The synthesis, crystal structures and magnetic properties of Cu-4(AsO4)(2)(O) and Ba2Cu7(AsO4)(6)

Two new compounds Cu-4(AsO4)(2)O (1) and Ba2Cu7As6O24 (2) were obtained from a mixture of CuO and As2O5 . xH(2)O heated to 1090 degrees C for 72 h in a BaCO3 flux. Compound 1 was obtained as dark green crystals, belonging to the orthorhombic crystal system. Compound 2 was obtained as light gray to colorless crystals belonging to the triclinic crystal system. The structure of 1 contains two copper ions with distorted trigonal bipyramidal geometries and a third with a distorted square pyramidal geometry. One ''oxide'' ion is surrounded by four copper ions in a distorted tetrahedral arrangement. Compound 2 consists of two chains with alternating units of trigonal bipyramidal CuO5 and square planar CuO4 units, with the oxygen atoms derived from the AsO4 groups. Magnetic susceptibility measurements of 1 and 2 both show effective magnetic moments, mu(eff) = 1.90-1.95 B.M./Cu at 25 degrees C, which is consistent with Cu2+ ions having one unpaired electron. Significant antiferromagnetic coupling was revealed by variable temperature measurements. At very low temperatures (15-25 K) samples of both 1 and 2 undergo a transition to a weak ferromagnetic state.