The mixed-valence iron compound Na0.1Fe7(PO4)6:: crystal structure and 57Fe Mossbauer 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 100K from singlecrystal X-ray diffraction data. The compound is triclinic, P-1, Z = 1, lattice parameters: a = 6.3944(9) Angstrom, b = 7.956(1) Angstrom, e = 9.364(1) Angstrom, alpha = 105.13(1)degrees , beta = 108.35(1)degrees, gamma = 101.64(1)degrees at 270 K and adopts the well-known howardevansitc structure type. Iron, being both in the divalent and the trivalent valence state, is ordered on the four symmetry non-equivalent iron positions [Fc 2 1on Fe(1) and Fe(3), Fe3+ on Fe(2) and Fe(4)]. Three of the four iron positions show octahedral oxygen atorn 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 Fe-57 Mossbauer 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 Mossbauer spectra. For the Fe3+ sites, the temperature-dependent variation of structural distortion parameters and the quadrupole splitting led to a clear doublet assignment. (C) 2003 Elsevier Inc. All rights reserved.