Neutron diffraction experiments were performed on polycrystalline UI 3 in the temperature range from 1.3 to 293K in order to clarify anomalies which were observed in previous measurements. The present neutron results prove the existence of three-dimensional long-range antiferromagnetic order in UI 3 below T N =(2.8+or-0.1)K. The magnetic structure corresponds to k vector ( 1 / 2 , 1 / 2 , 1 / 2 ), and the magnetic moments of U 3+ are oriented parallel to a. Similarly to MnO the U moments of adjacent ferromagnetic (111) layers are aligned in opposite directions. The ordered saturation moment mu U =(1.98+or-0.05) mu B of U 3+ (5f 3 , 4 I 9.2 ) at 1.3K indicates a substantial crystal-field reduction. Considerable antiferromagnetic short-range order is observed at temperatures above T N . The parameters of the PuBr 3 structure of UI 3 were refined by means of the profile method both at 293 and 4.2K.
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Murasik et al. (1981) studied this question.