Invar Model for δ-phase Pu: Thermal Expansion, Elastic, and Magnetic properties

We present a statistical mechanical model for the anomalous thermodynamic properties of FCC delta phase plutonium 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. Along with the usual Debye temperature and lattice Grüneisen constant, Θ and γ, the model includes two additional parameters, the energy spacing, ∆E, and an electronic Grüneisen constant, Γ. ∆E is 1400 K, and Γ is negative and depends strongly on Ga content. The model accounts satisfactorily for thermal expansion, and it accounts for the anomalous decrease of 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 dynamical mean-field theory (DMFT) for Pu. We found some evidence for the hypothetical two-level structure using inelastic neutron scattering.