On the Presence of Faces in the Habit of Growing Crystals

Presence and size of a face in the habit are conditioned by the value of relative growth velocity R_A/R_{crit A} with respect to the critical growth velocity. It has been established that during the growth of crystals only certain values of relative growth velocities R_A/R_{crit A} occur. Faces of KBC crystals realize values of R_A/R_{crit A} between 0 and 2. Unity is the demarcation level below which appearance and above which disappearance of faces takes place. Edges and corners of crystals are places where new faces appear during changes in growth conditions.     

On the Formation of Thick Sector Boundaries by Appearance and Disappearance of Faces of Crystals

Wide sector boundaries revealed in KBC crystals by chemical etching have been observed and analysed. It was established that the boundaries represent sectors of small faces appearing and vanishing in the habit during crystal growth. This nature of thick boundaries has been confirmed by the analysis of values of relative growth velocities of faces with respect to the critical velocities governing the presence of faces on growing crystals.     

The Nature of One-sided Intergrowths on (001) of Triclinic Centrosymmetric K2Cr2O7 (II)

SHUBNIKOV first observed in 1912 that centrosymmetric K₂Cr₂O₇ crystals (P1 ) exhibit a growth anomaly in highly supersaturated solutions. The (001) faces of these crystals are always smooth, the parallel opposite faces are always rough. Electron micrographs show these rough surfaces to be crystallites that have grown in stacks. In Part I of this publication, the crystallite orientation was determined using electron-induced diffraction methods. As a rule, these crystallites are parallel intergrowths. Only a small number of crystals near the surface of the host crystal are twinned according to [010]. Part II sought to ascertain the cause of these one-sided intergrowths. To determine the most favourable energetic arrangement of two intergrown individuals, interactions across the intergrowth face as a function of shifts parallel to (001) and to (001 ) were calculated, 64 theoretically possible cases were considered. Only in the [010]-twin and the seldom-occurring [11 0]-twin is an approximation of the 2/m symmetry of the macroscopic twin arrangements also found in the intergrowth structure, and this is the case only when the (001 ) faces of the host and guest crystal are intergrown.     

Investigations of impurity striations in potassium bichromate crystals

Impurity striations in potassium bichromate crystals (KBC, lopezite) formed during crystal growth from aqueous solution were revealed by chemical etching and analyzed. Striations were revealed as etch grooves, as rows of dislocation etch pits and as rows of flat‐bottomed etch pits. Various types and groups of striations have been visualized. Some striations were due to lateral segregation of impurities caused by convection flow of the mother solution, other were formed during growth stoppages whereas induced striations were generated by changes in hydrodynamical conditions. Growth rates changes resulted in zonal distribution of impurities, formation of planar lattice strain, rows of clusters of point defects and rows of dislocations. Generation of striations with different intensities in various sectors is a proof of the selective capture of impurities. Ratios of growth rates of various faces of KBC crystals growing in forced and free convection regimes were determined by induced striations. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of faces of K2Co x Ni1 – x(SO4)2 · 6H2O mixed crystals

Data on the morphology and normal growth rate of the (110) and (001) faces, velocities of step motion, and slopes of dislocation hillocks on the (001) face of K₂Co _x Ni₁– _x (SO₄)₂ · 6H₂O crystals at different supersaturations of solutions with a Co/Ni ratio equal to 1: 1 or 1: 2 have been obtained using a Michelson interferometer. The morphology of the (110) faces is found to be the same for solutions of both compositions. Powerful dislocation sources with large Burgers vectors dominate on the (001) face. The morphology of the (001) surface is rougher than that of (110), especially in a 1: 2 solution at high supersaturation. The (110) faces grow more slowly as compared with (001). The kinetic coefficients of steps on the (001) face are identical in the 1: 1 and 1: 2 solutions. The influence of the kinetic and morphological characteristics of (001) faces on the single-crystal quality is analyzed.

     

Growth and dissolution of BaFCl crystals

Single crystals of BaFCl have been growth by flux technique using BaF₂ and BaCl₂. Etching with formic acid revealed dislocation etch pits on (001) cleavage faces of the crystals, at room temperature. The influence of etching parameters such as undersaturation, temperature and concentration of poison in the etchant is studied. Decreasing the undersaturation of formic acid by reducing the percentage of water and increasing the temperature of the etchant were found not to have any effect on the morphology of etch pits. However, as the CdCl₂ poison concentration is gradually increased, the orientation of the pits change from 〈100〉 to 〈110〉 at high concentration.     

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: I. Growth habit and surface morphology

The effect of concentration of Mn(II) ions on the growth habit and the surface micromorphology of different as‐grown faces of ammonium oxalate monohydrate (AO) single crystals grown from aqueous solutions was studied at a constant temperature of 30 °C and predefined supersaturations up to 20%. It was observed that the growth habit and the surface morphology of the crystals strongly depend on the supersaturation used for growth and the impurity concentration in the solution. The experimental results were analysed in terms of connected nets determined from different projections of the structure of AO crystals. Analysis of the observations revealed that: (1) the directions of connected nets corresponding to basic growth units composed of single (NH₄)₂C₂O₄ · H₂O molecules are in excellent agreement with the low‐index crystallographic directions of the orientations of growth layers, (2) all faces appearing in the growth morphology of AO crystals are F faces, and (3) the {001} face growing from pure aqueous solutions is essentially a kinetically rough face but the presence of Mn(II) impurity leads to their appearance in the morphology due to increase in the strength of bonds of the connected nets composing the surface graph.     

Velocities of Disappearance and Lifetime of Faces of Growing Crystals

The formulae for absolute R_disap and relative R velocities of disappearance and lifetime τ of faces of growing crystals have been derived for stationary growth. It was shown that the quantities are determined by the relative growth velocity R_A/R_critA of the vanishing face A with respect to the critical growth velocity R_critA and by the geometry of a crystal expressed by the trigonometric functions of interfacial angles β and γ formed between face A and the adjacent faces. R increases and τ decreases with the increase in R_A/R_critA to certain limiting values. The calculations have been verified and illustrated by the experimental results for triclinic potassium bichromate (KBC) crystals. Results enable ones to predict values of velocities of disappearance and lifetimes of undesirable, supplementary faces of any real crystal.     

On the Growth Morphology of K2Cr2O7 (KBC) Crystals

The morphology of potassium dichromate (KBC) crystals grown under different conditions by solvent evaporation method is investigated. Observations of the examination of asgrown surfaces and inclusions, and of the realtion between dislocations and inclusions are presented. The results indicate that the change in the morphology of KBC crystals can be associated with the dislocation structure in the crystal.     

The catalytic effect of impurity. The effect of benzene, acetone, organic acids, iron, and chromium on growth of KH2PO4 and C8H5O4K crystals

The effect of benzene, acetone, acetic, formic, and oxalic acids, iron, and chromium on the growth kinetics of KH₂PO₄ and C₈H₅O₄ K crystal faces has been studied. It is found that low concentrations of organic impurities increase growth rates of some crystal faces (the so-called catalytic effect of impurity). This effect is rather weak in the case of organic acids, however, it is clearly seen in the presence of inorganic impurity (Fe³⁺). The analysis of two models of growth (dislocation-free and B + S) under the assumption that the main cause of the catalytic effect is a decrease in the step energy showed that the calculated and experimental data are consistent only under certain assumptions that should be additionally verified both theoretically and experimentally.     

A study of the rate-controlling stage of the process of crystal growth in solutions

The effective activation energies of crystal growth on the faces (100) of KCl, KBr, and KI crystals in water solutions have been determined. The values obtained have been compared with the respective crystal lattice energies according to Sletter and Mayer, as well as with the experimental ones. The activation energies of crystal growth are in strictest linear dependence on the heats of dehydration of the ions. This shows that the dehydration of the ions is most probably the rate-controlling stage of the process of crystal growth in solutions. This concept is supported by the results of an experimental study of the rates of growth on the faces (111) and (100) of NaClO₃ crystals.     

Growth of Potassium Sulphate Crystals by Silica Gel Technique

Single and transparent crystals of potassium sulphate up to 30 × 3 × 3 mm in size have been grown by silica gel technique, using various methods (two-layers three-layers and hybrid gel technique) which are described. The effects of the three differents alcohols (used as top solutions) upon the quality and nucleation of crystals are also reported. The “growth and equilibrium” morphologies and microstructures of the habit faces are discussed.     

Electron Optical Investigation of Growth-Induced Inhomogeneities of Synthetic Hg2Cl2 Monocrystals

The submicroscopic structure of synthetic Hg₂Cl₂ monocrystals was investigated by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM), indirect imaging of surfaces (Pt/C replica technique) as well as by application of supplementary methods. As already reported by NEUMANN et al., TEM-images show a periodic stripelike contrast whose period varies from 0.1 m̈m to 0.2 m̈m. It proved that the stripelike contrast arises from a periodic chemical inhomogeneity of the synthetic crystals. The chemical inhomogeneity probably consists in periodic deviations from the stoichiometric composition of Hg₂Cl₂. By reason of the orientation on {110}-, (001)-, and {114} faces a reconstruction of arrangement of the structures in the crystals is carried out. According to that, the stripes have to be interpreted as intersection lines of 2-dimensional formations with the corresponding faces.     

On crystal growth of orthorhombic alkali acid phthalates. An application of fourier transform of crystal morphology to tracht and habitus

The orthorhombic alkali acid phthalates crystallize in three different structure types. The structure of potassium acid phthalate (P2₁ab) can be regarded as a basic structure. The elementary cells of ammonium (Pcab) and sodium crystals (B2ab) contain two potassium structure units which are connected to each other along [001] by a symmetry centre in the first case and by a twofold rotation axis in the second case. It will be shown that close morphological relations between the crystals also can be observed. The morphological lattices of the substances agree fairly well with the centres of gravity of the phthalate ions (HP). The reciprocal crystals have mmm (KHP, NH₄HP) and mm2 symmetry (NaHP), respectively. The growth rates of the faces are proportional to the areas of the net plane meshes, divided by the number of the morphological units per growth layer. The thickness of the growth layer is determined by the morphological extinction rules. The specific surface energies are proportional to the surfaces of the reciprocal crystals parallel to the crystal faces, divided by the areas of the net plane meshes. The theoretical growth and equilibrium forms of the hydrogen phthalates are in good agreement with the experimental observations.     

Properties of Surface Layers in Single Crystals of Calomel

Within the framework of the systematic study of differences in physical properties of the calomel (Hg₂Cl₂) single crystals before and after surface treatment, the differences in the microhardness, the residual stress in the surface layer, and the chemical etching rate were investigated on the (110) and (001) crystal faces. It has turned out that Beilby layer (B-layer) formed during surface treatment is softer, and, on the cleavage surface is harder than the underlying bulk material. The thickness of the B-layer in these samples ranged between 5 anf 7 m̈m. Further it has been found that a tensile residual stress s̀ = 14 Nm^-2 prevails in the surface layer of a polished (110) face to a depth of 70 m̈m and, after grinding, to a depth from 90 to 200 m̈m. On the other hand, a compressive stress has been found in the natural cleavage plane. Experiments on etching the Hg₂Cl₂ crystals with HCl + HNO₃ etching solution have shown that the rate of chemical etching is 2 to 4 times larger on the (110) face than on the (001) face. Differences are very strongly dependent on the experimental conditions and on the real structure of a given surface.     

Flux growth of La‐doped lead zirconate stannate titanate antiferroelectric crystals

Relaxor antiferroelectric single crystals lead lanthanum zirconate stannate titanate (PLZST) with the composition around the morphotropic phase boundary (MPB) have been grown by flux method using 50 wt% PbO‐PbF₂‐B₂O₃ as a flux. The obtained crystals are light yellow in color. The XRD patterns revealed that the habitual faces of the obtained crystal are (001). The crystal morphology was studied and related to a layer growth mechanism controlled by two‐dimensional growth. The chemical composition of as‐grown crystal was analyzed by inductively coupled plasma atomic emission spectrometry (ICP), indicating a slight decrease of the amount of Ti compared to the starting materials. The result was verified by the XRD patterns with the phase transformation from the co‐existence of tetragonal and rhombohedra phases to the single tetragonal phase. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wachstumsformen von tetragonalen Erdalkaliwolframaten in Schmelzlösungen und deren Deutung

Investigations on solubility of CaWO₄ in NaCl melts (ANIKIN ) were extended to BaWO₄ and SrWO₄. – Crystals of tungstates of Ca, Ba, and Sr from NaCl and KCl fluxes respectively were grown at temperatures from 950 to 650°C at cooling rates from 2 to 50 grd. h⁻¹. Marked differences in the forms of these tungstates did not occur. Among many whiskers bipyramidal crystals grew with dendritic, sceletal and hollow transitions forms. The theoretically calculated morphological succession (PBC model) is realised by the faces of the whiskers and platelets ([110] and [100] zones). The remaining crystals – due to face specific adsorption – show other forms similar to those in nature. – Verneuil grown crystals, however, developed – free, from adsorption phenomena – {110} and {001} forms respectively.     

Characterization of L‐ascorbic acid single crystals grown from solution with different solvents

Single crystals of L‐ascorbic acid, popularly known as vitamin‐C, were grown from solutions with different solvents and solvent combinations by low temperature solution growth methods. The suitability of different solvents and solvent combinations such as acetone, ethanol, methanol, isopropyl alcohol, water, water + acetone (1:1), water + ethanol (3:1), water + isopropyl alcohol (3:1) and isopropyl alcohol + methanol (1:1) for crystal growth of L‐ascorbic acid was found out by assessing the solubility and crystallization behaviours. Solubility of L‐ascorbic acid in selected solvents and solvent combinations in a range of temperatures was determined by gravimetric method. Solution prepared with water, water + acetone (1:1), water + ethanol (3:1) and water + isopropyl alcohol (3:1) were yielded crystals with tabular, columnar and prismatic habits and their morphologies were evaluated by goniometry. Grown single crystals were characterized with various instrumental techniques such as powder X‐ray diffraction, Fourier infrared spectroscopy, optical transmittance study, differential scanning calorimetry and second harmonic generation studies. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study on the Growth of Triglycinsulphate Single Crystals

Crystallization of TGS at 52.0°C - above the transition point - has been studied in a wide range of supersaturation of the solution (σ = 0 to 10⁻²). The rates of growth of {110} and {001} faces were measured as a function of supersaturation at constant hydrodynamical conditions (Re = 3.4 · 10⁻³). Further, the influence of hydrodynamical conditions on the growth of {110} faces at constant supersaturation (σ = 4.2 · 10⁻³) was established. The parameters of the experimentally found dependences are determined on the basis of the surface-diffusion model of BURTON . CABRERA and FRANK . From these dependences follows that the growth rate of the {110} faces is already almost limited by the volume diffusion of TGS molecules towards the crystal surface, while in the case of {001} faces the surface diffusion mechanism of crystallization is clearly manifested. Dislocation densities in the crystals have been determined by means of etching technique. The number of dislocations increases with increasing supersaturation; hence, supersaturation of the solution together with the processes taking place in the regeneration zone surrounding the seed determine the number of dislocations in the crystal volume and thus the resulting structural perfection of single crystals. Investigation of the spontaneous redistribution of domains showed that the growth rate of TGS crystals influences the dielectric properties to much smaller extent than does chemical purity.     

Synthesis,growth, morphology and characterization of ferroelectric glycine phosphite single crystals

Glycine phosphite (NH₃CH₂COO.H₃PO₃), a potential ferroelectric material, was grown as single crystals from aqueous solutions by slow evaporation and slow cooling methods. Laboratory synthesized title compound was purified by recrystallization method and confirmed by Fourier transform infrared and Laser Raman studies. Temperature dependent solubility in double distilled water in the range between 288 and 328 K was determined by gravimetric method. Morphological importance of various growth faces were studied by optical goniometry. Powder x‐ray diffraction study performed on the grown crystals confirms the crystal system and lattice parameters of the unit cell. Optical transparency of the grown crystals in the ultraviolet–visible ‐near infrared region was studied by spectroscopic method. Thermal stability of the grown crystals in the temperature region above ambient until melting was studied using differential scanning calorimetry. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)