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

Abstract

We present a statistical mechanical model for the anomalous thermodynamic properties of fcc δ-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 Θ and lattice Grüneisen constant γ the model includes two additional parameters, the energy spacing ΔE and an electronic Grüneisen constant Γ. ΔE is 1400 K, and Γ 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.

Acknowledgements

We are pleased to thank Jason Cooley, Olle Eriksson, Walt Harrison, Sig Hecker, John Joyce, Jon Lawrence, Anna Llobet-Megias, Amy Ross, Jim Smith, Per Söderlind and Melissa Sweeney for discussions, the use of figures or other essential contributions. This research is sponsored by the US Department of Energy National Nuclear Security Administration and the Office of Science. The work has benefited from the use of the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory and of the LANSCE at Los Alamos National Laboratory. IPNS is funded by the US Department of Energy, BES-Materials Science, under contract W-31-109-ENG-38, and LANSCE is funded by the US Department of Energy under contract W-7405-ENG-36.