Synthesis, structure and magnetic characterisation of a new layered ammonium manganese(ii) diphosphate hydrate, (NH4)2[Mn3(P2O7)2(H2O)2]Electronic supplementary information (ESI) available: powder XRD data, atomic coordinates and thermal parameters, IR data, bond valence calculations, TGA. See http://www.rsc.org/suppdata/jm/b3/b304003h

A new layered ammonium manganese(II) diphosphate, (NH4)2[Mn3(P2O7)2(H2O)2], has been synthesised under solvothermal conditions at 433 K in ethylene glycol and the structure determined at 293 K using single-crystal X-ray diffraction data (Mr = 584.82, monoclinic, space group P21/a, a = 9.4610(8), b = 8.3565(7), c = 9.477(1) A, β = 99.908(9)°, V = 738.07 A3, Z = 2, R = 0.0351 and Rw = 0.0411 for 1262 observed data (I > 3(σ(I))). The structure consists of chains of cis- and trans-edge sharing MnO6 octahedra linked via P2O7 units to form layers of formula [Mn3P4O14(H2O)2]2− in the ab plane. Ammonium ions lie between the manganese-diphosphate layers. A network of interlayer and ammonium-layer based hydrogen bonding holds the structure together. Magnetic measurements indicate Curie–Weiss behaviour above 30 K with μeff = 5.74(1) μB and θ = −23(1) K, consistent with the presence of high-spin Mn2+ ions and antiferromagnetic interactions. However, the magnetic data reveal a spontaneous magnetisation at 5 K, indicating a canting of Mn2+ moments in the antiferromagnetic ground state. On heating (NH4)2[Mn3(P2O7)2(H2O)2] in water at 433 K under hydrothermal conditions, Mn5(HPO4)2(PO4)2·4H2O, synthetic hureaulite, is formed.