Niobium-containing compounds

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

Tetragonal syngony, a = 5.18 A, c = 10.05 A (after Bizot and Malek-Zadeh [77]). 

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Table 7. Niobium-containing compounds prepared from HF solutions. Reproduced from [61], D. V. Tsikaeva, A. I. 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.

The appearance of spontaneous polarization in the case of LuTaO is related to volumetric irregularities and ordering of the Li+ - Ta5+ dipoles, as is in the case of the similar niobium-containing compound Li4Nb04F. It can be assumed that the main difference between the two compounds is that the irregularities and the Li+ - Ta5+ dipoles are thermally more stable compared to the niobium-containing system. This increased stability of the dipoles leads to the reversible phase transition at 660°C. 

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 ... 

Three kinds of niobium-containing initial solutions were used, with different Nb F ratios 1 9, 1 18 and 1 6. The first two initial solutions contained mostly NbF6" ions, whereas the third was composed primarily of NbOF52 ions. Table 5 presents the composition of the compounds that were precipitated following the addition of certain alkali fluorides to the initial solutions. 

Nickel and copper containing compounds have been prepared in a similar manner. The phases obtained by the simultaneous fluorination of niobium oxide and other bivalent metal oxides were MHNbOF5, M21,Nb03F3 and M4UNb209, where M11 = Co Ni, Cu [129, 131],... 

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 radius of the second cation in known MuNbOFs, MU2Nb03F3 and Mul2Nb05F compounds containing bi- and trivalent metals, is usually similar to that of niobium s ionic radius. Such compounds cannot be considered as having an island-type structure and will be discussed later on. Only bismuth-containing compounds (Bi3+) display the presence of different cationic sublattices in their crystal structure. 

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. 

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. 

Table 49 lists the melting points of some niobium-containing fluoride compounds. 

THERMAL PROPERTIES AND DECOMPOSITION OF TANTALUM- AND NIOBIUM-CONTAINING FLUORIDE COMPOUNDS... 

The main problems encountered in the investigation of tantalum- and niobium-containing fluoride and oxyfluoride complexes are related to the tendency of the compounds to undergo hydrolysis, particularly at elevated/high temperatures. In addition, the interpretations of the observed effects are often nontrivial and unclear due to the relatively complicated inter-particular interactions and changes that occur under thermal treatment. From this point of view, vibration spectroscopy methods are of high importance due to the dependence of solid phase spectra on the temperature, which, above all, stems from the nature of such inter-ionic interactions [369]. 

Thus, it is possible to make a very important assumption that chain-type compounds decompose forming gaseous niobium-containing components, while island-type compounds release upon thermal decomposition only light atoms and molecules into the gaseous phase [383]. 

Oxyfluoride compounds whose crystal structure is made up of linked octahedrons decompose and release niobium-containing molecules into the gaseous phase ... 

The processing of tantalum and niobium begins with the fluorination of the raw material, which always consists of complex oxide compounds containing tantalum and niobium. The main types of tantalum- and niobium-containing minerals are discussed in Chapter 1, and typical compositions of such minerals are presented in Table 2. 

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... 

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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