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 5$f$ orbitals of Pu and its six different phases extends to its \ensuremath{\delta}-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 \ensuremath{\delta} alloys, including a set of high purity \ensuremath{\delta} Pu${}_{1\ensuremath{-}x}$Ga${}_{x}$ materials with 1.7 \ensuremath{\le} $x$ \ensuremath{\le} 6.4 at. % Ga that span the low [Ga] portion of the \ensuremath{\delta} region of the phase diagram across the \ensuremath{\sim}3.3 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 \ensuremath{\sim}3.3 at. % Ga are heterogeneous and in addition to the \ensuremath{\delta} phase also contain up to \ensuremath{\sim}20% of a novel, coexisting ``\ensuremath{\sigma}'' 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 \ensuremath{\sigma} structure forms in Ga-depleted domains that result from the Ga atoms in the \ensuremath{\delta} phase self-organizing into a quasi-intermetallic with a stoichiometry of Pu${}_{25\ensuremath{-}35}$Ga so that \ensuremath{\delta} Pu-Ga is neither a random solid solution nor the more stable Pu${}_{3}$Ga + \ensuremath{\alpha}. Above this 3.3 at. % Ga nominal composition, the \ensuremath{\delta} Pu-Ga alloy is homogeneous, and no \ensuremath{\sigma} 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.