PHYSICAL REVIEW B 89, 224102 (2014) Nanoscale heterogeneity, premartensitic nucleation, and a new plutonium structure in metastable δ fcc Pu-Ga alloys Steven D. Conradson, 1 , * Nicolas Bock, 2 Julio M. Castro, 3 Dylan R. Conradson, 2 Lawrence E. Cox, 4 Wojciech Dmowski, 5 David E. Dooley, 4 Takeshi Egami, 5 Francisco J. Espinosa-Faller, 6 Franz J. Freibert, 1 Angel J. Garcia-Adeva, 7 Nancy J. Hess, 8 Erik K. Holmstr ¨ om, 2 Rafael C. Howell, 1 Barbara Katz, 4 Jason C. Lashley, 1 Raymond J. Martinez, 1 David P. Moore, 1 Luis A. Morales, 1 J. David Olivas, 1 Ramiro A. Pereyra, 4 Michael Ramos, 1 Sven P. Rudin, 2 and Phillip M. Villella 1 1 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 2 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 3 Health, Safety, Radiation Protection Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 4 Nuclear Materials Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 5 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA 6 Departamento de F´ ısica para Ingenieros, Universidad Marista de Merida, Merida, Yucatan 97300, Mexico 7 Departamento de Fisica Aplicada I, E.T.S. Ingenieria de Bilbao, Universidad del Pais Vasco, Alda. Urquijo s/n, 48013 Bilbao, Spain 8 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA (Received 14 January 2014; revised manuscript received 13 May 2014; published 18 June 2014) 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 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 3.3 at. % Ga are heterogeneous and in addition to the δ phase also contain up to 20% 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. DOI: 10.1103/PhysRevB.89.224102 PACS number(s): 61.05.cj, 61.05.cf , 61.46.−w, 61.66.Dk I. INTRODUCTION Plutonium (Pu) is by consensus the most astonishing member of the actinides [1–6], the class of the elements in which the 5f electron shell is progressively filled. The heavier members of the actinide series (Am, Cm, and beyond) have larger atomic volumes that are almost independent of the 5f electron population. This behavior resembles those of the lanthanide elements; the 5f states are localized and do not participate in the bonding. In contrast, in the early part of this series (Th, Pa, U, and Np), the spatially extended 5f electrons contribute to the bonding between atoms to give high density materials with short interatomic distances. The 5f participation in bonding results in an atomic volume dependence on electron population similar to that of the transition metal series. In Pu, the 5f electrons are “on the edge” * Corresponding author: conradson@lanl.gov [3–5,7–15], and it is this unique 5f configuration that gives this element a host of unusual properties. Since the discovery of Pu in 1941, the element’s eccentricities have both awed and perplexed researchers [2,16]. Understanding its properties is indeed critical for the safe handling, use [17,18], and long-term storage of this highly toxic, radioactive, but technologically important material [2,16]. The complex and often unique properties of Pu have been well documented [19], especially with its recent renaissance in which the technology drivers of stockpile stewardship [2,16– 18] have stimulated intense investigation of the scientific issues [2–6]. The pure metal exhibits six solid-state phases with large volume expansions and subsequent contractions along the way to melting at a relatively low 650 °C to yield a higher density liquid than that of the solid from which it derives [2,4,19]: α (monoclinic) → β (monoclinic) → γ (orthorhombic) → δ (face-centered-cubic [fcc]) → δ ′ (face-centered-tetragonal [fct]) → ε (body-centered-cubic [bcc]) → liquid. There have also, however, been reports and suggestions of nanophase 1098-0121/2014/89(22)/224102(22) 224102-1 ©2014 American Physical Society