Crystallochemistry and structural studies of two newly CaSb0.50Fe1.50(PO4)3 and Ca0.50SbFe(PO4)3 Nasicon phases

Crystallographic structures of two new orthophosphates Ca0.50SbFe(PO4)(3) and CaSb0.50Fe1.50(PO4)(3) obtained by conventional solid state reaction techniques at 900 degrees C, were determined at room temperature from X-ray powder diffraction using Rietveld analysis. The two compounds belong to the Nasicon structural family. The space group is R (3) over bar for Ca0.50SbFe(PO4)(3) and R (3) over barc for CaSb0.50Fe1.50(PO4)(3). Hexagonal cell parameters for Ca0.50SbFe(PO4)(3) and CaSb0.50Fe1.50(PO4)(3) are: a=8.257(1) angstrom, c=22.276(2) angstrom, and a=8.514(1) angstrom, c=21.871(2) angstrom, respectively. Ca2+ and vacancies in {[Ca-0.50](3a)[rectangle(0.50)](3b)}(M1)SbFe(PO4)(3) are ordered within the two positions, 3a and 3b, of M1 sites. Structure refinements show also a quasi-ordered distribution of Sb5+ and Fe3+ ions within the Nasicon framework. Thus, in {[Ca-0.50](3a)[rectangle(0.50)](3b)}(M1)SbFe(PO4)(3), each Ca(3a)O6 octahedron shares two faces with two Fe3+O6 octahedra and each vacancy (rectangle O-(3b)(6)) site is located between two Sb5+O6 octahedra. In [Ca](M1)Sb0.50Fe1.50(PO4)(3) compound (R (3) over barc space group), all M1 sites are occupied by Ca2+ and the Sb5+ and Fe3+ ions are randomly distributed within the Nasicon framework. (c) 2006 International Centre for Diffraction Data.