Phase stability and phase transformations in Pu–Ga alloys

Plutonium is of interest because of its nuclear properties. However, it is its electronic structure and the resulting physical and chemical properties that make plutonium the most complex element in the periodic table. It exhibits six solid allotropes at ambient pressure and is notoriously unstable with temperature, pressure, chemical additions, and time. It undergoes virtually every type of phase transformation known. The addition of a few atomic percent gallium helps retain the face-centered cubic δ-phase to room temperature, avoiding transformation to a monoclinic phase with a huge volume contraction. We present previously unpublished experimental studies on phase stability and phase transformations in Pu–Ga alloys and compare these to available literature. However, we first review the physics of plutonium and the fcc δ-phase to help us understand why plutonium defies conventional metallurgical wisdom. Plutonium sits near the middle of the actinide series, which marks the emergence of 5f electrons in the valence shell. Right at plutonium the 5f electrons are caught in an abrupt transition between being bonding and being localized (chemically inert). In fact, in the δ-phase they appear to be in a unique state of being neither fully bonding nor localized, which leads to novel electronic interactions and physical behavior. We compare these interactions to two other unstable elements in the periodic table, namely iron and cerium, to get a better appreciation of peculiarities of plutonium.