Neutron diffraction study of thermal parameters in UX₃ compounds

The intensities of the weak superlattice reflections in the UX₃ compounds, where X = Ru, Rh, Ge, and Sn, have been measured by neutron diffraction. These compounds have the ordered AuCu₃ structure. Excellent fits to the observed intensities are obtained by introducing anisotropic second-order and fourth-order (anharmonic) terms to describe the probability distribution function at the X atom site. The ratio of the magnitude of the mean-square thermal vibration parallel to and perpendicular to the unique tetragonal axis of the X atom appears to depend on whether p bonding (UGe₃, USn₃) or d bonding (URu₃, URh₃) occurs with the U electrons. In UGe₃ data out to Q = 13 Å^-1 show the need to include a fourth-order anharmonic term. The form of this anharmonicity suggests an attractive potential between the nearest- neighbor U-X atoms.