The compound Tl3Nb5O11(PO4)(2) has been prepared both in crystalline and powdery forms by the usual ceramic method. Isotypic powdered A(3)Nb(5)O(11)(PO4)(2) (A = Na, K) compounds have been synthesized by "soft chemistry" using ion exchange. Tl3Nb5O11(PO4)(2) is rhombohedral (R (3) over barc) with a(H) = 13.014(2), c(H) = 53.614(9) Angstrom, Z = 18. Its crystal structure has been determined using single crystal X-ray data. Structure refinements carried out by the full matrix least squares method led to agreement factors R-1 = 0.0451, WR2(F-2) = 0.0987 and Goof = 2.003. The anionic framework [Nb5O11(PO4)(2)(3-)] built up of corner-shared NbO6 octahedra and PO4 tetrahedra and delimiting interconnected tunnels is derived from the pure octahedral pyrochlore structure. Unlike Rb+ in the isostructural compound Rb3Nb5O11(PO4)(2), each of the monovalent Tl+ cations is split up into several site positions close to one another. Ionic conductivity measurements on the A(3)Nb(5)O(11)(PO4)(2) (A = Tl, K, Na) compounds, using the complex impedance method, indicate that the sodium compound presents better ionic properties than the potassium and thallium compounds, as shown by a lower activation energy and a higher conductivity. (C) 2003 Elsevier Science Ltd. All rights reserved.