Structure and ionic conductivity of the lacunary apatite Pb6Ca2Na2(PO4)6

Polycrystalline Pb6Ca2Na2(PO4)6 (NCPbAp) was obtained via a solid state reaction. It was characterized by IR spectrometry and X-ray diffraction, its structure was refined using the Rietveld method. It crystallizes in the hexagonal system (s.g. P63/m, a=9.658(8) Å and c=7.081(6) Å) and presents a lacunary structure stabilized by the electron lone pair of lead (II) ions. At 720 K, an order–disorder phase transition probably of first order is observed either in conductivity or XRD studies versus temperature. Electric conductivity is dominated by the mobility of sodium in the tunnels.