Crystal Structure and Magnetic Study of a New Iron (III) Phosphate, Fe1.21PO4X (X = F, OH, H2O), Isostructural with 3MgSO4, Mg(OH)2, H2O

Fe3+1.21PO4X (X = F-, OH-, H2O), obtained by hydrothermal synthesis (200°C, 6 days) from a 1:1:2:40 mixture of Fe(NO3)3(H2O)9, P2O5, NH4F, H2O, is tetragonal (S.G. = I41/amd (No. 141)), a = 5.184(1) Å, c = 13.040(5) Å, V = 350.4(3) Å3, Z = 4. Its structure, isotypic with those of Lipscombite, Fe2-δPO4(OH), and 3MgSO4, Mg(OH)2, H2O, is built up from the interconnection of chains of face sharing anionic octahedra (5 × 14 Å2 structure type). The partial occupancy, close to 23, of these octahedra by Fe3+ is discussed. Antiferromagnetism (TN = 81 K) of Fe1.21PO4X was characterized by susceptibility measurements and Mössbauer spectroscopy. The magnetic behavior is analyzed in relation with the two types of disorder (vacancies and nature of X) occuring in the chains.