Regular Crystals Research Articles

Phason mechanism for diffusion in quasicrystalline structures

Specific properties of quasicrystals (QCs) with regards to their structures are discussed with emphasis on diffusion process. To fulfil this, regular mechanisms in the solids are introduced. The experimental results show that the predominant mechanism in quasicrystals is phason which is caused by the absence of proper matching in their structure and the vacancies in the lattice. Monolayers of Al were deposited on the surface of an i-Al-Pd-Mn quasicrystal at constant temperature and the diffusion of the elements into the surface measured by low energy ion scattering. According to our results the activation energy for Al diffusion in the i-Al-Pd-Mn quasicrystal is found to be smaller than that for Al diffusion in regular crystals. It was therefore concluded that a phason mechanism controlled the diffusion. The results of this work can be phason mechanism predominate which is agreement with the some theoretical scientists who believe to phason mechanism at least in low temperatures.

CVD growth of monolayer WS2 through controlled growth temperature and time

Recently, two-dimensional Transition metal dichalcogenides (TMDCs) have attracted widespread attention due to their unique electrical and optical properties. Specially, the monolayer WS2 has high photoluminescence intensity and short response time, and has high application potential in optoelectronic devices. Here, a method for synthesizing single-layer WS2 crystal by atmospheric pressure chemical vapor deposition (CVD) is reported. The triangular and regular hexagonal monolayer WS2 crystals are directly synthesized on Si/SiO2 substrate. Moreover, the measurement results show that the quality of the single-layer WS2 crystal can be further improved through adjusting the growth parameters, such as growth temperature and growth time.

On the fluoride-mediated synthesis and amine intercalation of large lamellar crystals of layered titanosilicate Na/K-JDF-L1

Fluoride-mediated synthesis in the inorganic hydrothermal system has been proposed for preparing large crystals of layered Na/K-JDF-L1 with lamellar morphology. The protonated JDF-L1 crystals derived from their layered precursors are structurally modified by 1,6-hexanediamine intercalation under mild hydrothermal conditions. All the materials mentioned are characterized by XRD, FESEM, EDX/ICP-AES, and TGA–DSC. It is concluded that the combination of fluorides with low alkalinity (pH ca.10.0) and crystallization temperature (200 °C) facilitates the oriented growth of metastable layered phase Na/K-JDF-L1, yielding regular lamellar crystals with square-sheet structure in two-dimensions of ca. 15 × 15 μm. Data on Na/K-JDF-L1 structure are presented for identification of the most probable tetragonal space group, P4212, corresponding to the highest crystallographic symmetry. It is also revealed that amine intercalation of relevant H-JDF-L1 by the hydrothermal process leads to the formation of layered titanosilicate materials with an ordered redistribution of the dominant interlayer space (d001) shifting from 10.674 to 15.620 A.

Organic Salts of Pharmaceutical Impurity p-Aminophenol.

The presence of impurities can drastically affect the efficacy and safety of pharmaceutical entities. p-Aminophenol (PAP) is one of the main impurities of paracetamol (PA) that can potentially show toxic effects such as maternal toxicity and nephrotoxicity. The removal of PAP from PA is challenging and difficult to achieve through regular crystallization approaches. In this regard, we report four new salts of PAP with salicylic acid (SA), oxalic acid (OX), l-tartaric acid (TA), and (1S)-(+)-10-camphorsulfonic acid (CSA). All the PAP salts were analyzed using single-crystal X-ray diffraction, powder X-ray diffraction, infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The presence of minute amounts of PAP in paracetamol solids gives a dark color to the product that was difficult to remove through crystallization. In our study, we found that the addition of small quantities of the aforementioned acids helps to remove PAP from PA during the filtration and washings. This shows that salt formation could be used to efficiently remove challenging impurities.

Quaternary Quasicrystal Alloys for Hydrogen Storage Technology

ABSTRACTThe Ti-Zr-Ni quasicrystal alloys have prospected to be one of the promising materials for hydrogen storage. This is because this type of quasicrystal contains 140 interstitial sites (T-sites) constituted in the Bergman Cluster that could accommodate hydrogen. The number of available sites is far greater than the number found in regular crystals, therefore the improvement of hydrogen storage capacity could be expected. For this study, we focus on the effect of substitution of Cr, in place of Ni in Ti-Zr-Ni amorphous and quasicrystal alloys. The studied samples are synthesized by the combination of mechanical alloying and sintering process. The subsequent measurements of electrochemical hydrogenation and dehydrogenation are carried out by a three-electrode cell at room temperature. The studied samples are structurally characterized by X-ray diffraction and their morphology is analyzed by scanning electron microscope and transmission electron microscope. The influence of the 4th substituted element on the possibility of a new-formed Cr quasicrystalline phase and the potential improvement of hydrogenation and dehydrogenation kinetics for both amorphous and quasicrystalline phase is evaluated. Our measurements showed the maximum discharge capacity achieved by Ti45Zr38Ni7Cr10 amorphous and Ti45Zr38Ni12Cr5 i-phase electrodes at a current density of 15 mA·g-1 to be 9.8 mAh·g-1and 55.2 mAh·g-1 respectively. The maximum estimated H/M value for the Ti45Zr38Ni12Cr5 i-phase electrode reached 1.36. These results are encouraging and show the merit of the usage of quasicrystals as hydrogen storage materials.

Influence of the exchange current density and overpotential for hydrogen evolution reaction on the shape of electrolytically produced disperse forms - Review

In this study, comprehensive survey of formation of disperse forms by the electrolysis from aqueous electrolytes and molten salt electrolysis has been presented. The shape of electrolitically formed disperse forms primarily depends on the nature of metals, determined by the exchange current density (j0) and overpotential for hydrogen evolution reaction as a parallel reaction to metal electrolysis. The decrease of the j0 value leads to a change of shape of dendrites from the needle-like and the 2D fern-like dendrites (metals characterized by high j0 values) to the 3D pine-like dendrites (metals characterized by medium j0 values). The appearing of a strong hydrogen evolution leads to formation of cauliflower-like and spongy-like forms (metals characterized by medium and low j0 values). The other disperse forms, such as regular and irregular crystals, granules, cobweb-like, filaments, mossy and boulders, usually feature metals characterized by the high j0 values. The globules and the carrot-like forms are a characteristic of metals with the medium j0 values. The very long needles were a product of molten salt electrolysis of magnesium nitrate hexahydrate. Depending on the shape of the disperse forms, i.e. whether they are formed without and with vigorous hydrogen evolution, formation of all disperse forms can be explained by either application of the general theory of disperse deposits formation or the concept of "effective overpotential". With the decrease of j0 value, the preferred orientation of the disperse forms changed from the strong (111) in the needle-like and the fern-like dendrites to randomly oriented crystallites in the 3D pine-like dendrites and the cauliflower-like and the spongy-like forms.

Universal temperature dependence of the thermal conductivity of clathrate compounds, molecular crystals, and glasses at low temperatures

A new approach is used to analyze the experimental data on the temperature dependence of low-temperature thermal conductivity κ(T) of some typical disordered complex crystals: clathrate hydrates (tetrahydrofuran, methane, xenon), Sr8Ga16Ge30 and p-type Ba8Ga16Ge30 clathrate compounds, YSZ ceramics, molecular structural glasses of 1-propanol, glycerol and D-ethanol exhibiting glass-like behavior of κ(T), and some representatives of regular complex crystals: tetrahydrofuran clathrate hydrate, n-type Ba8Ga16Ge30 clathrate compound, CsDy(MoO4)2 layered crystal, 1-propanol, and D-ethanol. A universal approach to normalizing low-temperature thermal conductivity is proposed, based on the theoretical model of hybridized Klinger–Kosevich excitations. The low-temperature universal behavior of the thermal conductivity of both crystalline and amorphous solids is shown to be a result of the same phenomenon: the hybridization of acoustic and low-lying optical branches.

Orientated Growth of Hemispherical Calcium Carbonate Beneath Protein Langmuir Monolayer

Oriented calcium carbonate crystals have been grown beneath bovine serum albumin (BSA) Langmuir monolayers at the air-solution interface from supersaturated calcium bicarbonate solutions. The calcium carbonate prepared was characterized by XRD, SEM, TEM and SAED. It was found the crystal morphologies changed from amorphous calcium carbonate (ACC) to regular hemispherical calcite crystals with the prolonging of the time. These observations suggest that the process is step by step, during which the orientation of the crystalline phase was controlled by the BSA Langmuir monolayer template. These results provide new insight into the proteins-biomineral interaction, which can be extended for shape control over other minerals or inorganic-organic hybrid materials.

Деканалирование высокоэнергетических частиц из плоскостных каналов кристалла. Упругие и неупругие процессы рассеяния

Based on the principles of nonequilibrium statistical thermodynamics, the kinetic theory of de-channeling of high-energy particles from planar channels of a crystal have been deduced. The de-channeling rate coefficient is considered from the point of view of the transition of a fast particle from the directional motion mode to the state of chaotic motion. The corresponding equation have been obtained that takes into account both the coherent nature of the diffraction of a particle beam in a regular single crystal and the inelastic scattering of channeled particles by electrons and phonons. An analytical solution of this equation have been obtained on the class of confluent hypergeometric function using which the de-channeling rate constant Re have been calculated. It have been shown that up to second-order terms in terms of the interaction potential the constant Re is inversely proportional to the relaxation time of the system tph. In calculating tph the polarization of space by a fast charged particle is not taken into account. The temperature and isotopic dependences of the de-channeling rate have been obtained.

TM band diagram of a waveguide in a honeycomb photonic lattice composed by triangular-shaped rods

Using the plane wave expansion and the supercell technique, we calculate the TM band diagram of a waveguide in a honeycomb lattice that composed rods with triangular cross-sections embedded in air. For this study, we use GaAs rods with a dielectric function dependent on pressure and temperature. We report the existence of a defect band within the photonic band gap that presents large confinement for small wave vector values along the line defect. However, for large wave vector values, the defect band approaches the regular crystal bands, thus scattering a greater amount of energy. When the triangles are rotated, greater confinement and smaller scattering of energy is observed than that for unrotated triangles. Additionally, we report that the TM band diagram and the defect band may be tuned to higher frequencies by increasing the applied pressure.

Electrochemical properties of powder-pressed Pb–Ag–PbO2 anodes

Abstract Pb–Ag–PbO2 composite anodes with different mass fractions (1%, 2%, 3%, 4% and 5%) of β-PbO2 were prepared by powder-pressed (PP) method. The galvanostatic polarization curves, Tafel curves and anodic polarization curves were tested in sulfuric acid solution. The morphologies and phase compositions of the anodic layers formed after galvanostatic polarization were investigated by using scanning electron microscope (SEM) and X-ray diffractometer (XRD), respectively. The results showed that β-PbO2 can improve the electrocatalytic activity of anodic oxide. The anode containing 3% β-PbO2 had the lowest overpotential of oxygen evolution reaction (OER) and the best corrosion resistance. The morphologies of the anode surfaces were gradually transformed from regular crystals to amorphous ones as the content of β-PbO2 increased in anodes.

Knot your regular crystal of atoms.

Analogs of Kelvin's “vortex atoms” are realized at microscopic scale in chiral liquid crystals

Hydrothermal stable ZIF-67 nanosheets via morphology regulation strategy to construct mixed-matrix membrane for gas separation

Mixed matrix membranes (MMMs) based on metal-organic frameworks (MOFs) have proven to have high separation performance, which has great potential in separation applications. However, unsatisfactory hydrothermal stability is the Achilles' heel of MOFs in practical applications, which seriously hinders the development of MOF based MMMs. Under the advantage of crystal anisotropy, a morphology regulation strategy without post-treatment is presented for the first time to efficiently improve the stability of MOF fillers in aqueous solution. Contrary to the regular dodecahedron ZIF-67 crystals, the [211] oriented ZIF-67 nanosheets (NS) can maintain an ordered microporous framework integrally in the aqueous solution of membrane preparation. The exposed [211] crystal face exhibits enhanced stability due to a lower proportion of Co–N bond and higher surface atomic density. The flake morphology of ZIF-67 with economic incorporation (5.0 wt %) offers the MMMs significantly enhanced CO2 permeability of 139.4 barrer and CO2/N2 ideal selectivity of 73.2, improved by 50.7% and 76.0% compared with the pristine polymer membrane. This strategy provides a new idea for improving the hydrothermal stability of MOF materials and extends their separation applications under aqueous conditions.

Formation of g‐C3N4 Nanotubes towards Superior Photocatalysis Performance

AbstractPoor charge separation efficiency seriously limits the application of graphitic carbon nitride (g‐C3N4) for water splitting. Herein, g‐C3N4 nanotubes were created to enhance charge carrier separation for superior H2 evolution via a thermal polymerizing reaction using melamine crystals prepared through a transitional metal derived re‐crystalline process. Melamine crystals were grown with help of transitional metal ions (Fe3+, Co2+, Ni2+, and Mn2+). As prepared crystals have regular shape with an average size of ∼20 μm. The growth of (002) facet was limited and another facets corresponding to (011), (210) and (311) promised. The tubular shape is firstly ascribed to the traces amount of transition metal ions in the crystal promise formation of tubular structure. Another key is that regular crystals with low surface energy slow down polycondensation reaction. The tubular g‐C3N4 revealed superior photocatalysis performance. Sample Fe3+ R‐650 CN prepared using Fe3+ ions modified crystals exhibited enhanced absorbance at 500 nm and decreased band gap. Tubular morphology accelerated electron transfer along the axial direction. The sample revealed the highest photocatalytic hydrogen evolution rate of 7538 μ mol h−1 g−1, which is nearly 13.5 times high compared with g‐C3N4 nanosheets prepared using similar conditions without transitional metal ions.

Structurally regular arrangement induced fluorescence enhancement and specific recognition for glutathione of a pyrene chalcone derivative

Glutathione (GSH) is an important antioxygen and free radical scavenger in the organism. Level of GSH in vivo is associated with many diseases and specific recognition for GSH is very important. Here, a pyrene chalcone derivative 1 1-(2-hydroxyphenyl)-3-(1-pyrenyl)-2-propen-1-one as specific probe for GSH was developed. The probe can give rise to rapid blue fluorescence enhancement for GSH based on Michael addition reaction in pure PBS solution with high sensitivity, fast response rate and high specificity. The compound also can be applied for GSH detection in HeLa cell. Simultaneously, the compound exhibits blue fluorescence emission enhancement in methanol-water (1:1, v/v) solution with fluorescence quantum yield being 0.45 due to the competition of water molecules for hydrogen bonds between hydroxyl and carbonyl and the formation of structurally regular rodlike crystals, which allows regulating fluorescence emission by different solvent condition.

Polarization-dependent non-uniform rotation rates of rhombohedral calcite

We demonstrate the polarization-dependent torque on rhombohedral calcite in an optical trap. Our precipitate technique produces regular crystals approximately 10μm on all edges. The regularity of the crystal shape makes it possible to visually identify the optical axis as well as the orientation of the polarization axes. When a rhombohedral crystal is trapped in an elliptically polarized beam, it orients itself with its optic axis approximately parallel to the beam axis. While in this orientation, the total torque increases and decreases relative to three extraordinary and ordinary axes of the crystal. We measure this axis-dependent calcite rotation through video analysis and model the dependence of the torque on the crystal orientation. The ability to predict the motion of calcite gives an analytical tool for applications such as fluid stirring or "lab on a chip" systems.

Gelation Phenomenon During Crystallization of Cefpiramide Sodium

Unlike the adverse effect caused by the gelation during crystallization process, gelation of cefpiramide sodium was found to provide ideal product properties, such as a larger and more regular crystal shape. The causes of the gelation phenomenon and the mechanism of gel–crystal transition during the crystallization of cefpiramide sodium were both investigated in this work. The gel was formed due to the strapping of the solvents by the networks of cefpiramide sodium molecules. The whole gel–crystal transition process was divided into the following three stages: (1) when the temperature decreased, the system reached a metastable-state gelation; (2) the initial microcrystal in the gel grew slowly because of the low supersaturation; and (3) the gel finally disappeared, and a larger and more regular crystal was formed. The Hansen solubility parameters were used to analyze the effects of the solvents on this gelation; the analysis results can serve as guidance for solvent screening in the actual production process.

Angular dependence of the optical reflectance in two-dimensional Si-ZnO photonic crystals

In this work, we present an experimental study of the angular dependence of the optical reflectance in two 2-dimensional photonic structures, built on a silicon substrate coated with a ZnO thin film. Photonic structures studied describe a regular square lattice with a lattice constant a = 1.05 μm and circular air holes with a radius r = 0.63a. Additionally, one of these photonic structures contains a pattern of nine quasi-circular micro-cavities embedded in a regular square lattice photonic crystal, describing a secondary square lattice. A comparison of the optical reflectance of the two photonic structures as a function of the incidence angle for transverse magnetic polarization mode in the visible (Vis) and near-infrared (NIR) ranges of the electromagnetic spectrum was accomplished. Results obtained revealed a strong angular dependence of the reflective optical properties of the two 2-dimensional photonic structures on the illumination incidence angle. The photonic structure with the array of resonant optical cavities presented a very unusual behavior regarding the reflectance of the substrate, this because its reflectance described specific modulations dependent on the incidence angle of illumination.

Blue shift effect on the reflectance in two-dimensional Si-ZnO photonic structure with patterned coupled optical micro-cavities

This research has revealed the possibility to induce a shift towards short wavelengths on the optical reflectance properties of a two-dimensional photonic crystal which include an embedded array of optical micro-cavities. This ability was attributed to the resonant effect produced by the micro-cavities array embedded in the regular photonic crystal and by the variation on the structural parameters that defined the photonic crystal under research. These results can be useful in future applications of solar cell systems and telecommunications, among others, which can be of great help to increase technological developments.

Magnons in a Quasicrystal: Propagation, Extinction, and Localization of Spin Waves in Fibonacci Structures

Magnonic quasicrystals exceed the possibilities of spin wave (SW) manipulation offered by regular magnonic crystals, because of their more complex SW spectra with fractal characteristics. Here, we report the direct x-ray microscopic observation of propagating SWs in a magnonic quasicrystal, consisting of dipolarly coupled permalloy nanowires arranged in a one-dimensional Fibonacci sequence. SWs from the first and second band as well as evanescent waves from the band gap between them are imaged. Moreover, additional mini-band gaps in the spectrum are demonstrated, directly indicating an influence of the quasiperiodicity of the system. The experimental results are interpreted using numerical calculations and we deduce a simple model to estimate the frequency position of the magnonic gaps in quasiperiodic structures. The demonstrated features of SW spectra in one-dimensional magnonic quasicrystals allows utilizing this class of metamaterials for magnonics and makes them an ideal basis for future applications.