Helimagnetic order in ferric arsenate, FeAsO4

High-resolution time of flight neutron powder diffractometry has been used to determine the antiferromagnetic structure of ferric arsenate at 4.2 K. T he structure is incommensurate with a magnetic propagation vector (0.04994( 3), 0, -0.24725(2)) in the a*-c* plane of the monoclinic unit cell. The spi ns of 4.01(2) mu(B) On the Fe3+ ions rotate in a plane containing b* and ma king an angle of 34.1(7)degrees to [001] in beta obtuse. The structure show s that the dominant antiferromagnetic coupling is between Fe3+ ions which a re linked through tetrahedral arsenate groups giving a frustrated exchange network.