Structure and spectroscopic characterization of the two PbSb0.5Fe1.5(PO4)3 and Pb0.5SbFe(PO4)3 phosphates with Nasicon type-structure

Two newly synthesised pb0.50sbfe(po4)3 [pb0.5] and pbsb0.50fe1.50(po4)3 [pb] phases were obtained by conventional solid state reaction techniques at 900°c in air atmosphere. their crystallographic structures were determined at room temperature from x-ray powder diffraction (xrpd) data using the rietveld analysis. both compounds belong to the nasicon structural family. [pb0.5] and [pb] crystallise in rhombohedral system with r3 and r 3c space group respectively. hexagonal cell parameters for [pb0.5] and [pb] are: a = 8.2397(1) å,c = 22.7801(2)å,and a = 8.313(1) å,c = 23.000(2) å,respectively. pb2+ and vacancies in [pb0.50]3a[0.50]3b m1sbfe(po4)3 are practically ordered within the two positions,3a and 3b,of m1 sites. structure refinements show also an ordered distribution of sb5+ and fe3+ ions within the nasicon framework. within the structure,each pb(3a)o6 octahedron shares two faces with two fe3+o6 octahedra and each vacancy ((3b)o6) site is located between two sb5+o6 octahedra. in [pb]m1sb0.50fe1.50(po4)3 compound,all m1 sites are occupied by pb2+ and the sb5+ and fe3+ ions are randomly distributed within the nasicon framework. a raman and infrared spectroscopic study was used to obtain further structural information about the nature of bonding in both selected compositions.