Acta Cryst. (2012). C68, i83-i85 [ doi:10.1107/S0108270112043016 ]
Abstract: Two new mixed-valence iron phosphates, namely heptairon pentaphosphate hydrogen phosphate, Fe6.67(PO4)5.35(HPO4)0.65, and heptairon tetraphosphate bis(hydrogen phosphate), Fe6.23(PO4)4.45(HPO4)1.55, have been synthesized hydrothermally at 973 K and 0.1 GPa. The structures are similar to that of FeII3FeIII4(PO4)6 and are characterized by infinite chains of Fe polyhedra parallel to the  direction. These chains are formed by the Fe1O6 and Fe2O6 octahedra, alternating with the Fe4O5 distorted pentagonal bipyramids, according to the stacking sequence Fe1-Fe1-Fe4-Fe2-Fe2. The Fe3O6 octahedra and PO4 tetrahedra connect the chains together. FeII is localized on the Fe3 and Fe4 sites, whereas FeIII 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 FeII + 2(PO43-) = vacancies + 2(HPO42-).
Formula: Fe6.67(PO4)5.35(HPO4)0.65 and Fe6.23(PO4)4.45(HPO4)1.55
Structure factor file (CIF format) (118.1 kbytes)
Structure factor file (CIF format) (116.8 kbytes)
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