Niobium-containing systems

It is important to note that the number of different compounds found in such systems increases significantly when moving along the sequence of alkali metals, from lithium to cesium, as does their thermal stability. This phenomenon is related to the systematic increase of both ionic radii and polarity of alkali metals ions when moving from lithium to cesium. 

Ternary melting diagrams of the KCl - NaF - K2NbF7 system (bottom). Reproduced from [300], L. A. Kamenskaya, V. I. Konstantinov, A. M. Matveev, Zh. Neorg. Khim. 17 (1972) 2567, Copyright 1972, with permission of Nauka (Russian Academy of Sciences) publishing. 

This interpretation correlates with data obtained for the ternary interconnecting system K+, NaV/CF, NbF72  

Kamenskaya [221] and Konstantinov [37] investigated the concentration field adjacent to the KF - NbF5 side of the ternary interconnected system K+, Nb57/02, F. Fig. 55 shows the projection of the crystallization fields. 

The largest crystallization field belongs to K.3Nb02F4, which undergoes congruent melting at 986°C. 

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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. 

Based on a comprehensive investigation of solubility isotherms and of conductometric and potentiometric titration in the NbF5 - HF - H2O system, Nicolaev and Buslaev [288] concluded that H2NbOF5 is the predominant niobium-containing form present in such solutions. 

The addition of alkali metal or ammonium fluorides reduce the acidity of the system and shift the equilibrium between the two ions toward the formation ofNbOFs2 ions [60,61]. The shift depends on the alkalinity of the cation. The more alkaline the cation is (higher atomic weight), the stronger the shift toward NbOF52 ion formation. Fig. 48 shows typical Raman spectra of niobium-containing solutions before and after such additions were made. 

The same analysis of the properties of niobium-containing melts [316, 317] shows that the KF - NbFs system exhibits behavior similar to that of the tantalum-containing melts. 

Using the same mechanism based on interactions (61) and (73), the peaks of the conductivity isotherms for niobium-containing molten systems KC1 -K2NbF7, KC1 - KF - K2NbF7 [317, 318], KC1 - NaF - K2NbF7 [319] can be explained. 

Phase transition irreversible, 225 order - disorder, 224-228 reversible, 225, 229, Physicochemical properties of ammonium hydrofluoride, 39 deviations from ideal, 149 ideal system, 148 NbF5 and TaFs, 25 niobium containing melts, 150 tantalum containing melts, 151 M5Nb3OFlg, 234-235 Piezoelectric properties, 245-247 Plasma chemical decomposition equipment, 311... 

Thus, the niobium hydrides in fact do exhibit the desired hydridic character they can reduce acetone but cannot react with CO reduction products. From the color, the absence of any NMR, and the formation of 1 mol of free cyclo-pentadiene, the niobium-containing product appears to be a monocyclopenta-dienyl Nb(IV) complex. The rather facile removal of one of the Cp groups appears somewhat surprising but has been observed in related systems. For example, alcoholysis of Cp2TiCl2 gives CpTiC OEt) plus cyclopentadiene, even at room temperature (26). 

Three conceptual steps can be discerned in the definition of the ionic structure of fluoride melts containing tantalum or niobium. Based on the very first thermodynamic calculations and melting diagram analysis, it was initially believed that the coordination numbers of tantalum and niobium, in a molten system containing alkali metal fluorides, increase up to 8. 

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