Abstract
A series of pyrochlores A(Nb,Ta)2(O,OH,F)6(nH2O), (A = NH4, K, Rb, Cs) have been synthesized by hydrolysis of aqueous solutions of oxofluoroniobic (H2NbOF5) and fluorotantalic (H2TaF7) acids. This new method allows preparation of nanocrystalline niobate pyrochlores (crystal size 10–20 nm) by simple precipitation with subsequent boiling; tantalate analogues can be obtained by soft hydrothermal treatment. The synthesized compounds include new pyrochlores NH4Nb2O5(OH,F), NH4Ta2O5(OH,F)·nH2O and K1–xNb2(O,OH)5F·nH2O as well as the already known CsNb2O5F, Rb1–xNb2(O,OH)5F, CsTa2O5F and their hydroxyl analogues. Ion-exchange experiments revealed that K0.7Nb2(O,OH)5F·nH2O exhibits strong selective uptake of Pb2+ from aqueous solutions, a feature not previously reported for pyrochlores. The facilitating role of fluoride ions in the synthesis of pyrochlores is discussed in comparison with zeolite synthesis, and the technological applicability of the new method is also presented in the paper.
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