X-RAY AND NEUTRON-DIFFRACTION STUDIES OF UPDSN

Crystal structure and magnetic interactions in the ternary compound UP dSn have been investigated by means of X-ray and neutron diffraction m easurements. In accordance with previous reports, except for the antif erro-paramagnetic transition at T-N approximate to 40 K, there is also a magnetic phase transition at approximately 25 K, first noted in the magnetic susceptibility behaviour. The magnetic structure with decrea sing temperature is thought to be associated with the rotation of the magnetic moment away from both the z and y axes. The occurrence of the non-collinear magnetic structure implies a crystal structure distorti on of the hexagonal unit cell into an orthorhombic one at T-N, which b ecomes much stronger with decreasing temperature. X-ray diffraction pa tterns obtained at T = 23 and 10 K are in accordance with the assumpti on that another monoclinic distortion takes place in the low temperatu re region. This is consistent with Robinson et al. and our neutron dif fraction investigations. At T = 25 K a remarkable change in the value of the ordered moment of uranium, mu, has been observed. The value of mu at the lowest measured temperature 2 K was found to be 2.20(5)mu(B) , while at T = 30 K it decreases to 1.42(10)mu(B). At temperatures bel ow 30 K the ac-susceptibility points to the coexistence of long-range order with some kind of spin-glass-like behaviour.