Unexpected elastic softening in δ-plutonium

Elastic-constant measurements on a Pu-2.4 at. % Ga quasiisotropic polycrystal reveal remarkable elastic softening as temperature increases from ambient to 500 K. Unexpected softening appears in both the bulk modulus B and the shear modulus G, thus in all quasiisotropic elastic stiffnesses such as the Young modulus and the Lame constants. The dB/dT slope gives a (lattice) Gruneisen parameter gamma=3.7, much higher than a typical fcc-metallic-element value of 2.4 +/- 0.5. Especially, this high gamma from dB/dT measurements disagrees strongly with the gamma=-0.26 +/- 0.5 from volume measurements. The dB/dT slope exceeds that measured previously at lower temperatures. Also, it exceeds that expected from high-temperature Debye-Waller-factor measurements. A two-level model used successfully previously to interpret this alloy's unusually low thermal expansion also describes the large dB/dT. We comment on possible explanations for plutonium's odd anharmonic behavior. These concepts include magnetism, 5f-electron localization-delocalization, and vibrational entropy. Our measurements on the Pu-Ga polycrystal agree remarkably well with a Kroner-theory average of previous measurements on a same-composition monocrystal.