A3Nb5O11(PO4)2 (A = Tl, K, Na) compounds: synthesis, crystal and vibrational characterization, conductivity study

The compound Tl3Nb5O11(PO4)2 has been prepared both in crystalline and powdery forms by the usual ceramic method. Isotypic powdered A3Nb5O11(PO4)2 (A=Na, K) compounds have been synthesized by “soft chemistry” using ion exchange. Tl3Nb5O11(PO4)2 is rhombohedral (R3̄c) with aH=13.014(2), cH=53.614(9) Å, 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 R1=0.0451, WR2(F2)=0.0987 and Goof=2.003. The anionic framework [Nb5O11(PO4)23−] 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 A3Nb5O11(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.