Tantalum-containing fluoride melts

The melting process of potassium fluorotantalate, K2TaF7, was investigated by IR emission spectroscopy using thick layers of the melt [356]. It should be mentioned that in some cases, if the temperature of the sample is high enough, the above method enables to obtain spectra of the material in solid state as well. 

The IR spectrum obtained at ambient temperature (Fig. 74, curve 1) shows the presence of a strong wide band at 535 cm 1 and a weak shoulder at about 643 cm 1. At the melting temperature (curve 2), the only discemable band observed is shifted slightly toward the higher wave numbers and occurs at 535-540 cm 1. This high frequency shift, which accompanies the melting, is related to changes in the distances between the central atom and the first and second coordination spheres, as illustrated in Fig. 75. 

Fluorine ions form the first coordination sphere around the tantalum ion, which is the central atom of the complex. Potassium ions form the second coordination sphere, which significantly affects the geometry and force field of the first coordination sphere. The melting of K2TaF7 leads to the dissociation of the compound into ions, as follows  

This dissociation is in effect an extension of the diameter d2 of the second coordination sphere and the subsequent decrease in the intrinsic interaction potential of the outer sphere. Therewith, the inter-spherical interaction potential between the central atom and the first coordination sphere increases, leading to shortening of the distance d, which in turn leads to an increase in the frequency of the Ta-F bond vibration. 

Further increase of the temperature leads to the appearance of a strong band at 605 cm 1 (see Fig. 74, curve 3) along with the band at 540 cm 1, which remains unchanged. Comparative analysis of the spectra shows that the above bands refer to the vibrations of the complex ions TaF6 and TaF72 , respectively. 

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

Investigation of fluoride melts containing tantalum or niobium is a very difficult procedure that involves high temperatures, corrosion activity and the... 

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. 

Physicochemical properties of fluoride melts containing tantalum... 

The method based on a net melt-holder is used successfully for materials with relatively low melting points and is less promising for the investigation of fluoride melts containing tantalum and niobium. 

Complexes in fluoride melts - tantalum-containing melts... 

Table 55 presents the results discussed above. Fluoride melts containing tantalum contain two types of complex ions, namely TaF6 and TaF72 . The equilibrium between the complexes depends on the concentration of fluoride ions in the system, but mostly upon the nature of the outer-sphere cations. The complex ionic structure of the melts can be adjusted by adding cations with a certain polarization potential. For instance, the presence of low polarization potential cations, such as cesium, leads primarily to the formation of TaF72 complexes, while the addition of cations with relatively high polarization potentials, such as lithium or sodium, shifts the equilibrium towards the formation of TaF6 ions. 

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