The effect of alloying on thermal displacements in uranium and plutonium alloys and the mechanism of stabilization of high-temperature phases by alloying

The effect of light alloying elements in heavy matrices (Mo and Nb in U; Al, Ga, and Ce in Pu) on the magnitude of mean-square thermal atomic displacements < u(2)> has been analyzed. It is shown that light atoms decrease < u(2)> due to the appearance of an additional line in the phonon spectrum at a frequency above the Debye frequency of the matrix. This results in a stabilization of high-temperature (cubic) polymorphic modifications that have high solubility of alloying elements. The proposed model of < u(2)> changes allows us to explain both the stability of gamma-uranium and delta-plutonium in alloys at room temperature and an increase in the melting temperature of these metals upon alloying.