Amorphous High Density Plutonium

Abstract

Metastable aluminum-alloyed $\delta$-plutonium shrinks rapidly and pure $\alpha$-plutonium swells rapidly at 4 K. At ambient temperature alloyed $\delta$-plutonium swells about $10^{-3}$ as fast as it shrinks at 4 K, but its bulk density decreases more slowly than would be inferred from the increase in its lattice parameter determined by X-ray diffraction. These results might be explained as the result of ingrowth of the opposite phases, but they have not been found in X-ray diffraction. The cryogenic results may be explained by ingrowth of an amorphous phase with density intermediate between those of $\alpha$ and $\delta$. This phase, when formed from alloyed $\delta$-plutonium, rapidly but not instantaneously anneals to $\delta$-plutonium at temperatures $\gtrapprox 100\,$K; when formed from pure $\alpha$-plutonium it anneals to $\alpha$ at similar temperatures. The room temperature discrepancy between growth of lattice parameter and length of $\delta$-plutonium is also explained by ingrowth of a denser amorphous phase that is continuously formed and annealed out.