Tantalum-containing compounds

High temperature modification, measured at 230°C (after Agulyansky et al. [148]). 

Korobeinikov, VIII Symposium on chemistry of inorganic fluorides -Abstracts, Polevskoy, Nauka, Moscow, 1987, p. 25. 

Proceedings of TIC symposium on tantalum and niobium, October 22nd - 25th 2000, San Francisco, California, USA, TIC Belgium, 2000, p. 87. 

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Island-type structures are inherent for compounds with X Me > 6. Typically X Me equals to 6, 7 or 8. We have found no documentation of niobium- or tantalum-containing compounds characterizing by X Me > 9. 

The resulting product depends on the precipitation conditions, and in particular, on the over-saturation level of the solution. Formation of ammonium oxyfluorometalate crystalline compounds occurs at a relatively low pH of the solution. From the standpoint of the interactions described in Equation (143), this means that the interaction between NH4F and Me205 (denoted as interaction 1) is stronger than the interaction denoted as interaction 2. In this case, subsequent processes of the hydroxide treatment lead to some defluorination of the product, but the performance of such processes is usually very problematic. Precipitation at high pH values leads to a strong oversaturation of niobium- or tantalum-containing compounds, which in turn... 

MCM-41 and silicalite-1 can be synthesized in the presence of niobium- and tantalum-containing compounds. The results indicated that Nb(V) and Ta(V) are well dispersed in the framework of silicalite-1 and in the amorphous walls of MCM-41 y-irradiation of activated niobium and tantalum molecular sieves show two radiation induced hole centers (V centers) located on Si-O-Si and M-O-Si (M = Nb, Ta) units. True isomorphous substitution as suggested in the literature for Ti(IV), however, is unlikely to be present Nevertheless, interesting chemical and catalytic properties can be expected from these systems and are subject to further studies... 

Table 4. Compounds precipitated from tantalum-containing solutions Reproducedfrom [61], D. V. Tsikaeva, A. /. Agulyansky, Y. I. Balabanov, V Y. Kuznetsov, V. T. Kalinnikov, Zh. Neorg. Khim. 34 (1989) 3046, Copyright 1989, with permission of Nauka (Russian Academy of Sciences) publishing.

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

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. 

Compounds of the same stoichiometry type usually have the same type crystal structure within the row of alkali metals K - Rb - Cs rarely the same type structure with sodium-containing analogues and never ciystallize similarly with lithium-containing compounds. The crystal structure analysis of different fluoride and oxyfluoride compounds clearly indicates that the steric similarity between all cations and tantalum or niobium must be taken into account when calculating the X Me ratio. 

Table 50 lists melting points of some tantalum-containing fluoride compounds. 

It is, therefore, required that all initial compounds be dried properly prior to performing the reduction. This procedure is not at all trivial and refers, first of all, to the diluent salts, and especially to potassium fluoride, KF, which is characterized by a strong hygroscopic property and a tendency to form stable crystal hydrates. The problem of contamination due to hydrolytic processes can usually be resolved in two manners. The first is to apply another tantalum-containing complex fluoride compound that does not undergo hydrolysis. The second involves the adjustment of the reduction process parameters and use of some additives that will "collect" the oxygen present, in the form of water, hydroxyl groups or other compounds. 

The first alkali metal-niobium-arsenic compounds were synthesized by accident while attempting the synthesis of alkali-metal main-group arsenides at relatively high temperature. It turns out that niobium and tantalum containers react readily... 

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