Rational ab initio modeling for low energy hydrogen-bonded phyllosilicate polytypes (original) (raw)
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Kaolin polytypes revisited ab initio
Acta Crystallographica Section B Structural Science, 2008
The well known 36 distinguishable transformations between adjacent kaolin layers are split into 20 energetically distinguishable transformations (EDT) and 16 enantiomorphic transformations, hereafter denoted EDT*. For infinitesimal energy contribution of interactions between non-adjacent layers, the lowest-energy models must result from either (a) repeated application of an EDT or (b) alternate application of an EDT and its EDT*. All modeling, quantum input preparation and interpretation was performed with Materials Toolkit, and quantum optimizations with VASP. Kaolinite and dickite are the lowest-energy models at zero temperature and pressure, whereas nacrite and HP-dickite are the lowest-enthalpy models under moderate pressures based on a rough enthalpy/pressure graph built from numbers given in the supplementary tables. Minor temperature dependence of this calculated 0 K graph would explain the bulk of the current observations regarding synthesis, diagenesis and transformation of...
Stability of 1-layer polytype of kaolin by means of molecular dynamics simulation
Clay science, 2002
An energy map for 1-layer kaolin polytypes was developed, plotting internal energies calculated from results of Molecular Dynamics (MD) simulations. The energy map indicates stabilities of various kaolin 1-layer polytypes by the internal energies as a function of the interlayer shift. The interlayer shifts were described on the basis of a pseudo-hexagonal lattice which contains an asymmetric unit of kaolinite structure. The MD simulations were carried out at grid points on a (001) plane of pseudo-hexagonal lattice with increments of 1/12 along each axis. In order to remain interlayer configurations in each structure, we adopted a constraint that the positions of non-hydrogen atoms had been fixed. The present energy map shows two energetic minima, which correspond to structures of kaolinite and its enantiomorph, and has no minimum at (1/3, 1/3) in fractional coordinates of a projection onto (001) plane of pseudo-hexagonal lattice, which had been clearly shown in the electrostatic pot...
Physics and Chemistry of Minerals, 2012
The high-pressure behavior of Keokuk kaolinite has been studied to 9.5 GPa by infrared spectroscopy using synchrotron radiation. The kaolinite-I ! kaolinite-II and kaolinite-II ! kaolinite-III transformations have clear spectroscopic expression, with discontinuities coinciding with the transformation pressures bracketed by X-ray diVraction (Welch and Crichton in Am Mineral 95:651-654, 2010). The experimental spectra have been interpreted from band assignments derived from density functional theory for the structures of kaolinite-II and kaolinite-III, using as starting models the ab initio structures reported by Mercier and Le Page (Acta Crystallogr A B64: 131-143, 2008, Mater Sci Technol 25:437-442, 2009) and unit-cell parameters from Welch and Crichton (Am Mineral 95:651-654, 2010). The relaxed theoretical structures are very similar to those reported by Mercier and Le Page (Acta Crystallogr A B64: 131-143, 2008, Mater Sci Technol 25:437-442, 2009) in their theoretical investigation of kaolinite polytypes at high pressure. The vibrational spectra calculated from the quantum-mechanical analysis allow band assignments of the IR spectra to be made and provide insights into the behavior of diVerent OH environments in the two high-pressure polytypes. The single perpendicular-interlayer OH group of kaolinite-III has a distinctive spectroscopic signature that is diagnostic of this polytype ( = 3,595 cm ¡1 at 9.5 GPa) and is sensitive to the compression/expansion of the interlayer space. This OH group also has a distinctive signature in the calculated spectra. The spectra collected on decompression are those of kaolinite-III and persist largely unchanged to 4.6 GPa, except for a continuous blue shift of the 3,595 cm ¡1 band to 3,613 cm ¡1 . Finally, kaolinite-I is recovered at 0.6 GPa, conWrming the kaolinite-III ! kaolinite-I transformation previously observed by X-ray diVraction, and the irreversibility of the kaolinite-II ! kaolinite-III transformation. The ambient spectra collected at the start and Wnish of the experiment are those of kaolinite-I, and start/Wnish band frequencies agree to within 6 cm ¡1 .
Theoretical Study of Kaolinite Structure; Energy Minimization and Crystal Properties
World Journal of Nano Science and Engineering, 2011
Computational energy minimization techniques have been used to study the structure and crystal properties of kaolinite. The full elastic tensors of the sheet silicates of clay have been derived with first-principles calculations based on density functional theory. All calculations were performed using GULP program.
Communication: Molecular simulation study of kaolinite intercalation with realistic layer size
The Journal of Chemical Physics, 2014
Intercalation phenomena of kaolinite in aqueous potassium acetate and in hexyl-amine solutions are studied by large scale molecular dynamics simulations. The simulated kaolinite particle is constructed from ~6.5×10 6 atoms, producing a particle size of ~100 nm × 100 nm × 10 nm. The simulation with potassium acetate results in a stable kaolinite-potassium acetate complex, with a basal spacing that is in close agreement with experimental data. The simulation with hexyl-amine shows signs of the experimentally observed delamination of kaolinite (the initial phase of the formation of nanoscrolls from the external layers).
Molecular simulation study of the curling behavior of the finite free-standing kaolinte layer
Computational Materials Science, 2021
Since kaolinite nanolayers potentially have many important applications, it is crucial to determine the factors that govern their curling behavior. The curling of a single-layer, free-standing kaolinite nanoparticle consisting of nearly 1 million atoms is studied with classical molecular dynamics simulation. Two up-to-date force fields are employed to describe the atomic interactions in the clay. The influence of force field details and of the use of different treatments of interactions (long range correction, potential cutoff radius) on the curling direction is systematically examined. That includes a practically infinite potential cutoff , which means that all atomic interactions are considered explicitly. For every inspected case, the structure is characterized by the overall shape of the particle, the axis of roll-up and particular bond lengths and angles. Both of the two possible curling directions are detected in the present simulations, which contradicts the crystallographic assumption that the constituent tetrahedral sheet can exclusively be on the concave side of curled kaolinite nanolayers.
Nanoscale structural and morphological features of kaolinite nanoscrolls
Applied Clay Science, 2020
One-dimensional kaolinite nanoscrolls have been arousing great interest due to their applicability in advanced materials for adsorption and slow release of reagents, as well as in nanoscale reactors and carriers. Production of high-quality halloysite-like nanoscrolls with controlled morphology, however, remains challenging as there is a lack of understanding of the curling process of kaolinite layers on the atomic scale. In the present work, the nanoscrolls were efficiently produced from the readily available natural kaolinite using a two-pot solvothermal exfoliation of the kaolinite-dimethyl sulfoxide, kaolinite-urea, and kaolinite-N-methylformamide precursors. The structures, sizes, shapes and crystallographic properties of the produced halloysite-like nanoscrolls were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and electron tomography in scanning transmission mode (STEM). In order to avoid structural disintegration of the highly electron beam-sensitive nanoscrolls, we used a low imaging current (70-240 e/A 2); as a result, the acquisition of structure images was possible. In contrast to the triclinic symmetry of kaolinite, electron diffraction patterns suggest a hexagonal symmetry of exfoliated layers. Both HRTEM and STEM tomography show partially and completely rolled up layers, with the axis of curvature being parallel to either the a or b axes. The methanolgrafted nanoscrolls have a variable but larger basal spacing (from 0.76 to 0.90 nm) than the value in ordered kaolinite (0.72 nm). Their external diameters range from 22 to 75 nm, lengths from 218 to 2287 nm, aspect ratios from 5 to 74, and the scrolls have recognizable chirality. Crystallographic image processing of HRTEM structure images suggest that the tetrahedral sheet can be on either the outer or the inner sides of the nanoscrolls. Molecular simulation results for the curled kaolinite layers are consistent in their details with the experimental observations, suggesting that the layers may roll up either way.
Molecular Dynamics Modeling of Kaolinite Particle Associations
The Journal of Physical Chemistry C, 2021
We developed a new procedure for calculating finite-size kaolinite particles, their associations with complex surface chemistry, and the natural flexibility of sheets within a particle using a large-scale atomic/molecular massively parallel simulator. For the first time, all possible particle associations previously described in the literature were obtained using an atomic method. The structural configurations obtained were shifted face-face, angular edge-edge, corner-corner, and shifted face-face-face booklet associations. The simulations showed that if the initial angle between two interacting particles is less than 45°, the particles will form layer-to-layer aggregates. If the angle is larger than 60°, the particles will form an angular arrangement. The densities of kaolinite arrangements with dense and loose packings were evaluated as a function of the structure. The densest structures, as expected, were the layered structures, with four and two layers. The density of the shifted face-face packing was about the same density as the two. The face-face-face association showed lower density, and the angular edge-edge association showed a 3 times lower density than the densest, four-layer structure.
O-HCl, at pH between 9 and 11, has been investigated at 200À250ºC from 1 to 180 days. The X-ray diffraction study of the K-bearing system indicates that the solid products formed were Li-A(BW) zeolite, crystalline silica, and randomly ordered chlorite-smectite mixed layers, dioctahedral on average, with a chlorite:smectite ratio of 1:4. Study by electron microscopy reveals, however, the presence of mixtures of very thin particles of smectite, di,trioctahedral chlorite and mixed-layer phases. The X-ray study of the solid products formed in the Na-bearing system reveals that the neo-formed phases were Li-A(BW) zeolite, analcime and partially ordered chlorite-vermiculite mixed-layers with high chlorite:vermiculite ratio. The transmission microscopic study of these phases shows the presence of dioctahedral 'vermiculitic' curved particles and di,trioctahedral chlorite-like particles. In both cases, long reaction times cause formation of a 7 Å phase with composition intermediate between kaolinite and serpentine. The composition of the solutions suggests that the stable phases would be chlorite/smectite mixed-layers, with compositions near tosudite in the case of the K-bearing reaction and both a dioctahedral phase (tosudite?) and a trioctahedral one (clinochlore) in the case of the Na-bearing reaction.