Investigation of Co6(OH)3(TeO3)4(OH)~0.9(H20): Synthesis, crystal and magnetic structures, magnetic and dielectric properties

Abstract

We have re-investigated the synthesis and the crystal structure of the non-centrosymmetric phase Co6(OH)3(TeO3)4(OH) published by Perez et al. in 1975 and we have obtained by hydrothermal synthesis the hydrated phase Co6(OH)3(TeO3)4(OH)~0.9(H20). The crystal structure related to the Ellenbergerite mineral was determined using single-crystal X-ray diffraction data. The non-centrosymmetric hexagonal structure (P63mc; a = 13.1640(7) Å; c = 5.0321(6) Å) is based on [CoO6] octahedra and [TeO3] polyhedra which formed a tunnel framework with a formal charge 1+ balanced by one OH- group located in large tunnels. Compared to Co6(OH)3(TeO3)4(OH) case we succeeded in locating the hydrogen atoms of the OH- groups bound to the Co and showed that a small amount of water resides also in the large tunnels with the OH- groups. The presence of water molecules was confirmed by infrared spectroscopy and thermogravimetric analyses. This phase behaves as a canted antiferromagnet below TN = 75.5 K. The average magnetic structure was solved from powder neutron diffraction. Antisymmetric Dzyaloshinsky-Moriya interactions in the (ab) plane have been highlighted and must be responsible for the spin canted antiferromagnetic ground state. Moreover, DFT calculations with an Ising model show the presence of a frustrated spin state for the two Co2+ cations involved in the antisymmetric Dzyaloshinsky-Moriya interaction. Under an applied magnetic field, a metamagnetic-like behaviour is observed with three different canted antiferromagnetic states. At low temperature, dielectric measurements evidence a magneto-structural coupling but no measurable magnetocapacitance.