The Synthesis, Crystal Structures and Magnetic Properties of Cu4(AsO4)2(O) and Ba2Cu7(AsO4)6

Abstract Two new compounds Cu4(AsO4)2O (1) and Ba2Cu7As6O24 (2) were obtained from a mixture of CuO and As2O5 · xH2O heated to 1090°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. Compounds 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, μeff1.90–1.95 B.M./Cu at 25°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.