Crystal structure and magnetic properties of a new two-dimensional S=1 quantum spin system Ni5(TeO3)4X2 (X = Cl, Br)

Two new transition-metal tellurium oxy-chlorides of the general formula Ni-5(TeO3)(4)X-2 (X = Cl, Br) have been isolated during the investigation of the ternary phase diagram NiO-NiCl2-TeO2. They crystallize in the monoclinic s stem, space group C2/c, and for the case of Ni-5(TeO3)(4)Cl-2 the unit cell is a = 19.5674(2) Angstrom, b = 5.2457(1) Angstrom, and c = 16.3084(1) Angstrom, with beta = 125.289(1). The structure is layered and built up of corner-connected [Ni5O17X2] entities, made of five nickel(II) octahedra associated by edge and face sharing. The tellurium(IV) atoms are fixed upon the nickel layers. Their lone pairs E and the halogen atoms are packed in a double layer perpendicular to the [010] direction. From the magnetic point of view, this system provides a new 2D S = 1 quantum spin system with antiferromagnetic superexchange interaction. The magnetic susceptibility shows anomalies pointing to magnetic ordering phenomena. The observed transition temperatures vary with the interlayer separation.