Nanoscale heterogeneity, premartensitic nucleation, and a new plutonium structure in metastableδfcc Pu-Ga alloys

Abstract

The scientifically fascinating question of the spatial extent and bonding of the 5f orbitals of Pu and its six different phases extends to its δ-retained alloys and the mechanism by which Ga and a number of other unrelated elements stabilize its low density face-centered-cubic (fcc) structure. This issue of phase stability is also important technologically because of its significance to Science-Based Stockpile Stewardship. Answering these questions requires information on the local order and structure around the Ga and its effects on the Pu. We have addressed this by characterizing the structures of a large number of Pu-Ga and two Pu-In and one Pu-Ce δ alloys, including a set of high purity δ Pu 1−x Ga x materials with 1.7 ࣘ x ࣘ 6.4 at. % Ga that span the low [Ga] portion of the δ region of the phase diagram across the ß3.3 at. % Ga metastability boundary, with extended x-ray absorption fine structure (EXAFS) spectroscopy that probes the element specific local structure, supplemented by x-ray pair distribution function analysis that gives the total local structure to longer distances, and x-ray diffraction that gives the long-range average structure of the periodic component of the materials. Detailed analyses indicate that the alloys at and below a nominal composition of ß3.3 at. % Ga are heterogeneous and in addition to the δ phase also contain up to ß20% of a novel, coexisting "σ " structure for Pu that forms in nanometer scale domains that are locally depleted in Ga. The invariance of the Ga EXAFS with composition indicates that this σ structure forms in Ga-depleted domains that result from the Ga atoms in the δ phase self-organizing into a quasi-intermetallic with a stoichiometry of Pu 25−35 Ga so that δ Pu-Ga is neither a random solid solution nor the more stable Pu 3 Ga + α. Above this 3.3 at. % Ga nominal composition, the δ Pu-Ga alloy is homogeneous, and no σ phase is present. These results that demonstrate that collective and cooperative behavior in the interactions between the alloy elements as well as local elastic forces are crucial in determining the properties of complex materials and contradict the conventional mechanism for martensitic transformations, in this case indicating that nucleation is not the rate limiting step.

Effect of temperature and radiation damage on the local atomic structure of metallic plutonium and related compounds

“…(The observation of such deviations in doped-semiconductors was an early triumph of the EXAFS technique [78]). This work was expanded and clarified by Conradson et al [19], demonstrating that the Ga-Pu distance remains constant within 0.01 Å from 1.7 to 6.5 at % Ga. Likewise, and perhaps more surprisingly, the Pu-Pu distance also remains constant within 0.01 Å over the same Ga concentration range.…”

“…Likewise, and perhaps more surprisingly, the Pu-Pu distance also remains constant within 0.01 Å over the same Ga concentration range. Such considerations are threads that run through several other subsequent EXAFS studies of elemental Pu by Conradson et al [19,79] and Allen et al [80,81].…”

“…Over the subsequent decade, Conradson and co-workers performed multiple studies of different materials with a range of Ga concentrations and ages/doses, that is, a range of time since the sample's most recent anneal. Some of this earlier work is available [82][83][84][85], but the complexity of the problem required a larger study, and consequently was not published until 2014 [79]. In this latter work, δ-Pu samples with ages up to 45 years were studied, including a variety of Ga concentrations and 1.7 at % Ga samples enriched with 238 Pu to act as 'accelerated' aged samples due to the higher activity of 238 Pu relative to the more common 239 Pu isotope.…”

“…Pair-distribution function analysis of total scattering data is such a local structure technique [14], but has only been applied sparingly to understanding radiation damage (e.g. [15][16][17][18]), especially in Pu materials [19]. On the other hand the EXAFS technique has been applied much more consistently to materials damaged by irradiation for the last 35 years, including a reasonable number on Pu metal and intermetallic studies, and so this review focuses on EXAFS studies.…”

First-principles DFT +U calculations on the energetics of Ga in Pu, Pu2O3 and PuO2

“…It is worth pointing out that many experimental and theoretical studies have been conducted on the fine structures of Pu-Ga alloy to understand the roles of Ga in the stabilization mechanism of this alloy. The incorporation site of Ga, bonding interaction between Pu and Ga, and the potential phase transformations (i.e., from d phase alloy to a phase and other intermetallic compounds such as Pu 3 Ga) have been at least partially understood [1,2,[7][8][9]. Knowledge from oxidation behavior of Pu shows that the oxidation rate of Pu-Ga alloy can be generally slower than that of unalloyed Pu.…”