases of niobium- and tantalum-containing formulary analogs have the same crystal structure type, at least at ambient temperature. 

The fact that tantalum and niobium complexes form in fluoride solutions not only supplements fundamental data on the coordination chemistry of fluoride compounds, but also has a broad practical importance. This type of solution is widely used in the technology of tantalum and niobium compounds in raw material digestion, liquid-liquid extraction, precipitation and re-pulping of hydroxides, and in the crystallization and re-crystallization of K-salts and other complex fluoride compounds. 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

The thermal decomposition of LiNbOF4 can be also represented based on the formation of a complex fluoride compound with partially reduced niobium. But the resulting products remain unchanged ... 

Stefanovich, Leonov and Venevtsev [417] describe a typical procedure of SHG measurement. The scheme of the SHG equipment enables to perform measurements at different temperatures, as shown in Fig. 96. SHG measurements of some tantalum and niobium complex fluoride and oxyfluoride compounds in the powdered form were reported in [206, 211] and some results are presented in Table 58. 

The validity of this approach can be demonstrated by the example of several complex fluoride compounds that exhibit ferroelectric properties, such as compounds that belong to the SrAlF5 family [402, 403]. The crystal structure of the compounds is made up of chains of fluoroaluminate octahedrons that are separated by another type of chains - ramified chains. Other examples are the compounds Sr3Fe2Fi2 and PbsWjOgFio. In this case, the chains of iron- or tungsten-containing octahedrons are separated from one another by isolated complexes with an octahedral configuration [423,424]. 

The fluorination process aims to decompose the material and convert tantalum and niobium oxides into complex fluoride compounds to be dissolved in aqueous solutions. The correct and successful performance of the decomposition process requires a clear understanding of the oxygen-fluorine substitution mechanism of the interaction itself. 

The next technological step is the purification and separation of tantalum and niobium in the form of complex fluoride compounds of tantalum and niobium. This process is performed using liquid-liquid extraction, using appropriate organic solvents. This technological step requires knowledge on... 

---