Niobium fluorides

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

Nevertheless, tantalum and niobium refining technology was, and remains, a part of fluorine chemistry, since its main processes are related to the chemistry of tantalum and niobium fluorides in solid, dissolved and molten states. 

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 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. 

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 ... 

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. 

CRYSTAL CHEMISTRY OF TANTALUM AND NIOBIUM FLUORIDE COMPOUNDS... 

The basis of the ciystal chemical classification of tantalum and niobium fluoride compounds was first formulated by Kuznetzov and Rogachov [232]. The main tenets of the classification are as follows ... 

Table 42. General classification of tantalum and niobium fluoride compounds...

This chapter focuses on tantalum and niobium fluoride complex ions that occur in solutions containing hydrofluoric acid. 

Raman spectra of fluoride solutions containing niobium were investigated by Keller [171]. Solutions were prepared by dissolving niobium fluoride compounds in solutions of hydrofluoride acid, HF, of different concentrations. 

Tantalum and niobium fluoride compounds that crystallize in coordination-type structures also seem to be perspective candidates for the investigation of ferroelectric properties. Ravez and Mogus-Milancovic [404] showed that some fluoride and oxyfluoride compounds with crystal structures similar to the Re03 type exhibit ferroelastic properties. For instance, ferroelastic properties were found in some solid solutions based on Nb02F and Ta02F [405,406]. 

Table 58. SHG intensity values (hdho, (SiO2)) °f some tantalum and niobium fluorides and oxyfluorides after normalization by alpha-quartz signal fu (SiO]). Measurements were taken before and after thermal treatment, up to extinction of signal.

This chapter is devoted to a discussion of the main steps in the currently-applied technology of tantalum and niobium compounds from the standpoint of the chemistry of complex tantalum and niobium fluoride compounds. 

It was proposed [445 - 447] that the dissolution of tantalum and niobium oxides in mixtures of hydrofluoric and sulfuric acids takes place through the formation of fluoride-sulfate complexes, at least during the initial steps of the interaction and at relatively low acid concentrations. Nevertheless, it was also assumed that both tantalum and niobium fluoride-sulfate complexes are prone to hydrolysis yielding pure fluoride complexes and sulfuric acid. No data was provided, however, to confirm the formation of fluoride sulfate complexes of tantalum and niobium in the solutions. 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

Modem requirements of the capacitor industry initiate further development of tantalum powder production processes. The tendency is to produce powder of higher purity with a higher specific charge and at lower cost. Further development of the processes can be successfully achieved based on current achievements in the chemistry of tantalum and niobium fluoride compounds. 

Density of melts containing niobium fluorides, 151 tantalum fluorides, 152, 165... 

Modem refining technology uses tantalum and niobium fluoride compounds, and includes fluorination of raw material, separation and purification of tantalum and niobium by liquid-liquid extraction from such fluoride solutions. Preparation of additional products and by-products is also related to the treatment of fluoride solutions oxide production is based on the hydrolysis of tantalum and niobium fluorides into hydroxides production of potassium fluorotantalate (K - salt) requires the precipitation of fine crystals and finishing avoiding hydrolysis. Tantalum metal production is related to the chemistry of fluoride melts and is performed by sodium reduction of fluoride melts. Thus, the refining technology of tantalum and niobium involves work with tantalum and niobium fluoride compounds in solid, dissolved and molten states. 

Also, the pentoxide may be produced by igniting niobium metal powder, niobium carbide, or niobium fluoride in oxygen. 

The hydrofluoric acid solution of niobic acid does not yield a precipitate on the addition of potassium fluoride (potassium niobium fluoride, K2NbF7, which is formed, being soluble in about 12-5 parts of water), whereas the hydrofluoric acid solution of tantalic add yields colourless, rhombic needles of potassium tantalum fluoride, K2TaF7 (which is soluble in about 150 parts of water under the same conditions), when treated with a saturated solution of potassium fluoride, carefully evaporated and cooled slowly. After removal of the tantalum, and with further concentration, any niobium present separates in pdates of potassium niobium oxyfluoride, K2Nb0F5.H20, if the hydrofluoric acid is not in excess, and in needles of potassium niobium fluoride, K2NbF7, if the hydrofluoric acid is in excess. 

Ammonium Niobium Fluoride.—(See below under Potassium Niobium Fluoride.)... 

Cadmium Niobium Fluoride, 3NbF5.5CdF2.5HF.28H20 or NbsCd5F25. 5HF.28HaO, is obtained in long, transparent prisms by the action of cadmium carbonate on a solution of niobic acid in concentrated hydrofluoric acid. It is insoluble in water. 

Caesium Niobium Fluoride, CsF.NbFs or CsNbF6, is obtained in fine needles by repeated crystallisation of csesium niobium oxyfluoride, CSjNbOFj, from hydrofluoric acid.2 Another csesium niobium fluoride having the probable composition 7CsF.NbFs or Cs7NbF5 has been prepared by the action of a solution of caesium hydroxide in hydrofluoric add on niobic acid in the same solvent.3... 

Cobalt Niobium Fluoride, 3NbFs.5CoF2.5HF.28H20 or Nb8Co5F2B. 

Copper Niobium Fluoride, NbFg.2CuF2.HF.9H2O or NbCuF9.HF. 9H20, forms large, dark blue crystals, which are obtained similarly to the cadmium salt. 

Ferrous Niobium Fluoride, 2NbF5.3FeF2.4HF.19H20 or NbaFe3F16. 4HF.19H20, is obtained in greenish-yellow, thin prisms by dissolving iron and niobic acid in equivalent proportions in hydrofluoric acid. 

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