Niobate and Tantalate Pyrochlores: Soft Synthesis by the Fluoride Route

A series of pyrochlores A(Nb,Ta)(2)(O,OH,F)(6)(nH(2)O), (A = NH(4), K, Rb, Cs) have been synthesized by hydrolysis of aqueous solutions of oxofluoroniobic (H(2)NbOF(5)) and fluorotantalic (H(2)TaF(7)) 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 NH(4)Nb(2)O(5)(OH,F), NH(4)Ta(2)O(5)(OH,F)center dot nH(2)O and K(1-x)Nb(2)(O,OH)(5)F center dot nH(2)O as well as the already known CsNb(2)O(5)F, Rb(1-x)Nb(2)(O,OH)(5)F, CsTa(2)O(5)F and their hydroxyl analogues. Ion-exchange experiments revealed that K(0.7)Nb(2)(O,OH)(5)F center dot nH(2)O exhibits strong selective uptake of Pb(2+) 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.