Atomic thermal vibrations of the light actinide elements

We have been using time-of-flight neutron powder diffraction to determine the Debye-Waller factors of the light actinide elements. The Debye-Waller factor is a measure of the mean-square atomic displacement that arises from the thermal motion of the atoms in any solid. Its temperature dependence determines a Debye-Waller temperature, ΘDW, that is characteristic of the elastic properties of the solid. The data are obtained by Rietveld analysis of neutron diffraction powder patterns obtained at several temperatures. We have analyzed the results for α-U, α-Np, α-Pu and γ-Pu0.95Al0.05. The measured temperature dependence of the ΘDW's can be interpreted with an ''effective'' Groneisen constant, whose numerical values are much too high to be explained with harmonic forces