Invar model for delta-phase Pu: thermal expansion, elastic and magnetic properties

摘要

We present a statistical mechanical model for the anomalous thermodynamic properties of fcc delta-phase Pu and Pu-Ga alloys. The model is based on the addition of a two-level 'Invar'-like electronic energy structure to the ordinary Debye model for the lattice, there is no assumption of magnetic character of these electronic levels. Together with the usual Debye temperature Theta and lattice Gruneisen constant gamma the model includes two additional parameters, the energy spacing Delta E and an electronic Gruneisen constant Gamma. Delta E is 1400 K, and Gamma is negative and depends strongly on the Ga content. The model accounts satisfactorily for thermal expansion, and it accounts for the anomalous decrease in the bulk modulus with temperature provided that one assumes zero elastic stiffness for the excited electronic state. This assumption is consistent with the results of the dynamic mean-field theory for Pu. We found some evidence for the hypothetical two-level structure using inelastic neutron scattering.