Vibrational order in Pu0.98Ga0.02

In an earlier paper (Lawson et al., 2000, Phil. Mag. B, 80, 53) we describe d the measurement and interpretation of Debye-Waller factors in a delta -ph ase Pu0.98Ga0.02 alloy. We concluded that the measurements could be describ ed using a Debye model with a strongly temperature-dependent Debye-Waller t emperature. In those experiments, we observed significant diffuse scatterin g in the neutron powder diffraction patterns. In this paper we describe a n ew technique for analysing diffuse scattering based on Rietveld analysis, a nd apply the technique to data for F-phase Pu0.98Ga0.02 and for Pb. The res ults from the correlations among lattice vibrations show that the diffuse s cattering is Warren-Borie scattering and that the diffuse scattering is in basic agreement with that expected from the Debye model except that the sca ttering from the nearest-neighbour correlations in Pu0.98Ga0.02 is anomalou s at high temperatures. We also discuss the observation of significant anis otropic broadening of the diffraction peaks for Pu0.98Ga0.02 at low tempera tures.