Synthesis and structure of two phosphates belonging to the ternary
Sb2O5-Fe2O3-P2O5 system are reported. Structures of both
(SbV0.50FeIIIe0.50)P2O7 phases, obtained by solid state reaction in air atmosphere at 950 °C and 900 °C, respectively, were determined at room temperature from X-ray powder diffraction using the Rietveld method.
Sb1.50Fe0.50(PO4)3 phosphate belongs to the Nasicon-type structure with R32 space group. Hexagonal cell parameters are ahex.=8.305(1) Å and chex.=22.035(2) Å. Rietveld refinement results show a 2-2 ordered distribution, along the c-axis, of X(1) and X(2) sites (crystallographic formula
[Sb0.88Fe0.12]X(1)[Fe0.38Sb0.62]X(2)(PO4)3) in the Nasicon framework.
(Sb0.50Fe0.50)P2O7 is isotypic with
β-SbP2O7 pyrophosphate [Pna21 space group; a=7.865(1) Å, b=15.699(2) Å, c=7.847(1) Å]. Its crystal structure is built up from corner-shared
FeO6 octahedra and
P2O7 groups (two group types). Each
P2O7 group shares its six vertices with three
SbO6 and three
FeO6 octahedra, and each octahedra is connected to six
P2O7 groups. A quasi 1-1 ordered distribution, along the b-axis, of
Sb5+ and Fe3+ ions in the pyrophosphate framework are observed.