Crystallization kinetics of MgSO4 · 7 H2O at higher ranges of the driving force

By means of batch crystallization technique the crystallization kinetics of MgSO₄ · 7 H₂O at 25 °C at higher ranges of the supersaturation from pure aqueous solutions was investigated. It was observed that the growth rate highly depends on the habit of crystals and on their ratio of length to width, respectively. This ratio is a function of the initial supersaturation of each crystal.     

Crystallization kinetics of MgSO4 · 7 H2O in presence of tenside

The effect of sodium dodecyl sulfate (SDS) on crystallization kinetics and crystal habit of MgSO₄ · 7 H₂O from aqueous solutions at 25 °C was investigated in batch experiments. It highly depends on the supersaturation level. Both increasing supersaturation and rising concentration of the tenside promote the production of needle-like crystals but the influence of the driving force is much more pronounced. SDS increases the crystallization rate and the linear crystal growth rate in length direction of the crystals. To a high degree it also influences properties of the crystallizing solution such as surface tension and viscosity.     

In situ AFM investigation of 2D nucleation on the (100) face of KDP

Surface morphology of the (100) face of potassium dihydrogen phosphate (KDP) crystals which were grown at different supersaturations at 25 °C was investigated by in situ atomic force microscopy (AFM). Various AFM images of 2D nucleation under different growth conditions were presented. It is found that the growth of KDP is controlled by polynuclear nucleation mechanism at the high supersaturation. With reduction of the supersaturation, the growth velocity of 2D nuclei becomes very slow and shows typical anisotropy. It is found that the process of coalescence of 2D nuclei does not lead to defect. The experiments show that the growth mechanism for KDP at 25 °C changes between step flow and 2D nucleation in the supersaturation range of 4.5‐5%. The triangular nuclei which are close to equilateral triangle are observed in the experiment at the supersaturation σ = 6% for the first time, showing typical anisotropic growth. Through observing the dissolution of 2D nuclei, the dissolving process can be regarded as the reverse process of growth. We also find that the microcrystals landing on the surface at σ = 9% would grow and coalesce with each other and there is no observable defect in the coalescence. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Various Growth Shapes of Na2S2O3 5H2O Crystals

The relation between the growth shapes and supersaturations and/or supercoolings was investigated in the Na₂S₂O₃ 5H₂O crystals: (i) The growth shapes changed from a hexagonal prismatic crystal, aggregate of platy crystals and to a spherulite with an increase of supersaturations and/or supercooloings. (ii) The deposition rate suddenly increased at a certain supersaturation and/or supercooling, where the growth shape changed from the hexagonal prism to the aggregate of platy crystals. (iii) The deposition rates of spherulites become higher with increasing the solute concentrations. (iv) The formation process of the spherulites were in situ observed by the Schlieren and/or Mach‐zehnder interferometer methods.     

Growth,Morphology, Structure and Properties of Na3BaCl5 2H2O Crystals

Single crystals of Na₃BaCl₅ 2H₂O were obtained from saturated aqueous solutions at two different temperatures (27° C and 32° C). The crystal size increases with increasing temperature. The crystals obtained were subjected to a systematic morphological, X-ray and thermal analyses. The study of as-grown crystal morphology and surface morphology of Na₃BaCl₅ 2H₂O crystals clearly indicate the requirement of a slow rate of evaporation at temperatures 27 to 28° C, moderate supersaturation in order to obtain high quality single crystals of Na₃BaCl₅ 2H₂O. The X-ray diffraction studies give the following cell parameters for the title compound: monoclinic, P21/n, a = 8.5773(9), b = 9.5502(4), c = 5.2873(3) A, β =92.069°, V = 432.8 A³, Z = 2. The study of TGA curves indicate the occurrence of dehydration process at around 200° C and not the decomposition of the compound. Similarly, the low temperature DSC study indicates a thermal anomaly at 267 K, and the high temperature DSC study (323 K to 873 K) indicates the transition from dihydrate to monohydrate and in turn anhydrous state at 454.6 K for Na₃BaCl₅ 2H₂O crystals. Thus both the TGA and DSC curves conclude that the Na₃BaCl₅ 2H₂O crystals contain two molecules of water of crystallization and its molecular formula is Na₃BaCl₅ 2H₂O.     

Crystallization and Characterization of Orthorhombic β‐MgSO4 · 7H2O

In solution, the growth rate and the crystal habit are influenced by a number of factors such as supersaturation, temperature, pH of the solution, cooling rate, agitation, viscosity, initial state of the seed crystal and the presence of impurities. The crystallization of orthorhombic β‐MgSO₄ · 7H₂O, from low temperature aqueous solution by slow cooling process was studied. The metastable zone width, the induction periods (τ) for different supersaturations and the effect of pH on the growth rate of the crystals were investigated. The increase of pH yielded bigger crystals. The structural, optical, thermal and mechanical properties of β‐MgSO₄ · 7H₂O have been studied using FT‐IR, X‐ray diffraction, TGA‐DTG and micro hardness analyses.     

Crystal Growth in Magnetic Fields (II) Crystallization of Co(NH4)2(SO4)2 · 6 H2O from Aqueous Solutions in High Magnetic Fields

The displacement rate of (110) single crystal faces of Co(NH₄)₂(SO₄)₂ · 6 H₂O has been measured in dependence on the supersaturation of aqueous solutions in the zero field and at magnetic flux densities of 7 T. The results show that in a given external field measurable effects near the equilibrium don't occur (thermostatic field effect). On the contrary a marked influence at extreme relative supersaturations can be observed (kinetic field effect).     

EPR spectra of irradiated Na2Cr2O7 · 2 H2O crystals

The EPR spectra of X-irradiated Na₂Cr₂O₇ · 2 H₂O crystals grown from the aqueous solution at room temperature are studied. Three groups of lines are detected, which marked conventionally as C₁, C₂ and C₃. Each of the C₁ and C₂ line groups is a superposition of two doublets. The lines are interpreted as due to the Cr⁵⁺ ions in the distorted (stretched) oxygenous tetrahedron. The doublet structure is due to hyperfine interaction with nuclei of 1/2 spin which are protons of water. It is shown that the C₁ and C₂ line groups are due to the centers of the same type with different orientations in the lattice. The principal g-tensor values for these centres were as follows: g_z = 1.916, g_y = 1.978, g_x = 1.986.     

The influence of concentration and supersaturation ratio of CuI·HI on CuI crystal growth by decomplexation method

The fundamental role of solubility and supersaturation of solute for crystal growth from solution has been widely realized. In order to optimize the process of CuI crystal growth by CuI·HI decomplexation in HI acid, the solubility and supersaturation curves of CuI in HI‐H₂O mixed solvent were measured, and then a modified concentration programming scheme was designed, which could grow high quality CuI single crystals of 2.5 mm on edge successfully. In this scheme, the concentration distribution of CuI·HI complex and HI acid in silica gel along their diffused direction were measured with spectrophotometer, and the evolution of CuI·HI complex supersaturation ratio was analysed. It was found that the excess CuI·HI complex concentration would lead to the high supersaturation ratio and the formation CuI dendrites. The condition for regular CuI single crystal growth in silica gel was measured as follow: when the crystal nuclei appears, the CuI·HI complex concentration in HI‐H₂O solvent should be kept in the range of 0.033–0.050 mol/L and its corresponding supersaturation ratio is 1.24–1.45, and then they should be respectively declined to a range of 0.025–0.033 mol/L and 1.14–1.26 at the stage of crystal growth. The results may provide a useful clue for further improvement of the experimental scheme. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Precipitation of Alkaline-Earth Metal Oxalate Powders from Aqueous Solution. Crystal Numbers and Final Sizes

The precipitation magnesium oxalate dihydrate, calcium oxalate monohydrate, strontium oxalate monohydrate and barium oxalate hemihydrate was studied from equivalent solutions of concentrations from 0.001 M to 0.5 M, at pHs from 7 to 6, by optical microscopy and other methods. Crystal growth started after induction periods: the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The crystal form and numbers of the final precipitates depended on the type and number of the nuclei (and crystallites) formed during the induction periods. Crystal numbers at medium to high supersaturation, increased with increasing initial mean metal oxalate concentrations according to the relation, N = N₁c; β was 5 for calcium oxalate precipitations and β was 6 for the other metal oxalate precipitations. The N₁ values increased in the order MgC₂O₄ · 2 H₂O < BaC₂O₄ · 1/2 H₂O < SrC₂O₄ · H₂O < CaC₂O₄ · H₂O. The final crystal lengths, in this supersaturation range, then decreased (from maximum values) with increasing initial concentrations according to the relation, l_fin = l₁/C_MOx^β, where γ was 1.3 to 1.6. For precipitations from solution of any concentration at any pH, smaller crystals were obtained in the precipitates of the metal oxalate of lower solubility.     

Precipitation of nesquehonite from homogeneous supersaturated solutions

The homogeneous (unseeded) precipitation of nesquehonite (MgCO₃·3H₂O) was studied over the temperature range of 10‐40 °C. Precipitation was triggered by the supersaturation created by mixing MgCl₂ solution (0.5‐1.5 M) with Na₂CO₃ solution in the same concentration range. The Meissner's method was adopted in the calculation of supersaturations during the MgCl₂‐Na₂CO₃ reaction to monitor the precipitation. Solids were identified using X‐ray diffraction (XRD) analysis and scanning electron microscope (SEM) images. In the temperature range of 10‐40 °C, MgCO₃·3H₂O with needle‐like or gel‐like morphology was precipitated. It was seen that the length, width and surface smoothness of the particles changed with reaction temperature and supersaturation. The supersaturation (S) was in the range of 1.09‐58.68 during titration of Na₂CO₃ solution. The dimension of the crystals increased with longer addition time (or lower initial concentration of reactant) at the same temperature. Slower addition via titration of 2 h followed by 2 h of equilibration at 40 °C proved successful in producing well developed needle‐like MgCO₃·3H₂O crystals of 30‐50 μm long and 3‐6 μm wide. MgCO₃·3H₂O obtained were calcined to produce highly pure magnesium oxide (MgO) at 800 °C. The morphology of MgO was similar to that of their corresponding precursors. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Elastic properties of the dithionates Na2S2O6 · 2 H2O,K2S2O6, Rb2S2O6, CaS2O6 · 4H2O and SrS2O6 · 4 H2O

Single crystals of the title compounds having optical quality and dimensions of several cm were grown from aqueous solutions. The elastic and thermoelastic constants were determined from ultrasonic resonance frequencies of thick plates. The true point symmetry of K₂S₂O₆ and Rb₂S₂O₆, which is screened by a hexagonal hypermorphy, could be clearly revealed to be trigonal (32) by the existence of the elastic constant c₁₄. In the case of CaS₂O₆ · 4H₂O and SrS₂O₆ · 4H₂O the constant c₁₄ of the specimens appeared too small to confirm the trigonal symmetry group required from electrooptic and non-linear optic effects unambiguously. The isotypy of K₂S₂O₂ and Rb₂S₂O₆ as well as that of CaS₂O₆ · 4H₂O and SrS₂O₆ · 4H₂O is confirmed by their elastic behaviour. The mean elastic stiffness of dithionates is closely related to that of the corresponding sulphates. In the vicinity of the second-order phase transition of K₂S₂O₆ near 235 K weak anomalies of the temperature derivatives of the longitudinal elastic stiffnesses are observed.     

Growth Rate anisotropy of the K2(S,Cr)O4 Mixed Crystals Grown from Aqueous Solution

The ratio of growth rates of the K₂(S,Cr)O₄ crystals in the [100] and [001] directions (the anisometry coefficient) is proportional to the initial effective supersaturation of the solutions from which the crystals were grown. The values of the factors of asymmetry and excess of distribution of the crystal anisometry coefficient are affected by the composition of solution. The change in the values of these factors is caused by the change in the value of the initial supersaturation of solution according to the specific features of the solubility diagram.     

Growth aspects of barium oxalate monohydrate single crystals in gel medium

Single crystals of barium oxalate monohydrate (BaC₂O₄.H₂O, BOM) were grown in pure form by controlled diffusion of Ba²⁺ using the gel technique at different temperatures. Starting from aqueous Ba²⁺ chloride (BaCl₂) and acetic acid (C₂H₂O₄) in gel, this method offers a low‐cost and an easiest alternative to other preparation methods for the production of barium oxalate bulky single crystals. The optimal conditions for the growth of BOM crystals in silica gel were found by investigating different growth parameters such as gel pH, gel aging and crystallization temperature. Irrespective of all such crystallization environments, growth rate of the crystals were initially less and then exhibited supersaturation effect leading to non‐linearity. Gel aging and temperature has profound effect on nucleation density that resulted less number of crystals of maximum size in the gel matrix. Perfect single crystals were grown on gels of higher pH. The macropore morphology and porosity was controlled by changing age of the gel. It has been found that temperature has a fabulous effect in controlling the nucleation density by altering the supersaturation conditions for the formation of critical nuclei. The entire growth kinetics informed that the grown crystals were derived by the one dimensional diffusion controlled process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Supersaturation and Temperature on the Growth Morphology of Ammonium Oxalate Monohydrate Crystals Obtained from Aqueous Solutions

The experimental results of a study of the effect of supersaturation and temperature on the growth morphology of ammonium oxalate monohydrate [(NH₄)₂C₂O₄H₂O; AO] single crystals obtained from aqueous solutions at 30 and 40 °C and supersaturation up to 9% are presented. The observations are analysed in terms of theoretical morphology, growth models and attachment energy for growth units in steps of growing faces.     

Study of growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

Experimental results of the dependence of linear growth rates of ammonium oxalate monohydrate [(NH₄)₂C₂O₄ · H₂O; AO] single crystals on solution supersaturation are presented. The AO crystals were grown by constant-temperature, constant-supersaturation method at 30 and 40 °C in the supersaturation range of 1–9%. It was observed that the supersaturation dependence of growth rates follows the parabolic growth law. Analysis of the supersaturation dependence of linear growth rates of AO crystals showed (1) that growth models involving surface diffusion and direct incorporation of growth units give kinetic parameters similar to those reported for other compounds grown from solutions, and (2) that the the BCF model of cooperating screw dislocations is also applicable. An inverse relationship between the estimated values of the length, L, of the line containing the dislocations and growth rate, R, and a direct relationship between L and interplanar distance, d_hkl, of the face {hkl} were found. Both these relationships are associated with the process of generation of screw dislocations in the growing layer.     

Polymorphic composition and morphology of calcium carbonate as a function of ultrasonic irradiation

This paper reports on the precipitation of CaCO₃ polymorphs, having various crystal morphologies under different conditions. In particular, systems that were subject to ultrasonic irradiation were compared to the corresponding reference systems in the absence of such a treatment. The application of ultrasonic irradiation predominantly resulted in a change of particle size distribution and polymorphic composition of the precipitate, in comparison to the reference systems. Thus, it was found that the supersaturation and temperature influenced the size distribution, in both the reference and sonicated systems. A mixture of calcite, vaterite and aragonite was obtained in all reference systems, at 25 °C. At this temperature, the sonication caused the vaterite content to increase, while aragonite was not detected. In reference and sonicated systems at 80 °C, only aragonite precipitated. The results also indicate that the principle parameter responsible for the morphology of vaterite was the initial supersaturation: at higher supersaturation spherical vaterite particles precipitated, while at lower supersaturation hexagonal platelets were obtained. The morphological investigations also indicated different mechanisms of vaterite formation in the systems in which precipitation was initiated at higher supersaturation: spherulitic growth of vaterite was observed in sonicated systems, while the aggregation of primary particles was predominant in the reference systems. At lower supersaturation, the effect of c(Ca²⁺)/c(CO₃²⁻) on the morphology of hexagonal platelets of vaterite was observed as well. By varying the c(Ca²⁺)/c(CO₃²⁻), significant changes of the polymorphic composition were observed only in the sonicated systems, at 25 °C.     

Optical properties of the germanate melilites Sr2MgGe2O7, Sr2ZnGe2O7 and Ba2ZnGe2O7

Refractive indices and their dispersion in the wavelength range from 365 nm to 2325 nm and transmission ranges of the tetragonal melilite‐type germanates Sr₂MgGe₂O₇, Sr₂ZnGe₂O₇ and Ba₂ZnGe₂O₇ were determined. The uniaxial positive crystals Sr₂MgGe₂O₇ and Ba₂ZnGe₂O₇ both offer the possibility for phase matched second harmonic generation, a detailed analysis of phase matching conditions is given. The refractive indices of Sr₂ZnGe₂O₇ show an isoindex (isotropic) point at 467 nm. The investigation was performed on Czochralski grown large single crystals. The crystal structure of all three germanates were determined by means of X ‐ray diffraction. The results corroborate unmodulated melilite‐type structures at room temperature. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the Growth Morphology of Ammonium Oxalate Monohydrate Crystals Obtained from Aqueous Solutions

The experimental results of the growth morphology of ammonium oxalate monohydrate [(NH₄)₂C₂O₄ · H₂O; AO] single crystals obtained from aqueous solutions at 30 and 40 °C and supersaturation up to 9% are presented. The observations are compared with the theoretical morphology predicted by PBC analysis and Braivais-Donnay-Harker law.     

Hydrothermal synthesis and formation of 4ZnO·B2O3·H2O whiskers via a new route

4ZnO·B₂O₃·H₂O whiskers were prepared from 2ZnO·3B₂O₃·3.5H₂O under hydrothermal process at 160 °C for 10 h. The synthesized product was characterized by XRD, SEM, TG‐DSC and FT‐IR. SEM results showed that the synthesized 4ZnO·B₂O₃·H₂O whiskers' length was about 3–10 μm and the diameter was 0.2–0.3 μm. Further study on the whiskers' growth process and mechanism showed that the formation of the whiskers went through three stages and the morphology of 4ZnO·B₂O₃·H₂O changed from irregular particles to rod‐like structures and finally to whiskers. The variation of the morphology of the 4ZnO·B₂O₃·H₂O whisker with the concentration of the starting material was investigated. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)