The heteropolyanions of W, Mo, and V, which have found numerous applications, are formed simply by acidification of solutions of their oxoanions. Under similar conditions, these oxoanion precursors are not available for Nb, and Nb-oxo chemistry is dominated by formation of the Lindquist ion [Nb6O19]8- only. However, heteropolyniobate formation is favored in hydrothermal reactions of aqueous, alkaline precursor mixtures. Here we give two examples of heteropolyniobates formed by this general reaction type: K12[Ti2O2][SiNb12O40].16H2O [1], which contains chains of silicododecaniobate Keggin ions, and Na14[H2Si4Nb16O56].45.5H2O [2], a new heteropolyanion structure type.
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Nyman et al. (2002) studied this question.