Ionic conductivity and phase transition in Pb4.8Bi1.6Na3.6(PO4)6, an apatite-type compound

Bismuth sodium lead phosphate, Pb4.8Bi1.6Na3.6(PO4)6, a compound having an apatite structure, was obtained via a solid-state reaction. Determination of the structure by X-ray diffraction on a single crystal gave the following results: hexagonal, P63/m, a = 9.6545(11) Å, c = 7.1457(5) Å, R = 0.03 and Rw = 0.09. The presence of Bi3+ and Pb2+ ions, carrying a lone electron pair, at (6h) sites helped to stabilize the apatite structure in spite of the anionic vacancies in the center of the tunnel. The compound was characterized by chemical analysis, differential thermal analysis, thermal expansion of the cell, and infrared, impedance and 23Na NMR spectroscopy. The thermal behavior revealed two breaks attributed to order–disorder transitions. One of them was ascribed to a reordering of cations from the (6h) site to the center of the tunnel. Conductivity was dominated by hopping mechanisms.