Fluorides compounds

Following Bartlett s discovery of xenon hexafluoroplatinate(VI), xenon and fluorine were found to combine to give several volatile, essentially covalent fluorides, and at least one fluoride of krypton has been obtained. From the xenon fluorides, compounds containing xenon-oxygen bonds have been made much of the known chemistry of xenon is set out in Figure 12.1. 

Volatile impurities, eg, F2, HF, CIF, and CI2, in halogen fluoride compounds are most easily deterrnined by gas chromatography (109—111). The use of Ftoroplast adsorbents to determine certain volatile impurities to a detection limit of 0.01% has been described (112—114). Free halogen and haHde concentrations can be deterrnined by wet chemical analysis of hydrolyzed halogen fluoride compounds. 

The aluminum halides and aluminum nitrates have similar properties with the exception of the family of aluminum fluoride compounds which are discussed elsewhere (see Fluorine compounds, inorganic). Of the remaining members ia this aluminum haUde family, chloride derivatives are the most commercially important alurniaum brornide /7727-/3 -JJ AlBr, alumiaum iodide [7784-23-8] and alumiaum nitrate [13473-90-0] are of... 

Complexing agents, which act as buffers to help control the pH and maintain control over the free metal—salt ions available to the solution and hence the ion concentration, include citric acid, sodium citrate, and sodium acetate potassium tartrate ammonium chloride. Stabilizers, which act as catalytic inhibitors that retard the spontaneous decomposition of the bath, include fluoride compounds thiourea, sodium cyanide, and urea. Stabilizers are typically not present in amounts exceeding 10 ppm. The pH of the bath is adjusted. 

For halides the cation should have a charge of 2+ rather than 4+ for tetrahedral coordination. The only fluoride compound capable of containing two-coordinate F and four-coordinate cations is Bep2. For ZrF, the radius ratio rule predicts that Zr" " is eight-coordinate if all fluorine atoms are two - c o o rdinate. 

Nitric acid acidulation NO, gaseous fluoride compounds Scrubber, addition of urea... 

Data based on soluble fluoride increased values for insoluble fluoride compounds. 1 ppm wt = 1 mg/kg. 

Hydrogen fluoride reacts witlr metal carbonates, oxides, and hydroxides. Accumulation of these fluoride compounds can render valves and other close-fitting moving parts inoperable in a process system, causing possible equipment or process failures. Hydrogen fluoride also attacks glass, silicate ceramics, leather, natural rubber, and wood, but does not promote their combustion. 

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. 

As opposed to the oxides, fluoride compounds are characterized by the formation of mostly ionic bonds. This peculiarity is related to fluorine s high electronegativity. 

Despite the relatively short history of the chemistry of fluoride compounds, several thousands of binary and ternary fluoride compounds have been described, and their systematization is well developed [39 - 41]. Significant progress was achieved in the study of the crystal chemistry of fluoride compounds thanks to the ionic character of their chemical bonds and corresponding simplicity of their ciystal structure. The structure of these kinds of compounds is defined primarily by the geometry and the energy of mainly... 

On the other hand, fluorine s high electronegativity and its ability to form mostly ionic chemical bonds, provide materials with several useful properties. First, compared to oxides, fluoride compounds have a wide forbidden zone and as a result, have low electroconductivity. In addition, fluorides are characterized by a high transparency in a wide optical range that allows for their application in the manufacturing of electro-optical devices that operate in the UV region [42,43]. 

At the same time, the relatively low energy and ionic character of the chemical bonds between metal and fluorine cause some difficulties in the application of fluoride compounds. First, fluorides typically have a tendency towards thermolysis and hygroscopicity. In addition, fluoride compounds usually display relatively low temperatures of electrostatic and magnetic ordering. 

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

In summary, investigations in the area of the chemistry of tantalum and niobium fluoride compounds will advance tantalum-niobium metallurgy and promote the development of new materials for modem applications. 

The variety of fluoride compounds that exist and the wide spectrum of their preparation methods are related to the properties of fluorine, and above all to fluorine s high electronegativity. Low dissociation energy of the fluorine molecule, F2, relatively high energies of bond formation found in most fluoride compounds, as well as fluorine s strong oxidizing ability lead, in some cases, to spontaneous fluorination. 

The synthesis of tantalum and niobium fluoride compounds is, above all, related to the fluorination of metals or oxides. Table 3 presents a thermodynamic analysis of fluorination processes at ambient temperature as performed by Rakov [51, 52]. It is obvious that the fluorination of both metals and oxides of niobium and tantalum can take place even at low temperatures, whereas fluorination using ammonium fluoride and ammonium hydrofluoride can be performed only at higher temperatures. 

Synthesis of fluoride compounds is performed in various media, such as aqueous solutions, non-aqueous systems and heterogeneous interactions. 

Precipitation of fluoride compounds from solutions of hydrofluoric acid, HF, is performed by the addition of certain soluble compounds to solutions containing niobium or tantalum. Initial solutions can be prepared by dissolving metals or oxides of tantalum or niobium in HF solution. Naturally, a higher concentration of HF leads to a higher dissolution rate, but it is recommended to use a commercial 40-48% HF acid. A 70% HF solution is also available, but it is usually heavily contaminated by H2SiF6 and other impurities, and the handling of such solutions is extremely dangerous. 

Synthesis of the compounds from such HF solutions is performed by adding soluble fluoride compounds to the tantalum or niobium solution or by recrystallization of prepared fluoride compounds from water or HF solutions of different concentrations. In the first case, the composition of the compounds obtained depends on the ratio between Ta/Nb and the added metal and on the initial concentration of the HF used, whereas in the second case, it depends only on the HF concentration. 

Potassium-containing tantalum and niobium fluoride compounds can be precipitated from HF solutions as described previously (see Fig. 3 and 4). Ritchie and Mitra [59] described the synthesis of K.2TaF7 in an HF solution, based on the following interaction (5), using TaCl5 as a precursor ... 

Attempts to obtain fluoride compounds of niobium and tantalum with alkali earth and some transitional metals were made as early as one hundred years ago, but synthesis and identification methods were described only at later times. 

No available data was found on the precipitation from fluoride solutions of niobium and tantalum fluoride compounds containing tri- and tetravalent metals. 

The results available on the synthesis of niobium and tantalum fluoride compounds from aqueous solutions are in good correlation with the concept of... 

Anhydrous hydrogen fluoride is widely used for the synthesis of fluoride compounds [68]. In particular, NaNbF6 and Na2NbF7 were prepared using a solution of anhydrous hydrogen fluoride, HF, containing dissolved NbF5 and NaF [69]. 

Another anhydrous solvent that is frequently used for the synthesis of tantalum and niobium fluoride compounds is bromine trifluoride, BrF3. At ambient temperature, bromine trifluoride is light yellow liquid characterized by a boiling point of 126°C, a melting point of 9°C and a density of 2.84 g/cm3 at melting temperature. 

The most universal method for the synthesis of tantalum and niobium fluoride compounds is based on direct interaction between their pentafluorides, TaF5 or NbFs, and fluorides of other metals. Some physical-chemical properties of these compounds are presented in Table 8 [71, 72]. 

The formation of (NH4)3NbOF6 and its decomposition products define the composition of the niobium fluoride compounds that can be prepared by the hydrofluoride method [123-125]. 

Thus, the sequence of phases that are formed in hydrofluoride synthesis of lithium-niobium fluoride compounds is (NH4)3NbOF6 - (NH NbOFs -LiNbOF4-Li2NbOF5. 

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. 

CRYSTAL CHEMISTRY OF TANTALUM AND NIOBIUM FLUORIDE COMPOUNDS... 

Crystal chemistry of Ta and Nb fluorides - Compounds with chain-type structures... 

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