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The connection between solution structure, particle forces, and emergent phenomena at solid-liquid interfaces remains ambiguous. In this case study on boehmite aggregation, we established a connection between interfacial solution structure, emerging hydration forces between two approaching particles, and the resulting structure and kinetics of particle aggregation. In contrast to expectations from continuum-based theories, we observed a nonmonotonic dependence of the aggregation rate on the concentration of sodium chloride, nitrate, or nitrite, decreasing by 15-fold in 4 molal compared to 1 molal solutions. These results are accompanied by an increase in repulsive hydration forces and interfacial oscillatory features from 0.27-0.31 nm in 0.01 molal to 0.38-0.52 nm in 2 molal. Moreover, molecular dynamics (MD) simulations indicated that these changes correspond to enhanced ion correlations near the interface and produced loosely bound aggregates that retain electrolyte between the particles. We anticipate that these results will enable the prediction of particle aggregation, attachment, and assembly, with broad relevance to interfacial phenomena.

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We present a dataset of structural relaxations of bulk ternary transition metal dichalcogenides (TMDs) computed via plane-wave density functional theory (DFT). We examined combinations of up to two chalcogenides with seven transition metals from groups 4-6 in octahedral (1T) or trigonal prismatic (2H) coordination. The full dataset consists of 672 unique stoichiometries, with a total of 50,337 individual configurations generated during structural relaxation. Our motivations for building this dataset are (1) to develop a training set for the generation of machine and deep learning models and (2) to obtain structural minima over a range of stoichiometries to support future electronic analyses. We provide the dataset as individual VASP xml files as well as all configurations encountered during relaxations collated into an ASE database with the corresponding total energy and atomic forces. In this report, we discuss the dataset in more detail and highlight interesting structural and electronic features of the relaxed structures.

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Incorporation of a Ba impurity in amorphous calcium carbonate (ACC) is shown with ab initio molecular dynamics simulations to have a long-range effect on its atomic-level structure and to be energetically favoured relative to incorporation in crystalline calcium carbonate polymorphs. The ability of carbonate ions to rotate and of ACC to undergo local density changes explain ACC's propensity for incorporating divalent metal impurities with a wide range of ionic radii. These findings provide an atomic-level basis for understanding the significant effects of low concentrations of impurities on the structure of ACC.

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Water is known to play a controlling role in directing mineralization pathways and stabilizing metastable amorphous intermediates in hydrous carbonate mineral MCO3·nH2O systems, where M2+ is a divalent metal cation. Despite this recognition, the nature of the controls on crystallization is poorly understood, largely owing to the difficulty in characterizing the dynamically disordered structures of amorphous intermediates at the atomic scale. Here, we present a series of atomistic models, derived from ab initio molecular dynamics simulation, across a range of experimentally relevant cations (M = Ca, Mg, Sr) and hydration levels (0 ≤ n ≤ 2). Theoretical simulations of the dependence of the X-ray pair distribution function on the hydration level n show good agreement with available experimental data and thus provide further evidence for a lack of significant nanoscale structure in amorphous carbonates. Upon dehydration, the metal coordination number does not change significantly, but the relative extent of water dissociation increases, indicating that a thermodynamic driving force exists for water dissociation to accompany dehydration. Mg strongly favors monodentate conformation of carbonate ligands and shows a marked preference to exchange monodentate carbonate O for water O upon hydration, whereas Ca and Sr exchange mono- and bidentate carbonate ligands with comparable frequency. Water forms an extensive hydrogen bond network among both water and carbonate groups that exhibits frequent proton transfers for all three cations considered suggesting that proton mobility is likely predominantly due to water dissociation and proton transfer reactions rather than molecular water diffusion.

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HYPOTHESIS: The precipitation and dissolution of aluminum-bearing mineral phases in aqueous systems often proceed via changes in both aluminum coordination number and connectivity, complicating molecular-scale interpretation of the transformation mechanism. Here, the thermally induced transformation of crystalline sodium aluminum salt hydrate, a phase comprised of monomeric octahedrally coordinated aluminate which is of relevance to industrial aluminum processing, has been studied. Because intermediate aluminum coordination states during melting have not previously been detected, it is hypothesized that the transition to lower coordinated aluminum ions occurs within ahighly disordered quasi-two-dimensional phase at the solid-solution interface. EXPERIMENTS AND SIMULATIONS: In situ X-ray diffraction (XRD), Raman and27Al nuclear magnetic resonance (NMR) spectroscopy were used to monitor the melting transition of nonasodium aluminate hydrate (NSA, Na9[Al(OH)6]2·3(OH)·6H2O). A mechanistic interpretation was developed based on complementary classical molecular dynamics (CMD) simulations including enhanced sampling. A reactive forcefield was developed to bridge speciation in the solution and in the solid phase. FINDINGS: In contrast to classical dissolution, aluminum coordination change proceeds through a dynamically stabilized ensemble of intermediate states in a disordered layer at the solid-solution interface. In both melting and dissolution of NSA, octahedral, monomeric aluminum transition through an intermediate of pentahedral coordination. The intermediate dehydroxylates to form tetrahedral aluminate (Al(OH)4-) in the liquid phase. This coordination change is concomitant with a breaking of the ionic aluminate-sodium ionlinkages. The solution phase Al(OH)4- ions subsequently polymerize into polynuclear aluminate ions. However, there are some differences between bulk melting and interfacial dissolution, with the onset of the surface-controlled process occurring at a lower temperature (∼30 °C) and the coordination change taking place more gradually as a function of temperature. This work to determine the local structure and dynamics of aluminum in the disordered layer provides a new basis to understand mechanisms controlling aluminum phase transformations in highly alkaline solutions.

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Gibbsite [α-Al(OH)3] is the solubility limiting phase for aluminum across a wide pH range, and it is a common mineral phase with many industrial applications. The growth mechanism of this layered-structure material, however, remains incompletely understood. Synthesis of gibbsite at low to circumneutral pH yields nanoplates with substantial interlayer disorder. Here we examine defects in this material in detail, and the effects of recrystallization in highly alkaline sodium hydroxide solution at 80 °C. We employed a multimodal approach, including scanning electron microscopy, magic-angle spinning nuclear magnetic resonance (MAS-NMR), Raman and infrared spectroscopies, X-ray diffraction (XRD), and X-ray total scattering pair distribution function (XPDF) analysis to characterize the ageing of the nanoplates over several days. XRD and XPDF indicate that gibbsite nanoplates precipitated at circumneutral pH contain dense, truncated sheets imparting a local difference in interlayer distance. These interlayer defects appear well described by flat Al13 aluminum hydroxide nanoclusters nearly isostructural with gibbsite sheets present under synthesis conditions and trapped as interlayer inclusions during growth. Ageing at elevated temperature in alkaline solutions gradually improves crystallinity, showing a gradual increase in H-bonding between interlayer OH groups. Between 7 to 8 vol% of the initial gibbsite nanoparticles exhibit this defect, with the majority of differences disappearing after 2-4 hours of recrystallization in alkaline solution. The results not only identify the source of disorder in gibbsite formed under acidic/neutral conditions but also point to a possible cluster-mediated growth mechanism evident through inclusion of relict oligomers with gibbsite-like topology trapped in the interlayer spaces.

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Aluminate salts precipitated from caustic alkaline solutions exhibit a correlation between the anionic speciation and the identity of the alkali cation in the precipitate, with the aluminate ions occurring either in monomeric (Al(OH)4-) or dimeric (Al2O(OH)62-) forms. The origin of this correlation is poorly understood as are the roles that oligomeric aluminate species play in determining the solution structure, prenucleation clusters, and precipitation pathways. Characterization of aluminate solution speciation with vibrational spectroscopy results in spectra that are difficult to interpret because the ions access a diverse and dynamic configurational space. To investigate the Al(OH)4- and Al2O(OH)62- anions within a well-defined crystal lattice, inelastic neutron scattering (INS) and Raman spectroscopic data were collected and simulated by density functional theory for K2[Al2O(OH)6], Rb2[Al2O(OH)6], and Cs[Al(OH) 4]·2H2O. These structures capture archetypal solution aluminate species: the first two salts contain dimeric Al2O(OH)62- anions, while the third contains the monomeric Al(OH)4- anion. Comparisons were made to the INS and Raman spectra of sodium aluminate solutions frozen in a glassy state. In contrast to solution systems, the crystal lattice of the salts results in well-defined vibrations and associated resolved bands in the INS spectra. The use of a theory-guided analysis of the INS of this solid alkaline aluminate series revealed that differences were related to the nature of the hydrogen-bonding network and showed that INS is a sensitive probe of the degree of completeness and strength of the bond network in hydrogen-bonded materials. Results suggest that the ionic size may explain cation-specific differences in crystallization pathways in alkaline aluminate salts.

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Two-dimensional materials exhibit properties promising for novel applications. Topologically protected states at their edges can be harnessed for use in quantum devices. We use ab initio simulations to examine properties of edges in 1T'-WTe2 monolayers, known to exhibit topological order, and their interactions with Cu atoms. Comparison of (010)-oriented edges that have the same composition but different terminations shows that, as the number of Cu atoms increases, their thermodynamically preferred arrangement depends on the details of the edge structure. Cu atoms aggregate into a cluster at the most stable edge; while the cluster is nonmagnetic, it spin-polarizes the W atoms along the edge, which removes the topological protection. At the metastable edge, Cu atoms form a chain incorporated into the WTe2 lattice; the topological state is preserved in spite of the dramatic edge restructuring. This suggests that exploiting interactions of metal species with metastable edge terminations can provide a path toward noninvasive interfaces.

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Hematite (α-Fe2O3) exerts a strong control over the transport of minor but critical metals in the environment and is used in multiple industrial applications; the photocatalysis community has explored the properties of hematite nanoparticles over a wide range of transition metal dopants. Nonetheless, simplistic assumptions are used to rationalize the local coordination environment of impurities in hematite. Here, we use ab initio molecular dynamics (AIMD)-guided structural analysis to model the extended X-ray absorption fine structure (EXAFS) of Cu2+- and Zn2+-doped hematite nanoparticles. Specific defect-impurity associations were identified, and the local coordination environments of Cu and Zn both displayed considerable configurational disorder that, in aggregate, approached Jahn-Teller-like distortion for Cu but, in contrast, maintained hematite-like symmetry for Zn. This study highlights the role of defects in accommodating impurities in a nominally low-entropy phase and the limits to traditional shell-by-shell fitting of EXAFS for dopants/impurities in unprecedented bonding environments.

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Tetrahedrally coordinated aluminate Al(OH)4- and dialuminate Al2O(OH)62- anions are considered to be major species in aluminum-rich alkaline solutions. However, their relative abundance remains difficult to spectroscopically quantify due to local structure similarities and poorly understood effects arising from extent of polymerization and counter-cations. To help unravel these relationships here we report detailed characterization of three solid-phase analogues as structurally and compositionally well-defined reference materials. We successfully synthesized a cesium salt of the aluminate monomer, CsAl(OH)4·2H2O, for comparison to potassium and rubidium salts of the aluminate dimer, K2Al2O(OH)6, and Rb2Al2O(OH)6, respectively. Single crystal and powder X-ray diffraction methods clearly reveal the structure and purity of these materials for which a combination of 27Al MAS-NMR, Al K-edge X-ray absorption and Raman/IR spectroscopies was then used to fingerprint the two major tetrahedrally coordinated Al species. The resulting insights into the effect of Al-O-Al bridge formation between aluminate tetrahedra on spectroscopic features may also be generalized to the many materials that are based on this motif.

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Zn is an essential micronutrient that is often limited in tropical, lateritic soils in part because it is sequestered in nominally refractory iron oxide phases. Stable phases such as goethite and hematite, however, can undergo reductive recrystallization without a phase change under circumneutral pH conditions and release metal impurities such as Zn into aqueous solutions. Further, the process appears to be driven by Fe vacancies. In this contribution, we used ab initio molecular dynamics informed extended X-ray absorption fine structure spectra to show that Zn incorporated in the structure of hematite is associated with coupled O-Fe and protonated Fe vacancies, providing a potential link between crystal chemistry and the bioavailability of Zn.

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The development of advanced functional nanomaterials for selective adsorption in complex chemical environments requires partner studies of binding mechanisms. Motivated by observations of selective Cr(III) adsorption on boehmite nanoplates (γ-AlOOH) in highly caustic multicomponent solutions of nuclear tank waste, here we unravel the adsorption mechanism in molecular detail. We examined Cr(III) adsorption to synthetic boehmite nanoplates in sodium hydroxide solutions up to 3 M, using a combination of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), scanning/transmission electron microscopy (S/TEM), electron energy loss spectroscopy (EELS), high-resolution atomic force microscopy (HR-AFM), time-of-fight secondary ion mass spectrometry (ToF-SIMS), Cr K-edge X-ray absorption near edge structure (XANES)/extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR). Adsorption isotherms and kinetics were successfully fit to Langmuir and pseudo-second-order kinetic models, respectively, consistent with monotonic uptake of Cr(OH)4- monomers until saturation coverage of approximately half the aluminum surface site density. High resolution AFM revealed monolayer cluster self-assembly on the (010) basal surfaces with increasing Cr(III) loading, possessing a structural motif similar to guyanaite (ß-CrOOH), stabilized by corner-sharing Cr-O-Cr bonds and attached to the surface with edge-sharing Cr-O-Al bonds. The selective uptake appears related to short-range surface templating effects, with bridging metal connections likely enabled by hydroxyl anion ligand exchange reactions at the surface. Such a cluster formation mechanism, which stops short of more laterally extensive heteroepitaxy, could be a metal uptake discrimination mechanism more prevalent than currently recognized.

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Aluminum-bearing minerals show different hydrogen evolution and dissolution properties when subjected to radiation, but the complicated sequence of events following interaction with high-energy radiation is not understood. To gain insight into the possible mechanisms of hydrogen production in nanoparticulate minerals, we study the electronic response and determine the bandgap energies of three common aluminum-bearing minerals with varying hydrogen content: gibbsite (Al(OH)3), boehmite (AlOOH), and alumina (Al2O3) using electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and first-principles electronic structure calculations employing hybrid density functionals. We find that the amount of hydrogen has only a small effect on the number and spectrum of photoexcitations in this class of materials. Electronic structure calculations demonstrate that low energy electrons are isotropically mobile, while holes in the valence band are likely constrained to move in layers. Furthermore, holes in the valence band of boehmite are found to be significantly more mobile than those in gibbsite, suggesting that the differences in radiolytic and dissolution behavior are related to hole transport.

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Boehmite (γ-AlOOH) and gibbsite (α-Al-(OH)3) are important archetype (oxy)hydroxides of aluminum in nature that also play diverse roles across a plethora of industrial applications. Developing the ability to understand and predict the properties and characteristics of these materials, on the basis of their natural growth or synthesis pathways, is an important fundamental science enterprise with wide-ranging impacts. The present study describes bulk and surface characteristics of these novel materials in comprehensive detail, using a collectively sophisticated set of experimental capabilities, including a range of conventional laboratory solids analyses and national user facility analyses such as synchrotron X-ray absorption and scattering spectroscopies as well as small-angle neutron scattering. Their thermal stability is investigated using in situ temperature-dependent Raman spectroscopy. These pure and effectively defect-free materials are ideal for synthesis of advanced alumina products.

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We present a theoretical framework for calculating probe-position-dependent electron energy-loss near-edge structure for the scanning transmission electron microscope by combining density functional theory with dynamical scattering theory. We show how simpler approaches to calculating near-edge structure fail to include the fundamental physics needed to understand the evolution of near-edge structure as a function of probe position and investigate the dependence of near-edge structure on probe size. It is within this framework that density functional theory should be presented, in order to ensure that variations of near-edge structure are truly due to local electronic structure and how much from the diffraction and focusing of the electron beam.

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Using a combination of Z-contrast imaging and atomically resolved electron energy-loss spectroscopy on a scanning transmission electron microscope, we show that the chemical bonding of individual impurity atoms can be deduced experimentally. We find that when a Si atom is bonded with four atoms at a double-vacancy site in graphene, Si 3d orbitals contribute significantly to the bonding, resulting in a planar sp(2) d-like hybridization, whereas threefold coordinated Si in graphene adopts the preferred sp(3) hybridization. The conclusions are confirmed by first-principles calculations and demonstrate that chemical bonding of two-dimensional materials can now be explored at the single impurity level.

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Certain cobalt oxides are known to exhibit ordered Co spin states, as determined from macroscopic techniques. Here we report real-space atomic-resolution imaging of Co spin-state ordering in nanopockets of La(0.5)Sr(0.5)CoO(3-δ) thin films. Unlike the bulk material, where no Co spin-state ordering is found, thin films present a strain-induced domain structure due to oxygen vacancy ordering, inside of which some nanometer sized domains show high-spin Co ions in the planes containing O vacancies and low-spin Co ions in the stoichiometric planes. First-principles calculations provide support for this interpretation.

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We briefly review our implementation of the real-space Green's function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye-Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.

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A new strategy is presented to reduce primary X-ray damage in macromolecular crystallography. The strategy is based on separating the diffracting and damaged regions as much as feasible. The source of the radiation damage to macromolecular crystals is from two primary mechanisms: the direct excitations of electrons by absorption, and inelastic scattering of the X-rays. The first produces photoelectrons with their accompanying Auger electrons from relaxation of the core hole and the second creates Compton electrons. The properties of these two mechanisms and calculations of primary X-ray damage quantify how to modify the spatial distribution of X-rays to reduce the deleterious effects of radiation damage. By focusing the incident X-rays into vertical stripes, it is estimated that the survival (the time during which quality diffraction data can be obtained with a given X-ray flux) of large crystals can be increased by at least a factor of 1.6, while for very small platelet crystals the survival can be increased by up to a factor of 14.

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