25 research outputs found
The Effect of the Environment on alpha-Al_2O_3 (0001) Surface Structures
We report that calculating the Gibbs free energy of the alpha-Al_2O_3 (0001)
surfaces in equilibrium with a realistic environment containing both oxygen and
hydrogen species is essential for obtaining theoretical predictions consistent
with experimental observations. Using density-functional theory we find that
even under conditions of high oxygen partial pressure, the metal terminated
surface is surprisingly stable. An oxygen terminated alpha-Al_2O_3 (0001)
surface becomes stable only if hydrogen is present on the surface. In addition,
including hydrogen on the surface resolves discrepancies between previous
theoretical work and experimental results with respect to the magnitude and
direction of surface relaxations.Comment: 4 pages including 2 figures. Submitted to Phys. Rev. Lett. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
EMSL Geochemistry, Biogeochemistry and Subsurface Science-Science Theme Advisory Panel Meeting
This report covers the topics of discussion and the recommendations of the panel members. On December 8 and 9, 2010, the Geochemistry, Biogeochemistry, and Subsurface Science (GBSS) Science Theme Advisory Panel (STAP) convened for a more in-depth exploration of the five Science Theme focus areas developed at a similar meeting held in 2009. The goal for the fiscal year (FY) 2011 meeting was to identify potential topical areas for science campaigns, necessary experimental development needs, and scientific members for potential research teams. After a review of the current science in each of the five focus areas, the 2010 STAP discussions successfully led to the identification of one well focused campaign idea in pore-scale modeling and five longer-term potential research campaign ideas that would likely require additional workshops to identify specific research thrusts. These five campaign areas can be grouped into two categories: (1) the application of advanced high-resolution, high mass accuracy experimental techniques to elucidate the interplay between geochemistry and microbial communities in terrestrial ecosystems and (2) coupled computation/experimental investigations of the electron transfer reactions either between mineral surfaces and outer membranes of microbial cells or between the outer and inner membranes of microbial cells
Data Infrastructure to Meet the Prediction Challenge
The Problem: Computer models and simulations can yield inconsistent and often contradictory results; the Need: Experimental data infrastructure for validation of models and simulation to drive the science forward
Ab Initio Thermodynamics of Hydrated Calcium Carbonates and Calcium Analogues of Magnesium Carbonates: Implications for Carbonate Crystallization Pathways
Formation of calcium carbonate and
its hydrates are important for
a wide variety of geological, biological, and technological concerns.
Recent studies have determined that formation of anhydrous crystalline
calcite, aragonite, and vaterite can involve a complex series of nonclassical
pathways in which the hydrated polymorphs monohydrocalcite (CaCO<sub>3</sub>·H<sub>2</sub>O), ikaite (CaCO<sub>3</sub>·6H<sub>2</sub>O), and amorphous calcium carbonate (ACC) play key roles and
in some instances are stable or metastable endproducts. The stages
of nucleation and crystallization along these pathways are not well
understood, nor is how what is learned in an aqueous environment transfers
to CO<sub>2</sub>-rich conditions. In this work ab initio thermodynamics
based on density-functional theory and experimental chemical potentials
for H<sub>2</sub>O-rich and CO<sub>2</sub>-rich systems are used to
determine the stability of calcium carbonate polymorphs as a function
of environmental conditions. In water-saturated supercritical CO<sub>2</sub>, formation of ikaite and monohydrocalcite are both highly
exothermic, yet metastable to calcite, and are therefore likely intermediates
upon carbonation of CaO and Ca(OH)<sub>2</sub> according to the Ostwald
step rule. Hence low energy nonclassical crystallization pathways
that utilize these intermediates are available for calcite formation
in CO<sub>2</sub>-rich environments as well as aqueous systems, particularly
in water-saturated systems even though water is less than only 1%
by mass. Formation free energies calculated for Ca analogues of nesquehonite
(MgCO<sub>3</sub>·3H<sub>2</sub>O), lansfordite (MgCO<sub>3</sub>·5H<sub>2</sub>O), hydromagnesite (Mg<sub>5</sub>(CO<sub>3</sub>)<sub>4</sub>(OH)<sub>2</sub>·4H<sub>2</sub>O), and pokrovskite
(Mg<sub>2</sub>CO<sub>3</sub>(OH)<sub>2</sub>) are exothermic in both
aqueous and water-saturated scCO<sub>2</sub> from 273 to 373 K, but
they are always metastable with respect to the observed Ca minerals.
Hence they may form prenucleation clusters, transient intermediates,
or localized coordination arrangements trapped in hydrated ACC, but
will never be observed in nature. The arrangement of CaCO<sub>3</sub>·6H<sub>2</sub>O complexes in ikaite is proposed as the structure
of prenucleation clusters
Symétries et asymétries dans un tutorat distanciel : typologie et conséquences pour les participants
communication acceptée par le comité scientifique du colloqueInternational audienceNous nous proposons de contribuer à la réflexion autour de la question du tutorat distanciel et en particulier lorsque celui-ci est assuré par des étudiants pour d'autres étudiants. Nous nous intéressons particulièrement aux symétries et asymétries présentes dans tout dispositif de formation et à la manière dont elles sont perçues ou non pas les participants et gérées par les uns et les autres dans le déroulement d'un tutorat
Ab Initio Thermodynamic Model for Magnesium Carbonates and Hydrates
An
ab initio thermodynamic framework for predicting properties
of hydrated magnesium carbonate minerals has been developed using
density-functional theory linked to macroscopic thermodynamics through
the experimental chemical potentials for MgO, water, and CO<sub>2</sub>. Including semiempirical dispersion via the Grimme method and small
corrections to the generalized gradient approximation of Perdew, Burke,
and Ernzerhof for the heat of formation yields a model with quantitative
agreement for the benchmark minerals brucite, magnesite, nesquehonite,
and hydromagnesite. The model shows how small differences in experimental
conditions determine whether nesquehonite, hydromagnesite, or magnesite
is the result of laboratory synthesis from carbonation of brucite,
and what transformations are expected to occur on geological time
scales. Because of the reliance on parameter-free first-principles
methods, the model is reliably extensible to experimental conditions
not readily accessible to experiment and to any mineral composition
for which the structure is known or can be hypothesized, including
structures containing defects, substitutions, or transitional structures
during solid state transformations induced by temperature changes
or processes such as water, CO<sub>2</sub>, or O<sub>2</sub> diffusion.
Demonstrated applications of the ab initio thermodynamic framework
include an independent means to evaluate differences in thermodynamic
data for lansfordite, predicting the properties of Mg analogues of
Ca-based hydrated carbonates monohydrocalcite and ikaite, which have
not been observed in nature, and an estimation of the thermodynamics
of barringtonite from the stoichiometry and a single experimental
observation
Molecular-level understanding of environmental interfaces using density functional theory modeling
AbstractThe ability to apply existing density functional theory-based modeling techniques to timely research problems in environmental chemistry is demonstrated by an ab initio thermodynamics investigation of stable hydrated oxide surface models and a comparative reactivity study of Pb(II) adsorption on two water-mineral interfaces with a common geometry, but distinct electronic structure. We emphasize the unique considerations required to produce chemically reasonable structural models for hydrated surfaces and surface complex structures, as well as how to use experimental insights to limit the extensive configuration space encountered in complex hydrated models relative to theoretical surface science done under idealized, ultra-high vacuum conditions