Synthesis and structural characterization of ASnFe(PO_4)_3 (A = Na_2, Ca,Cd) phosphates with the Nasicon type structure

摘要

The crystal structures of ASnFe(PO_4)_3 (A = Na_2, Ca, Cd) phases, obtained by conventional solid state reaction techniques at (950-1000℃), were determined at room temperature from X-ray powder diffraction (XRD) using Rietveld analysis. The three materials exhibit the Nasicon-type structure (R3c 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(l) A, c = 22.151(2) A; a = 8.569(1) A, c = 22.037(2) A and a = 8.587(1) A, c = 21.653(2) A, respectively. Structural refinements show a partial occupancy of M1 (Na(1)) and M2 (Na(2)) sites in Na_2SnFe(PO_4)_3 leading to the cationic distribution [Na_(1.22)□_(1.78)]_(M2) [Na_(0.78)□_(0.22)]_(M1)SnFe(PO_4)_3. Ca~(2+) ions are distributed only in the Ml site of [□_3]_(M2)[Ca]_(M1)SnFe(PO_4)_3. From XRD data, it is difficult to unambiguously distinguish between Cd~(2+) and Sn~(4+) ions in CdSnFe(PO_4)_3. Nevertheless the overall set of cation-anion distances within the Nasicon framework clearly shows that the cationic distribution can be illustrated by the [□_3]_(M2)[Cd]_(M1) SnFe(PO_4)_3 crystallographic formula. Distortion within the [Sn(Fe)(PO_4)_3] frameworks, in ASnFe(PO_4)_3 (A = Na_2, Ca,Cd) phases, is shown to be related to the M1 site size.