Basal Face Research Articles

Morphology and Agglomeration Control of LiMnPO4 Micro- and Nanocrystals

Microwave-assisted hydrothermal synthesis was used to grow LiMnPO4 micro- and nanocrystals from acetate precursors. By appropriate adjustment of the precursor concentration and the pH-value of the reactant, the product composition and purity along with the crystal size can be manipulated, resulting in particle-dimensions from around 10 μm down to a few 100 nm. Prisms and plates with hexagonal basal faces as well as cuboid and rod-like particles were produced. The effects on the crystal morphology as well as on the materials texture and agglomeration tendency are discussed and a comprehensive agglomeration phase diagram is constructed.

Phase Equilibrium System of CsCl–TbCl3–HCl(∼7.6 mass %)–H2O at 298.2 ± 0.1 K and Fluorescent and Thermal Properties of Its Solid-Phase Compounds in the System

The solubility data of the quaternary system CsCl–TbCl3–HCl(∼7.6 mass %)–H2O at 298.2 K were determined, and the corresponding phase diagram according to central projection data on the trigonal basal face CsCl–TbCl3–H2O was plotted. The diagram includes three invariant points and four crystallization fields corresponding to CsCl, Cs5TbCl8·6H2O, Cs2TbCl5·6H2O, and TbCl3·6H2O. Two new solid-phase compounds of Cs5TbCl8·6H2O and Cs2TbCl5·6H2O, which are congruently soluble in the system, were isolated and characterized by the methods of powder X-ray diffraction, thermogravimetric, and differential thermal analysis. The standard molar enthalpies of solution for the two compounds of Cs5TbCl8·6H2O and Cs2TbCl5·6H2O were measured to be (60.64 ± 0.28) and (14.08 ± 0.45) kJ·mol–1 by heat conduction microcalorimetry under the conditions of infinite dilution, and their standard molar enthalpies of formation were calculated to be −(5087.1 ± 1.7) kJ·mol–1 and −(3764.2 ± 1.0) kJ·mol–1. The fluorescence excitation and emission spectra of Cs5TbCl8·6H2O and Cs2TbCl5·6H2O were measured. The results show that upconversion spectra exhibit for Cs5TbCl8·6H2O at 500 nm and 545 nm and for Cs2TbCl5·6H2O at 495 nm and 545 nm excited at 750 nm, and the upconversion intensity increases with the increase of TbCl3 contents in CsCl.

Adsorption of haloadamantanes on the basal face of graphite

The graphite surface exhibits different structural selectivity to isomeric haloadamantanes depending on the kind and number of halogen atoms in the molecule. In the series of fluoro-, chloro-, bromo-, and iodoadamantanes, the selectivity of separation of the respective isomers on columns packed with graphitized thermal soot increases. The equilibrium adsorption characteristics of haloadamantanes (adsorption equilibrium constants, heats and entropies of adsorption), calculated within the framework of molecular-statistical theory, agree well with the experimental values and allow comprehensive description of the adsorbed state of these compounds on the graphite surface. The results of molecular-statistical calculations allow evaluating the possibility of haloadamantane separation on columns with graphitized thermal soot under the conditions of equilibrium gas-adsorption chromatography.

How do Quasi-Liquid Layers Emerge from Ice Crystal Surfaces?

Ice crystal surfaces melt at temperatures below 0 °C, and then quasi-liquid layers (QLLs) are formed. However, revealing the dynamic behavior of QLLs, which dominates the surface properties of ice crystals at temperatures near the melting point, remains an experimental challenge. Here, we demonstrate the similarities and differences in the generation mechanisms of two types of QLL phases, which show different morphologies and dynamics. We directly visualized the appearance of round liquidlike droplets (α-QLLs) and thin liquidlike layers (β-QLLs) on ice basal faces by advanced optical microscopy, which can allow visualization of the individual elementary steps on basal faces. We found that α-QLLs always appear at outcrops of dislocations, and that β-QLLs emerge from crystal surfaces where many microdefects are embedded. These results clearly demonstrate the similar function that strain induces the appearance of both types of QLLs. We also found that β-QLLs are spontaneously formed at interfaces between basal faces and α-QLLs, when the diameter of the α-QLLs becomes larger than several tens of micrometers. This result arose from the different structures of α- and β-QLLs: the β-QLLs probably have a structure intermediate between those of basal faces and α-QLLs, resulting in a reduction of the total interfacial free energy.

Spontaneous inversion of in-plane polarity ofa-oriented GaN domains laterally overgrown on patternedr-plane sapphire substrates

Spontaneously regulated in-plane polarity inversion ofa-oriented GaN domains has been demonstrated for the first time. Crystallographic analysis revealed that each domain grown on circular-hole-patternedr-plane sapphire substrates has basal faces with oppositely oriented in-plane polarity. The inverted orientation of in-plane polarity on the opposite basal faces is not due to merging between in-plane polarity-inverted domains nucleated on the patternedr-plane sapphire substrate, but it was found to be due to spontaneous formation of an inversion domain boundary on the growth fronts of existing domains. This result provides new insights into controlling the in-plane polarity ofa-oriented GaN, because the nucleation of in-plane polarity-inverted domains ofa-oriented GaN onr-plane sapphire is symmetrically not allowed.

Holistic approach to dissolution kinetics: linking direction-specific microscopic fluxes, local mass transport effects and global macroscopic rates from gypsum etch pit analysis

Dissolution processes at single crystal surfaces often involve the initial formation and expansion of localized, characteristic (faceted) etch-pits at defects, in an otherwise comparatively unreactive surface. Using natural gypsum single crystal as an example, a simple but powerful morphological analysis of these characteristic etch pit features is proposed that allows important questions concerning dissolution kinetics to be addressed. Significantly, quantitative mass transport associated with reactive microscale interfaces in quiescent solution (well known in the field of electrochemistry at ultramicroelectrodes) allows the relative importance of diffusion compared to surface kinetics to be assessed. Furthermore, because such mass transport rates are high, much faster surface kinetics can be determined than with existing dissolution methods. For the case of gypsum, surface processes are found to dominate the kinetics at early stages of the dissolution process (small etch pits) on the cleaved (010) surface. However, the contribution from mass transport becomes more important with time due to the increased area of the reactive zones and associated decrease in mass transport rate. Significantly, spatial heterogeneities in both surface kinetics and mass transport effects are identified, and the morphology of the characteristic etch features reveal direction-dependent dissolution kinetics that can be quantified. Effective dissolution velocities normal to the main basal (010) face are determined, along with velocities for the movement of [001] and [100] oriented steps. Inert electrolyte enhances dissolution velocities in all directions (salting in), but a striking new observation is that the effect is direction-dependent. Studies of common ion effects reveal that Ca(2+) has a much greater impact in reducing dissolution rates compared to SO(4)(2-). With this approach, the new microscopic observations can be further analysed to obtain macroscopic dissolution rates, which are found to be wholly consistent with previous bulk measurements. The studies are thus important in bridging the gap between microscopic phenomena and macroscopic measurements.

The microscopic features of heterogeneous ice nucleation may affect the macroscopic morphology of atmospheric ice crystals

It is surprisingly difficult to freeze water. Almost all ice that forms under "mild" conditions (temperatures > -40 degrees C) requires the presence of a nucleating agent--a solid particle that facilitates the freezing process--such as clay mineral dust, soot or bacteria. In a computer simulation, the presence of such ice nucleating agents does not necessarily alleviate the difficulties associated with forming ice on accessible timescales. Nevertheless, in this work we present results from molecular dynamics simulations in which we systematically compare homogeneous and heterogeneous ice nucleation, using the atmospherically important clay mineral kaolinite as our model ice nucleating agent. From our simulations, we do indeed find that kaolinite is an excellent ice nucleating agent but that contrary to conventional thought, non-basal faces of ice can nucleate at the basal face of kaolinite. We see that in the liquid phase, the kaolinite surface has a drastic effect on the density profile of water, with water forming a dense, tightly bound first contact layer. Monitoring the time evolution of the water density reveals that changes away from the interface may play an important role in the nucleation mechanism. The findings from this work suggest that heterogeneous ice nucleating agents may not only enhance the ice nucleation rate, but also alter the macroscopic structure of the ice crystals that form.

Understanding anisotropic growth behavior of hexagonal ice on a molecular scale: A molecular dynamics simulation study

Although distinct growth behaviors on different faces of hexagonal ice have long been suggested, their understanding on a molecular scale has been hampered due to experimental difficulties near interfaces. We present a molecular dynamics simulation study to unravel the molecular origin of anisotropy in the growth kinetics of hexagonal ice by visualizing the formation of transient water structures in the growing ice interface. During ice growth, the formation of transient structures and their rearrangement to the final ice configuration are observed irrespective of growth direction. However, we find that their structure and duration differ significantly depending on growth direction. In the direction perpendicular to the basal face of hexagonal ice along which growth occurs most slowly, a two-dimensional transient structure, which is formed by competing hexagonal and cubic arrangements within the same layer, persists for a significant period of time, contrasted with short-lived transient structures in other directions. This observation of such transient water structures and their rearrangement during ice growth provides a clear explanation of different growth rates on each face of hexagonal ice on a molecular scale.

Car-Parrinello molecular dynamics study of the uranyl behaviour at the gibbsite/water interface

The uranyl cation UO(2)(2+) adsorption on the basal face of gibbsite is studied via Car-Parrinello molecular dynamics. In a first step, we study the water sorption on a gibbsite surface. Three different sorption modes are observed and their hydrogen bond patterns are, respectively, characterized. Then we investigate the sorption properties of an uranyl cation, in the presence of water. In order to take into account the protonation state of the (001) gibbsite face, both a neutral (001) face and a locally deprotonated (001) face are modeled. In the first case, three adsorbed uranyl complexes (1 outer sphere and 2 inner spheres) with similar stabilities are identified. In the second case, when the gibbsite face is locally deprotonated, two adsorbed complexes (1 inner sphere and 1 outer one) are characterized. The inner sphere complex appears to be the most strongly linked to the gibbsite face.

Adsorption of nitrobenzene, aniline and nitroanilines on the surface of a basal face of graphite

Thermodynamic characteristics of the adsorption of nitrobenzene, aniline, and isomeric nitro-anilines are determined experimentally and from molecular statistics in the range of the closest packing of a graphite surface according to equilibrium gas adsorption chromatography. It is found that graphitized thermal carbon black is characterized by high selectivity upon the chromatographic separation of isomeric nitro-anilines (retention increases in the series of ortho, meta, and para isomers). Using the atom-atomic approximation of the semi-empirical molecular statistical theory of adsorption, parameters are calculated for the potential function of the pair intermolecular interaction of the NO2 group in nitrobenzene and the N atom in aniline with the C atoms in a basal face of graphite. The effect of the NH2 and NO2 groups on the parameters of adsorption for isomeric nitroanilines as a result of direct polar conjugation is demonstrated and must be assumed when performing molecular statistical calculations.

Anisotropy in the crystal growth of hexagonal ice, Ih

Growth of ice crystals has attracted attention because ice and water are ubiquitous in the environment and play critical roles in natural processes. Hexagonal ice, I(h), is the most common form of ice among 15 known crystalline phases of ice. In this work we report the results of an extensive and systematic molecular dynamics study of the temperature dependence of the crystal growth on the three primary crystal faces of hexagonal ice, the basal {0001} face, the prism {1010} face, and the secondary prism {1120} face, utilizing the TIP4P-2005 water model. New insights into the nature of its anisotropic growth are uncovered. It is demonstrated that the ice growth is indeed anisotropic; the growth and melting of the basal face are the slowest of the three faces, its maximum growth rates being 31% and 43% slower, respectively, than those of the prism and the secondary prism faces. It is also shown that application of periodic boundary conditions can lead to varying size effect for different orientations of an ice crystal caused by the anisotropic physical properties of the crystal, and results in measurably different thermodynamic melting temperatures in three systems of similar, yet moderate, size. Evidence obtained here provides the grounds on which to clarify the current understanding of ice growth on the secondary prism face of ice. We also revisit the effect of the integration time step on the crystal growth of ice in a more thorough and systematic way. Careful evaluation demonstrates that increasing the integration time step size measurably affects the free energy of the bulk phases and shifts the temperature dependence of the growth rate curve to lower temperatures by approximately 1 K when the step is changed from 1 fs to 2 fs, and by 3 K when 3 fs steps are used. A thorough investigation of the numerical aspects of the simulations exposes important consequences of the simulation parameter choices upon the delicate dynamic balance that is involved in ice crystal growth.

Carbon alloy formation during graphite pulse laser melting in a medium with pressure of ∼10 MPa

The growth of metastable carbon (MC) from the melt formed by melting of high oriented pyrolitic graphite (HOPG) with a laser pulse in helium is discussed. It has been found that in the melt of the HOPG the basal face grows in the form of stepped hillocks nucleating on screw dislocations, which does not necessitate substantial overcooling of the liquid. On the contrary, the carbon alloy from MCs with different phase compositions (diamond included) is formed from the melt of the HOPG prism face. The only explanation of this fact is homogeneous nucleation in a high supercooled melt. The numerical simulation of the heating process showed that homogeneous nucleation is related to the second solidification front directed from the melt-helium interface toward the principle solidification front moving from the bottom of a liquid bath toward its surface. Correlation between the formation of the second front with laser-induced electromagnetic waves on the melt surface (SEWs) manifesting themselves as periodic surface structures in the solidified melt is demonstrated.

Ice Growth from the Vapor at −5°C

Abstract Results are summarized and illustrated from a long series of experiments on ice growth from the vapor, nearly all in a very small range of conditions: −5°C, slightly below liquid water saturation, with minimal environmental gradients and no imposed ventilation. The temperature was chosen because c-axis ice needles grow in a narrow temperature interval there, which coincides with the temperature at which the Hallett–Mossop ice multiplication process operates most effectively, and one may suspect that this coincidence is likely to be meaningful. The ice growth habit is poorly reproducible in these conditions, dictating many runs with little change. Growth as plates can persist for hours, and two distinct types of needle growth occur, called sheath needles and sharp needles. Both are distinct from thin columns in that they taper to a point, with no discernible basal face. Both deviate slightly from parallel to the c axis. Sharp needles have been reported before, but only as occurring with an applied high DC voltage. New crystal orientations nucleate occasionally at the tips of the sharp needles; this also has been seen before in the presence of strong electric fields. There appears to be an ice multiplication mechanism in these conditions that does not involve riming.

Molecular Dynamics Simulation Study of the Interfacial Structure and Differential Capacitance of Alkylimidazolium Bis(trifluoromethanesulfonyl)imide [Cnmim][TFSI] Ionic Liquids at Graphite Electrodes

The dependence on electrode potential of the interfacial structure and differential capacitance (DC) for 1-alkyl-3-methyimidazolium bis(trifluoromethanesulfonyl)imide ([Cnmim][TFSI], n = 2, 4, 6, and 8) ionic liquids (IL) near basal (flat) and prismatic edge face (rough) graphite electrodes was investigated here with atomistic simulations. Overall camel-shaped DCs were observed for both surfaces. The prismatic graphite generated systematically larger capacitances than the atomically flat basal face. Although on the flat electrodes the DC is almost constant at electrode potential bellow saturation (i.e., roughly within ±2 V), on the prismatic edge face the DC showed large amplitude changes between minima and maxima. This trend in DC was explained from the dependence versus potential of the structure and composition of the interfacial electrolyte layer; specifically, faster counterions accumulation and ion segregation in the interfacial layer are observed for atomically corrugated electrode surfaces as compared to the flat ones. Surprisingly, the increase of the charge-neutral alkyl tail length of the cation resulted only in a small reduction in DC, indicating ions ability to rearrange/reorient charge-caring groups such that it maximizes the counterions charge near the surface. This finding shows a promising route for optimization of ions structure to achieve the desired/optimal properties of electrolyte (e.g., low melting point and viscosity) without significant reduction of energy density storage capabilities.

Shrinkage Characteristics of Experimental Polymer Containing Composites under Controlled Light Curing Modes

The adsorption of polymethylmethacrylate polymer of different molecular weight at the aerosil/ethyleneglycol- or 1,3 butanediol-dimethacrylate interfaces was determined to provide microstructured networks. Their structural characteristics were determined to be controlled by the amount of polymer initially supplied to the system. The sediment (the settled phase) characteristics, determined as a function of the polymer concentration and the rate of the polymerization shrinkage determined for composite resins, obtained by extrusion of the sediment after centrifugation, were found to be correlated. The specific role of the adsorbed polymer was found to be differently perturbed with the supplementary supply of dimethacrylate based monomer additives. Particularly, the bisphenol A dimethacrylate that generated crystals within the sediment was found to impede the shrinkage along the crystal lateral faces and strongly limit the shrinkage along its basal faces. Addition of ethyleneglycol- or polyethylene-glycoldimethacrylate monomers was determined to modify the sedimentation characteristics of the aerosil suspension and the shrinkage properties of the composites. Finally, the effects of stepwise light curing methods with prolonged lighting-off periods were investigated and found to modify the development and the final values of the composite shrinkage.

Biomimetic type morphologies of calcium carbonate grown in absence of additives

This report demonstrates how typical particle morphologies documented in biomimetic mineralization studies of calcium carbonate will precipitate also from solutions without adding modulating additives, at supersaturation levels below the solubility level of amorphous calcium carbonate (ACC). In the literature, hexagonal plates and flower structures of vaterite, as well as dumbbell structures of aragonite are explained by non-classical aggregation mechanisms from precursor crystals, assisted and stabilized by biomolecules, ions and templates, or by transformation from ACC. By performing experiments at both depleting and constant low supersaturation ratios for a range of temperatures, we show that the vaterite morphology changes from hexagonal monocrystalline plates expressing the basal (001) faces, to dendritic flower shapes and finally to spherulites, as a function of increasing supersaturation. Aragonite goes through a similar transition from monocrystalline elongated structures to polycrystalline dumbbells and spherical structures. We conclude that the key to understand the shape development is quantification of the activity based supersaturation and the realization that calcium carbonate forms along classical crystallization pathways. The higher number of crystals required for aggregation based growth is not favoured at these low supersaturation values. Dislocation and surface nucleation driven crystal growth is responsible for faceted morphologies at moderate supersaturation, whereas dendritic and spherulitic growth patterns appear due to interface instability at higher driving forces.

Hydrogen peroxide triggered morphological evolution of barium sulfate crystals under basic conditions

Inorganic small molecule H2O2 was utilized as an additive to control the crystallization of BaSO4. The evolution from simple flake-like to pillow morphologies shaped with dominant (hk0) planes and (001) plane was observed by just increasing the pH value and the H2O2 concentration in the reaction solution. Accompanying the change, the hexagonal (001) plane becomes the main face with the side faces transforming into the (210) and (100) faces. The inhibition on the [001] direction results in the appearance of a (001) face and the faster growth in the [010] direction influences the shape of the (001) plane to be hexagonal, elongated along the [010] direction. The crystalline morphologies of urchin-like BaSO4 at different mineralization times were investigated by SEM and a four-step evolution mechanism was proposed. Selective adsorption of HO2− originated from the decomposition of H2O2 over the BaSO4 particles, following gradual O2 evolution, and the formation of BaO2 were considered to be the key factors in the structural evolution. The structural units of urchin-like BaSO4 were strips with hexagonal basal faces, which could be seen as defect-free one-dimensional single crystals. The continuous variation of the morphology captured in this work may provide a deeper understanding of the formation mechanism of one-dimensional barium sulfate fibers. In addition, the results indicate that H2O2 may act as an additive for the crystallization of minerals. Because the final decomposition products are H2O and O2, H2O2 is especially suitable for directing the crystallization demanding high purity of the products.

Molecular Dynamics Simulation Studies of the Structure of a Mixed Carbonate/LiPF6 Electrolyte near Graphite Surface as a Function of Electrode Potential

Molecular dynamics (MD) simulations of an electrolyte comprised of ethylene carbonate (EC), dimethyl carbonate (DMC), and LiPF6 salt near the basal face of graphite electrodes have been performed as a function of electrode potential. Upon charging of the electrodes, the less polar DMC molecule is partially replaced in the interfacial electrolyte layer by the more polar EC. At negative potentials, the carbonyl groups from the carbonate molecules are repelled from the surface, while at positive potentials, we find a substantial enrichment of the surface with carbonyl groups. PF6– rapidly accumulates at the positive electrode with increasing potential and vacates the negative electrode with increasing negative potential. In contrast, Li+ concentration in the interfacial layer is found to be only weakly dependent on potential except at very large negative potentials. Hence, both composition of the electrolyte at the electrode surface and solvent environment around Li+ are observed to vary dramatically with the applied potential with important implications for oxidation/reduction of the electrolyte and the process of Li+ intercalation/deintercation.

Mandibles and labrum-epipharynx of tiger beetles: basic structure and evolution (Coleoptera, Carabidae, Cicindelitae)

Using for comparison with, and as outgroups for, supertribe Cicindelitae, we describe and illustrate the mandibles and labrum-epipharynx of the basal geadephagans Trachypachus gibbsii LeConte, 1861 (family Trachypachidae), and family Carabidae: Pelophila rudis (LeConte, 1863) (supertribe Nebriitae, tribe Pelophilini) and Ceroglossus chilensis (Eschscholtz, 1829) (supertribe Carabitae, tribe Ceroglossini). The range and pattern of variation in structure of mandibles and labrum-epipharynx within the supertribe Cicindelitae was assessed using scanning-electron (SEM) images of these structures in nine exemplar taxa: Amblycheila baroni (Rivers, 1890), Omus californicus (Eschscholtz, 1829) and Picnochile fallaciosa (Chevrolat, 1854) (representing the Amblycheilini); Manticora tuberculata (DeGeer, 1778) (representing the Manticorini): Tetracha carolina (Linnaeus, 1767) (representing the Megacephalini); Pogonostoma chalybeum (Klug, 1835) (representing the Collyridini); and Therates basalis Dejean, 1826, Oxycheila species, and Cicindela longilabris Say, 1824 (representing the Cicindelini). An evolutionary transformation series was postulated for the mandibles and labrum-epipharynx, based on a reconstructed phylogenetic sequence, which, in turn, was based on morphological and DNAevidence.Principal features of the transformation series for the mandibles included development of a densely setose basal face; wide quadridentate retinaculum; a lengthened incisor tooth; a multidentate terebra (one to five teeth; two-three most frequent), followed by subsequent loss of one or more such teeth; development of a diastema in the occlusal surface; development and subsequent loss of scrobal setae, and reduction and loss of the scrobe. Principal features of the transformation series for the labrum included evolution of form from transverse, sub-rectangular to elongate almost square, to triangular; position and number of setae evolved from dorsal to insertion on the apical margin, the number increased from 8-10 to as many as 36, and decreased to as few as four. The epipharynx broadened evolutionarily, the pedium evolving in form from narrow, triangular and nearly flat, to broad, palatiform, and markedly convex; anterior parapedial setae both increased and decreased in number, and in orientation, from a row parallel to the parapedial ridge to a setal row extended forward at about a right angle to the latter.

Single-scattering of preferentially oriented ice crystals at centimeter and millimeter wavelengths

The scattering properties of frozen cloud and precipitation particles at centimeter and millimeter wavelengths are fundamental for active and passive microwave remote sensing. So far the scattering parameters (scattering-, absorption-, and backscattering cross sections as well as asymmetry factor) have been calculated for randomly oriented particles only. However, some particle types, especially planar crystals, exhibit a preferred horizontal orientation. In this study the scattering and absorption parameters of four different planar ice crystal types have been modeled and exemplarily calculated from 1GHz to 300GHz with a freely available software code implementing the Discrete Dipole Approximation (DDA). It is presented how scattering and absorption of horizontally oriented ice crystals turn out for nadir-looking and inclined satellite sensors. The results show that small deviations around the horizontal orientation, as they are reported by relevant papers on cloud microphysics, have only marginal influence. Among all scattering parameters, the backscattering cross section of oriented crystals can differ most with respect to a random orientation. Especially if the incident wave direction is normal to the basal face, the parameter was found to be up to 20 times higher. Therefore the consideration of the orientation especially can be important for active microwave sensors. The more so if horizontally oriented particles and a nadir looking active sensor are involved. Furthermore the existing compilations for the index of refraction of ice at microwave frequencies have been reviewed.