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 (TeO 3 ) 4 X 2 (X = Cl, Br) have been isolated during the investigation of the ternary phase diagram NiO−NiCl 2 −TeO 2 . They crystallize in the monoclinic system, space group C 2/ c, and for the case of Ni 5 (TeO 3 ) 4 Cl 2 the unit cell is a = 19.5674(2) Å, b = 5.2457(1) Å, and c = 16.3084(1) Å, with β = 125.289(1)°. The structure is layered and built up of corner-connected [Ni 5 O 17 X 2 ] 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.