MAGNETOSTRUCTURAL DISTORTIONS IN THE NONCOLLINEAR HEXAGONAL ANTIFERROMAGNET UPDSN

The intermetallic compound UPdSn has been studied by means of high-res olution neutron powder diffraction at low temperature, and both orthor hombic and monoclinic structural distortions have been observed. In ad dition to confirming the previously published noncollinear antiferroma gnetic structures with these symmetries, full Rietveld refinements inc luding the magnetic cross sections have been made on the new data, joi ntly with older low-resolution data. The structure is monoclinic below 25 K with space group P2(1) and magnetic symmetry P(C)2(1), orthorhom bic between 25 and 40 K with structural space group Cmc2(1) and magnet ic space group P(C)m'c2(1), and paramagnetic above 40 K with space gro up P6(3)mc. The temperature variations of the orthorhombicity (b/squar e-root 3 - a) and monoclinicity (gamma - 90) parameters have been extr acted and the monoclinicity is linearly coupled to the x-component of the uranium magnetic moment mu(x), the 'magnetic monoclinicity' order parameter. In contrast, the orthorhombicity seems to be coupled to the total uranium magnetic moment. The sign of the coupling between mu(x) and gamma-90 has also been determined. It is positive, that is the pr ojected moments prefer to point across the shorter diagonal of the mon oclinic basal plane, rather than the longer diagonal.