Neutron diffraction study of U 0.5Np 0.5O 2

Abstract

A neutron powder diffraction study of U 0.5Np 0.5O 2 has been performed recently at the D2 spectrometer of the ILL high flux reactor. Neutron diffraction patterns were recorded for the temperature range 3 ≤ T ≤ K. This compound crystallizes in the CaF 2type structure. The lattice parameter has been found to be a = 5.4624(3) Å in the temperature range 3 ≤ T ≤ 67 K. A magnetic ordering was observed below T N≅ 12 K, in agreement with the previous Mössbauer experiments [1]. Surprisingly the observed magnetic order is different from the one previously found in UO 2 [2]. Actually only one weak magnetic reflection quite near to the 1 2 , 1 2 , 1 2 line referred to the nuclear unit cell was observed; the high Miller indice reflections are probably too small due to the magnetic form factor to be seen. The magnetic order is compatible with the ordered moment −0.6μ B per Np atom - observed by Mössbauer resonance. A striking fact is the large width of the magnetic reflection which decreases only slightly with T remaining higher by a factor of about 5 as compared to the width of the 1,1,1 nuclear reflection; this fact is probably to be attributed to a short range of the magnetic order (a correlation length of about 20 Å has been found). Moreover clustering effects of Np and U ions should be excluded.