THE ANTIFERROMAGNETIC STRUCTURE OF COPPER TUNGSTATE, CUWO4

Copper tungstate becomes antiferromagnetically ordered below 23.0(2) K and its magnetic structure has been determined from single-crystal unpolarized neutron diffraction measurements at 5 K. The chemical unit cell is triclinic P1BAR, with a = 4.694(1), b = 5.830(1), c = 4.877(1) angstrom, alpha = 91.64(1), beta = 92.41(2) and tau = 82.91(1)-degrees at 15 K. The magnetic propagation vector is (1/200) and the magnetic space group P2a1BAR. The two equivalent copper ions within the unit cell have magnetic moments of 0.67(1) mu-B aligned ferromagnetically at the spherical polar angles THETA = 121(2)-degrees and phi = 52(2)-degrees, the polar axis being parallel to c and phi being measured from the c-a* plane. This direction coincides, within experimental error, with the axis of elongation of the Jahn-Teller distorted octahedron of oxygen atoms about the Cu2+ ion. A multipole refinement of the moment distribution, to order two on quantum axes Z parallel to the moment direction and X, Y directed towards the four close oxygen neighbours, shows that Y20 is the only significant multipole. Its sign indicates that the moment distribution approximates to an oblate ellipsoid of revolution with its axis parallel to Z. Only two of the six oxygen neighbours of the Cu2+ ion carry a significant transferred moment of 0.06(1) mu-B. These observations are related to the exchange paths and covalency in the compound.