In pure plutonium, the monoclinic α phase is stable at room temperature. However, the high temperature face-centered cubic δ phase can be stabilized at room temperature by adding a small amount of a so-called deltagen element such as Ga, Al, Am, or Ce. The radioactive decay of plutonium leads to the apparition of Am, U, He, and defects such as vacancies and interstitial defects. That is at the origin of some physical properties' evolution, such as swelling (elongation of material).
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