Microtopographical and Raman spectral studies on calcium sulphate dihydrate (gypsum) crystals grown in silica gel in the presence and absence of barium chloride as additive

A procedure for the growth of calcium sulphate dihydrate (gypsum) crystals, CaSO₄ · 2H₂O, by a gel technique in the presence of barium chloride as an additive as well as without the additive, is described. Optimum conditions for the growth of good quality crystals are worked out. Needle-shaped crystals with well-defined prism faces are obtained. Microtopographical studies of the crystals are carried out. The observed surface structures on the prism faces of the crystals include vertical striations and parallelogram-shaped, oriented growth hillocks. Etch patterns on these faces are described and discussed. Reciprocity of the growth and etch mechanisms is established. Laser Raman spectral studies of these crystals are also made. Splitting of the non-degenerate fundamental is observed in the spectrum of the crystals grown in the presence of the additive. The present studies indicate that the additive used enhances the quality of the crystals.     

Surface structural investigations on (100) and (110) faces of flux-grown lanthanum borate crystals

Results of topographical studies carried out on (100) and (110) faces of lanthanum borate crystals grown from the PbO-B₂O₃ flux system are illustrated and discussed. The habit faces display the formation of cavities, microcrystals, elliptical etch pits, elliptical hillocks, circular hillocks and irregular structures. Also described are a various number of elevated structures identified as impurity phases. Energy dispersive X-ray (EDAX) studies have confirmed that these impurity phases in the growth of lanthanum borate (LaBO₃) crystals are enriched by lead (Pb). The habit faces also exhibit some elevated regions which are reported to be more imperfect in comparison with others. It is inferred that independent growth on the habit faces has taken place during the last stage of crystal growth by a two-dimensional nucleation mechanism.     

New seed geometry for growth of low dislocation synthetic quartz

A method to grow low dislocation density synthetic quartz by using a special cutting seed geometry is reported. With this method, a relatively high dislocation density seed material is allowable. For such a purpose, a seed of new geometry was prepared and grown in a standard hydrothermal growth condition, long in Y-direction with multiple V-shaped notches made on Z-face. The characterization study was conducted by X-ray topography. The results showed new growth regions, equal to the numbers of V-shaped notches made in the seed and usually not found in the conventional Y- and Z-bar synthetic quartz crystals. Each new growth region is composed of two sectors of distinct textures. Soon they disappear due to their high growth velocity, and they are replaced by the so-called Z-region. However, the growth process of these new sectors grown perpendicular to the internal faces of the V-shaped notches played an important role in inhibiting the propagation of the dislocation originally present in the seed into the grown Z-region.     

Hydrothermal synthesis of cuboidal PbSe microcrystals via an EDTA reduction route

Cuboidal lead selenide (PbSe) microcrystals have been successfully synthesized via a novel EDTA reduction route in which EDTA molecules serve as both chelating reagent and reductant. Hereafter EDTA is the abbreviation for ethylenediaminetetraacetic acid tetrasodium salt dihydrate (C₁₀H₁₂N₂Na₄O₈·2H₂O). The unique cuboidal PbSe microcrystals have eight symmetric arms along the < 111> directions. The edge of a cuboidal PbSe microcrystal extends outwards from its core with a tiny cubic center leaving step-like faces. The whole synthetic system provides an appropriate crystal growth environment for the formation of these microcrystals. Furthermore cuboidal PbSe microcrystals with different concave faces can be obtained at different reaction stages.     

若干晶体结晶习性的形成机理

本文根据在热液条件下生长水晶和钛酸钠晶体的电泳实验，提出热液条件下，上述晶体的生长基元具有负离子配位多面体的结构形式。由于溶液过饱和度在变化，这些生长基元的维度也随之变化。不同维度的生长基元往晶体各族晶面上叠合速率的比例也发生相应改变。它直接反映在晶体结晶形貌上。本文根据不同维度生长基元在各族晶面上的叠合速率解释了水晶和钛酸钠的结晶习性形成机理。     

A novel growth method for organic nonlinear optical crystals

A novel crystal growth method called seed-orientated undercooled melt growth is proposed for growth of organic nonlinear optical (NLO) crystals. The main advantage of this method is that organic NLO crystals can be grown along a given direction and the optional crystal faces (h k l) may be grown directly in a crystallizer. The grown crystals with determinate crystal faces are possible to apply as nonlinear optical devices regardless of subsequent cutting and polishing. Using this method, the different crystal faces of benzophenone crystals were successfully grown in a quartz crystallizer, and the orientation of the grown crystal faces were measured by an X-ray angular instrument.     

A Technique for Examination of the Edge Faces of Tabular Microcrystals Applied to Silver-Bromide Grains for Evidence of Twinning†

By coating spherical microcrystals on to a glass support prior to a coating of tabular micro-crystals, many of the latter are lilted so that the topography of their edge faces is readily visible in carbon-replica electron micrographs. Electron micrographs are reproduced of tabular silver-bromide microcrystals (grains) from a photographic emulsion examined in tliis way, which show that the edge faces of many grains are of the form to be expected if twinning had occurred during grain growth. The significance of certain findings is discussed briefly.     

Internal structure of insulating and semiconducting synthetic diamonds

From optical studies on polished cross sections of synthetic diamond crystals the diamond growth on three different crystallographic planes can be distinguished within the crystals. Diamond growth on the cubic, octahedral, and dodecahedral faces each have a characteristic abrasion resistance, colour, and inclusion content. The differences in abrasion resistance do not appear to be dependent on the content of dispersed nitrogen. The characteristic growth on each crystal face allows the relative growth history of the three faces to be reconstructed.     

Growth of gallium orthophosphate single crystals in acidic hydrothermal solutions

The hydrothermal growth of GaPO₄ single crystals has been studied in several solutions. Among many solutions, H₃PO₄, HCl and H₂SO₄ solutions were found to be effective solvents for the growth of GaPO₄ single crystals. Single crystals have been hydrothermally grown at temperatures over the range 210 to 290 °C in these solutions with seed crystals. HCl solution was found to be the most effective solvent in which to grow large single crystals. Morphologies of crystals grown at temperatures below 200 °C tended to be bounded by small major rhombohedral (1 0 ˉ1 1) faces. In the temperature range from 210 to 430°C, the crystals have morphologies bounded by prism (1 0 ˉ1 0), small major rhombohedral (1 0 ˉ1 1) and minor rhombohedral (0 1 ˉ1 1) faces, and grew with well developed basal (0 0 0 1) faces with increase in the growth temperature. Single crystals of GaPO₄ with lower dislocation density have been hydrothermally grown at 210 to 290°C in 3m H₃PO₄ solution.     

Surface studies on as-grown (111) faces of sodium bromate crystals

Single crystals of sodium bromate are grown at various supersaturations ranging from 3% to 8%. Surface studies have been carried out on as-grown and etched (111) faces of these crystals. Typical and systematically oriented growth hillocks are observed almost on all the faces. Further dislocation studies are made to understand the growth history of these crystals. These studies suggest that the crystals grow by 2D-growth mechanism. In addition to this, studies are also conducted on the formation of overgrowths and inclusions in these crystals.     

Typical hillocks and spirals on flux-grown DyFeO3 crystal surfaces

Multi-sided and almost rectangular growth hillocks on (1 1 0) faces of flux-grown DyFeO₃ crystals are illustrated. Some such hillocks exhibit spiral growth layers originating from the summits of hillocks. Spiral growth layers originating from more than one initiation centre interact, giving rise to closed loop, interlocked and interlaced or other complex growth formations. The origin of the multi-sided hillocks is attributed to preferential growth at the sites of screw dislocations in DyFeO₃ crystals. Defects other than screw dislocations also stimulate growth on (1 1 0) faces of DyFeO₃ crystals. An example of this is offered. The mechanism of independent growth on (1 1 0) faces of these crystals is discussed.     

The lattice constants of a-quartz

Data relating to the lattice constants of quartz are reviewed and it is shown that hydrogen and most other impurities increase the a lattice constant and decrease the axial ratio c/a. The available data can be described by c/a=1.10013–0.20 (a–a ₀) where a ₀ is the lattice constant of the pure material. The values of a ₀ at 25° C are deduced to be 4.9127 and 4.9134 Å, respectively, for natural and synthetic quartz with uncertainties in the last figure of about 1. This difference cannot be attributed to the large hydrogen content of the synthetic quartz since it is shown that this has a negligible effect on a. Neither is it due to differences of growth mechanism since crystals of synthetic quartz grown on different faces behave identically. Thus the differences between natural and synthetic quartz must be attributed to differences in the growth conditions (temperature, pressure and mineralizer). This attribution can account for the spread of natural quartz data.Values of the cell parameters are suggested which allow interplanar and Bragg angles to be calculated with uncertainties of between 0.001 and 0.003°. Thus for decreasingly pure grades of quartz, natural quartz has a rising from 4.9129 to 4.9138 Å associated with a fall of c/a from 1.10012 to 1.10004. Uncertainties in the a values range from 2×10^–4 to 5×10^–4 Å and uncertainties in the c/a ratio range from 4×10^–5 to 12×10^–5.     

Polarization Structures in Parhelic Circles and in 120 degrees Parhelia

Parhelic circles due to plate-oriented crystals (hence, with main axes vertical) and 120 degrees parhelia change in position when viewed through a rotating polarizer. The parhelic circle moves vertically; its largest shift is found at an azimuthal distance between 90 degrees and 120 degrees from the Sun. The 120 degrees parhelia move both vertically and horizontally. The magnitudes of the shifts are between 0.1 degrees and 0.3 degrees , depending on solar elevation. The mechanism is polarization-sensitive internal reflection by prism faces of the ice crystals. We outline the theory and present three visual and one instrumental observation of the displacements of these halos in polarized light.     

Alpha-Quartz 2. Crystal chemistry,and nature and distribution of impurities

Crystal chemistry of alpha-quartz is discussed in relation to the nature, concentration and distribution of the common impurities. Two schemes for incorporation of impurities in the host structure,viz., the charge compensation model for untwinned natural quartz, and the broken-bond model for synthetic quartz to accomodate excess impurity residue after charge compensation, are critically reviewed. Another model for the presence of Al-H defects independent of alkali association in natural quartz is suggested. It is pointed out that the growth pressure influences the nature and distribution of impurities as well as generation of planar and line defects in synthetic quartz. It is also established that mechanicalQ (which is a measure of the acoustic loss) of synthetic crystals grown at low pressure deteriorates with increase in impurity content in excess of what is required for satisfying the charge compensation rule.     

Formation of mushroom-like ZnO microcrystals through a solution calcination process

Mushroom-like ZnO microcrystals have been prepared via a solution calcination route, using Zn(NO₃)₂ as Zn source in the absence of any surfactants, templates or catalysts. This is the first example to prepare mushroom-like crystals as semiconductors, which are expected to show particular physical properties. The ZnO products were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and photoluminescence (PL) spectrum measurements. A suitable concentration of Zn(NO₃)₂ solution was important for the growth of the mushroom-like products. The reported synthetic procedure is straightforward and inexpensive, and thus can be readily adopted to produce large quantities of mushroom-like ZnO microcrystals.     

Defect characterization of KTP single crystals

Potassium titanyl phosphate (KTP) is a relatively new nonlinear optical material with excellent combination of physical properties. This paper presents the combined etching and X-ray topographic studies carried out on KTP crystals with a view to characterizing their defects. KTP crystals employed in this investigation were grown from flux. Optical microscopic study of habit faces revealed growth layers and growth hillocks on (100) and (011) faces respectively. Etching of (011) habit faces proved that growth hillocks corresponded to the emergence point of dislocation out crops on these faces. The suitability of the new etchant to reveal dislocation was confirmed by etching the matched pairs obtained by cleaving. The defects present in the crystal were also studied by X-ray topography. The defect configuration in these crystals is characteristic of crystals grown from solution. The dislocations arc predominantly linear with their origin either at the nucleation centre or inclusions. In general, grown crystals were found to have low dislocation density and often large volumes of crystals free from dislocation could be obtained.     

Halo polarization profiles and the interfacial angles of ice crystals

Polarization and radiance of various types of refraction halo in ice-crystal swarms that extend to ground level were measured as a function of scattering angle. Simultaneously, samples of the crystals that produce these halos were collected and replicated. The halo polarization peaks are wider than the Fraunhofer theory of diffraction predicts for the observed size distribution of the replicated crystals. The explanation we put forward is that the angles between crystal prism faces are not always exact integer multiples of 60°, and the basal faces are not always exactly parallel, as is usually assumed. The collected crystals confirm this. The widths of the halo polarization peaks can be explained if the distributions of the interfacial angles around their means reach their half-maximum values at a deviation of 0.49° ± 0.05°. This corresponds to a deviation of 0.35° ± 0.03° of the face normals from their crystallographic positions. The presence of variation in interfacial angles in low-level halos seems to ari e from the fact that the crystals are growing. Some hitherto unexplained features in halo displays can be understood by considering variations in the interfacial angles.     

Growth mechanism of NaBrO3 crystals from aqueous solutions

To study the growth mechanism of {111} faces of NaBrO₃, crystals were grown at different supersaturations ranging from 2% to 8%. The growth mechanisms were investigated based on the growth rate versus supersaturation relation and from the surface features observed on {111} faces. The growth mechanism of these crystals appear to be due to 2D nucleation. The growth rate curve has been further investigated using Ohara and Reid equations. Polynucleation model in two-dimensional nucleation growth theory is suggested as the most possible growth mechanism for these crystals in the present supersaturation range.     

Optical materials based on molecular nano/microcrystals and ultrathin films

Methodologies that we developed recently for the fabrication of molecular crystals with size variation in the nano to micro regime and polyelectrolyte templated mono and multilayer Langmuir-Blodgett films, are reviewed. The electronic absorption and strong fluorescence in the molecular nano/microcrystals are found to be size-dependent. Crystal structure and computational investigations provide a unified model to explain these observations. Role of polyelectrolyte templating in achieving stable and enhanced optical second harmonic generation response from LB films based on a hemicyanine amphiphile is highlighted.