23 research outputs found
Oxygen Vacancy Formation Energy in Metal Oxides: High Throughput Computational Studies and Machine Learning Predictions
The oxygen vacancy formation energy () governs defect dynamics
and is a useful metric to perform materials selection for a variety of
applications. However, density functional theory (DFT) calculations of come at a greater computational cost than the typical bulk calculations
available in materials databases due to the involvement of multiple
vacancy-containing supercells. As a result, available repositories of direct
calculations of remain relatively scarce, and the development
of machine learning models capable of delivering accurate predictions is of
interest. In the present, work we address both such points. We first report the
results of new high-throughput DFT calculations of oxygen vacancy formation
energies of the different unique oxygen sites in over 1000 different oxide
materials, which together form the largest dataset of directly computed oxygen
vacancy formation energies to date, to our knowledge. We then utilize the
resulting dataset of 2500 values to train random forest
models with different sets of features, examining both novel features
introduced in this work and ones previously employed in the literature. We
demonstrate the benefits of including features that contain information
specific to the vacancy site and account for both cation identity and oxidation
state, and achieve a mean absolute error upon prediction of 0.3 eV/O,
which is comparable to the accuracy observed upon comparison of DFT
computations of oxygen vacancy formation energy and experimental results.
Finally, we demonstrate the predictive power of the developed models in the
search for new compounds for solar-thermochemical water-splitting applications,
finding over 250 new AABBO double perovskite
candidates
Phase Stability of Lead Phosphate Apatite PbCu(PO)O, PbCu(PO)(OH), and PbCu(PO)
Recently, Cu-substituted lead apatite LK-99 was reported to have
room-temperature ambient-pressure superconductivity. Here we utilize density
functional theory (DFT) total energy and harmonic phonon calculations to
investigate the thermodynamic and dynamic stability of two lead phosphate
apatites in their pure and Cu-substituted structures. Though
Pb(PO)O and Pb(PO)(OH) are found to be
thermodynamically stable (i.e., on the T=0K ground state convex hull), their
Cu-substituted counterparts are above the convex hull. Harmonic phonon
calculations reveal dynamic instabilities in all four of these structures.
Oxygen vacancy formation energies demonstrate that the addition of Cu dopant
substituting for Pb increases the likelihood of the formation of oxygen
vacancies on the anion site. We propose a new possible phase in this system,
PbCu(PO), where two monovalent Cu atoms are substituted for two
Pb(1) atoms and the anion oxygen is removed. We also propose several reaction
pathways for PbCu(PO)O and PbCu(PO), and found that
both of these two structures are likely to be synthesized under a 1:1 ratio of
reactants PbSO and CuP. Our work provides a thorough foundation for
the thermodynamic and dynamic stabilities of LK-99 related compounds and we
propose several possible novel synthesis reaction pathways and a new predicted
structure for future studies
Transport of larvae and detritus across the surf zone of a steep reflective pocket beach
The article of record as published may be found at http://dx.doi.org/10.3354/meps11223Larvae of many intertidal species develop offshore and must cross the surf zone to complete their onshore migration to adult habitats. Depending on hydrodynamics, the surf zone may limit this migration, especially on reflective rocky shores. As a logistically tractable analog of a rocky shore environment, we carried out a comprehensive biological and physical study of the hydrodynamics of a steep reflective sandy beach. Holoplankton and precompetent larval invertebrates were much less abundant within the surf zone than offshore, and their concentrations inside and outside the surf zone were not significantly correlated, suggesting that they were not entering the surf zone. Persistent offshore flow throughout the water column at the outer edge of the surf zone may prevent these organisms from entering the surf zone. In contrast, the concentrations of detritus and a competent larval invertebrate (i.e. cyprids), while also not significantly correlated with concentrations offshore, were frequently more concentrated in the surf zone than offshore. Within the surf zone, the concentration of detritus was significantly correlated with concentrations of competent larval invertebrates (barnacles, gastropods, polychaetes, and bopyrid amphipod) and organisms that may be associated with detritus (amphipods and harpacticoid copepods). These concentrations were significantly negatively correlated with average daily wave height. We hypothesize that detritus and larvae enter the surf zone near the bottom during calm wave conditions by a process of near-bottom streaming. Near-bottom streaming is associated with all surf zones and may be a general mechanism for onshore transport of larvae close to the coast.National Science FoundationGrant NSD-OCE#09273
Surf zones regulate larval supply and zooplankton subsidies to nearshore communities
The article of record as published may be located at http://doi.org/10.1002/lno.10609Surf zone hydrodynamics vary along shorelines potentially affecting the delivery of larvae and zooplankton
subsidies to intertidal communities, and, hence, the intensity of postsettlement interactions, growth and
reproduction of filter-feeding foundation species and planktivorous fishes. We investigated the ability of zooplankton
assemblages to enter the wide surf zone of the rip-channeled, more dissipative beach at Sand City,
California, and the narrow surf zone of the steep reflective beach at nearby Carmel River State Beach. Every
day for a month, we surveyed zooplankton inside and outside the surf zone and concomitant larval settlement
of the dominant invertebrate onshore at each site in this upwelling regime. At the more dissipative
surf zone, all zooplankters were far more concentrated inside than outside the surf zone. Many taxa increased
in the surf zone and the predominant invertebrate on beaches, Emerita analoga, settled abundantly when prevailing
northwesterly winds relaxed and waves were small. At the reflective surf zone, concentrations of zooplankters
of most taxa were far greater outside than inside the surf zone, and many taxa increased in the surf
zone when waves were small. Twice as many taxa were positively correlated inside and outside the surf zone
at the dissipative than the reflective surf zone, indicating that zooplankters were more freely exchanged
although behavior also played a role. Thus, spatial and temporal variation in surf zone hydrodynamics may
regulate subsidies of zooplankton food and larval recruits to nearshore communities with potential cascading
effects on community dynamics and structure.Bodega Marine Laboratory and the Oregon Institute of Marine BiologyNSF-OCE-092719
Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities.
Larvae of intertidal species develop at sea and must return to adult habitats to replenish populations. Similarly, nutrients, detritus and plankton provide important subsidies spurring growth and reproduction of macroalgae and filter-feeding invertebrates that form the foundation of intertidal communities. Together, these factors determine the density and intensity of interactions among community members. We hypothesized that spatial variation in surfzone hydrodynamics affects the delivery of plankton subsidies. We compared entire zooplankton communities inside and outside the surf zone daily while monitoring physical conditions for one month each at two shores with different surfzone characteristics. Opposite cross-shore distributions of larvae and other zooplankters occurred at the two sites: zooplankton was much more abundant inside the mildly sloping dissipative surf zone (DSZ) with rip currents and was more abundant outside the steep reflective surf zone (RSZ). Biophysical numerical simulations demonstrated that zooplankters were concentrated in rip channels of the DSZ and were mostly unable to enter the RSZ, indicating the hydrodynamic processes behind the observed spatial variation of zooplankters in the surf zone. Differences in the concentration of larvae and other zooplankters between the inner shelf and surf zone may be an underappreciated, key determinant of spatial variation in inshore communities
Persistent Differences in Horizontal Gradients in Phytoplankton Concentration Maintained by Surf Zone Hydrodynamics
The article of record as published may be located at http://link.springer.com/article/10.1007/s12237-017-0278-2Surf zones, regions of breaking waves, are at the
interface between the shore and coastal ocean. Surf zone hydrodynamics
may affect delivery of phytoplankton subsidies
to the intertidal zone. Over a month of daily sampling at an
intermediate surf zone with bathymetric rip currents and a
reflective surf zone, we measured surf zone hydrodynamics
and compared concentrations of coastal phytoplankton taxa in
the surf zones to concentrations offshore. At the intermediate
surf zone, ~80% of the variability in the concentration of
coastal phytoplankton taxa within the surf zone was explained
by their variation offshore; however, concentrations were
much higher and lower than those offshore in samples from
a bathymetric rip current and over the adjacent shoal, respectively.
Hydrodynamics at this intermediate surf zone did not
hinder the delivery of coastal phytoplankton to the surf zone,
but the bathymetric rip current system appeared to redistribute phytoplankton concentrating them within eddies. At the reflective
shore, we sampled surf zones at a beach and two
adjacent rocky intertidal sites. Concentrations of typical coastal
phytoplankton taxa were usually an order of magnitude or
more lower than those offshore, even when offshore samples
were collected just 20 m beyond the breakers. The phytoplankton
assemblages inside and outside the surf zone often
appeared to be disconnected. Surf zone hydrodynamics at the
steep, reflective shore coupled with low phytoplankton concentrations
in near-surface water appeared to limit delivery of
phytoplankton subsidies to the surf zone. Surf zone hydrodynamics
may be a key factor in the alongshore variation in
phytoplankton subsidies to coastal communities.NSF-OCE no. 09273
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Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities.
Larvae of intertidal species develop at sea and must return to adult habitats to replenish populations. Similarly, nutrients, detritus and plankton provide important subsidies spurring growth and reproduction of macroalgae and filter-feeding invertebrates that form the foundation of intertidal communities. Together, these factors determine the density and intensity of interactions among community members. We hypothesized that spatial variation in surfzone hydrodynamics affects the delivery of plankton subsidies. We compared entire zooplankton communities inside and outside the surf zone daily while monitoring physical conditions for one month each at two shores with different surfzone characteristics. Opposite cross-shore distributions of larvae and other zooplankters occurred at the two sites: zooplankton was much more abundant inside the mildly sloping dissipative surf zone (DSZ) with rip currents and was more abundant outside the steep reflective surf zone (RSZ). Biophysical numerical simulations demonstrated that zooplankters were concentrated in rip channels of the DSZ and were mostly unable to enter the RSZ, indicating the hydrodynamic processes behind the observed spatial variation of zooplankters in the surf zone. Differences in the concentration of larvae and other zooplankters between the inner shelf and surf zone may be an underappreciated, key determinant of spatial variation in inshore communities
A dataset of DFT-computed oxygen vacancy formation energies of metal oxides
<p>This dataset contains DFT-computed oxygen vacancy formation energies of different oxygen sites in over 1000 metal oxides present on the Open Quantum Materials Database (OQMD). Entries in the Data.csv file are indexed using the OQMD entry ID of the pristine structure, and list, alongside the oxygen vacancy formation energy, a number of other properties used as features in machine learning models. An exemplary code allowing to test the performance of a random forest regressor using different sets of features to predict the vacancy formation energy, as a function of training set size, is provided in Test_Models.py. More details on the dataset and machine learning models can be found in: arXiv:2309.01160. </p><p> </p><p><strong>Acknowledgments</strong></p><p>The work was funded by the U.S. Department of Energy under Grant DE-EE0008089. S. G. acknowledges the Air Force Office of Scientific Research for support under Award No. FA9550-18-1-0136 (OQMD database). A. G. acknowledges the Center for Hierarchical Materials Design (ChiMaD) under Award No. 70NANB19H005 (ML models). A. J.A. S.-C. acknowledges the financial support to National Agency for Research and Development (ANID)/DOCTORADO BECAS CHILE/2018 - 56180024. A. J.A. S.-C. and T. L. acknowledge funding from the Toyota Research Institute through the Accelerated Materials Design and Discovery program (ML representations). The data was produced relying on the computing power provided by Quest high performance computing facility at Northwestern University.</p>
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Variation in the abundance of Pseudo-nitzschia and domoic acid with surf zone type.
Most harmful algal blooms (HAB) originate away from the shore and, for them to endanger human health, they must be first transported to shore after which they must enter the surf zone where they can be feed upon by filter feeders. The last step in this sequence, entrance into the surf zone, depends on surf zone hydrodynamics. During two 30-day periods, we sampled Pseudo-nitzschia and particulate domoic acid (pDA) in and offshore of a more dissipative surf zone at Sand City, California (2010) and sampled Pseudo-nitzschia in and out of reflective surf zones at a beach and rocky shores at Carmel River State Beach, California (2011). At Sand City, we measured domoic acid in sand crabs, Emerita analoga. In the more dissipative surf zone, concentrations of Pseudo-nitzschia and pDA were an order of magnitude higher in samples from a rip current than in samples collected just seaward of the surf zone and were 1000 times more abundant than in samples from the shoals separating rip currents. Domoic acid was present in all the Emerita samples and varied directly with the concentration of pDA and Pseudo-nitzschia in the rip current. In the more reflective surf zones, Pseudo-nitzschia concentrations were 1-2 orders of magnitude lower than in samples from 125 and 20m from shore. Surf zone hydrodynamics affects the ingress of Pseudo-nitzschia into surf zones and the exposure of intertidal organisms to HABs on the inner shelf
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Beyond connectivity: how empirical methods can quantify population persistence to improve marine protected-area design.
Demographic connectivity is a fundamental process influencing the dynamics and persistence of spatially structured populations. Consequently, quantifying connectivity is essential for properly designing networks of protected areas so that they achieve their core ecological objective of maintaining population persistence. Recently, many empirical studies in marine systems have provided essential, and historically challenging to obtain, data on patterns of larval dispersal and export from marine protected areas (MPAs). Here, we review the empirical studies that have directly quantified the origins and destinations of individual larvae and assess those studies' relevance to the theory of population persistence and MPA design objectives. We found that empirical studies often do not measure or present quantities that are relevant to assessing population persistence, even though most studies were motivated or contextualized by MPA applications. Persistence of spatial populations, like nonspatial populations, depends on replacement, whether individuals reproduce enough in their lifetime to replace themselves. In spatial populations, one needs to account for the effect of larval dispersal on future recruitment back to the local population through local retention and other connectivity pathways. The most commonly reported descriptor of larval dispersal was the fraction of recruitment from local origin (self-recruitment). Self-recruitment does not inform persistence-based MPA design because it is a fraction of those arriving, not a fraction of those leaving (local retention), so contains no information on replacement. Some studies presented connectivity matrices, which can inform assessments of persistence with additional knowledge of survival and fecundity after recruitment. Some studies collected data in addition to larval dispersal that could inform assessments of population persistence but which were not presented in that way. We describe how three pieces of empirical information are needed to fully describe population persistence in a network of MPAs: (1) lifetime fecundity, (2) the proportion of larvae that are locally retained (or the full connectivity matrix), and (3) survival rate after recruitment. We conclude by linking theory and data to provide detailed guidance to empiricists and practitioners on field sampling design and data presentation that better informs the MPA objective of population persistence