Plutonium Elastic Moduli, Electron Localization, and Temperature

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1104-NN08-07

Plutonium Elastic Moduli, Electron Localization, and Temperature Albert Migliori, Izabella Mihut-Stroe, and Jon B. Betts National High Magnetic Field Lab, Los Alamos National Laboratory, MS E 536, Los Alamos, NM, 87545 ABSTRACT In almost all materials, compression is accompanied naturally by stiffening. Even in materials with zero or negative thermal expansion, where warming is accompanied by volume contraction it is the volume change that primarily controls elastic stiffness. Not so in the metal plutonium. In plutonium, alloying with gallium can change the sign of thermal expansion, but for the positivethermal-expansion monoclinic phase as well as the face-centered-cubic phase with either sign of thermal expansion, and the orthorhombic phase, recent measurements of elastic moduli show soften on warming by an order of magnitude more than expected, the shear and compressional moduli track, and volume seems irrelevant. These effects point toward a novel mechanism for electron localization, and have important implication for the pressure dependence of the bulk compressibility. INTRODUCTION An analysis and discussion of the temperature dependence of the elastic moduli of α, γ, and δ plutonium with implications for electron localization based on this study of the moduli of pure γPu and previous studies of α-Pu and gallium-stabilized δ-Pu is presented. Resonant ultrasound spectroscopy (RUS) measurements of the elastic moduli of α−Pu [1] were made previously on very-high-purity, very-low-inclusion 99.96 wt. % electrorefined fine-grained polycrystal239Pu. δ-Pu moduli were measured previously on a 239Pu 2.36 at. % Ga polycrystal [1]. Here previous results are discussed in light of new γ-Pu measurements briefly described here, made on the same specimen as used previously [Error! Bookmark not defined.]. EXPERIMENT The bulk B and shear G elastic moduli for all three phases are plotted in Fig. 1. Note that although the α−Pu and γ−Pu measurements were made on the same specimen during the same run by warming at about 2K/h, some curious behavior was noted. The resonances obtained for the α−Pu phase

were sharp and low in noise, enabling very accurate determination of elastic moduli. As we warmed into the β−Pu phase, the transition was about 1K wide, and the resonances essentially smeared out and disappeared. No elastic moduli could be obtained. Upon continued warming into the γ−Pu phase, again, with a 1K-wide transition, sharp resonances reappeared and we were able to obtain moduli again. DISCUSSION Most of the elastic moduli of these Pu allotropes that we measured exhibit extreme softening on warming by an order of magnitude more than expected for ordinary metals with similar Debye temperatures (the exception is the bulk modulus of γ-Pu). Surprisingly, the fractional softening (dlnCij/dT) of the shear moduli are nearly the same for all three phases, while the fractional softening of the bulk moduli of the δ and α phase are the same as the shear moduli, but not for γ. That this fractional softening is nearly the same