Showing papers on "Crystal published in 2006"

Three-dimensional mapping of a deformation field inside a nanocrystal

Abstract: Synchrotron X-ray radiation, produced by electron accelerators at central facilities, can now be produced in extremely narrow coherent beams. When these X-rays illuminate a crystal of nanometre dimensions a diffraction pattern emerges that is highly resolved. This provides a powerful new tool for structural analysis, as the fine features of the diffraction pattern can be interpreted in terms of sub-atomic distortions within the crystal attributable to its contact with an external support. Coherent X-ray diffraction patterns derived from third-generation synchrotron radiation sources can lead to quantitative three-dimensional imaging of lattice strain on the nanometre scale. Coherent X-ray diffraction imaging is a rapidly advancing form of microscopy: diffraction patterns, measured using the latest third-generation synchrotron radiation sources, can be inverted to obtain full three-dimensional images of the interior density within nanocrystals1,2,3. Diffraction from an ideal crystal lattice results in an identical copy of this continuous diffraction pattern at every Bragg peak. This symmetry is broken by the presence of strain fields, which arise from the epitaxial contact forces that are inevitable whenever nanocrystals are prepared on a substrate4. When strain is present, the diffraction copies at different Bragg peaks are no longer identical and contain additional information, appearing as broken local inversion symmetry about each Bragg point. Here we show that one such pattern can nevertheless be inverted to obtain a ‘complex’ crystal density, whose phase encodes a projection of the lattice deformation. A lead nanocrystal was crystallized in ultrahigh vacuum from a droplet on a silica substrate and equilibrated close to its melting point. A three-dimensional image of the density, obtained by inversion of the coherent X-ray diffraction, shows the expected facetted morphology, but in addition reveals a real-space phase that is consistent with the three-dimensional evolution of a deformation field arising from interfacial contact forces. Quantitative three-dimensional imaging of lattice strain on the nanometre scale will have profound consequences for our fundamental understanding of grain interactions and defects in crystalline materials4. Our method of measuring and inverting diffraction patterns from nanocrystals represents a vital step towards the ultimate goal of atomic resolution single-molecule imaging that is a prominent justification for development of X-ray free-electron lasers5,6,7.

Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting

Abstract: Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity--ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {1010} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.

Complete band gaps in two-dimensional phononic crystal slabs

Abstract: The propagation of acoustic waves in a phononic crystal slab consisting of piezoelectric inclusions placed periodically in an isotropic host material is analyzed. Numerical examples are obtained for a square lattice of quartz cylinders embedded in an epoxy matrix. It is found that several complete band gaps with a variable bandwidth exist for elastic waves of any polarization and incidence. In addition to the filling fraction, it is found that a key parameter for the existence and the width of these complete band gaps is the ratio of the slab thickness, d, to the lattice period, a. Especially, we have explored how these absolute band gaps close up as the parameter d/a increases. Significantly, it is observed that the band gaps of a phononic crystal slab are distinct from those of bulk acoustic waves propagating in the plane of an infinite two-dimensional phononic crystal with the same composition. The band gaps of the slab are strongly affected by the presence of cutoff frequency modes that cannot be excited in infinite media.

Structural and magnetic properties of uniform magnetite nanoparticles prepared by high temperature decomposition of organic precursors

Abstract: Magnetite nanoparticles (Fe3O4) of three different sizes below the limit for single domain magnetic behaviour have been obtained by thermal decomposition of an iron precursor in an organic medium in the presence of a surfactant. Good agreement between mean particle size obtained by TEM, crystal size calculated from x-ray diffraction and magnetic diameter calculated from magnetization curves measured at room temperature shows that the samples consist of uniform, crystalline and isolated magnetite nanoparticles with sizes between 5 and 11 nm. High saturation magnetization and high initial susceptibility values have been found, the latter decreasing as the particle size decreases. The main contribution to the anisotropy is magnetocrystalline and shape anisotropy, since surface anisotropy is suppressed by the oleic acid molecules which are covalently bonded to the nanoparticle surface.

Elucidating the effect of additives on the growth and stability of Cu2O surfaces via shape transformation of pre-grown crystals

Abstract: A new strategy of using pre-grown crystals to study preferential adsorption of various additives is demonstrated for the electrocrystallization of Cu2O. In this method, micron-size Cu2O crystals with well-defined cubic and octahedral shapes were first electrochemically grown, and their crystallization was resumed in a medium containing the additive to be investigated (e.g., Na+, NH4+, SO42-, Cl-, dodecyl sulfate). This method makes it possible to systematically study the interaction of additives with specific planes (e.g., {100} of a cube and {111} of an octahedron) already present. By observing shape transformation over time, the relative stabilities of {100}, {111}, and {110} planes of Cu2O in various growth media could be determined. During this study, a general scheme of forming new crystal shapes containing crystallographic planes that cannot be directly stabilized by preferential adsorption alone was also established (i.e., rhombicuboctahedral shape of Cu2O containing {110} planes). This method can ...

Evidence for complete surface wave band gap in a piezoelectric phononic crystal

Abstract: A complete surface acoustic wave band gap is found experimentally in a two-dimensional square-lattice piezoelectric phononic crystal etched in lithium niobate. Propagation in the phononic crystal is studied by direct generation and detection of surface waves using interdigital transducers. The complete band gap extends from 203 to 226 MHZ, in good agreement with theoretical predictions. Near the upper edge of the complete band gap, it is observed that radiation to the bulk of the substrate dominates. This observation is explained by introducing the concept of the sound line.

Analysis of amorphous and nanocrystalline solids from their X-ray diffraction patterns.

Abstract: The purpose of this paper is to provide a physical description of the amorphous state for pharmaceutical materials and to investigate the pharmaceutical implications. Techniques to elucidate structural differences in pharmaceutical solids exhibiting characteristic X-ray amorphous powder patterns are also presented. The X-ray amorphous powder diffraction patterns of microcrystalline cellulose, indomethacin, and piroxicam were measured with laboratory XRPD instrumentation. Analysis of the data were carried out using a combination of direct methods, such as pair distribution functions (PDF), and indirect material modeling techniques including Rietveld, total scattering, and amorphous packing. The observation of X-ray amorphous powder patterns may indicate the presence of amorphous, glassy or disordered nanocrystalline material in the sample. Rietveld modeling of microcrystalline cellulose (Avicel® PH102) indicates that it is predominantly disordered crystalline cellulose Form Iβ with some amorphous contribution. The average crystallite size of the disordered nanocrystalline cellulose was determined to be 10.9 nm. Total scattering modeling of ground samples of α, γ, and δ crystal forms of indomethacin in combination with analysis of the PDFs provided a quantitative picture of the local structure during various stages of grinding. For all three polymorphs, with increased grinding time, a two-phase system, consisting of amorphous and crystalline material, continually transformed to a completely random close packed (RCP) amorphous structure. The same pattern of transformation was detected for the Form I polymorph of piroxicam. However, grinding of Form II of piroxicam initially produced a disordered phase that maintained the local packing of Form II but over a very short nanometer length scale. The initial disordered phase is consistent with continuous random network (CRN) glass material. This initial disordered phase was maintained to a critical point when a transition to a completely amorphous RCP structure occurred. Treating X-ray amorphous powder patterns with different solid-state models, ranging from disordered nanocrystalline to glassy and amorphous, resulted in the assignment of structures in each of the systems examined. The pharmaceutical implications with respect to the stability of the solid are discussed.

Inorganic Scintillators for Detector Systems: Physical Principles and Crystal Engineering

Abstract: Scintillation and Inorganic Scintillators.- How User's Requirements Influence the Development of a Scintillator.- Scintillation Mechanisms in Inorganic Scintillators.- Influence of the Crystal Structure Defects on Scintillation Properties.- Crystal Engineering.- Two Examples of Recent Crystal Development.

Deformation and stress from in-pore drying-induced crystallization of salt

Abstract: The deformation and the fracture of porous solids from internal crystallization of salt is explored in the framework of the thermodynamics of unsaturated brittle poroelasticity. In the first place the usual theory of crystal growth in confined conditions is further developed in order to include both the deformation and the drying of the porous solid. The thermodynamics reveals the existence of a dilation coefficient associated with the crystallization process, and provides a solute crystal equilibrium condition which involves the relative humidity, the supersaturation, and the salt characteristics. This thermodynamic condition and the mechanical equilibrium of the solution crystal interface combine to give the current crystallization pore radius. Upscaling this information at the macroscopic scale, and taking into account the salt mass supplied by the invading solution, the approach leads to a quantitative analysis of the role of the pore size distribution on the crystal growth under repeated imbibition drying cycles. The deformation and the fracture of the porous solid from drying-induced crystallization are then considered in the context of brittle poroelasticity. The current unsaturated macroscopic poroelastic properties are upscaled from the microscopic elastic properties of the solid matrix and from the current liquid, crystal and gas saturations. The adoption of a fracture criterion based on the elastic energy that the solid matrix can ultimately store finally leads to the determination of how long a stone can resist repeated cycles of drying-induced crystallization of salt.

Structural basis for the fast phase change of Ge2Sb2Te5: Ring statistics analogy between the crystal and amorphous states

Abstract: The three-dimensional atomic configuration of amorphous Ge2Sb2Te5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge2Sb2Te5 can be regarded as “even-numbered ring structure,” because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge–Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge2Sb2Te5 is the key for the fast crystallization speed of the material.

Structure−Property Correlations in Bending and Brittle Organic Crystals

Abstract: Bending of crystals of molecular solids occurs when the strength of intermolecular interactions in orthogonal directions is significantly different. We report here a survey of 60 molecular crystals and establish a causative correlation between bending and crystal packing. This group contains crystals with 4 and 8 A crystal axes and includes 1D, 2D, 3D, isostructural, polymorphic, stacked, interlocked, single, and multicomponent crystals and solvates. We found that 17 of these 60 crystals may be bent, whereas the rest are brittle and cannot be bent plastically. The bending crystals could be deformed into many shapes; sometimes, they could even be flattened upon themselves without breakage. A model for bending is proposed using the information obtained from X-ray diffraction, face indexing, and mechanical property measurements on both bending and non-bending (brittle) crystals. The bending and brittleness of these molecular crystals are discussed in comparison with the deformation behavior of metals. Molecu...

Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition

Abstract: We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe (a-GeTe) films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous to crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed which is compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for.

2,6-Bis[2-(4-pentylphenyl)vinyl]anthracene: a stable and high charge mobility organic semiconductor with densely packed crystal structure.

Abstract: The development of new organic semiconductors with improved electrical performance and enhanced environmental stability is the focus of considerable research activity. This paper presents the design, synthesis, optical and electrochemical characterization, crystal packing, modeling and thin film morphology, and organic thin film field effect transistor (OTFT) device data analysis for a novel 2,6-bis[2-(4-pentylphenyl)vinyl]anthracene (DPPVAnt) organic semiconductor. We observed a hole mobility of up to 1.28 cm2/V.s and on/off current ratios greater than 107 for OTFTs fabricated using DPPVAnt as an active semiconductor layer. The mobility value is comparable to that of the current best p-type semiconductor pentacene-based device performance. In addition, we found a very interesting relationship between the charge mobility and molecule crystal packing in addition to the thin film orientation and morphology of the semiconductor as determined from single-crystal molecule packing study, thin film X-ray diffraction, and AFM measurements. The high performance of the semiconductor ranks among the best performing p-type organic semiconductors reported so far and will be a very good candidate for applications in organic electronic devices.

Enhancement of crystalline perfection by organic dopants in ZTS, ADP and KHP crystals as investigated by high‐resolution XRD and SEM

Abstract: To reveal the influence of complexing agents on crystalline perfection, tristhiourea zinc(II) sulfate (ZTS), ammonium dihydrogen phosphate (ADP) and potassium hydrogen phthalate (KHP) crystals grown by slow-evaporation solution growth technique using low concentrations (5 x 10 -3 M) of dopants like ethylenediamminetetraacetic acid (EDTA) and 1,10-phenanthroline (phen) were characterized by high-resolution X-ray diffractometry (XRD) and scanning electron microscopy (SEM). High-resolution diffraction curves (DCs) recorded for ZTS and ADP crystals doped with EDTA show that the specimen contains an epilayer, as observed by the additional peak in the DC, whereas undoped specimens do not have such additional peaks. On etching the surface layer, the additional peak due to the epilayer disappears and a very sharp DC is obtained, with full width at half-maximum (FWHM) of less than 10 arcsec, as expected from the plane wave dynamical theory of X-ray diffraction for an ideally perfect crystal. SEM micrographs also confirm the existence of an epilayer in doped specimens. The ZTS specimen has a layer with a rough surface morphology, having randomly oriented needles, whereas the ADP specimen contains a layer with dendric structure. In contrast to ADP and ZTS crystals, the DC of phen-doped KHP shows no additional peak, but it is quite broad (FWHM = 28 arcsec) with a high value of integrated intensity, p (area under the DC). The broadness of the DC and the high value of p indicate the formation of a mosaic layer on the surface of the crystal. However, similar to ADP and ZTS, the DC recorded after etching the surface layer of the KHP specimen shows a very sharp peak with an FWHM of 8 arcsec. An SEM photograph of phen-doped KHP shows deep cracks on the surface, confirming the mosaicity. After removing the surface layer, the SEM pictures reveal a smooth surface. A similar trend is observed with other complexing agents, like oxalic acid, bipy and picolinic acid. However, only typical examples are described in the present article where the effects were observed prominently. The investigations on ZTS, ADP and KHP crystals, employing high-resolution XRD and SEM studies, revealed that some organic dopants added to the solution during the growth lead to the formation of a surface layer, due to complexation of these dopants with the trace metal ion impurities present in the solution, which prevents the entry of impurities, including the solvent, into the crystal, thereby assisting crystal growth with high crystalline perfection. The influence of organic dopants on the second harmonic generation efficiency is also investigated.

Effects of crystal size and Si/Al ratio on the surface properties of H-ZSM-5 zeolites

Abstract: Highly crystalline H-ZSM-5 zeolite samples with crystal sizes from 0.5 to 5 μm and Si/Al ratios ranging from 20 to 70 have been prepared and characterized by IR spectroscopy, SEM and XRD techniques. The surface properties of these materials have been compared with those of conventional small crystal size materials by using FT-IR spectroscopy of the surface hydroxyl groups and of adsorbed carbon monoxide. The external surface has been investigated using pivalonitrile (2,2-dimethyl-propionitrile) as an adsorption probe. Data show that the basic structure of the zeolite active sites do not depend significantly on the crystal size. Bronsted acid sites of high acid strength are constituted by bridging OH groups located inside the zeolite channels, while Lewis acid sites and weakly acidic silanol groups are found on the external surface in all cases.

Theory of the helical spin crystal: a candidate for the partially ordered state of MnSi.

Abstract: MnSi is an itinerant magnet which at low temperatures develops a helical spin-density wave. Under pressure it undergoes a transition into an unusual partially ordered state whose nature is debated. Here we propose that the helical spin crystal (the magnetic analog of a solid) is a useful starting point to understand partial order in MnSi. We consider different helical spin crystals and determine conditions under which they may be energetically favored. The most promising candidate has bcc structure and is reminiscent of the blue phase of liquid crystals in that it has line nodes of magnetization protected by symmetry. We introduce a Landau theory to study the properties of these states, in particular, the effect of crystal anisotropy, magnetic field, and disorder. These results compare favorably with existing data on MnSi from neutron scattering and magnetic field studies. Future experiments to test this scenario are also proposed.

Crystals and Crystal Structures

Abstract: Preface. 1 Crystals and crystal structures. 1.1 Crystal families and crystal systems. 1.2 Morphology and crystal classes. 1.3 The determination of crystal structures. 1.4 The description of crystal structures. 1.5 The cubic close-packed (A1) structure of copper. 1.6 The body-centred cubic (A2) structure of tungsten. 1.7 The hexagonal (A3) structure of magnesium. 1.8 The halite structure. 1.9 The rutile structure. 1.10 The fluorite structure. 1.11 The structure of urea. 1.12 The density of a crystal. Answers to introductory questions. Problems and Exercises. 2 Lattices, planes and directions. 2.1 Two-dimensional lattices. 2.2 Unit cells. 2.3 The reciprocal lattice in two dimensions. 2.4 Three-dimensional lattices. 2.5 Alternative unit cells. 2.6 The reciprocal lattice in three dimensions. 2.7 Lattice planes and Miller indices. 2.8 Hexagonal lattices and Miller-Bravais indices. 2.9 Miller indices and planes in crystals. 2.10 Directions in lattices. 2.11 Lattice geometry. Answers to introductory questions. Problems and Exercises. 3 Two-dimensional patterns and tiling. 3.1 The symmetry of an isolated shape: point symmetry. 3.2 Rotation symmetry of a plane lattice. 3.3 The symmetry of the plane lattices. 3.4 The ten plane crystallographic point symmetry groups. 3.5 The symmetry of patterns: the 17 plane groups. 3.6 Two-dimensional 'crystal structures'. 3.7 General and special positions. 3.8 Tesselations. Answers to introductory questions. Problems and Exercises. 4 Symmetry in three dimensions. 4.1 The symmetry of an object: point symmetry. 4.2 Axes of inversion: rotoinversion. 4.3 Axes of inversion: rotoreflection. 4.4 The Hermann-Mauguin symbols for point groups. 4.5 The symmetry of the Bravais lattices. 4.6 The crystallographic point groups. 4.7 Point groups and physical properties. 4.8 Dielectric properties. 4.9 Refractive index. 4.10 Optical activity. 4.11 Chiral molecules. 4.12 Second harmonic generation. 4.13 Magnetic point groups and colour symmetry. Answers to introductory questions. Problems and Exercises. 5 Building crystal structures from lattices and space groups. 5.1 Symmetry of three-dimensional patterns: space groups. 5.2 The crystallographic space groups. 5.3 Space group symmetry symbols. 5.4 The graphical representation of the space groups. 5.5 Building a structure from a space group. 5.6 The structure of diopside, CaMgSi2O6. 5.7 The structure of alanine, C3H7NO2 . 6 Diffraction and crystal structures. 6.1 The position of diffracted beams: Bragg's law. 6.2 The geometry of the diffraction pattern. 6.3 Particle size. 6.4 The intensities of diffracted beams. 6.5 The atomic scattering factor. 6.6 The structure factor. 6.7 Structure factors and intensities. 6.8 Numerical evaluation of structure factors. 6.9 Symmetry and reflection intensities. 6.10 The temperature factor. 6.11 Powder X-ray diffraction. 6.12 Electron microscopy and structure images. 6.13 Structure determination using X-ray diffraction. 6.14 Neutron diffraction. 6.15 Protein crystallography. 6.16 Solving the phase problem. 6.17 Photonic crystals. Answers to introductory questions. Problems and Exercises. 7 The depiction of crystal structures. 7.1 The size of atoms. 7.2 Sphere packing. 7.3 Metallic radii. 7.4 Ionic radii. 7.5 Covalent radii. 7.6 Van der Waals radii. 7.7 Ionic structures and structure building rules. 7.8 The bond valence model. 7.9 Structures in terms of non-metal (anion) packing. 7.10 Structures in terms of metal (cation) packing. 7.11 Cation-centred polyhedral representations of crystals. 7.12 Anion-centred polyhedral representations of crystals 7.13 Structures as nets. 7.14 The depiction of organic structures. 7.15 The representation of protein structures. Answers to introductory questions. Problems and Exercises. 8 Defects, modulated structures and quasicrystals. 8.1 Defects and occupancy factors. 8.2 Defects and unit cell parameters. 8.3 Defects and density. 8.4 Modular structures. 8.5 Polytypes. 8.6 Crystallographic shear phases. 8.7 Planar intergrowths and polysomes. 8.8 Incommensurately modulated structures. 8.9 Quasicrystals. Answers to introductory questions. Problems and Exercises. Appendices. Appendix 1 Vector addition and subtraction. Appendix 2 Data for some inorganic crystal structures. Appendix 3 Schoenflies symbols. Appendix 4 The 230 space groups. Appendix 5 Complex numbers. Appendix 6 Complex amplitudes. Answers to problems and exercises. Bibliography. Formula index. Subject index.

Gallium nitride crystals and wafers and method of making

Abstract: A GaN crystal having up to about 5 mole percent of at least one of aluminum, indium, and combinations thereof. The GaN crystal has at least one grain having a diameter greater than 2 mm, a dislocation density less than about 104 cm−2, and is substantially free of tilt boundaries.

Microwave dielectric losses caused by lattice defects

Abstract: Dielectric loss tangent at microwave frequency is mainly determined by the anharmonic terms in the crystal's potential energy. In addition, there is a kind of lattice defect that increases the dielectric loss tangent seriously. This paper presents the experimental results for two materials; the system Ba(Zn,Ta)O 3 -BaZrO 3 and (Zr,Sn)TiO 4 . The dielectric loss tangents of the system Ba(Zn,Ta)O 3 -BaZrO 3 increases seriously when the B-site ions distribute disorderedly in the crystal. The doping of oxygen vacancies and acceptor ions in (Zr,Sn)TiO 4 increase tan δ by the way they increase the gradient and intercept of linear frequency dependency of tan δ. These experimental results are reasonably explained by Schlomann's theory. He predicted that the dielectric loss tangent increases when the ions are distributed disorderedly in a way that they break the periodic arrangement of charges in the crystal, and that the increase of tan δ is negligible if the disordered charge distribution maintains the charge neutrality within a short range of the lattice constant in the crystal.

Phase behaviour of hard spheres confined between parallel hard plates: manipulation of colloidal crystal structures by confinement.

Abstract: We study the phase behaviour of hard spheres confined between two parallel hard plates using extensive computer simulations. We determine the full equilibrium phase diagram for arbitrary densities and plate separations from one to five hard-sphere diameters using free energy calculations. We find a first-order fluid-solid transition, which corresponds to either capillary freezing or melting depending on the plate separation. The coexisting solid phase consists of crystalline layers with either triangular ([Formula: see text]) or square ([Formula: see text]) symmetry. Increasing the plate separation, we find a sequence of crystal structures from [Formula: see text], where n is the number of crystal layers, in agreement with experiments on colloids. At high densities, the transition between square to triangular phases is interrupted by intermediate structures, e.g., prism, buckled, and rhombic phases.

Assisted Desolvation as a Key Kinetic Step for Crystal Growth

Abstract: The crystallization of materials from a supersaturated solution is a fundamental chemical process. Although several very successful models that provide a qualitative understanding of the crystal growth process exist, in most cases the atomistic detail of crystal growth is not fully understood. In this work, molecular dynamics simulations of the morphologically most important surfaces of barite in contact with a supersaturated solution have been performed. The simulations show that an ordered and tightly bound layer of water molecules is present on the crystal surface. The approach of an ion to the surface requires desolvation of both the surface and the ion itself leading to an activated process that is rate limiting for two-dimensional nucleation to occur. However, desolvation on specific surfaces can be assisted by anions adsorbed on the crystal surface. This hypothesis, corroborated by crystallization and scanning electron microscopy studies, allows the rationalization of the morphology of barite crystals grown at different supersaturations.

Theoretical study of N-doped TiO2 rutile crystals.

Abstract: The N-doping effects on the electronic and optical properties of TiO2 rutile crystal have been studied using density functional theory (DFT). The calculations of several possible N-doped structures show that band gaps have little reduction but some N 2p states lie within the band gap in the substitutional N to O structure and interstitial N-doped rutile supercell, which results in the reduction of the photon-transition energy and absorption of visible light. In contrast, substitutional N to Ti doped model has a significant band-gap narrowing. The results maybe clarify confusions in nitrogen-doped TiO2 rutile crystal.

Estimation of the Elastic Modulus of Cellulose Crystal by Molecular Mechanics Simulation

Abstract: The elastic modulus of natural cellulose crystal was estimated by the molecular simulation technique. Values between 124 and 155 GPa were derived for the reasonable cellulose Iβ crystal model that were nearly equal to the observed value of 138 GPa. While the second-generation force fields were found to be superior to the first-generation ones for the optimization of cellulose structure, neither of these was good enough to achieve the structural optimization. They were, however, adequate for estimating the mechanical properties of cellulose, especially the second-generation force fields. The lateral (that is, intermolecular) interactions between cellulose chains were found to play an important role in the expression of the mechanical properties of cellulose crystal.