Basal Face Research Articles

Preparation of Y-SiAlON rod-like crystals and whiskers by combustion synthesis

By combustion synthesis, Y (α+β)-SiAlON ceramic powders consisting of rod-like crystals were prepared. Effect of starting composition on the phase assemblage of combustion products was studied. It was found that with the increase of oxygen content, the phase assemblage of products varied from single-phase α-SiAlON to β-SiAlON gradually. Novel β-SiAlON whiskers were also observed in the combustion products. From the whisker morphology, it was suggested that both VLS and VS mechanisms operated here. It was also noticed that the whiskers could nucleate either on the basal face or on the side faces of as-existed prismatic rod-like crystals.

Detection efficiency of fission tracks in apatite related to mica external detectors by the spike irradiation technique

Detection efficiency of fission tracks in non-annealed and annealed Durango apatite crystals related to mica external detectors for fission track dating was estimated experimentally using the spike irradiation technique [Suzuki, 2002a. Abstracts of 21st International Conference on Nuclear Tracks in Solids, p. 22]. The relation of the fission track density between apatite samples and mica external detectors is an important parameter for the determination of fission track ages using an absolute approach especially for crystalline minerals. Spike irradiation of uranium-235-induced fission fragments was carried out using a high uranium content glass, GE1497, fixed firmly on the sample surface. Experimental works were carried out for sections parallel and perpendicular to the crystallographic c -axis of apatite. The detection efficiency of fission tracks in non-annealed apatite related to the mica external detectors was estimated to be 0.90 for the prism face and 0.99 for the basal face. The fission track age of Durango apatite averaged for the prism and basal faces was estimated to be 30.6 ( ± 2.5 ) Ma, which is concordant with reference values reported previously, using the detection efficiency data with a B 0 value of the CN5 monitor glass and a U-238 spontaneous decay constant value of 8.46 × 10 - 17 a - 1 . The results also indicate that the B 0 age calibration method can be used effectively for the absolute evaluation of fission track ages of apatite.

Experimental leaching of atacamite, chrysocolla and malachite: Relationship between copper retention and cation exchange capacity

Copper retention due to chemical and physical interaction with reactive gangue has been studied during sulphuric acid leaching of atacamite, chrysocolla and malachite, using a synthetic rock (57% quartz, 42% reactive gangue and 1% ore). Smectite- and mordenite-rich gangues represent the highest copper retention whereas the lowest is found for kaolinites and illite. During leaching, smectites show the largest change (increase) in volume; no volume change occurred for kaolinites, zeolite and illite. Kaolinite-rich gangue forms barriers due to de-lamination demonstrated by acid accumulation at the top of the columns. The X-ray diffraction patterns of smectite, illite and kaolinite in the residues demonstrate the diminution or complete loss of basal crystal faces accompanied by the neo-formation of alunogen, meta-alunogen, coquimbite and goldichite. These new phases contribute to a temporary reduction of permeability. For all the synthetic rocks assayed, the copper retention is directly proportional to cation exchange capacity (CEC) of the starting materials, and corroborated by the CEC of Cu of the residues determined without water elution prior to analysis. This test used represents a predictive tool to evaluate the potential Cu retention and gangues with elevated CEC.

Surface morphology and crystal growth mechanism of gibbsite in industrial Bayer liquors

Atomic force and scanning electron microscopy have been used to study the surface topography and crystal growth mechanism of gibbsite crystals in industrial Bayer liquors. AFM images of the as-grown basal face of industrially produced gibbsite show that the surface is rough and contains diamond shaped features, whereas freshly cleaved basal planes show steps of one unit cell height. An array of steps terminating within the crystal was imaged, indicating the presence of an emergent screw dislocation. Typical AFM images of the prismatic faces contain molecular steps parallel to the basal face. Growth on single faces of synthetic crystals has been observed by a series of ex situ growth experiments in both industrial (impure) and pure synthetic liquor at constant supersaturation and temperature. The results reveal that growth in plant liquor is almost inhibited on the prismatic faces, while further immersion in pure solution induces growth by formation of prism-shaped nuclei. This may suggest that organics from plant liquor adsorb strongly to the growth sites on the prismatic faces, preventing the incorporation of the growth units, and further growth in pure solution occurs by nucleation and spreading of surface features. Growth on the basal faces of gibbsite in plant liquor occurs by a birth and spread mechanism, which may suggest that the interaction of process liquor organics within process liquor with the basal face is different than their interaction with the prismatic faces.

Synthesis of monoamine derivatives of [Ir 4(CO) 12]. X-ray crystal structure of [Ir 4(CO) 11(4-picoline)

The mono-substituted amine derivatives [Ir 4(CO) 11L] (L = pyridine ( 1), 4-methylpyridine ( 2), 4- ter-butyl pyridine ( 3), 3,5-dimethylpyridine ( 4), 3,4-dimethylpyridine ( 5)) were obtained by the reaction of [Ir 4(CO) 11Br] − with the corresponding aromatic amine. In the solid state, cluster 2 has an approximate C s symmetry with all terminal ligands as shown by an X-ray analysis. In solution, this unbridged structure is in dynamic equilibrium with two other isomeric forms having three edge-bridging CO’s on a common basal face and the amine ligand coordinated in axial or in radial position relative to this face.

Ice nucleation on BaF2(111)

The mechanism of heterogeneous ice nucleation on inorganic substrates is not well understood despite work on AgI and other materials over the past 50 years. We have selected BaF(2) as a model substrate for study since its (111) surface makes a near perfect match with the lattice of the basal face of I(h) ice and would appear to be an ideal nucleating agent. Two series of experiments were undertaken. In one, nucleation of thin film water formed from deposition of vapor on BaF(2)(111) faces was explored with the finding that supercooling to -30 degrees C was required before freezing occurred. In the other series, nucleation of liquid water on submerged BaF(2) crystals was studied. Here supercooling to -15 degrees C was needed before ice formed. The reason why BaF(2) is such a poor nucleating agent contains clues to realistic mechanisms of heterogeneous nucleation. Our explanation of these results follows the model of Fletcher [J. Chem. Phys. 29, 572 (1958)] who showed that heterogeneous ice nucleating ability depends on how well ice wets a substrate. In this view, a smooth BaF(2)(111) face is poor at nucleation because ice only partially wets its surface. In an extension of Fletcher's model, our calculations, consistent with the experimental results demonstrate that the pitting of a submerged BaF(2) crystal dramatically improves its ice nucleating ability.

Preparation of an ordered zeolite MFI film by epitaxial growth

An ordered zeolite MFI film has been prepared on the substrate of a {001} basal face of high-quality large silicalite-1 crystals by epitaxial growth.

Experimental and theoretical study of intramolecular exchange in Ir2Rh2(CO)12and Ir4(CO)11(µ-SO2)

As observed by variable-temperature and -pressure 13C NMR, intramolecular carbonyl scrambling in Ir2Rh2(CO)12 and Ir4(CO)11(micro-SO2) proceeds via a 'change of basal face' mechanism. In both cases the site exchange process has a positive activation volume suggesting that the transition states contain longer M-M distances compared to ground states of Cs symmetry. Transition state structures have been located by density functional calculations including relativistic effects. These structures contain a new symmetry plane which interchanges the indistinguishable starting and final geometries. Both transition state structures contain one significantly elongated M-M distance, bearing the bridging ligand unaffected by the site exchange. Differences in molecular volumes of ground and transition state geometries as calculated from Connolly surfaces and electron densities confirm the volume expansion in both cases. The sign of the activation volume is therefore a good criterion for distinguishing between the two main site exchange processes occurring in tetrahedral d9 carbonyl clusters, i.e. the 'change of basal face' process and the 'merry-go-round' process, as the latter presents a negative activation volume.

THE RELATIONSHIP BETWEEN GLOBAL AND REGIONAL DISTRIBUTION DIMINISHES AMONG PHYLOGENETICALLY BASAL SPECIES

Phylogenetic legacy and phylogenetic trends affect the ecology of species-except, apparently, for the width of their distribution. As a result, "macroecological" patterns of species distributions emerge constantly in phylogenetically very distinct species assemblages. The width of the global distribution of species, for instance, constantly correlates positively to the width of their regional distribution. However, such patterns primarily reflect the phylogenetically derived species that dominate most assemblages. Basal species, in contrast, might show different macroecological patterns. We tested the hypothesis that the correlation between global and regional distributions of species diminishes among the phylogenetically basal species. We considered central European higher plants and defined global distribution as the occupancy of global floristic zones, regional distribution as the grid occupancy in Eastern Germany, and phylogenetic position as the rank distance to tree base. We also took into account a number of confounding variables. We found that, across all lineages, the global/regional correlation diminished among basal species. We then reanalyzed 19 lineages separately and always found the same pattern. The pattern reflected both increases in global distributions and decreases in regional distributions among basal species. The results indicate that many basal species face a risk of global or at least regional extinction, but have escaped the downward spiral of mutually reinforcing extinction risks at multiple scales. We suggest that many basal species had much time to expand their global ranges but are presently displaced locally by more derived species. Overall, the study shows that macroecological patterns may not be static and universal, but may undergo macroevolutionary trends. Analyses of macroecological patterns across a phylogeny may thus provide insights into macroevolutionary processes.

PREDICTION OF CRYSTAL MORPHOLOGY OF COMPLEX URANYL-SHEET MINERALS. II. OBSERVATIONS

The morphology of basal faces of uranyl-sheet minerals is predicted by considering the bond-valence deficiency of the chains of polyhedra, the shift between the layers, and the arrangement of the interstitial complexes between the layers. The sheet structural units of schoepite, [(UO2)8O2(OH)12](H2O)12, and fourmarierite, Pb[(UO2)4O3(OH)4](H2O)4, have the same underlying anion-topology. The stabilities of their edges differ because adjacent layers in schoepite are shifted by one chain of polyhedra, whereas adjacent layers are not shifted in fourmarierite. The minerals becquerelite, [7] Ca(H2O)4[(UO2)3O2(OH)3]2 (H2O)4, compreignacite, [7] K2(H2O)3 [(UO2)3O2(OH)3]2(H2O)4, billietite, [10] Ba(H2O)4[(UO2)3O2(OH)3]2 (H2O)3, protasite, [10] Ba(H2O)3[(UO2)3O3(OH)2], and masuyite, [10] Pb(H2O)3[(UO2)3O3(OH)2], all have structural units with the same basic aniontopology. The stabilities of their edges differ because of the different arrangements of interstitial species. In becquerelite, the interstitial cations are arranged in rows parallel to [010], whereas in billietite and compreignacite, the interstitial cations are arranged in rows parallel to [100]. For masuyite and protasite, the stability of their edges is defined by large differences in the bond-valence deficiencies of the chains of polyhedra with left and right terminations. The stabilities of edges in curite, Pb3[(UO2)8O8(OH)6](H2O)3, are dictated by the low bond-valence deficiencies of the chains of polyhedra parallel to [001] and [011], and by the arrangement of cations in rows parallel to [001]. Minerals of the carnotite group contain the structural unit [(UO2)2(V2O8)] 2‐ , and the stability of the edges of their sheets is determined by the low bond-valence deficiency of chains of polyhedra parallel to [010 and [110]. Minerals of the uranophane group are based on the [(UO2)SiO3(OH)] ‐ sheet, and the stabilities of their edges are more strongly affected by the bond-valence deficiency of the chains of polyhedra than by the arrangement of the interstitial cations, and by the shift in the sheets. All predictions are compared with the corresponding morphologies of crystals recorded by Atomic Force Microscopy (AFM), on images of crystals, and on crystal drawings from the (mainly older) mineralogical literature. All predictions are in good agreement with the observed morphology of the basal faces.

Growth of nordstrandite: effect of carboxylates on surface morphology

We studied the effect of carboxylates on the crystallization of nordstrandite precipitated from ethylenediamine solutions by examining the nanometer scale step structure of the basal faces with atomic force microscopy (AFM). The crystals grown in the presence of carboxylates are more rounded in shape than those grown without added carboxylates, with irregular growth in the direction perpendicular to the basal faces and small protuberances from the crystals. AFM images show monolayer steps on the surfaces of the crystals. These steps are composed of straight segments down to a horizontal resolution of a few nanometers. A survey of the step segments showed that the steps formed in the presence of carboxylates have shorter segments. For the crystals grown in the control solutions, two-dimensional nucleation is evident in a few places on the surfaces, and concentric terraces are present in many cases. For the crystals grown in carboxylate solutions, two-dimensional nucleation had occurred more randomly and evenly on the surfaces, with smaller nuclei. These differences in step morphology are due to the adsorption of the carboxylates at the steps, which affected the step edge surface energy.

Dioxycarbene complexes derived from reactions of tetrairidium carbonyl clusters with oxirane: X-ray crystal structures of and

Some novel cyclic-dioxycarbene derivatives of general formula ▪ (L = P t Bu 3, n = 1 ( 2), PPh 3: n = 1 ( 3), 2 ( 4)) and ▪ (L–L = PPh 2PCH 2PPh 2, n = 1 ( 5), norbornadiene, n = 1 ( 6) and 1,5-cyclooctadiene, n = 1 ( 7), 2 ( 8)) have been obtained by reaction of oxirane ▪ with the tetrairidium cluster derivatives [Ir 4(CO) 11(L)] and [Ir 4(CO) 10(L–L)] in the presence of bromide ion as catalyst. Elemental analysis, IR, and NMR spectra ( 1H, 31P{ 1H}, 13C{ 1H}), and for compounds 2 and 5 also the X-ray crystal structures, were carried out for their characterisation. All the derivatives have 3 edge-bridging CO’s on the basal face of the iridium tetrahedron with non-CO ligands in axial and/or radial positions. For the mixed-ligand cluster compounds, two or three stereoisomers were observed in solution by 1H, 31P and 13C NMR spectroscopies at low temperature. All these clusters are fluxional at room temperature.

Growth of nordstrandite: effect of chloride on surface morphology

The effect of chloride on the growth of the Al(OH) 3 mineral nordstrandite in ethylenediamine solutions has been investigated. Atomic force microscopy (AFM) observations revealed a difference in the morphology of monolayer steps on the basal faces of the crystals. The steps are made up of straight segments, down to a horizontal resolution of a few nanometers. These segments are shorter for the crystals precipitated in the presence of chloride. However, they show no difference in crystallographic orientation compared to those of the crystals synthesized without added chloride. Double steps are more prevalent for crystals synthesized with higher chloride concentrations. For crystals synthesized in the control solutions, two-dimensional nucleation on the surfaces only occurred at a few points and concentric steps were commonly observed. However, for crystals synthesized in the presence of chloride, the nucleation sites were more evenly spread. The observations suggest that the differences in the step morphology between the nordstrandite crystals synthesized in the presence of chloride and those synthesized in the control solutions are due to the adsorption of chloride at the steps. For all twinned crystals, two-dimensional nucleation occurred preferentially at point sources within the re-entrant angles. Spiral growth was only observed in one case.

Screw Dislocations on Polycenes: A Requirement for Crystallization

The morphology and growth behavior of naphthalene, anthracene, and tetracene is studied both experimentally and by computer simulations. We will show that the attachment energy morphology prediction for all compounds gives approximately the correct habit, but that this is merely coincidental. Our Monte Carlo simulations and vapor growth experiments show that the thickness growth of the crystals perpendicular to the {001} faces is due to spirals on that face. If these screw dislocations were not present, the basal faces would not grow and no crystal would be formed at moderate driving force. At very high driving forces, very thin plates without screw dislocations were observed, with a morphology that is in agreement with the results of our Monte Carlo simulations.

THE RELATIONSHIP BETWEEN GLOBAL AND REGIONAL DISTRIBUTION DIMINISHES AMONG PHYLOGENETICALLY BASAL SPECIES

Phylogenetic legacy and phylogenetic trends affect the ecology of species—except, apparently, for the width of their distribution. As a result, “macroecological” patterns of species distributions emerge constantly in phylogenetically very distinct species assemblages. The width of the global distribution of species, for instance, constantly correlates positively to the width of their regional distribution. However, such patterns primarily reflect the phylogenetically derived species that dominate most assemblages. Basal species, in contrast, might show different macroecological patterns. We tested the hypothesis that the correlation between global and regional distributions of species diminishes among the phylogenetically basal species. We considered central European higher plants and defined global distribution as the occupancy of global floristic zones, regional distribution as the grid occupancy in Eastern Germany, and phylogenetic position as the rank distance to tree base. We also took into account a number of confounding variables. We found that, across all lineages, the global/regional correlation diminished among basal species. We then reanalyzed 19 lineages separately and always found the same pattern. The pattern reflected both increases in global distributions and decreases in regional distributions among basal species. The results indicate that many basal species face a risk of global or at least regional extinction, but have escaped the downward spiral of mutually reinforcing extinction risks at multiple scales. We suggest that many basal species had much time to expand their global ranges but are presently displaced locally by more derived species. Overall, the study shows that macroecological patterns may not be static and universal, but may undergo macroevolutionary trends. Analyses of macroecological patterns across a phylogeny may thus provide insights into macroevolutionary processes.

Atomic force microscopy study of the growth mechanism of gibbsite crystals

A combination of atomic force microscopy (AFM) and scanning electron microscopy (SEM) has been used to investigate the crystal growth mechanism of gibbsite (aluminium trihydroxide) in pure solutions. Under the conditions studied, the growth on the basal face of synthetic gibbsite prepared from sodium aluminate solution proceeds by a continuous birth and spread mechanism. The nuclei formed on the surface of the basal face of gibbsite grow both laterally and vertically, with lateral growth being much faster than vertical growth. Moreover, a remarkable cyclical, smooth→rough→smooth→rough process has been directly observed. The effect of alkali ions on the crystal growth of gibbsite has been investigated. Curved features and a growth hillock were observed for the first time on the basal face of gibbsite prepared from potassium aluminate solution; whereas, steps terminating within the plane and hexagonal shaped features tilted at an angle and related to the symmetry of the face were imaged on the basal faces of gibbsite prepared from sodium aluminate solution. The results suggest faster growth on the basal face of gibbsite prepared from potassium aluminate compared to that prepared from sodium aluminate, leading to the observed elongated morphology.

Coprecipitation of Fe(II–III) hydroxycarbonate green rust stabilised by phosphate adsorption

[Fe II 4Fe III 2(OH) 12] 2+•[CO 3, nH 2O] 2− green rust one, GR1(CO 3 2−), was synthesised by coprecipitation in the presence of phosphate by dissolving a Na 2HPO 4 salt. In its absence, the resulting product is a mixture of Fe(II–III) hydroxysalt, magnetite Fe 3O 4 and ferrous hydroxide Fe(OH) 2. In the presence of phosphate as low as {[PO 4] min 0/[Fe]}∼1%, the resulting product is GR1(CO 3 2−) alone. No P was detected by TEM-EDX when the basal (0001) crystal faces were analysed, showing that no P was inserted in the GR. In contrast, very low quantities of P were detected when the analyses were performed on the lateral faces of the GR crystals. Analysis of P in solution by ICP-AES evaluates the quantity of phosphate adsorbed species. A model representing the precipitate and its surface showed that the quantity of adsorbed phosphate has the same order of magnitude than the number of Fe atoms present on the lateral faces. The preference of phosphate species for lateral faces is due to the fact that the {10 1 ̄ 0} faces contain mono- and di-coordinated OH −surface groups. PO 4 3− adsorbed on the lateral faces may act as a barrier that slows down the release in solution of the CO 3 2− species preventing GR1(CO 3 2−) to transform into the mixture of Fe 3O 4 and Fe(OH) 2.

Reactions of Diphosphine-Stabilized Tetracobalt Carbonyl Clusters with −Si(OR)3-Functionalized Alkynes

Short-bite ligand cluster stabilization is achieved in the tetrahedral clusters [Co4(μ-CO)3 (CO)7(μ-dppy)] (1a−c), obtained in high yields by reactions of [Co4(CO)12] with 1 equiv of the diphosphine ligands dppy, Ph2PCH2PPh2 (dppm), Ph2PNHPPh2 (dppa), or (Ph2P)2N(CH2)3Si(OEt)3 (dppaSi), respectively. The structure of 1a has been determined by X-ray diffraction, and the P atoms occupy axial positions on the basal face, transoid to a Co−Co bond. Clusters 1a−c were reacted with phenylacetylene to afford the corresponding butterfly clusters [Co4(μ-CO)2(CO)6(μ-dppy)(μ4-η2-PhC2H)] (2a−c) by insertion of the alkyne into a Co−Co bond of the precursor. This was established by an X-ray diffraction study of [Co4(μ-CO)2(CO)6(μ-dppm)(μ4-η2-PhC2H)] (2a). In an alternative synthetic procedure, the alkyne cluster [Co4(CO)10(μ4-η2-PhC2H)] (3), prepared from [Co4(CO)12] and phenylacetylene, was reacted with the diphosphines dppy. This led in good yields to butterfly clusters, isomeric with 2a−c in terms of the position of ...

Surface Adsorption and Trapping of Xe on Hexagonal Ice at 180 K by Molecular Dynamics Simulations

Classical molecular dynamics (MD) simulations of Xe on the basal (0001) face of hexagonal ice at 180 K have been performed in order to investigate the mechanism of adsorption and the initial stage of absorption of a van der Waals particle into crystalline ice. The potential of mean force (PMF) as a function of the relative position of the Xe atom perpendicular to the ice surface is found to increase abruptly as the particle propagates from the disordered outermost region to the deeper hexagonally ordered region. A set of local minima observed in the PMF appears to correspond directly to the layer-by-layer crystal structure of hexagonal ice. Along with the unbound state, the first two minima (with similar free energies) that correspond to the accommodation of the guest particle on and inside the disordered outermost bilayer, respectively, are found to be populated during the MD runs. The multiple transitions of the system among these three states are also observed in accord with the PMF profile, which also suggests that the penetration of any ordered hexagonal bilayer by the Xe atom is unlikely. Furthermore, MD simulations of pristine ice (i.e., without Xe) over a 3-ns simulation period show that the initially perfect-ordered hexagonal crystalline structure of the outermost bilayer undergoes transformation to a noncrystalline phase, in which fragmented domains with hexagonal ordering persist. Moreover, the accommodation of the Xe atom inside the outermost bilayer could facilitate further disordering of the hexagonal structure of this bilayer with the formation of a completely disordered Xe−ice surface phase.

Growth of platelets under diffusion conditions. Growth kinetics of platelets obtained on the zinc single crystals and determination of the timing of their appearance (II)

The kinetics of growth of platelets on the pyramidal { 1 0 1 ̄ 1 } and prismatic { 1 0 1 ̄ 0 } faces of zinc single crystals and the alteration with time t of the edge length a of the {0 0 0 1} basal face of the same crystals in the presence of argon were investigated. The measurements of the edge a with time show that at the moment when the critical size a k is reached, the transition from kinetic to diffusion regimes starts. The comparison between the time when the visible platelets appeared on the zinc crystals and the a 2 vs t curves revealed that the first platelets grow exactly at the transition between the two regimes. It has been established that during the initial stage, when the size L is small, the platelets grow at a constant rate. During further growth, as the size L increases and reaches the critical dimension L k , a change in the L 2 vs t curves appears: the linear curve is transformed into a parabolic one. In conformity with the morphological theory of Chernov in the isotropic case, it is concluded that the platelets grow in a kinetic and a diffusion regime. This effect of transition from the kinetic to the diffusion regime during the growth of platelets, similar to the effect observed during the growth of zinc single crystals, is a common phenomenon, which results from the diffusion conditions under which platelets and single crystals grow.