Pyramidal Faces Research Articles

Rate of decay of specific surface area of snow during isothermal experiments and morphological changes studied by scanning electron microscopy

The quantification of the specific surface area (SSA) of snow crystals and of its variation during metamorphism are essential to understand and model the exchange of reactive gases between the snowpack and the atmosphere. Therefore, the decay rate of SSA of five fresh snow samples was studied in the laboratory at –4, –10, and –15°C under isothermal conditions in closed systems. The time-evolution of the snow SSA can in all cases be very well described by an empirical law of the form, SSA = – A log(t + Δt) + B, where A, B, and Δt are adjustable parameters. B seems to be closely related to the initial SSA of the snow, and A describes the SSA decay rate. Our preliminary findings at –15°C suggest that a linear relationship exists between A and B, so that it may be possible to predict the decay rate of snow SSA from its initial value. For the first time, images obtained from scanning electron microscopy show that crystal rounding of snow is the main process taking place during isothermal metamorphism. New grain boundaries also form. More surprising, however, was the formation of new basal, prismatic, and pyramidal crystal faces, sometimes with very sharp angles, especially at –15°C. The growth of facets with sharp angles is not fully explained by current theories of snow metamorphism and has not been observed before. PACS Nos.: 68.35Md, 68.37Hk, 81.20Ev, 81.05Rm

From atomic kinks to mesoscopic surface patterns: Ionic layers on vicinal metal surfaces

A highly regular assembly of three-sided pyramids can be fabricated by growing the ionic insulator NaCl on the kinked metal surface Cu(532). Only two pyramid faces are covered by NaCl, resulting in an overall surface structure which is modulated in surface chemical behavior. Scanning tunneling microscopy shows that the underlying restructuring mechanism can be attributed to a clear-cut criterion for enhanced interface stability mediated by electrostatic interactions. This criterion provides a generally applicable guideline to create nano- to mesoscopic surface structures by design.

Synchrotron radiography and x-ray topography studies of hexagonal habitus SiC bulk crystals

Phase-sensitive synchrotron radiation (SR) radiography was combined with x-ray diffraction topography to study structural defects of SiC crystals. The particular bulk SiC crystals examined had a low micropipe density and a hexagonal habitus composed of prismatic, pyramidal, and basal faces well developed. X-ray diffraction topography images of the sliced (0001) wafers, which were formed due to the complex lattice distortions associated with defective boundaries, demonstrated the existence of two-dimensional defective boundaries in the radial direction, normal to the (0001) planes. In particular, those parallel to the 〈1120〉 directions extended rather far from the seed. On the other hand, by phase-sensitive SR radiography the effect of micropipe collection was detected. Micropipes grouped mostly in the vicinities of the defective boundaries but rarely appeared between groups. Some general remarks about possible reasons for the development of such peculiar defect structures were made.

In situ measurement of pH and supersaturation-dependent growth kinetics of the prismatic and pyramidal facets of KDP crystals

The effect of pH and supersaturation on the growth kinetics of the pyramidal (1 0 1) and prismatic (1 0 0) faces of KDP crystals is reported. Growth rates of these faces at the microscopic level are measured using the laser-polarisation–interference technique, and a comparative analysis is carried out. In the light of these results, an attempt has been made to clear the ambiguity surrounding the effect of pH on the growth rates of KDP crystals along the a- and the c-axis. We have found that the growth rate of (1 0 1) as well as (1 0 0) face is maximum at stoichiometric pH of 4.2 (at all supercoolings considered), and it decreases on either side of this value. This is in agreement with the results of Efremova et al. (Crystallogr. Rep. 38(5) (1993) 674), and contradictory to those of Wang et al. (J. Crystal Growth 83 (1987) 471), Marecek et al. (Kristall Technik 4(1) (1969) 39) and Rashkovich et al. (J. Crystal Growth 151 (1995) 145) where it is reported that growth rate of (1 0 0) face increases if we go above the stoichiometric pH. For performing these on-line and in situ studies, a novel crystal-growth cell, a special seed-orienting mechanism for mounting the desired crystal face in the laser beam path, and a data acquisition system were developed. The shadowgraphy technique was employed for making an accurate measurement of the saturation temperature.

Structures of quartz (100)- and (101)-water interfaces determined by x-ray reflectivity and atomic force microscopy of natural growth surfaces

The structures of prismatic (100) and pyramidal (101) growth faces of natural quartz crystals, and their modification upon annealing at T ≤ 400°C were investigated ex situ by atomic force microscopy (AFM) and in water by high-resolution X-ray reflectivity. AFM images revealed the presence of ∼ 0.1 to 1 μm-wide flat terraces delimited by steps of one to several unit cells in height. These steps follow approximately directions given by the intersection of growth faces. Modeling of X-ray reflectivity data indicates that surface silica groups on flat terraces have only one free Si-O bond each (presumably hydroxylated), except for some having two free Si-O bonds observed on a single (100) surface. Vertical relaxation of atomic positions (< 0.4 Å for terminal oxygens and < 0.2 Å for silicon and oxygen atoms fully coordinated to structural tetrahedra) is limited to a depth of 14 Å. Electron density profiles for all measured interfaces are consistent with a single layer of adsorbed water, with no evidence for additional organization of water molecules into distinct layers extending into the bulk solution. Similar interfacial structures were observed for natural and annealed surfaces of identical crystallographic orientation, indicating that extensive reconstruction of the silica network at the quartz surface did not occur under the annealing conditions.

Hexagonal shaped ice spicules in frozen antifreeze protein solutions

In the presence of antifreeze proteins from both Antarctic and Arctic fishes, water freezes in the form of long c-axis spikes or spicular-like crystals. Transmission electron microscopy of the Pt/C replicas of the freeze fractured spicular ice in a small capillary revealed the presence of many hexagonally shaped structures whose cross-sectional dimensions were between 0.5 and 10 μm. Well-defined parallel faces were associated with most fractured and etched spicules. When fracture planes occurred near the tip of a spicule, well-defined pyramidal faces were apparent. Steps were sometimes associated with these pyramidal spicular crystal faces. On some of the replicas obvious roughening of certain crystal faces of the spicule was observed, suggesting that the antifreeze proteins may have adsorbed to those faces.

Distinct Growth Phenomena Observed on Zinc Cadmium Thiocyanate Crystals by Atomic Force Microscopy

Atomic force microscopy is used to investigate the surface morphology of the prismatic (100) face of ZCTC crystal grown at 30°C at a supersaturation of 0.16. This surface is distinctly formed by periodic macrosteps that advance along different directions and join with each other leading to the interlaced growth layers with an inclination of about 137°. These two macrostep trains well correspond to the pyramidal faces of (011) and (011) in orientation, therefore they probably propagate from the edges of these faces. The macrosteps are practically formed by highly dense steps at the front with regular elementary steps in between. The alternation of macrosteps and elementary steps vividly reflects Chernov's kinematic waves of steps theory (CHERNOV, (1984)) on a nanometer scale. Wide indentations and long clefts are generated at the macrosteps. The former is generated by two-dimensional nucleation growth at a relatively faster growth rate than that of the underlying layer. The latter is probably caused by step trains generated by individual growth sources that have not merged.

Morphology and Polarity of GaN Single Crystals Synthesized by the Na Flux Method

The correlation between morphology and polarity was investigated for GaN single crystals synthesized by the Na flux method. The polarity was identified by using anomalous X-ray dispersion. Colorless transparent prismatic single crystals were classified into three types. The first had a smooth basal plane face and no hexagonal pyramidal faces, the second had smooth pyramidal faces, and the third had a rough basal plane face and rough pyramidal faces. The prismatic crystals with smooth surfaces had N-polarity and grew in the −c direction, whereas the crystals having rough surfaces had Ga-polarity and grew in the +c direction. In colorless transparent platelet single crystals, one side of the basal planes had a mirror smooth surface, while the other side had many step edges and hexagonal pits. The smooth basal plane and the rough basal plane were the (0001̄) N-face and (0001) Ga-face, respectively. Black pyramidal crystals had a smooth (0001) basal plane face corresponding to the Ga-face.

Raman scattering investigation of the effect of EDTA additives on growth habit of KDP

KDP crystal is grown from solutions in the presence of doping Ethylenediamine tetra acetic acid (EDTA) in the concentration range of 0–500 ppm, and the growth rates of prismatic and pyramidal faces of KDP crystal are measured, respectively. The Raman spectrums of crystal growth solution in different growth layers with and without EDTA are investigated. Experiment results show that the macroscopic growth habit of KDP crystal will change obviously by adding a small quantity of EDTA into the growth solution. For the prismatic faces, it has been found that additive EDTA is favorable to the formation of growth units and thus it can enhance the crystal growth rate. For the pyramidal faces, EDTA additive also has an enhancing effect on the growth rate, but this effect is not considerable compared to prismatic faces. Our research results also show that EDTA does not affect the crystalline quality.

Optical characterization of GaAs pyramid microstructures formed by molecular beam epitaxial regrowth on pre-patterned substrates

Arrays of GaAs pyramids with square (001) bases of length 1–5 μm have been fabricated by molecular beam epitaxy regrowth on pre-patterned GaAs (001) substrates. The optical properties of the pyramid faces have been studied by microreflection and microtransmission imaging measurements with light (λ=900–1000 nm) incident through the pyramid base. Digitized charge coupled device images indicate that total internal reflection occurs at the {110} pyramid facets and that their reflectivities are greater than 80%, provided overgrowth of the facets does not occur. These properties suggest that such structures may be suitable as the top mirror in novel micron-scale vertical microcavity devices.

A new equilibrium form of zircon crystal

The surface tensions of the {100}, {110}, {101} and {211} faces are calculated by using Machenzie’s method. A new equilibrium form of zircon crystal is then derived on the basis of Wulf’s Law. This theoretic form consists of {100} prismatic faces and {211} pyramidal faces, which is different from that of the prediction by the periodic bond chain theory. The discovery of the equilibrium form of zircon crystal provides a clue for understanding of the morphology of zircon crystals formed in deep magma chamber, indicating that zircon morphology is an indicator of crystallization conditions.

Fluctuation and dipolar interaction effects on the pinning of domain walls

We discuss the effect of the dipolar interaction on the pinning of domain walls. Domain walls are usually pinned near the boundaries between grains. Magnetic charges accumulated at the domain wall make the wall more unstable and easier to depin. We discuss how the grain-orientaion and thermal fluctuations affect these magnetic charges and hence the depinning of the domain walls. Our results are illustrated by finite temperature Monte Carlo simulation on periodic arrays of large cells separated by walls consisting of faces of pyramids.

Study on K(DxH1-x)2PO4 Crystals: Growth Habit, Optical Properties and their Improvement by Thermal-Conditioning

Phosphite, which often exists in growth solutions obtained directly from commercial P2O5 , was found to have significant inhibiting effects on the growth of pyramidal face of KDP crystals. K(DxH1-x)2PO4 (referred to as DKDP) crystals with different deuterium fraction x were grown and the optical performances were investigated. The absorption coefficients at 1.05 μm decreases monotonically with the increase of x. The transmission threshold shift from 1.65μm at x=0 to 2.10 μm at x=0.96. The high temperature phase transition temperature and latent heat were measured using the method of differential scanning calorimetry (DSC). Thermal conditioning experiments were carried out at 180°C and 140°C for KDP and DKDP, respectively. After conditioning, a different degree of improvement was observed in the optical homogeneity of the samples, while the laser damage threshold and light absorption coefficient showed no significant change.

Effects of anions on rapid growth and growth habit of KDP crystals

The synthesis of KDP from its raw material has been found exist in the growth solution. In the crystal growth experiment, significant extension of specific faces was observed at low dopant concentration. At high doping concentration, the growth rate of the whole crystal decreased with no significant habit modification. The inhibiting effects of phosphite and other H-bonding anionic ions on the growth of pyramidal faces are discussed. Rapid growth rate experiments have been carried out with purified material and an averaged growth rate of 18.6mm/day was obtained.

Growth of the {101} face of KDP crystals in the presence of dye Chicago Sky Blue

ABSTRACTIn situ interference technique is used for a study of morphology and growth kinetics of a pyramidal face of KDP crystals with dye Chicago Sky Blue. It is shown that the impurity is captured by a crystal when its concentration in the solution exceeds a certain temperature depended value. In the presence of impurity the face loses its stability with increasing of supersaturation. Further increase of supersaturation leads to the restore and subsequent loss of stability. The experimental results are interpreted in the framework of earlier developed model for a crystal growth in the presence of the easily desorbed mobile impurity. The parameters characterizing the adsorption behavior of mobile impurity and its capture have been determined.

KDP kinetics and dislocation efficiency of growth

A macroscopic growth kinetic study has been carried out by taking advantage of simultaneous observations on a number of crystals (three in our experiments), growing under the same conditions. A large number of measurements on kinetic data for R 〈0 0 1〉 (σ) were found in three separate experiments of KDP growth in a dynamic regime, using the temperature lowering method. Macroscopic growth rates were compared with those from the literature. Arrhenius corrections were made at 40°C. The large scatter of experimental data, particularly for σ≈0.02–0.05 is discussed in relation with ε R the growth efficiency. Growth rate of the pyramidal faces has distinct parabolic dependencies on supersaturation, for several efficiencies of growth. Burgers vector and the linear dimension of the dominant centres of dislocations during the growth are discussed.

Models for the pinning of domain walls in Sm(Co,Fe,Cu,Zr)z type magnets at finite temperatures

We discuss simulation results of three different models for the pinning of the domain walls in permanent magnets as a function of temperature. They correspond to pinning of the wall by periodic arrays of planes, faces of cubes and pyramids. The pinning potential can repel or attract the domain walls. The simulation results are compared with recent experimental data for the 2:17 magnets.

The effects of anionic impurities on the growth habit and optical properties of KDP

Some anionic impurities (sodium metaphosphate, potasium pyrophosphate, formic acid, acetic acid, oxalic acid and tyrosine) were found to have an inhibiting effect on the growth of KDP crystals. Complete stoppage of pyramidal face growth was observed in the presence of high concentration of both [H x− n (PO 3) x ] n− and [H 2P 2O 7] 2−, and in these cases the crystal habit was changed into a closed octahedron and the whole crystal stopped growing. It was proposed that the inhibition is caused by the adsorption of the anions at the growing crystal surface, especially the pyramidal surface through the formation of H-bond between the anions and the H 2PO 4 − groups. The anions have a harmful effect on the laser damage threshold due to possible incorporation.

Structural evolution of lithium niobate deposited on sapphire (0 0 0 1): from early islands to continuous films

Lithium niobate films were deposited on sapphire substrates at temperature T d=750°C by pulsed laser deposition. Their structure was investigated at different stages of growth by transmission electron microscopy. The epitaxial relationship already found by X-ray diffraction has been confirmed to be (0 0 0 1) Al 2 O 3 ‖(0 0 0 1) LiNbO 3 and [1 1 2 ̄ 0] Al 2 O 3 ‖[1 1 2 ̄ 0] LiNbO 3 . A second cristallographic variant, rotated by 60° about c -axis, was also present in some samples. At the early stages of growth, lithium niobate constitutes pyramidal islands with {0 1 1̄ 2} faces that have mean height of 40 nm and are misoriented from each other by 0.5° on average. At the coalescence stage, surface migration of already deposited and incoming atoms from the source leads to the disappearing of pyramidal faces and the formation of 20 nm height plateaux; the misorientation between initial nuclei is accommodated by formation of subgrain boundaries. Continuous films have a mosaic texture also inherited from the initial misorientation of islands. They display mechanical twins and cracks induced by tensile stresses occurring at the cooling stage from deposition to room temperature. Cracks have variable width according to the film thickness and they are oriented preferentially along 〈1 0 1̄ 0〉 directions. Twins are located at the border of cracks and seem to be preferential sites for their initiation.

Halo arcs from airborne, pyramidal ice crystals falling with their c axes in vertical orientation

Many halo arcs are caused by pyramidal crystals that have {1 0 -1 1} crystal faces. We treat halo arcs arising from pyramidal crystals that fall in the air with their c axes vertically oriented. To our knowledge only 6 of the 12 possible halo phenomena that belong to this category have been dealt with in the literature. Surprisingly the yet undiscussed halos are predicted to be of comparable intensity with those already treated. They are produced by reflections from pyramidal crystal basal faces. A theoretical summary and computer simulations are presented of the mentioned 12 halo phenomena and of the individual arcs into which they break in the sky. We give an overview to the current level of documentation of these phenomena by listing the first published photographs of each phenomenon known to the authors.