title: Fe6.67(PO4)5.35(HPO4)0.65 and Fe6.23(PO4)4.45(HPO4)1.55: two new mixed-valence iron phosphates.
abstract: Two new mixed-valence iron phosphates, namely heptairon pentaphosphate hydrogen phosphate, Fe(6.67)(PO(4))(5.35)(HPO(4))(0.65), and heptairon tetraphosphate bis(hydrogen phosphate), Fe(6.23)(PO(4))(4.45)(HPO(4))(1.55), have been synthesized hydrothermally at 973 K and 0.1 GPa. The structures are similar to that of Fe(II)(3)Fe(III)(4)(PO(4))(6) and are characterized by infinite chains of Fe polyhedra parallel to the [101] direction. These chains are formed by the Fe1O(6) and Fe2O(6) octahedra, alternating with the Fe4O(5) distorted pentagonal bipyramids, according to the stacking sequence ...Fe1-Fe1-Fe4-Fe2-Fe2.... The Fe3O(6) octahedra and PO(4) tetrahedra connect the chains together. Fe(II) is localized on the Fe3 and Fe4 sites, whereas Fe(III) is found in the Fe1 and Fe2 sites, according to bond-valence calculations. Refined site occupancies indicate the presence of vacancies on the Fe4 site, explained by the substitution mechanism Fe(II) + 2(PO(4)(3-)) = vacancies + 2(HPO(4)(2-)).
authors.affiliation: Laboratory of Mineralogy, University of Liège, Belgium.
authors.forename: Fabrice
authors.initials: F
authors.lastname: Dal Bo
authors.forename: Frédéric
authors.initials: F
authors.lastname: Hatert