The charged surfaces of micaceous minerals, especially illite, regulate the mobility of the major radioisotopes of Cs (Cs, Cs,Cs) in the geosphere. Despite the long history of Cs adsorption studies, the nature of the illite surface sites remains incompletely understood. To address this problem, we present atomistic simulations of Cs competition with Na for three candidate illite adsorption sites - edge, basal plane, and interlayer. Our simulation results are broadly consistent with affinities and selectivities that have been inferred from surface complexation models. Cation exchange on the basal planes is thermodynamically ideal, but exchange on edge surfaces and within interlayers shows complex, thermodynamically non-ideal behavior. The basal planes are weakly Cs-selective, while edges and interlayers have much higher affinity for Cs. The dynamics of NaCs exchange are rapid for both cations on the basal planes, but considerably slower for Cs localized on edge surfaces. In addition to new insights into Cs adsorption and exchange with Na on illite, we report the development of a methodology capable of simulating fully-flexible clay mineral nanoparticles with stable edge surfaces using a well-tested interatomic potential model.

Using atomistic modeling, we show that restructuring of the network of interconnected ligaments causes coarsening in a model of nanoporous gold. The restructuring arises from the collapse of some ligaments onto neighboring ones and is enabled by localized plasticity at ligaments and nodes. This mechanism may explain the occurrence of enclosed voids and reduction in volume in nanoporous metals during their synthesis. An expression is developed for the critical ligament radius below which coarsening by network restructuring may occur spontaneously, setting a lower limit to the ligament dimensions of nanofoams.

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K and has high affinity for Cs. Although exchange of interlayer K for Cs is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs-K exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs for the smaller K significantly lowers the migration barrier of neighboring K, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.

Atomistic simulations are used to study the formation, migration, and clustering of delocalized vacancies and interstitials at a model fcc-bcc semicoherent interface formed by adjacent layers of Cu and Nb. These defects migrate between interfacial trapping sites through a multi-step mechanism that may be described using dislocation mechanics. Similar mechanisms operate in the formation, migration, and dissociation of interfacial point defect clusters. Effective migration rates may be computed using the harmonic approximation of transition state theory with a temperature dependent prefactor. Our results demonstrate that delocalized vacancies and interstitials at some interfaces may be viewed as genuine defects, albeit governed by mechanisms of higher complexity than conventional point defects in crystalline solids.2

Vacancies and interstitials absorbed at Cu-Nb interfaces are shown to migrate by a multistage process involving the thermally-activated formation, motion, and annihilation of kinks and jogs on interface misfit dislocations. This mechanism, including the energy along the entire migration path, can be described quantitatively within dislocation theory, suggesting that analysis of misfit dislocation networks may enable prediction of point defect behaviors at semicoherent heterointerfaces.

Patterning is a familiar approach for imparting novel functionalities to free surfaces. We extend the patterning paradigm to interfaces between crystalline solids. Many interfaces have non-uniform internal structures comprised of misfit dislocations, which in turn govern interface properties. We develop and validate a computational strategy for designing interfaces with controlled misfit dislocation patterns by tailoring interface crystallography and composition. Our approach relies on a novel method for predicting the internal structure of interfaces: rather than obtaining it from resource-intensive atomistic simulations, we compute it using an efficient reduced order model based on anisotropic elasticity theory. Moreover, our strategy incorporates interface synthesis as a constraint on the design process. As an illustration, we apply our approach to the design of interfaces with rapid, 1-D point defect diffusion. Patterned interfaces may be integrated into the microstructure of composite materials, markedly improving performance.

We report results of molecular dynamics simulations that reveal fcc→ hcp martensitic transformations in biaxially strained ultrathin films of face-centered cubic metals. We find that martensites nucleate at the surface and grow into the bulk of the film due to dislocation glide; in this process, the magnitudes of the relative atomic slip displacements are identical to those proposed for Bain transformations. Mechanical stability analysis shows that the onset of the phase transformation is consistent with the onset of a shearing instability of the thin film.

Interfaces, grain boundaries, and dislocations are known to have significant impact on the transport properties of materials. Even so, it is still not clear how the structure of interfaces influences the mobility and concentration of carriers that are responsible for transport. Using low angle twist grain boundaries in MgO as a model system, we examine the structural and kinetic properties of vacancies. These boundaries are characterized by a network of screw dislocations. Vacancies of both types, Mg and O, are strongly attracted to the dislocation network, residing preferentially at the misfit dislocation intersections (MDIs). However, the vacancies can lower their energy by splitting into two parts, which then repel each other along the dislocation line between two MDIs, further lowering their energy. This dissociated structure has important consequences for transport, as the free energy of the dissociated vacancies decreases with decreasing twist angle, leading to an increase in the net migration barrier for diffusion as revealed by molecular dynamics simulations. Similar behavior is observed in BaO and NaCl, highlighting the generality of the behavior. Finally, we analyze the structure of the dissociated vacancies as a pair of jogs on the dislocation and construct a model containing electrostatic and elastic contributions that qualitatively describe the energetics of the dissociated vacancy. Our results represent the first validation of a mechanism for vacancy dissociation on screw dislocations in ionic materials first discussed by Thomson and Balluffi in 1962.