Growth of lanthanum tartrate crystals in silica gel

The spherulitic, dendritic and single-crystal growth of hydrated lanthanum tartrate by controlled diffusion in silica gels is reported. The influence of growth parameters, e.g. reactant concentrations, gel pH, gel ageing, on the size and nucleation density of crystals has been studied. Operative mechanisms of crystallization, results of growth kinetics and morphology of crystals are discussed. The adsorption property of the gel is found to play a vital role during the crystallization of lanthanum tartrate crystals, Parabolic kinetics, characteristic of a one-dimensional diffusion-controlled process, for single crystals is observed to be obeyed in case of variation of upper reactant concentration.     

Growth of strontium tartrate tetrahydrate single crystals in silica gels

Growth of single crystals of strontium tartrate tetrahydrate by controlled diffusion in silica gels has been narrated. In the field of material science, there is always a keen and competitive race to grow perfect single crystals with sufficient purity and perfection. Successful attempts to larger as well as more perfect crystals of SrTr are described in this paper and thus probably the largest (about 2 in. across) ever grown size of gel-grown products is reported here. The effect of several parameters, e.g. crystallization apparatus, environmental temperature, reactant concentration, gel density, gel-ageing and gel pH, on the size, quality and morphology of the crystals has also been studied.     

Morphological aspects of some symmetrical ammonium hydrogen tartrate crystals grown by silica gel technique

Ammonium hydrogen tartrate single crystals have been prepared by the reaction of NH₄Cl and tartaric acid in silica gel. The morphology of some as-grown symmetrical crystals has been studied by optical microscopy. The growth morphology studies indicate that the grown crystals are needle-shaped, orthorhombic disphenoidal and tetrahedral disphenoidal. The growth mechanism has been assessed.     

Growth of BaMoO4 and BaCO3 crystals in silica gel media

Barium molybdate, BaMoO₄, and barium carbonate, BaCO₃, crystals have been grown in sodium metasilicate gels. The influence of concentration of reactant solutions and pH of gels on the nucleation and growth, and on the penetration depth of the growth front of BaMoO₄ crystals in the test tubes were primarily studied. Combinations of pH 6 gel mixture with the 0.4 m and 0.6 m supernatant BaCl₂ and Na₂MoO₄ solutions resulted in the best size of crystals, inter-crystalline separation and clarity of the gel media. Two best test tube experimental combinations of hybrid growth were repeated in the U-shaped glass tubes to obtain further information. Depending on the growth sites in the tubes, different morphologies of the crystals were observed. The bulk morphologies of these crystals were studied by scanning electron microscopy. Growth of BaCO₃ crystals was performed only as exploratory work in the test tubes.At Universidad Central de Venzuela only.     

Thermal behaviour of strontium tartrate single crystals grown in gel

Thermal behaviour of strontium tartrate crystals grown with the aid of sodium metasilicate gel is investigated using thermogravimetry (TG) and differential thermal analysis (DTA). Effect of magnetic field and dopant (Pb)²⁺ on the crystal stability is also studied using thermal analysis. This study reveals that water molecules are locked up in the lattice with different strengths in the grown crystals.     

Growth of larger and more perfect single crystals of KClO4 in silica gel

As part of a continuing effort on crystal growth in gels, this paper describes the growth of large transparent single crystals of KClO₄ by the improved design. The effect of various anionic and cationic components on nucleation, growth and quality of these crystals has been studied. It has been found that a combination of KNO₃ and HClO₄ as the reactants resulted in the best crystals in terms of crystal size, quality and inter-crystalline separation. The effect of various impurities on nucleation, growth and quality of these crystals has been studied. It has been found that the impurities which enhance the solubility of KClO₄ have a positive effect on the size and quality of the crystals. A new etchant, consisting of concentrated HCl and H₂SO₄ in the volume ratio 2:3 has been found to reveal as-grown as well as freshly introduced dislocations.     

Dielectric and thermal studies on gel grown strontium tartrate pentahydrate crystals

Results of dielectric and thermal studies on strontium tartrate pentahydrate crystals are described. The value of dielectric constant is shown to be independent of temperature till 360 K at all the frequencies (110–700 kHz) of the applied a.c. field. It increases abruptly achieving a peak value of 25.5 at 100 kHz; the peak value being strongly dependent on frequency. In the temperature range, 87 < T < 117°C, the value of ɛ′ falls suggesting a transition at around 100°C or so. The dielectric constant, ɛ′, of the material is shown to be frequency dependent but temperature independent in the pre- or post-T _c range 87 < T < 117°C, suggesting that the contribution towards polarization may be due to ionic or space charge polarization which gets eliminated at higher frequencies. The ferroelectric transition is supported by the results of thermoanalytical studies. It is explained that crystallographic change due to polymorphic phase transition may be occurring in the material, besides the change due to loss of water molecules, which leads to the dielectric anomaly at around 100°C. Coats-Redfern approximation method is applied for obtaining non-isothermal kinetic parameters leading to calculation of activation energies corresponding to three decomposition stages of material in the temperature ranging from 379–1113 K.     

Some structural aspects of neodymium praseodymium oxalate single crystals grown in hydro silica gels

The mixed crystals of neodymium praseodymium oxalate are grown by the diffusion of a mixture of aqueous solutions of neodymium nitrate and praseodymium nitrate (as an upper reactant) into the set gel embedded with oxalic acid. By varying the concentration (by volume) of rare earth nitrates in the upper reactant, the incorporation of Nd and Pr in the mixed crystals has been studied. Tabular crystals with the well defined hexagonal basal planes are observed in the mixed crystals of varying concentrations. X-ray diffraction patterns of these powdered samples reveal that these mixed crystals are ‘isostructural’, while IR and FTIR spectra establish the presence of oxalate groups. TGA and DSC analyses show the correctness of the chemical formula for the mixed crystals, by the release of water molecules (endothermic) and of CO and CO₂ (exothermic), with the rare earth oxides as the stable residue. X-ray fluorescence (XRF) and energy dispersive X-ray analyses (EDAX) establish the presence of heavy rare earth elements qualitatively and to a good extent quantitatively. X-ray photoelectron spectroscopic (XPS) studies confirm the presence of rare earth elements (Nd and Pr) as their respective oxides. The findings of these techniques of characterization are in excellent agreement with the proposed empirical structure for the mixed rare earth oxalates. The implications are discussed.     

Characterization and thermal behaviour of lanthanum tartrate crystals grown from silica gels

Results obtained on characterization of lanthanum tartrate crystals, grown by the gel method, using chemical analysis, x-ray and electron diffraction, infra-red and mass spectroscopy are reported. The thermal behaviour is studied using DTA, TGA and DTG. The decomposition pattern is reported to be typical of a hydrated metal tartrate. Kinetic parameters like order of reaction, frequency factor and activation energy are evaluated. Contracting cylinder kinetic model is found to be the best fit for the decomposition processes involved. Magnetic susceptibility measurements indicate the material to be diamagnetic.     

Dielectric and thermal characteristics of gel grown single crystals of ytterbium tartrate trihydrate

Dielectric and thermal characteristics of gel grown single crystals of ytterbium tartrate trihydrate have been carried out. The dielectric constant has been measured as a function of frequency in the range 2 kHz–1 MHz and temperature range 30–300 °C. The dielectric constant increases with temperature, attains a peak near 215 °C, and then decreases as the temperature exceeds 215 °C. The dielectric anomaly at 215 °C is suggested to be due to phase transition brought about in the material. The dielectric behaviour of the material is correlated with the results on thermal analysis. Thermogravimetric and differential thermal analysis have been used to study the thermal characteristics of the material. The experimental results show that the material is thermally stable up to 200 °C. The decomposition process occurs in two stages until ytterbium oxide is formed at 700 °C. The non-isothermal kinetic parameters e.g., activation energy and the frequency factor have been evaluated for each stage of thermal decomposition by using the integral method, applying the Coats–Redfern approximation.     

Characterization and thermal behaviour of gel grown mixed rare-earth (Didymium) tartrate crystals

Sherulites, crystal aggregates and platelets of Di₂ (C₄H₄O₆)₃ · 5H₂O mixed crystals were grown in gel using the single tube diffusion method. The material was characterized by using different techniques such as chemical analysis, EDAX, X-ray and electron diffraction, infrared and mass spectroscopy. The thermal behaviour of the material was studied using differential thermal analysis, DTG, thermogravimetric analysis and differential scanning calorimetry. The material is thermally unstable and starts decomposing at 50‡ C. Thermal analysis results indicate the application of the contracting cylindrical kinetic model for the solid state reactions involved in the decomposition process. The values of kinetic parameters, e.g. order of reaction, activation energy and frequency factor are worked out. The implications are discussed.