CU-4(ASO4)(2)(O) - A NEW COPPER ARSENATE WITH UNUSUAL LOW-TEMPERATUREMAGNETIC-PROPERTIES

The new copper oxide orthoarsenate, Cu-4(AsO4)(2)O, was obtained from a mixture CuO and As2O5 . xH(2)O in a BaCO3 heated to 1090 degrees C. The material contains two copper ions in distorted trigonal bipyramida l CuO5 geometries with a third copper ion in a distorted square pyrami dal CuO5 geometry. The free ''oxide'' ion is surrounded by four copper ions in a distorted tetrahedral arrangement. The copper ions form. ex tended two dimensional layers via a network of Cu-O-Cu bridges. The la yers are interconnected via Cu-O-As-O-Cu bridging units. Magnetic susc eptibility measurements show significant antiferromagnetic coupling of the unpaired electrons and an unusual transition into a weak ferromag netic state in the temperature range 15-25 K.