Tantalates, alkali

Dioxytetrafluoroniobates (-tantalates) of alkali metals for the general M3Me02F4 type also belong to the family characterized by X Me=6. Structural characteristics of the compounds are collected in Table 26. 

Regarding dioxytrifluoroniobates (-tantalates) of alkali metals, information on the synthesis of K2Nb02F3 and K2Ta02F3 is available in [213] and [214], respectively. 

Tantalum powder is produced by reduction of potassium heptafluoro-tantalate, K2TaF7, dissolved in a molten mixture of alkali halides. The reduction is performed at high temperatures using molten sodium. The process and product performance are very sensitive to the melt composition. There is no doubt that effective process control and development of powders with improved properties require an understanding of the complex fluoride chemistry of the melts. For instance, it is very important to take into account that changes both in the concentration of potassium heptafluorotantalate and in the composition of the background melt (molten alkali halides) can initiate cardinal changes in the complex structure of the melt itself. 

Fig. 7.42 Ta (6.2 keV) Mosshauer spectra of alkali tantalates obtained at room temperature with a source of in W (single crystal). The centers (isomer shifts) of the quadmpole split spectra are indicated by arrows (from [186])...

H. Kato, A. Kudo, Water splitting into H2 and O2 on alkali tantalate photocatalysts ATa03 (A — Li, Na, and K),... 

Kato H, Kudo A (2001) Water splitting into H2 and O2 on Alkali tantalate photocatalysts ATaOa. J Phys ChemB 105 4285-4292... 

Tantalic Acid and Tantalates. Tantalic acid [75397-94-3], Ta2Os H20, is the name of the white insoluble precipitate formed by hydrolysis of alkali hydroxide or alkali carbonate fusions containing tantalum, or by adding ammonia to an acidic solution containing tantalum ions. Tantalic acid is characterized by a high surface acidity, affording it potential use as a catalyst. 

In addition to potassium hydrogen sulphate and sodium hydrogen sulphate8 for opening up the ore, potassium carbonate,9 sodium peroxide, and alkali hydroxides 10 have been employed. The use of potassium hydroxide is preferred in the case of a high-grade ore of low titanium content 11 it has the advantage over sodium hydroxide that potassium tantalates are soluble in solutions which contain excess of the alkali, whereas sodium tantalates are insoluble. 

The vapours of the alkali metals are without action even at 2000° C.2 Boiling solutions of the alkalis attack tantalum slowly fusion with caustic potash in air yields a tantalate. 

Tantalum pentoxide is insoluble in solutions of caustic alkalis and alkali carbonates, but on being fused with these substances reacts to produce the alkali tantalates. Tantalates of the metals are obtained by double decomposition, using a soluble alkali tantalate and a soluble salt of the metal. Tantalates of the alkaline earths have also been obtained by fusing tantalum pentoxide with the chloride of the alkaline earth. 

The soluble alkali tantalates undergo ready hydrolysis when their aqueous solutions are boiled, precipitation of a more add salt takes place and some alkali base passes into solution. Separation of the base in this manner is naturally favoured by the presence of acids, and even so weak an acid as carbon dioxide or hydrogen sulphide pretipitates tantalic add or an acid salt with the stronger adds (sulphurous acid, sulphuric add, hydrochloric acid, nitric add, etc., but not with hydrofluoric add) predpitation of tantalic add takes place readily, but excess of the strong mineral add redissolves the predpitate. Potassium chloride... 

Measurements of the electrical conductivity of solutions of alkali tantalates have been made,1 but no definite conclusions are deducible as to the complexity of the ions present. 

Niobate and tantalate isopolyanions can be obtained by fusing the oxides in an excess of alkali hydroxide or carbonate and dissolving the melts in water. The solutions are stable only at higher pH precipitation occurs below pH 7 for niobates and 10 for tantalates. At pH > 14.5, mononuclear Nb02(0H)5" anions are likely to be present in dilute aqueous solution. At lower pH the only species that appear to be present in solution are the ions (x = 0, 1, 2, or 3),4 despite... 

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