The mixed-valence iron compound Na0.1Fe7(PO4)6: crystal structure and 57Fe Mössbauer spectroscopy between 80 and 295 K.

The crystal structure of the synthetic iron phosphate Na0.10(1)Fe6.99(1)(P1.00(1)O4)6 has been refined at 270 and 100 K from single-crystal X-ray diffraction data. The compound is triclinic, P−1, Z=1, lattice parameters: a=6.3944(9) Å, b=7.956(1) Å, c=9.364(1) Å, α=105.13(1)°, β=108.35(1)°, γ=101.64(1)° at 270 K and adopts the well-known howardevansite structure type. Iron, being both in the divalent and the trivalent valence state, is ordered on the four symmetry non-equivalent iron positions [Fe2+ on Fe(1) and Fe(3), Fe3+ on Fe(2) and Fe(4)]. Three of the four iron positions show octahedral oxygen atom coordination, the fourth one, which is occupied by Fe2+, is five-fold coordinated. The structure consists of crankshafts (buckled chains) of edge sharing Fe-oxygen polyhedra, passing through the unit cell in [101] direction. Structural investigation at 100 K shows no change of symmetry. The valence state and distribution of iron was determined by 57Fe Mössbauer spectroscopy. The compound shows 4 subspectra in agreement with the four different Fe sites. The assignment of the Fe2+ doublets to the Fe(1) and Fe(3) sites is trivial due to the 2:1 stoichiometry, also found in the Mössbauer spectra. For the Fe3+ sites, the temperature-dependent variation of structural distortion parameters and the quadrupole splitting led to a clear doublet assignment. [Copyright &y& Elsevier]