Understanding the phonon spectrum of plutonium

The recently observed vibration spectrum of face-centered-cubic plutonium is interpreted in terms of a simple inclusion of electron correlations in a Friedel Model of the density of f-electron states, and an empty-core pseudopotential treatment of the three free electrons per atom. This model has given reasonable accounts of the volume-dependence of the total energy and of the experimental density of electronic states. It yields a volume-dependent energy, plus a two-body central-force interaction V(d) between atoms, which enters the vibration spectrum only through the two parameters, V-(1)=dpartial derivativeV/partial derivatived and V-(2)=d(2)partial derivative(2)V/partial derivatived(2), evaluated at the nearest-neighbor distance. Using existing parameters yields the three elastic constants in crude accord with experiment but peak vibrational frequencies too low and an instability against shear modes in the [111] and [110] directions. Moderate scaling of the two parameters brings the vibration spectrum into reasonable accord. It also produces the strong fluctuation in the [110] T-1 mode, suggesting it is not from Kohn anomalies, which are absent from the model.