Energetics Of Migration Research Articles

(Invited) Ab Initio Studies of Strain Effects on Perovskite Oxygen Vacancy Formation and Migration Energetics

Oxygen active materials are capable of rapidly transporting oxygen and exchanging it with the environment and have a wide range of applications, including solid oxide fuel cells, gas separation membranes, oxygen sensors, chemical looping devices, and memristors. Recent work has shown that epitaxial strain can play a significant role in altering the transport of oxygen through oxygen active materials, particularly in fluorite structured compounds. However, transition metal perovskites are an important class of oxygen active materials that have received only limited attention. In this work we use ab initio methods to model the coupling of biaxial epitaxial strain with oxygen vacancy formation and migration energetics in a range of transition metal perovskites. We focus on materials of the form LaBO3, where B = [Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga]. We predict the formation volume and strain response of oxygen vacancy formation energy, demonstrating that non-linear coupling of strain and vacancy formation can yield responses qualitatively different than those expected from simple elasticity arguments. In particular, as first observed in Ref. [1], we show that in some cases the vacancy can weaken the bonding and lower the elastic constants, which allows the formation of a vacancy to reduce the strain energy. This effect leads to stabilization of the vacancy under both compressive and tensile strain, even though the vacancy has positive formation volume. We also predict the migration volume and the strain response of oxygen migration energetics, here finding that values are quite consistent with a simple elastic strain model.[2] We find that tensile (compressive) biaxial strain reduces (enhances) the oxygen vacancy migration barrier approximately linearly across the systems studied (Figure 1). The average decrease in migration energy is 66 meV per percent strain for a single selected hop, with a low of 36 and a high of 89 meV decrease in migration barrier per percent strain across all the systems studied. In general we find that the oxygen transport kinetics can be altered by orders of magnitude in perovskites through strain effects on both vacancy content and migration energetics. These effects could play a significant role on materials performance at interfaces and in thin-film devices. [1] W. Donner, C. L. Chen, M. Liu, A. J. Jacobson, Y. L. Lee, M. Gadre, and D. Morgan, Epitaxial Strain-Induced Chemical Ordering in La0.5Sr0.5CoO3-delta Films on SrTiO3, Chemistry of Materials 23, p. 984-988 (2011). [2] T. Mayeshiba and D. Morgan, Strain Effects on Oxygen Migration in Perovskites, Phys. Chem. Chem. Phys. 17, p. 2715-2721 (2015 ). Figure 1

Enhanced interfacial proton migration on BaZr(Y)O3 by molten carbonate: A first principles study

Enhanced proton conductivity of Y-doped BaZrO3 (BZY) by molten carbonate (MC) has recently been reported. To understand the phenomenon, the present work investigates the pathways and energetics of proton migration along ZrO2-terminated (100) surface of BaZr(Y)O3 without and with the presence of MC from a density functional theory (DFT) perspective. The calculations explicitly show that the proton migration on BaZrO3 prefers a curved pathway with an energy barrier of 0.567eV. In the presence of carbonate-ion, the proton migration barrier is remarkably reduced to 0.332eV, which is in excellent agreement with 0.33eV of the experimental activation energy for conductivity in the BZY-MC electrolyte. Overall, the enhanced proton conductivity in the BZY-MC composite electrolyte is originated from facile interfacial proton migration between BZY and MC phase.

Energetics of Photoinduced Charge Migration within the Tryptophan Tetrad of an Animal (6-4) Photolyase.

Cryptochromes and photolyases are flavoproteins that undergo cascades of electron/hole transfers after excitation of the flavin cofactor. It was recently discovered that animal (6-4) photolyases, as well as animal cryptochromes, feature a chain of four tryptophan residues, while other members of the family contain merely a tryptophan triad. Transient absorption spectroscopy measurements on Xenopus laevis (6-4) photolyase have shown that the fourth residue is effectively involved in photoreduction but at the same time could not unequivocally ascertain the final redox state of this residue. In this article, polarizable molecular dynamics simulations and constrained density functional theory calculations are carried out to reveal the energetics of charge migration along the tryptophan tetrad. Migration toward the fourth tryptophan is found to be thermodynamically favorable. Electron transfer mechanisms are sought either through an incoherent hopping mechanism or through a multiple sites tunneling process. The Jortner-Bixon formulation of electron transfer (ET) theory is employed to characterize the hopping mechanism. The interplay between electron transfer and relaxation of protein and solvent is analyzed in detail. Our simulations confirm that ET in (6-4) photolyase proceeds out of equilibrium. Multiple site tunneling is modeled with the recently proposed flickering resonance mechanism. Given the position of energy levels and the distribution of electronic coupling values, tunneling over three tryptophan residues may become competitive in some cases, although a hopping mechanism is likely to be the dominant channel. For both reactive channels, computed rates are very sensitive to the starting protein configuration, suggesting that both can take place and eventually be mixed, depending on the state of the system when photoexcitation takes place.

Li Intercalation into a β-MnO2 Grain Boundary.

MnO2 is well-known for its technological applications including Li ion, Li-air batteries, and electrochemical capacitors. Compared to the bulk material, nanostructuring of rutile (β-)MnO2 has been shown to vastly improve its electrochemical properties and performance. While the bulk material cannot readily intercalate Li, nanostructured mesoporous samples exhibit good Li intercalation. This observation is not yet fully understood. In this work, we use state-of-the-art theoretical techniques to investigate Li intercalation and migration at the β-MnO2 Σ 5(210)/[001] grain boundary (GB). We show how large tunnel structures in the GB can promote Li intercalation with voltages of up to 3.83 eV compared to the experimental value of 3.00 eV. Conversely, small tunnel structures resulting from overcoordination of ions at the GB can hinder Li intercalation with significantly reduced voltages. The size and shape of these tunnels also strongly influence the energetics of Li migration with energy barriers ranging from 0.15 to 0.89 eV, compared to a value for the bulk of 0.17 eV. Our results illustrate how GBs with large, open tunnel structures may promote electrochemical performance and could be a contributing factor to the excellent performance of nanostructured β-MnO2.

Structural stability and energetics of grain boundary triple junctions in face centered cubic materials

We present a systematic study to elucidate the role of triple junctions (TJs) and their constituent grain boundaries on the structural stability of nanocrystalline materials. Using atomistic simulations along with the nudge elastic band calculations, we explored the atomic structural and thermodynamic properties of TJs in three different fcc materials. We found that the magnitude of excess energy at a TJ was directly related to the atomic density of the metal. Further, the vacancy binding and migration energetics in the vicinity of the TJ were examined as they play a crucial role in the structural stability of NC materials. The resolved line tension which takes into account the stress buildup at the TJ was found to be a good measure in predicting the vacancy binding tendency near the TJ. The activation energy for vacancy migration along the TJ was directly correlated with the measured excess energy. Finally, we show that the resistance for vacancy diffusion increased for TJs with larger excess stored energy and the defect mobility at some TJs is slower than their constituent GBs. Hence, our results have general implications on the diffusional process in NC materials and provide new insight into stabilizing NC materials with tailored TJs.

First-Principles Insight into the Hydration Ability and Proton Conduction of the Solid State Proton Conductor, Y and Sn Co-Doped BaZrO3

Y and Sn co-doped BaZrO3 (BZSY) has recently been shown to exhibit superior hydration ability and improved power output performance compared to those of the traditional solid oxide fuel cell (SOFC) electrolyte, Y-doped BaZrO3. The fact that BZSY is also chemically stable in both H2O and CO2 atmospheres further illustrates the great potential for BZSY as a future electrolyte material in proton-conducting SOFCs. In this work, we conducted comprehensive density functional theory calculations to analyze the energetics of hydration and proton migration in BZSY. The energy of hydration is calculated at four different locations in the cell to better assess the specific contributions of each defect type. For all locations tested, hydration is found to be strongly exothermic and is most favorable when Y ions are in the vicinity. The calculated energies of hydration (−1.72 to −1.11 eV) are significantly lower than any previously calculated for acceptor-doped BaZrO3-based electrolytes. Nudged elastic band calculations confirm low proton reorientation energies (0.08–0.24 eV) and low intraoctahedral hopping energies (0.21–0.41 eV) and that these diffusion barriers are at minimum when the proton is migrating to an oxygen ion in a YO6 octahedron. Our results show how the synergy of the Sn and Y dopant ions produces the excellent hydration and conduction performance of BZSY and fully support its potential application in next-generation SOFCs.

Theoretical investigation of thermodynamic stability and mobility of the intrinsic point defects in Ti3AC2 (A = Si, Al).

Nano-laminated Ti3AC2 (A = Si, Al) are highlighted as nuclear materials for a generation IV (GIV) reactor because they show high tolerance to radiation damage and remain crystalline under irradiation of high fluence heavy ions. In this paper, the energetics of formation and migration of intrinsic point defects are predicted by density functional theory calculations. We find that the space near the A atomic plane acts as a point defect sink and can accommodate lattice disorder. The migration energy barriers of Si/Al vacancy and TiSi anti-site defects along the atomic plane A are in the range of 0.3 to 0.9 eV, indicating their high mobility and the fast recovery of Si/Al Frenkel defects and Ti-A antisite pairs after irradiation. This layered structure induced large disorder accommodation and fast defect recovery must play an important role in the micro-structural response of Ti3AC2 to irradiation.

Optimal migration energetics of humpback whales and the implications of disturbance.

Whales migrate long distances and reproduce on a finite store of energy. Budgeting the use of this limited energy reserve is an important factor to ensure survival over the period of migration and to maximize reproductive investment. For some whales, migration routes are closely associated with coastal areas, exposing animals to high levels of human activity. It is currently unclear how various forms of human activity may disturb whales during migration, how this might impact their energy balance and how this could translate into long-term demographic changes. Here, we develop a theoretical bioenergetic model for migrating humpback whales to investigate the optimal migration strategy that minimizes energy use. The average migration velocity was an important driver of the total energy used by a whale, and an optimal velocity of 1.1 m s(-1) was determined. This optimal velocity is comparable to documented observed migration speeds, suggesting that whales migrate at a speed that conserves energy. Furthermore, the amount of resting time during migration was influenced by both transport costs and feeding rates. We simulated hypothetical disturbances to the optimal migration strategy in two ways, by altering average velocity to represent changes in behavioural activity and by increasing total travelled distance to represent displacement along the migration route. In both cases, disturbance increased overall energy use, with implications for the growth potential of calves.

Theoretical investigation of thermodynamic stability and mobility of the oxygen vacancy in ThO2–UO2solid solutions

The thermodynamic stability and the migration energy barriers of oxygen vacancies in ThO2-UO2 solid solutions are investigated by density functional theory calculations. In pure ThO2, the formation energy of the oxygen vacancy is 7.58 eV and 1.46 eV under O rich and O poor conditions, respectively, while its migration energy barrier is 1.97 eV. The addition of UO2 into ThO2 significantly decreases the energetics of formation and migration of the oxygen vacancy. Among the range of UO2-ThO2 solid solutions studied in this work, UO2 exhibits the lowest formation energy (5.99 eV and -0.13 eV under O rich and O poor conditions, respectively) and Th0.25U0.75O2 exhibits the lowest migration energy barrier (∼1 eV). Furthermore, by considering chemical potential, the phase diagram of the oxygen vacancy as a function of both temperature and oxygen partial pressure is shown, which could help to gain experimental control over oxygen vacancy concentration.

Investigation of oxygen point defects in cubic ZrO2 by density functional theory

The energetics of formation and migration of the oxygen vacancy and interstitial in cubic ZrO2 are investigated by density functional theory calculations. In an O-rich environment, the negatively charged oxygen interstitial is the most dominant defect whereas, the positively charged oxygen vacancy is the most dominant defect under O-poor conditions. Oxygen interstitial migration occurs by the interstitialcy and the direct interstitial mechanisms, with calculated migration energy barriers of 2.94eV and 2.15eV, respectively. For the oxygen vacancy, diffusion is preferred along the 〈100〉 direction, and the calculated energy barriers are 0.26eV for VO2+, 0.27eV for VO1+ and 0.54eV for VO0. These results indicate that oxygen diffusivity is higher through the vacancy-migration mechanism.

Accommodation, Accumulation, and Migration of Defects in Ti3SiC2 and Ti3AlC2 MAX Phases

We have determined the energetics of defect formation and migration in Mn+1AXn phases with M = Ti, A = Si or Al, X = C, and n = 3 using density functional theory calculations. In the Ti3SiC2 structure, the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular configuration (C–Si = 1.889 Å) and to two apical Ti atoms (C–Ti = 2.057 Å). This carbon–metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti–Si, 17.3 eV for Ti–C, and 6.1 eV for Si–C. Broadly similar behavior was found for Frenkel and antisite defect energies in the Ti3AlC2 structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti3AlC2 is different and the energy is lower compared with the Ti3SiC2 structure. This effect, together with the observation of a stable C interstitial defect coordinated by three silicon species and two titanium species in Ti3SiC2, will have important implications for radiation damage response in these materials.

Premigratory fat metabolism in hummingbirds: A Rumsfeldian approach

Abstract Hummingbird migration is a remarkable feat, given the small body sizes of migratory species, their high metabolic rates during flight and the long distances traveled using fat to fuel the effort. Equally remarkable is the ability of premigratory hummingbirds in the wild to accumulate fat, synthesized from sugar, at rates as high as 10% of body mass per day. This paper summarizes, using Rumsfeldian terminology, “known knowns” concerning the energetics of hummingbird migration and premigratory fattening. Energy metabolism during hover-feeding on floral nectar is fueled directly by dietary sugar through the pathway recently named the “sugar oxidation cascade”. However, flight without feeding for more than a few minutes requires shifting to fat as a fuel. It is proposed that behavior and metabolic fuel choice are coadapted to maximize the rate of fat deposition during premigratory fattening. The hummingbird liver appears to possess extraordinarily high capacities for fatty acid synthesis. The analysis of “known knowns” leads to identification of “known unknowns”, e.g., the fates of dietary glucose and fructose, the regulation of fat metabolism and metabolic interactions between liver and adipose tissue. The history of science behooves recognition of “unknown unknowns” that, when discovered serendipitously, might shed new light on fundamental mechanisms as well as human pathological conditions.

Pathway and energetics of xenon migration in uranium dioxide

Using a combination of density functional theory (DFT), classical potentials, molecular dynamics, and nudged elastic band (NEB) calculations, we explore the diffusion of xenon in uranium dioxide (UO${}_{2}$). We compare migration barriers of empirical potentials with DFT by performing NEB calculations and subsequently we use the DFT-validated empirical potentials to calculate vacancy clusters, with and without xenon, to determine the migration path and barrier of xenon in bulk UO${}_{2}$. We find the following: (i) Two empirical potentials out of four tested agree qualitatively with DFT derived energetics for Schottky defect migration; (ii) through the use of molecular dynamics with empirical potentials, we have found a path for the diffusion of xenon-tetravacancy clusters (Xe$+2{V}_{\mathrm{U}}+2$${V}_{\mathrm{O}}$); (iii) this path has an energy barrier significantly lower than previously reported paths by nearly 1 eV; (iv) we examine the physical contributions to the migration pathway and find the barrier is largely electrostatic and that xenon contributes very little to the barrier height; (v) once a uranium vacancy attaches to a xenon-Schottky defect, the resulting xenon-tetravacancy cluster is strongly bound; and (vi) as xenon in a tetravacancy, a xenon-double Schottky defect can diffuse in a concerted manor with a comparable barrier to xenon in a tetravacancy, but two of the oxygen vacancies are only weakly bound to the defect.

Effect of anomalous compressibility on Fe diffusion in ferropericlase throughout the spin crossover in the lower mantle

The transport of iron in the deep earth is important for understanding chemical equilibration between deep earth phases and between the core and mantle. However, iron transport modeling is complicated by its changes in spin state with pressure and temperature. Recent studies (Saha et al., 2011; Ammann et al., 2011) calculated the diffusivity of iron in ferropericlase as a function of iron's spin state but did not include the coupling of iron migration energetics to the observed anomalous bulk modulus softening associated with the spin transition (Wentzcovitch et al., 2009; Crowhurst et al., 2008). Here we calculate the diffusivity of iron in ferropericlase throughout the lower mantle using a model for both iron's spin and the anomalous bulk modulus softening to provide a complete model of iron diffusion in ferropericlase. Including the bulk modulus softening increases iron diffusivities by a factor of thirty relative to values without this effect at some depths on the geotherm.

Configurational disorder effects on adatom mobilities on Ti1−xAlxN(001) surfaces from first principles

We use metastable NaCl-structure Ti0.5Al0.5N alloys to probe effects of configurational disorder on adatom surface diffusion dynamics which control phase stability and nanostructural evolution during film growth. First-principles calculations were employed to obtain potential energy maps of Ti and Al adsorption on an ordered TiN(001) reference surface and a disordered Ti0.5Al0.5N(001) solid-solution surface. The energetics of adatom migration on these surfaces are determined and compared in order to isolate effects of configurational disorder. The results show that alloy surface disorder dramatically reduces Ti adatom mobilities. Al adatoms, in sharp contrast, experience only small disorder-induced differences in migration dynamics.

Diffusion of ion implanted indium and silver in ZnO crystals

ABSTRACTWe report on diffusion behavior for ion implanted indium and silver atoms in ZnO crystals. Both In and Ag ions were implanted at room temperature at 7-10° relative to c-axis to avoid channeling effects during implantation. In ions were implanted at four different energies (40, 100, 200, and 350 keV, respectively) and doses (4.20×1013, 6.70×1013, 8.10×1013 and 3.10×1014 /cm2, respectively), resulting in a total dose of 5 ×1014 /cm2. For another set of ZnO samples, Ag ions were implanted at energies 30, 75, 150, and 350 keV at doses 3.3×1013, 4.2×1013, 8.3×1013 and 3.4×1014 /cm2, respectively, to reach a total dose of 5×1014 /cm2. Both In and Ag implants resulted in a uniform concentration profile of the implanted dopants from surface to depth ~ 150 nm. The samples were annealed for 30 minutes at temperatures between 850-1050 °C in an oxygen gas flow. The distributions of In and Ag atoms, either aligned or nonaligned along the crystalline directions, were measured by Rutherford backscattering combined with ion channeling. The diffusivities for nonaligned (interstitial) and aligned (substitutional) dopants atoms were determined to vary with annealing temperature via the Arrhenius relationship. The diffusion activation energies (Ea) along the <10-11> direction for substitutional impurity atoms were lower than those for interstitial dopants atoms e.g., in the case of In, Ea ~ 1.52 eV for <10-11> aligned In atoms and Ea ~ 2.61 eV for interstitial In atoms between <10-11> atomic rows and in the case of Ag, Ea ~ 1.77 eV for the interstitial Ag atoms between the <10-11> atomic rows and 1.11 eV for <10-11> aligned Ag atoms. The diffusion activation energies showed a different trend for the two dopants as measured along the <0001> crystalline direction. For Ag implanted in ZnO, the activation energy of Ea ~ 0.91 eV for the aligned Ag atoms along <0001> direction and Ea ~ 1.55 eV were found for the interstitial Ag atoms, whereas in the case of In along the <0001> direction, the interstitial In was found to migrate with a higher activation energy (Ea ~ 1.78 eV) than the substitutional In (Ea ~1.42 eV). These results will be compared with first-principle calculations for understanding the energetics of defect formation and migration in both n- and p-type doping cases.

A single point defect diffusion near a Cu (100) surface

As part of our investigations on the disordering of metal surfaces, we report the results of our calculations of the energetics of formation and migration of a single point defect (vacancy and interstitial) in the Cu (100) surface using molecular dynamics method. We used the embedded-atom method to describe inter-atomic interactions. Defect formation and migration energies have been calculated at the surface as well as in the near-surface region. Vacancy and interstitial formation and migration energies converge to bulk values already at a few layers below the surface. We find that the surface vacancy and interstitial formation and migration energies are all lower than their bulk values. For the inter-layer migration, a point defect in each of the first six layers is favorable to migrate into the corresponding upper layer, especially for the first three layers.

Going the Distance

One challenge to studying migration energetics: Not every bird will fly for hours in a wind tunnel.

Ion irradiation of the TiO2 polymorphs and cassiterite

Thin crystals of rutile, brookite, anatase, and cassiterite were irradiated in situ in the transmission electron microscope using 1.0 MeV Kr ions at 50-300 K. Synthetic rutile and natural cassiterite, with 0.1-0.2 wt% impurities, remain crystalline up to a fluence of 5 x 10{sup 15} ions cm{sup -2} without evidence for amorphization at 50 K. Natural brookite and anatase, with 0.3-0.5 wt% impurities, become amorphous at fluences of 8.1 x 10{sup 14} and 2.3 x 10{sup 14} ions cm{sup -2}, respectively. We have also studied two natural rutile samples containing {approx}1.7 and 1.2 wt% impurities. These samples became amorphous at 9.2 x 10{sup 14} and 8.6 x 10{sup 14} ions cm{sup -2} at 50 K, respectively. Further analyses of the fluence-temperature data for natural brookite, rutile, and anatase give critical amorphization temperatures of 168 {+-} 11, 209 {+-} 8, and 242 {+-} 6 K, respectively. Results are briefly discussed with respect to several criteria for radiation resistance, including aspects of the structure, bonding, and energetics of defect formation and migration.

Large-scale variation in seasonal swimming patterns of plaice in the North Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 392:167-178 (2009) - DOI: https://doi.org/10.3354/meps08213 Large-scale variation in seasonal swimming patterns of plaice in the North Sea E. Hunter1,*, R. J. Cotton2, J. D. Metcalfe1, J. D. Reynolds3 1Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK 2Mathematical Sciences Unit, Health & Safety Laboratory, Harpur Hill, Buxton, Derbyshire SK17 9JN, UK 3Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada *Email: ewan.hunter@cefas.co.uk ABSTRACT: We examined the mid-water swimming activity of 139 mature female plaice Pleuronectes platessa L. These were tagged with electronic data storage tags (DST1s and DST3s) to test whether swimming at different times of the year differed among areas of the North Sea with average tidal current velocities ranging from fast (West), to intermediate (East), to slow (North). Longer swimming duration and more tidal activity were predicted for the western group, where fast-flowing tidal currents allow efficient selective tidal stream transport. Individual depth data were converted into binary records representing either swimming or resting; repeated patterns of swimming were analysed according to cycle-length frequencies. Most swimming occurred during expected times of migration and spawning (October to March). Plaice infrequently spent >5 h in mid-water, and rarely left the sea-bed during summer. DST1 tagged plaice (West only) spent the longest times swimming (p > 0.001), but there was no significant effect of Area for DST3s (all areas), suggesting that swimming plays an important behavioural role in migration in addition to transport between feeding and spawning areas. Tidal patterns of activity occurred in all 3 sub-stocks, predominantly during the migratory period (albeit at a significantly lower frequency in the North). These data provide one of few examples where the annual behaviour patterns of a fish stock have been recorded across a large part of its geographical range. The results have important implications for understanding the energetics of fish migration and the availability of demersal stocks to capture by commercial and survey vessels. KEY WORDS: Plaice · Migration · Data storage tags · Swimming · Behaviour · Fisheries Full text in pdf format PreviousNextCite this article as: Hunter E, Cotton RJ, Metcalfe JD, Reynolds JD (2009) Large-scale variation in seasonal swimming patterns of plaice in the North Sea. Mar Ecol Prog Ser 392:167-178. https://doi.org/10.3354/meps08213 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 392. Online publication date: October 19, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.