Poisson’s ratio: key to electron localization in plutonium

Focusing on plutonium, we show that a familiar usually-ignored elastic parameter---the dimensionless Poisson ratio ($\nu )$---helps understand a knotty unsolved problem: itinerant-localized 5f electrons. A simple electrostatic electron-gas model predicts that the bulk modulus $B$ and shear modulus $G$ decrease as electrons change from itinerant to localized (the free-electron-gas density decreases), but that the Poisson ratio remains unchanged. From the bulk modulus, the model predicts approximate divalency for plutonium, implying a 5$f^{4}$ localized-electron configuration. We deduce that warming plutonium from 0 to 300 K causes 0.24 electrons/atom to change from itinerant state to localized. Sufficient alloying with Ga or Al suppresses the itinerant---localized transition, causing the Poisson ratio to show near-typical temperature dependence. This work was carried out at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation, the State of Florida and DOE.