Positron annihilation spectroscopy of the equilibrium vacancy ensemble in aluminium

Fluss, M. J.; Berko, S.; Chakraborty, Bulbul; Hoffmann, K; Lippel, P. H.; Siegel, Richard W.

doi:10.1088/0305-4608/14/12/008

Cited by 34 publications

(20 citation statements)

References 32 publications

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“…This agrees with previous DFT calculations which also found a similar negative binding energy, -0.05 eV using 15 LDA and -0.08 eV using 7 GGA (PW91). Thus the disagreement between previous calculations and the original interpretation of experimental data 11,16 , which gave positive binding, is likely not a result of DFT approximations. Our results are consistent with the reinterpretation 7 of the data.…”

Section: Defects In Aluminummentioning

confidence: 63%

Diffusion quantum Monte Carlo study of the equation of state and point defects in aluminum

2012

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The many-body diffusion quantum Monte Carlo (DMC) method with twist-averaged boundary conditions is used to calculate the ground-state equation of state and the energetics of point defects in fcc aluminum, using supercells up to 1331 atoms. The DMC equilibrium lattice constant differs from experiment by 0.008Å or 0.2%, while the cohesive energy using DMC with backflow wave functions with improved nodal surfaces differs by 27 meV. DMC calculated defect formation and migration energies agree with available experimental data, except for the nearest-neighbor divacancy, which is found to be energetically unstable in agreement with previous density functional theory (DFT) calculations. DMC and DFT calculations of vacancy defects are in reasonably close agreement. Self-interstitial formation energies have larger differences between DMC and DFT, of up to 0.33eV, at the tetrahedral site. We also computed formation energies of helium interstitial defects where energies differed by up to 0.34eV, also at the tetrahedral site. The close agreement with available experiment demonstrates that DMC can be used as a predictive method to obtain benchmark energetics of defects in metals.

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Section: Defects In Aluminummentioning

confidence: 63%

Diffusion quantum Monte Carlo study of the equation of state and point defects in aluminum

2012

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“…This corresponds to realistic vacancy concentrations of a few parts per million. 19 The boundary conditions for all simulations are chosen such that the electronic fields decay to bulk values on the boundaries of the sample. Numerical parameters were chosen to keep the error in the formation energy due to discretization and coarse-graining to be less than 0.01 eV.…”

Section: Fig 1 ͑Color͒ ͑A͒mentioning

confidence: 99%

“…17,18 A challenge in studying defects in solids, and especially vacancies, is their extremely small concentrations which are typically a few parts per million in aluminum. 19 Therefore any realistic calculation of vacancies and their interaction has to involve millions of atoms. Unfortunately, performing electronic structure calculations with such numbers of atoms remained beyond reach until the recent development of the quasicontinuum orbital-free density-functional method ͑QCOFDFT͒.…”

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confidence: 99%

Vacancy clustering and prismatic dislocation loop formation in aluminum

2007

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The formation of prismatic dislocation loops is an important factor leading to radiation damage of metals. However, the formation mechanism and the size of the smallest stable loop has remained unclear. In this Rapid Communication, we use electronic structure calculations with millions of atoms to address this problem in aluminum. Our results show that there is a cascade of larger and larger vacancy clusters with smaller and smaller energy. Further, the results show that a seven vacancy cluster on the ͑111͒ plane can collapse into a stable prismatic loop. This supports the long-standing hypothesis that vacancy clustering leads to a prismatic loop, and that these loops can be stable at extremely small sizes. Finally our results show that it is important to conduct calculations using realistic concentrations ͑computational cell size͒ to obtain physically meaningful results. The embrittlement of metals subjected to radiation is a long-standing problem in various applications including nuclear reactors. As the irradiation dose increases above a certain threshold, a significant population of prismatic dislocation loops ͑dislocation loops whose Burgers vector has a component normal to their plane͒ has been experimentally observed to arise in metals. [1][2][3][4][5] It is widely believed these prismatic loops form through the clustering of vacancies that are generated randomly by irradiation.6 Specifically, the vacancies diffuse and eventually cluster on specific planes. Once there is a large enough planar cluster, the atoms on the two faces collapse onto each other leaving behind a prismatic dislocation loop.However, the formation mechanism and the size of the smallest stable loop remain unclear: there is no direct experimental observation of the process, and the theoretical investigations are inconclusive. Recent molecular dynamics simulations 7 support the hypothesized mechanism for iron, but these calculations used the Finnis-Sinclair empirical atomistic potentials whose validity is uncertain in situations involving changing atomic bonds. 8 In contrast, calculations for aluminum using quantum mechanical density-functional theory 9,10 show that divacancies-a complex of two vacancies-are either energetically unfavorable if they are aligned along the ͗110͘ direction or barely favorable with negligible binding energy if aligned along ͗100͘. If two vacancies can barely bind, it seems doubtful that they can be stable and grow to form clusters that can turn into prismatic loops. While these density-functional theory ͑DFT͒ methods are far more accurate, the computational effort is extremely large and consequently these calculations were limited to less than 100 atoms. This corresponds to an unphysically high concentration of vacancies. Furthermore, the results are in variance with experiments 11,12 that are indicative of a high binding energy of divacancies and a significant concentration of divacancies, especially at elevated temperatures.We study vacancy clustering and prismatic loops by performing electronic structure c...

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“…12͒. Positronannihilation 2D-ACAR studies in aluminum 47,48 show a large increase of the ACAR peak height with increasing temperature. Those studies show that at 613 K, i.e., about 2/3 of the FIG.…”

Section: Discussionmentioning

confidence: 94%

Electronic structure and orientation relationship of Li nanoclusters embedded in MgO studied by depth-selective positron annihilation two-dimensional angular correlation

et al. 2002

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Quantum-confined positrons are sensitive probes for determining the electronic structure of nanoclusters embedded in materials. In this work, a depth-selective positron annihilation 2D-ACAR ͑two-dimensional angular correlation of annihilation radiation͒ method is used to determine the electronic structure of Li nanoclusters formed by implantation of 10 16 -cm Ϫ2 30-keV 6 Li ions in MgO ͑100͒ and ͑110͒ crystals and by subsequent annealing at 950 K. Owing to the difference between the positron affinities of lithium and MgO, the Li nanoclusters act as quantum dots for positrons. 2D-ACAR distributions for different projections reveal a semicoherent fitting of the embedded metallic Li nanoclusters to the host MgO lattice. Ab initio KorringaKohn-Rostoker calculations of the momentum density show that the anisotropies of the experimental distributions are consistent with an fcc crystal structure of the Li nanoclusters. The observed reduction of the width of the experimental 2D-ACAR distribution is attributed to positron trapping in vacancies associated with Li clusters. This work proposes a method for studying the electronic structure of metallic quantum dots embedded in an insulating material.

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Positron annihilation spectroscopy of the equilibrium vacancy ensemble in aluminium

Cited by 34 publications

References 32 publications

Diffusion quantum Monte Carlo study of the equation of state and point defects in aluminum

Diffusion quantum Monte Carlo study of the equation of state and point defects in aluminum

Vacancy clustering and prismatic dislocation loop formation in aluminum

Electronic structure and orientation relationship of Li nanoclusters embedded in MgO studied by depth-selective positron annihilation two-dimensional angular correlation

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