Dielectric,ferroelectric and optical behaviour of terbium hydrogen tartrate trihydrate crystals

Gel diffusion technique, using agar–agar as gel medium, has been employed to obtain single crystals of terbium hydrogen tartrate trihydrate. The grown crystals are characterised by X-ray diffraction, scanning electron microscopy, CHN technique, thermogravimetric methods and UV–Vis spectroscopy. Dielectric, ferroelectric and optical studies on this metal–organic compound have been carried out. The dielectric constant has been measured as a function of temperature and frequency in the ranges of 20–250 °C and 20 Hz–3 MHz, respectively. The study of dielectric behaviour as a function of temperature reveals two dielectric anomalies at 95 and 198 °C. The dielectric anomaly at 95 °C is suggested to be due to ferroelectric phase transition brought about in the material. The study of polarisation versus electric field shows a hysteresis loop which thereby confirms the ferroelectric nature of terbium hydrogen tartrate trihydrate crystals. The dielectric anomaly at 198 °C is suggested to be due to loss of water molecules in the compound. The results of thermal study show that the material is thermally stable up to temperature of about 200 °C. Optical studies show that the terbium hydrogen tartrate trihydrate crystal has good transparency in the entire visible and infra red range of the spectrum.     

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.     

Dielectric behaviour of strontium tartrate single crystals

Strontium tartrate trihydrate (STT) crystals have been grown in silica hydrogel. Various polarization mechanisms such as atomic polarization of lattice, orientational polarization of dipoles and space charge polarization in the grown crystals have been understood using results of the measurements of dielectric constant (έ′) and dielectric loss (tan δ) as functions of frequency and temperature. Ion core type polarization is seen in the temperature range 75–180°C, and above 180°C, there is interfacial polarization for relatively lower frequency range. One observes dielectric dispersion at lower frequency presumably due to domain wall relaxation.     

Characterization and thermal behaviour of gel-grown gadolinium tartrate crystals

Gadolinium tartrate crystals in the form of spheru!itoe were synthesized by using a controlled diffusion system in sillica gel. Characterization of the material was performed by utilizing the techniques of chemical analysis, X-ray and electron diffraction, infrared and mass spectroscopy, and by As thermal behaviour. The material turns out to be a dihvdrate and the chemical composition vvoethus established as Gd₂(C₄H₄O₆)₃ · 2H₂O. The data obtained from the thermal analysis show the tendency of the material to decompose, and this is further confirmed by mass spectroscopy The decomposition process is completed in four steps until gadolinium oxide is obtained at 840* C. The energetics of the reactions at each stage of decomposition have been examined and mechanisms for the decomposition reactions are proposed.     

Dielectric and magnetic studies of gel-grown copper malonate trihydrate crystals

Single crystals of copper malonate trihydrate were grown in silica gel by slow diffusion of copper nitrate by sodium metasilicate gel impregnated with malonic acid. The grown crystal was subjected to dielectric studies and magnetic studies. The dielectric properties of the crystal were analyzed as a function of frequency and temperature. A vibrating sample magnetometer was used to determine the hysteresis properties of the crystals by measuring the magnetic moment for different applied fields. The crystal is found to be weakly ferromagnetic.     

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.     

Synthesis,characterization and properties of ferroelectric rubidium hydrogen tartrate single crystals

Rubidium hydrogen tartrate single crystals were grown in silica hydrogel. Controlled reaction was employed between tartaric acid and rubidium chloride solutions by slow diffusion process in the gel medium. Colourless transparent crystals were grown at room temperature. The average density of crystals measured pyknometrically was 2·263 g cm⁻³. Thermal stability of the materials was studied. Chemical etching was employed to determine the perfection of these crystals.     

Gel growth and characterization of pure and vanadyl-doped strontium tartrate tetrahydrate single crystals

Pure and vanadyl-doped single crystals of SrC₄H₄O₆·4H₂O (SrTr·4H₂O) were grown by controlled diffusion in silica gel. The crystals were characterized by EPR and electronic absorption techniques. The angular variation EPR spectra showed that the VO²⁺ ion has a fixed orientation in the lattice. Theg andA tensors and their direction cosines were evaluated from the EPR analysis. The above parameters for the observed sites were analysed assuming a spin Hamiltonian corresponding to a tetragonal symmetry. Electronic absorption studies carried out at 300 K as well as at 80 K showed three characteristic absorption bands at 12 500, 13 201 and 18 348 cm^–1 which were assigned to crystal field (d-d) transitions and the site symmetry of the ion has been estimated to be tetragonal. To confirm the ascribed transitions, the band positions were calculated by solving the energy expressions corresponding to the tetragonal symmetry and compared with the observed band positions. The crystal field parameters (Dq) and tetragonal distortion parameters (Ds andDt) were obtained from the best fit of the observed and calculated band positions. The EPR and electronic spectral results showed that the VO²⁺ ion has occupied the interstitial site having C_4V symmetry.     

Indentation study of cadmium oxalate trihydrate single crystals

Vickers microhardness indentation studies have been made on the as-grown (¯2¯21), (110) and (001) planes, and on the only cleavage plane (1¯10) of gel-grown cadmium oxalate trihydrate single crystals. The material chipping off and slip lines observed around indentation figures have been discussed. The anisotropy in microhardness of the crystals is indicated.     

Growth and characterization of pure and lithium doped strontium tartrate tetrahydrate crystals by solution-gel technique

The effect of lithium ion as dopant on the size and transparency of strontium tartrate tetrahydrate (SrC₄H₄O₆·4H₂O) crystals are presented in this paper. Growth of single crystals of undoped and lithium doped strontium tartrate tetrahydrate by controlled diffusion of strontium nitrate into the gel charged with tartaric acid at room temperature are narrated. The lithium ion enhances the size and transparency of the doped crystals. The crystal structure of the compound was confirmed by X-ray diffractometry and dopant concentration with ICP-atomic emission spectrometer system. Thermal decomposition of the grown crystals is investigated by TGA and DTA studies. The FTIR spectra of pure and doped crystals are recorded and analysed. Kurtz powder technique has been used to test SHG efficiency of the crystals.     

Growth of praseodymium tartrate crystals in silica gel

The growth of praseodymium tartrate crystals in the system Pr(NO₃)₃-Na₂ SiO₃-C₄H₆O₆, using a single-tube-single-gel technique is described. The growth conditions are delineated and a spherulitic morphology is reported. The spikes attached to the spherulites are single crystals of praseodymium tartrate. The mechanisms of crystallization for various types of spherulites are described. The information presented contributes to the understanding of spherulitic growth in general, and that of praseodymium tartrate in particular.     

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.     

Dielectric properties of perovskite crystals

The soft mode dynamical model has been used to study the dielectric properties of Perovskite-type crystals. The model Hamiltonian proposed by Pytte has been modified and designed in terms of creation and annihilation operators. The correlations appearing in the dynamical equation have been evaluated using double time thermal retarded Green’s function and Dyson’s equation. Without any decoupling the higher order correlations have been evaluated using the renormalized Hamiltonian and thus, all possible interactions among phonons have been taken into account. The expressions for phonon frequencies and widths have beenMcalculated. Using appropriate parameters the softening of different modes at different transition temperatures give rise to a series of transitions from cubic to tetragonal, orthorhombic or trigonal phases. The significantly temperature-dependent modes are considered responsible for damping constant, dielectric constant, tangent loss and attenuation constant for these crystals. The dielectric properties are directly related to the optical phonon frequencies and widths and acoustic attenuation to the acoustic and optical phonon widths. Using suitable approximations, the model explains the experimental results on dielectric properties and acoustic attenuation reported for LiNbO₃, SrTiO₃, BaTiO₃ and LaAlO₃.     

On the growth of calcium tartrate tetrahydrate single crystals

Calcium tartrate single crystals were grown using silica gel as the growth medium. Calcium formate mixed with formic acid was taken as the supernatant solution. It was observed that the nucleation density was reduced and the size of the crystals was improved to a large extent compared to the conventional way of growing calcium tartrate crystals with calcium chloride. The role played by formate-formic acid on the growth of crystals is discussed. The grown crystals were characterized by atomic absorption spectroscopy (AAS), X-ray diffraction analysis (XRD), microhardness measurement, Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG) and differential thermal analysis (DTA). The results obtained are compared with the previous work.     

Slip systems in gadolinium gallium garnet single crystals

Gd₃Ga₅O₁₂ single crystals with compression axes of different orientations have been deformed under creep conditions in air at 1450° C and 1550° C (0.86 T _M and 0.92 T _M, respectively, where T _M is the melting temperature). After a few per cent of permanent strain the deformation substructure has been studied by optical and transmission electron microscopy. Etching and bi-refringence patterns indicate that slip on {110}, {112} and {123} can be activated depending on the orientation of the compression load. Dislocations with a/2 111 Burgers vectors have been observed to glide in {110 planes. They exhibit a segmented aspect suggesting their dissociation out of the glide plane.