![]() This collective and cooperative behavior involving the Ga and other defects in terms of a tendency to aggregate into domains with structures that differ from the δ host and the resulting nanoscale heterogeneity also appears to play an important role in the observation of analogous locally ordered structures in aged materials. We find that metastable alloys contain tens of percents of a novel, “σ”, Pu structure that we attribute to rearrangement of the Ga-depleted regions after the self-organization of the Ga to form quasi-intermetallic Pu25–35Ga. X-ray diffraction that gives the long-range average structure of the periodic component of the materials and pair distribution functions analysis and X-ray absorption fine structure that give the overall and the element specific local structure have been used to examine a variety of new and aged materials, including a set of high purity δ Pu1–xGax alloys with 1.7 ≤ x ≤ 6.4 atom % Ga that span the low portion of the δ region of the phase diagram across the 3.3 atom % Ga metastability boundary, a 1.7 atom % Ga alloy that was enriched with Pu238 to accelerate the aging process, and others. The local order and structure and the role of the Ga are crucial to understanding the phase stability and the aging effects. Δ Pu–Ga alloys and their response to self-irradiation are important scientifically because of the unique complexity of Pu and technologically because of their importance in Science Based Stockpile Stewardship. These findings also pose the question of whether these behaviors are specific to Pu or are instead an attribute common to the broader class of complex functional materials such as stripes in mixed valence correlated transition metal oxides, 20,96−100 most of which also display inhomogeneities in the form of bulk compositions and/or charge distributions at variance with the stoichiometries of their unit cells. 8,9,27, 28 Measurements on a defined set of laboratory-prepared materials made from ultrapure Pu now unequivocally demonstrate this heterogeneity in newly prepared metastable alloys while results from aged samples demonstrate the importance of the heterogeneity in aging processes in Pu. It is thus consistent with Pu alloys exhibiting unusual structural behavior of this type, including the possibilities of coexisting ordered structures in δ-stabilized Pu−Ga and enhanced homogeneity on aging that have been suggested based on earlier extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray pair distribution function (pdf) measurements.
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