Neutron diffraction studies of the actinide oxides I. Uranium dioxide and thorium dioxide at room temperature

Three-dimensional neutron structure factors have been measured on single crystals grown from the melt and from solution. The Bragg reflexions divide into three groups, strong, medium and weak: the strong and medium reflexions are affected by extinction, which is particularly severe for solution-grown crystals, and the weak reflexions by double Bragg scattering. Least-squares analysis of experimental data corrected for these systematic errors confirms that the oxides have the fluorite structure at room temperature and gives precise values for the Debye-Waller factors of the heavy-metal and oxygen atoms and for the ratios of their nuclear coherent scattering amplitudes.