ATOMIC THERMAL VIBRATIONS OF THE LIGHT ACTINIDE ELEMENTS

We have been using time-of-flight neutron powder diffraction to determ ine the Debye-Waller factors of the light actinide elements. The Debye -Waller factor is a measure of the mean-square atomic displacement tha t arises from the thermal motion of the atoms in any solid. Its temper ature dependence determines a Debye-Waller temperature Theta(DW) 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 alp ha-U, alpha-Np, alpha-Pu and delta-Pu0.95Al0.05. The measured temperat ure dependence of the Theta(DW) can be interpreted with an ''effective '' Grueisen constant, whose numerical values are much too high to be e xplained with harmonic forces.