  Two new transition-metal tellurium oxy-chlorides of the general formula
Ni_5(TeO_3)_4X_2 (X=Cl, Br) have been isolated during the investigation of the
ternary phase diagram NiO-NiCl2-TeO2. They crystallize in the monoclinic
system, space group C2/c, and for the case of Ni_5(TeO_3)_4Cl_2 the unit cell
is a=19.5674(2) A, b=5.2457(1) A, c=16.3084(1) A, with beta=125.289(1). The
structure is layered and built up of corner connected [Ni_5O_17X_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.
