Synthesis, structure and magnetic characterisation of a new layered ammonium manganese(ii) diphosphate hydrate, (NH4)2[Mn3(P2O7)2(H2O)2]

Abstract

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 (M_r = 584.82, monoclinic, space group P2₁/a, a = 9.4610(8), b = 8.3565(7), c = 9.477(1) Å, β = 99.908(9)°, V = 738.07 ų, Z = 2, R = 0.0351 and R_w = 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 μ_eff = 5.74(1) μ_B 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.