Synthesis and structural characterization of ASnFe(PO4)3 (A=Na2,Ca,Cd) phosphates with the Nasicon type structure

The crystal structures of ASnFe(Po-4)(3) (A= Na-2, Ca, Cd) phases, obtained by conventional solid state reaction techniques at (950-1000 degrees C), were determined at room temperature from X-ray powder diffraction (XRD) using Rietveld analysis. The three materials exhibit the Nasicon-type structure (R (3) over barc space group, Z=6) with a random distribution of Sn(Fe) within the framework. Hexagonal cell parameters when A=Na-2, Ca and Cd are: a = 8.628(1) angstrom, c = 22.151(2) angstrom; a =8.569(1) angstrom, c = 22.037(2) angstrom and a = 8.587(1) angstrom, c = 21.653(2) angstrom, respectively. Structural refinements show a partial occupancy of M1 (Na(1)) and M2 (Na(2)) sites in Na2SnFe(PO4)(3) leading to the cationic distribution [Na-1.22 square(1.78)](M2)[Na-0.78 square(0.22)](M1)SnFe(PO4)(3). Ca2+ ions are distributed only in the M1 site of [square(3)](M2)[Ca](M1)SnFe(PO4)(3). From XRD data, it is difficult to unambiguously distinguish between Cd2+ and Sn4+ ions in CdSnFe(PO4)(3). Nevertheless the overall set of cation-anion distances within the Nasicon framework clearly shows that the cationic distribution can be illustrated by the [square(3)](M2)[Cd](M1)SnFe(PO4)(3) crystallographic formula. Distortion within the [Sn(Fe)(PO4)(3)] frameworks, in ASnFe(PO4)(3) (A = Na-2,Ca,Cd) phases, is shown to be related to the M1 site size. (c) 2004 International Centre for Diffraction Data.