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The charged surfaces of micaceous minerals, especially illite, regulate the mobility of the major radioisotopes of Cs (134Cs, 135Cs, 137Cs) 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.

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.

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.

Results are reported of a systematic atomic-scale computational analysis of strain relaxation mechanisms and the associated defect dynamics in nanometer-scale thin or ultrathin Cu films that are subjected to a broad range of biaxial tensile strains. The films contain pre-existing voids and the film planes are oriented normal to the [formula] crystallographic direction. The analysis is based on isothermal-isostrain molecular-dynamics simulations according to an embedded-atom-method parameterization for Cu and employing multimillion-atom slab supercells. In addition to an initial elastic response for an applied biaxial strain level [variant_greek_epsilon]<2%, our analysis reveals three regimes in the thin-film mechanical response as [variant_greek_epsilon] increases. For 2%≤[variant_greek_epsilon]≤6%, biaxial strain relaxation is dominated by emission and propagation of dislocations (plastic flow) from the surface of the void accompanied by ductile void growth. For 6%<[variant_greek_epsilon]<10%, the biaxial strain in the thin film is relaxed by both ductile void growth and emission of dislocations from the surfaces of the thin film. For [variant_greek_epsilon]≥10%, strain relaxation is dominated by dislocation emission from the surfaces of the thin film, leading to a structural transformation from the face-centered cubic to a hexagonal close-packed phase. The defect nucleation mechanisms and the high-strain response of the thin films are found to be significantly different from those observed in <111>-oriented Cu thin films [M. R. Gungor and D. Maroudas, J. Appl. Phys. 97, 113527 (2005); M. R. Gungor and D. Maroudas, Appl. Phys. Lett. 87, 171913 (2005)]. [ABSTRACT FROM AUTHOR]

We report results of large-scale molecular-dynamics simulations of dynamic deformation under biaxial tensile strain of prestrained single-crystalline nanometer-scale-thick face-centered cubic (fcc) copper films. Our results show that stacking faults, which are abundantly present in fcc metals, may play a significant role in the dissociation, cross slip, and eventual annihilation of dislocations in small-volume structures of fcc metals. The underlying mechanisms are mediated by interactions within and between extended dislocations that lead to annihilation of Shockley partial dislocations or formation of perfect dislocations. Our findings demonstrate dislocation starvation in small-volume structures with ultrathin film geometry, governed by a mechanism other than dislocation escape to free surfaces, and underline the significant role of geometry in determining the mechanical response of metallic small-volume structures. [ABSTRACT FROM AUTHOR]

We report results of a detailed systematic computational analysis of strain relaxation mechanisms and the associated defect dynamics in ultrathin, i.e., a few nanometers thick, Cu films subjected to a broad range of biaxial tensile strains. The analysis is based on isothermal-isostrain molecular-dynamics simulations of the response of Cu films that are oriented normal to the [111] crystallographic direction using an embedded-atom-method parametrization for Cu and multimillion-atom slab supercells. Our analysis reveals five regimes in the thin film’s mechanical response with increasing strain. Within the considered strain range, after an elastic response up to a biaxial strain level [variant_greek_epsilon]=5.5%, the strain in the metallic thin film is relaxed by plastic deformation. At low levels of the applied biaxial strain above the yield strain ([variant_greek_epsilon]∼6%), threading dislocation nucleation at the surface of the thin film in conjunction with vacancy cluster formation in the film leads eventually to the formation of voids that extend across the thickness of the film. For 6%<[variant_greek_epsilon]<8%, dislocations are emitted uniformly from the thin-film surface, inhibiting the nucleation of voids. For [variant_greek_epsilon]≥8%, in addition to nucleation of dislocations from the film surface, dislocation loops are generated in the bulk of the film and grow to intersect the thin-film surface. For [variant_greek_epsilon]≥10%, a high density of point defects in the film leads to nucleation of Frank partial dislocations that dissociate to form stacking fault tetrahedra. In addition, dislocation-dislocation interactions due to the high dislocation density lead to the formation of Lomer–Cottrell dislocation locks and complex stable dislocation junctions that act as obstacles to dislocation glide. As a result of these defect mechanisms, nanoscale domains are formed in the crystalline film with an average domain size of 1.5 nm and low-angle misorientations. [ABSTRACT FROM AUTHOR]

The study results of a systematic atomic-scale computational analysis of strain relaxation mechanisms and the related defect dynamics in nanometer-scale thin or ultrathin Cu films that are subjected to a broad range of biaxial tensile strains are reported. The defect nucleation mechanisms and the high-strain response of the thin films are found to be significantly different from those observed in -oriented Cu thin films.

We report the results of a theoretical analysis for the kinetics of strain relaxation in Si1-xGex thin films grown epitaxially on Si(100) substrates. The analysis is based on a properly parametrized dislocation mean-field theoretical model describing plastic deformation dynamics due to threading dislocation propagation and addresses strain relaxation kinetics during both epitaxial growth and thermal annealing, including post-implantation annealing. Theoretical predictions for strain relaxation as a function of film thickness in Si0.80Ge0.20/Si(100) samples annealed after epitaxial growth either unimplanted or after He ion implantation are in excellent agreement with experimental measurements [J. Cai et al., J. Appl. Phys. 95, 5347 (2004)]. [ABSTRACT FROM AUTHOR]