BACO2SI2O7 - A NEW ONE-DIMENSIONAL ANTIFERROMAGNET BASED ON CHAINS OFOXIDE BRIDGED COO4 TETRAHEDRA

The new compound BaCo2Si2O7 was obtained by heating the mixture BaCO3, CoCO3 and SiO2 to 1300-degrees-C for 48 h. The material was character ized by single-crystal X-ray diffraction and magnetic susceptibility m easurements. Each cobalt ion is surrounded by four oxygen atoms arrang ed in a distorted tetrahedral shape. These tetrahedra are linked into chains through the sharing of one of the oxygen atoms. The CoO4 chains are cross-linked by bridging Si2O7 groups. The barium ions occupy cha nnels that run parallel to the crystallographic c-axis. The effective magnetic moment, mu(eff) = 4.70 B.M., at 25-degrees-C is consistent wi th Co2+ in a tetrahedral environment having three unpaired electrons. Antiferromagnetic coupling of the electrons occurs between the nearest neighbour cobalt ions, as indicated by the sign of the Weiss constant , theta(p) = -46 K, and by the presence of a broad maximum in the susc eptibility curve. At 26 K it undergoes a three-dimensional magnetic tr ansition into a field-induced weak ferromagnet that is characteristic of a one-dimensional metamagnet.