Thermodynamics of the Metal Carbonates and Bicarbonates of Mn, Co, Ni, Cu, and Zn Relevant to Mineral Energetics.

The gas phase heats of formation of ground-state MCO, M(HCO), and M(HCO)(OH), where M = Mn, Co, Ni, Cu, and Zn, have been predicted using the correlated molecular orbital theory at the CCSD(T) level extrapolated to the complete basis set limit using the Feller-Peterson-Dixon (FPD) approach. Cohesive energies of the carbonates were predicted based on the calculated gas phase and experimental solid heats of formation. Coulombic dissociation energies (CDEs) between metal cations and anions show a near-linear correlation with Shannon metal cation atomic radii, yet no correlation is found with the hardness of these cations.

Optimization of Thermal Conductance at Interfaces Using Machine Learning Algorithms.

Optimization of thermal transport across the interface of two different materials is critical to micro-/nanoscale electronic, photonic, and phononic devices. Although several examples of compositional intermixing at the interfaces having a positive effect on interfacial thermal conductance (ITC) have been reported, an optimum arrangement has not yet been determined because of the large number of potential atomic configurations and the significant computational cost of evaluation. On the other hand, computation-driven materials design efforts are rising in popularity and importance.

In situ imaging of amorphous intermediates during brucite carbonation in supercritical CO.

Progress in understanding crystallization pathways depends on the ability to unravel relationships between intermediates and final crystalline products at the nanoscale, which is a particular challenge at elevated pressure and temperature. Here we exploit a high-pressure atomic force microscope to directly visualize brucite carbonation in water-bearing supercritical carbon dioxide (scCO) at 90 bar and 50 °C. On introduction of water-saturated scCO, in situ visualization revealed initial dissolution followed by nanoparticle nucleation consistent with amorphous magnesium carbonate (AMC) on the surface.

Ab initio thermodynamics reveals the nanocomposite structure of ferrihydrite.

Ferrihydrite is a poorly crystalline iron oxyhydroxide nanomineral that serves a critical role as the most bioavailable form of ferric iron for living systems. However, its atomic structure and composition remain unclear due in part to ambiguities in interpretation of X-ray scattering results. Prevailing models so far have not considered the prospect that at the level of individual nanoparticles multiple X-ray indistinguishable phases could coexist.

Thermodynamics of Metal Carbonates and Bicarbonates and Their Hydrates for Mg, Ca, Fe, and Cd Relevant to Mineral Energetics.

The heats of formation of the carbonate, bicarbonate, and bicarbonate/hydroxide metal complexes, including hydrates of Mg, Ca, Fe, and Cd, and the oxides, dichlorides, and dihydroxides are predicted from atomization energies using correlated molecular orbital theory at the CCSD(T) level extrapolated to the complete basis set limit following the Feller-Peterson-Dixon (FPD) approach. Using the calculated gas phase values and the available experimental solid-state values, we predicted the cohesive energies of selective minerals. The gas phase decomposition energies of MO, CO, and HO follow the order Mg ≈ Ca > Cd ≈ Fe and correlate with the hardness of the metal +2 ions.

Quantifying the Impact of Magnesium on the Stability and Water Binding Energy of Hydrated Calcium Carbonates by Ab Initio Thermodynamics.

Determining conditions that drive carbonate formation is important for many phenomena such as paleoindicators for mineral deposition, the carbon cycle, biomineralization, and industrial applications such as scale inhibition and manufacturing of cement and concrete. Magnesium and incorporated water have been observed to play critical roles in nonclassical crystallization pathways of calcium carbonate, the dominant carbonate found in nature, through promoting formation of low energy metastable intermediates such as monohydrocalcite (CaCO·HO), ikaite (CaCO·6HO), and amorphous calcium carbonate (CaCO·HO). The impact of Mg on the thermodynamics and water binding ability of these hydrated intermediates is challenging to measure and is not understood at the molecular level.

Quantifying small changes in uranium oxidation states using XPS of a shallow core level.

The U 4f line is commonly used to determine uranium oxidation states with X-ray photoelectron spectroscopy (XPS). In contrast, the XPS of the shallow core-levels of uranium are rarely recorded. Nonetheless, theory has shown that the U 5d (and 5p) multiplet structure is very sensitive to oxidation state.

Oxidative Corrosion of the UO (001) Surface by Nonclassical Diffusion.

Uranium oxide is central to every stage of the nuclear fuel cycle, from mining through fuel fabrication and use, to waste disposal and environmental cleanup. Its chemical and mechanical stability are intricately linked to the concentration of interstitial O atoms within the structure and the oxidation state of U. We have previously shown that, during corrosion of the UO (111) surface under either 1 atm of O gas or oxygenated water at room temperature, oxygen interstitials diffuse into the substrate to form a superlattice with three-layer periodicity.

Ab Initio Thermodynamics and the Relationship between Octahedral Distortion, Lattice Structure, and Proton Substitution Defects in Malachite/Rosasite Group Endmember Pokrovskite MgCO(OH).

Divalent metal hydroxycarbonates with MCO(OH) stoichiometry are widely used in industry and are abundant in nature as the malachite/rosasite group of minerals. Essential to their performance as catalytic precursors and in nanoelectronics, these materials and minerals exhibit a high degree of cation ordering in mixed metal systems due to differences in distortion of the octahedral metal sites. Density-functional theory (DFT) calculations on pokrovskite MgCO(OH) in the rosasite structure and Mg analogues of monoclinic and orthorhombic forms of malachite determine that the octahedral sites are innately distorted, and that d Cu(II) Jahn-Teller distortion accommodates this distortion rather than causes it, leading to the significant preference of Cu for the type I octahedral sites.

UO(2) Oxidative Corrosion by Nonclassical Diffusion.

Using x-ray scattering, spectroscopy, and density-functional theory, we determine the structure of the oxidation front when a UO(2) (111) surface is exposed to oxygen at ambient conditions. In contrast to classical diffusion and previously reported bulk UO(2+x) structures, we find oxygen interstitials order into a nanoscale superlattice with three-layer periodicity and uranium in three oxidation states: IV, V, and VI. This oscillatory diffusion profile is driven by the nature of the electron transfer process, and has implications for understanding the initial stages of oxidative corrosion in materials at the atomistic level.

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 CO2. 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.

Structural basis of pathway-dependent force profiles in stretched DNA.

Understanding the mechanical properties that determine the flexibility of DNA is important, as DNA must bend and/or stretch in order to function biologically. Recent single-molecule experiments have shown that above a certain loading rate double-stranded DNA is more stable when stretched from the 3' termini than when stretched from the 5' termini. Unfortunately these experiments cannot provide insight into the structural basis for this behavior.

Electronically induced atom motion in engineered CoCun nanostructures.

We have measured the quantum yield for exciting the motion of a single Co atom in CoCu(n) linear molecules constructed on a Cu(111) surface. The Co atom switched between two lattice positions during electron excitation from the tip of a scanning tunneling microscope. The tip location with highest probability for inducing motion was consistent with the position of an active state identified through electronic structure calculations.