Structures of two newly synthesized A0.50SbFe(PO4)3 (A=Mn, Cd) Nasicon phases

Crystal structures of A(0.50)SbFe(PO4)(3) (A = Mn, Cd) phases, obtained by solid state reaction at 920 degrees C, were determined at room temperature from X-ray powder diffraction (XRD) using the Rietveld method. The structures of the two samples are of the Nasicon-type with the R3 space group. Hexagonal cell parameters for A = Mn and Cd are: a = 8.375(1) angstrom, c = 21.597(2) angstrom and a = 8.313(1) angstrom, c = 21.996(2) angstrom, respectively. From XRD data, it is difficult to unambiguously distinguish between Cd2+ and Sb5+ ions in Cd0.50SbFe(PO4)(3) and between Mn2+ and Fe3+ cations in Mn0.50SbFe(PO4)(3). Nevertheless the overall set of cation-anion distances within the Nasicon framework clearly shows that the cation distribution can be illustrated by the {[A(0.50)](3a)[square(0.50)](3b)}(M1)SbFe(PO4)(3) (A = Mn, Cd) crystallographic formula. The divalent A(2+) cations and vacancies are ordered within the two positions, 3a and 3b, of the M1 sites. Structure refinements show also a quasi-ordered distribution of Sb5+ and Fe3+ ions within the Nasicon framework. Thus, each A((3a))O(6) (A = Mn, Cd) octahedron shares two faces with two Fe3+O6 octahedra and each vacancy (square O-(3b)(6)) site is located between two Sb5+O6 octahedra. (c) 2005 International Centre for Diffraction Data.