Plutonium has a hidden ghost state that only appears at temperatures near absolute zero and vanishes the moment it warms up.
April 29, 2026
Original Paper
Amorphous High Density Plutonium
SSRN · 6659813
The Takeaway
Plutonium is already famous for being the most complex element, with six different crystal structures that change with heat. This research found a seventh state where the atoms are completely disorganized like a liquid, but frozen in place. This amorphous phase is invisible to standard X-ray scans because it has no regular pattern. It only exists at extreme cryogenic temperatures and explains why the metal's density changes in such a bizarre way. Discovering this hidden phase helps scientists better understand the volatile nature of nuclear materials.
From the abstract
Metastable aluminum-alloyed $\delta$-plutonium shrinks rapidly and pure $\alpha$-plutonium swells rapidly at 4 K. At ambient temperature alloyed $\delta$-plutonium slowly swells, but its bulk density decreases more slowly than would be inferred from the increase in its lattice parameter. 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 w