Hydrothermal synthesis, X-ray structure and complex magnetic behaviour of Ba4(C2O4)Cl2[[Fe(C2O4)(OH)]4].

Hydrothermal reaction of iron(III) chloride, barium chloride and sodium oxalate in a narrow stoichiometry range produces the title compound Ba4(C2O4)Cl2[[Fe(C2O4)(OH)]4] (1). This new iron(II) oxalate crystallises in the tetragonal space group P42/mnm: a = 13.811(3), c = 7.026(2) A. The structure consists of parallel chains of mu2-hydroxy-bridged iron(II) ions. These are connected by bridging oxalates to form an anionic framework with large channels that contain the remaining barium, chloride and oxalate counter ions. Magnetisation studies on an oriented single crystal of 1 revealed a magnetic phase transition at 32 K and a strong easy-plane anisotropy at all temperatures. Above Tc the compound behaves as an S = 2XY antiferromagnetic chain, showing a broad maximum in the susceptibility at about 70 K. We determined the intrachain coupling J and the interchain coupling J' to be -7 cm(-1) and +0.4 cm(-1), respectively. The low-temperature phase is an ordered antiferromagnetic state. Zero- and longitudinal-field muon spin-rotation/relaxation studies support this interpretation; below Tc oscillations in the muon spin-autocorrelation function are observed giving unambiguous evidence for a non-zero sublattice magnetisation and proof of a long-range magnetically ordered state.