Chapter 3. Experimental Procedure and Characterization Techniques

Transcription

1 Chapter 3 Experimental Procedure and Characterization Techniques 3.1 Introduction Amino acids are biologically important organic compounds composed of amine (-NH2) and carboxylic acid (COOH) functional groups, along with a side-chain specific to each amino acid. The fundamental elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, though other elements are found in the side-chains of certain amino acids. The amino acids are (Wagner., et al 1983) classified according to the core structural functional groups locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, ph level, and side-chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). The side chains of the amino acids can be divided into two major classes, one is non-polar side chains and other one is polar side chain. Non polar side chain consists mainly of hydrocarbon alkyl or aromatic groups. Amino acids are crystalline solids able to dissolve in water. Meanwhile, they only dissolve sparingly in organic solvents and the extent of their solubility depends on the size and nature of side chain. Amino acids feature very high melting point upto 200 C -300 C,and other properties vary for each particular amino acid. L-Alanine is an α-amino acid with the chemical formula HO2CCH(NH2)CH3.Other name of thel- Alanine is 2-Aminopropanoic acid. It is classified as a non-polar amino acid. The α-carbon atom of L-Alanine is bound with a methyl group (-CH3), making it one of the simplest α-amino acids with respect to molecular structure and also resulting in L-Alanine is being classified as an aliphatic amino acid. The methyl group of L-Alanine is non-reactive and is thus almost never directly involved in protein function and is shown in figure

2 Figure 3.1 Structure of L-Alanine molecule L-Alanine can be prepared by the condensation of acetaldehyde with ammonium chloride in the presence of sodium cyanide by the Strecker reaction, or by the ammonolysis of 2bromopropanoic acid (E.C. Kendall, et al 1929). 41

3 L-Cysteine is an α-amino acid with the chemical formula HO2 CCH (NH2) CH2 SH. It is a semi-essential amino acid. Other name of the L-Cysteine is 2-Amino-3-sulfhydrylpropanoic acid. The Cysteine thiol group is nucleophilic and can be easily oxidized.because of its high reactivity; the thiol group of amino acid Cysteine has numerous biological functions. The structure of the L-Cysteine is shown in figure 3.2 Figure 3.2. Structure of L-Cysteine molecule Barium nitrate with chemical formula Ba(NO3)2 is a salt composed of barium and the nitrate ion. Barium nitrate exists as a white solid at room temperature. It is soluble in water. It occurs naturally as the rare mineral nitro barite. Barium nitrate's properties make it suitable for use in various military applications. Barium nitrate is usually prepared by one of below mentioned two process. The first involves dissolving small chunks of barium carbonate in nitric acid, allowing any iron impurities to precipitate, then filtered, evaporated and crystallized. The second requires combining barium chloride with a heated solution of sodium nitrate, causing barium nitrate crystals to separate from the mixture. It forms colourless anhydrous crystals. The structure of the barium nitrate is shown in figure

4 Figure 3.3 Structure of Barium nitrate Lithium nitrate is an inorganic compound with the formula LiNO3. It is the lithium salt of nitric acid and oxidizing agent mixtures with alkyl esters may explode owing to the formation of alkyl nitrates. It is made by reacting lithium carbonate or lithium hydroxide with nitric acid. The structure of the Lithium nitrate is shown in figure 3.4. Lithium nitrate can be synthesized by reacting nitric acid and lithium carbonate. Li2CO3 + 2 HNO3 2 LiNO3 + H2O + CO2 (Solid Phase) The properties of L-Alanine, L-Cysteine,Barium nitrate and Lithium nitrate are given in table 3.1 Figure 3.4. Structure of LiNO3 molecule 43

5 Table 3.1 Properties of L-Alanine, L-Cysteine, Barium nitrate and Lithium nitrate Properties L-Alanine L-Cysteine Barrium Lithium Nitrate Nitrate Molecular C3H7NO2 C3H7NO2S Ba(NO3)2 LiNO3 Molar mass g mol g mol g mol g mol-1 Appearance White powder White powder White crystals White solid Melting point 258 C 240 C 592 C 255 C Solubility Soluble in water Soluble in water Soluble in water Soluble in water formula 3.2. Crystal Growth of L-Alanine doped with LiNO3, Ba (NO3)2 The starting material was synthesized by taking LAlanine (Merck India Ltd) and Lithium nitrate (Merck India Ltd) in a 1:1 stoichiometric ratio. The calculated amount of Lithium nitrate was first dissolved in deionized water. L-Alanine was then added to the solution. The solution was agitated with a magnetic stirring device for two hours continuously and filtered after complete dissolution of the starting materials. The solution thus prepared was allowed to evaporate at room temperature and allowed to crystallize by slow evaporation of solvent at 32 C. Well-defined single LALN crystals of good transparency were collected in about five weeks. A transparent single crystal of size upto 2 2 1mm 3 was harvested and is shown in figure 3.5. L-Alanine and Barium Nitrate (Merck India Ltd) was taken in equimolar ratio. Barium Nitrate was first dissolved in the deionized water and L-Alanine was then added to the solution. The solution was agitated with a magnetic stirring device for 2 hours continuously and filtered after complete dissolution of the starting materials. The solution thus prepared was allowed to evaporate at room temperature for around 4 days. A good transparent LABN crystal was harvested and is shown in figure

6 Figure 3.5.LABN crystal Figure 3.6.LALN crystal 45

7 3.3. Crystal Growth of L-Cysteine doped with LiNo3, Ba(No3)2 L-Cysteine (Merck India Ltd) and Hydrochloric acid (Merck India Ltd) was taken in equimolar ratio.l-cysteine was first dissolved in double distilled water. Hydrochloric acid was.then added to the solution. The starting solution was agitated with a magnetic stirrer for two hours continuously and solution was flitted twice to remove the suspended impurities. The homogeneous solution was allowed to evaporate at room temperature and good optical qualit y crystals were harvested from the solution in a growth period of 45 days is shown in figure 3.7 The starting material was synthesized by taking L-Cysteine and Barium Nitrate in appropriate ratio. High qualtity Barium Nitrate (Merck India Ltd) was first dissolved in the dilute Hydrochloric acid (Merck India Ltd) and L-Cysteine (Merck India Ltd) was then added to the solution. The solution was agitated with a magnetic stirring for an hour continuously and filtered by Wattman filter paper no 40 after complete dissolution of the starting materials. The homogeneous solution thus prepared was allowed to evaporate at room temperature and a crystal of good optical quality was harvested from the solution in a growth period of 13 days and is shown in figure 3.8. High quality L-Cysteine (Merck India Ltd) and Lithium Nitrate (SRL Ltd) was taken in appropriate ratio. Lithium Nitrate was dissolved in the dilute Hydrochloric acid (Merck India Ltd).After that L-Cysteine was then added to the solution. The solution was agitated with a magnetic stirring for an hour continuously and then filtered by Wattman filter paper no 40 after complete dissolution of the starting materials. The prepared homogeneous solution was allowed to evaporate at room temperature and good optical quality crystals harvested from the solution in a growth period of 7 days is shown in figure

8 Figure Figure 3.7. LCH crystal 3.8. LCHBN crystal 47

9 Figure 3.9. LCHLN crystal 3.4. Characterization Techniques Characterization is a tool for the measurement of physical and chemical properties of material. Characterization provides basis for understanding and improving the characteristics of material for specific applications. The grown single crystals have been subjected to various characterization studies. The transmission properties of the crystals were examined by UV-Vis Spectrometer in the range 200nm-900nm. FTIR spectra analysis was carried out to confirm the various functional groups presence in the compound. The X-ray diffraction analysis on the grown crystal was used to confirm the crystalline nature of the grown crystals. The grown crystal has been subjected to single crystal X-ray diffraction study to obtain the crystallographic data and identification of the unit cell parameters. Mechanical strength of the material was carried out using Vickers microhardness tester fitted with diamond indenter. 48

10 The second harmonic generation behaviour of the powdered material was tested by using the Kurtz and Perry method. Thermo gravimetric analysis (TG) was carried out between C in nitrogen atmosphere at a heating rate of 10 C/min using TA instruments NETZSCH STA 409PC/PG and NETZSCH STA 449F3 to determine the thermal stability of the grown crystals. Dielecrtic constant and dielectric loss of the grown crystal were studied by HIOKI 3532 LCR HITESTER instrument Magnetic properties of the grown LCH, LCHBN, LCHLN, LABN and LALN crystals have been analyzed by using a Vibrating Sample Magnetometer (VSM) upto 250 C UV-Visible spectroscopy Ultraviolet -visible spectroscopy (UV-Vis) is also known as electronic spectroscopy. Ultraviolet (200nm-400nm) and visible absorption spectroscopy is the measurement of the attenuation of a beam of light after it passes through a sample of after reflection from a sample surface and schematic diagram is shown in figure It uses light in the visible and adjacent near ultraviolet (UV) and near infrared (NR) ranges. In this region of energy space molecules undergo electronic transitions. Molecules containing π-electrons or non-bonding electrons (n-electrons) can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals. The more easily excited the electrons (i.e. lower energy gap between the HOMO and the LUMO), the longer the wavelength of light it can absorb. Every time a molecule has a bond, the atoms in a bond have their atomic orbitals merged to form molecular orbitals which can be occupied by electrons of different energy levels. Ground state molecular orbitals can be excited to anti-bonding molecular orbitals. 49

11 Figure 3.10 Photograph of UV-Visible Spectrometer The electrons in a molecule can be of one of three types: namely σ (single bond), π (multiplebond), or non-bonding (n- caused by lone pairs). These electrons when imparted with energy in the form of light radiation get excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and the resulting species is known as the excited state or anti-bonding state. 1. σ-bond electrons have the lowest energy level and are the most stable electrons. These would require a lot of energy to be displaced to higher energy levels. As a result these electrons generally absorb light in the lower wavelengths of the ultraviolet light and these transitions are rare. 50

12 2. π- bond electrons have much higher energy levels for the ground state. These electrons are therefore relatively unstable and can be excited more easily and would require lesser energy for excitation. These electrons would therefore absorb energy in the ultraviolet and visible light radiations. 3. n-electrons or non-bonding electrons are generally electrons belonging to lone pairs of atoms. These are of higher energy levels than π-electrons and can be excited by ultraviolet and visible light as well. Most of the absorption in the ultraviolet-visible spectroscopy occurs due to π-electron transitions or n-electron transitions. Each electronic state is well defined for a particular system. The different transition between the bonding and anti-bonding electronic states is shown in figure Figure 3.11 Energy level diagram with electronic transitions When a sample is exposed to light energy that matches the energy difference between a possible electronic transition within the molecule, a fraction of the light energy would be absorbed by the molecule and the electrons would be promoted to the higher energy state orbital. Samples were placed in a transparent cell, known as a cuvette. The sample holders was the rectangular shaped quartz or glass cells of about 10mm path length. Transmitted light radiation is received at the photomuliplier tube alternately from the reference and the sample beams. A photoelectric signal timing system is synchronized with the alternate pulse which permits the 51

13 comparison of signals from the two beams. The difference between the two signals is recorded with the help of a interfaced with a PC-XT and stored there for easy reference.samples in solid form, powder, pellets was dissolved in suitable solvents to form the contents of the sample cell and the solvents was taken in the reference cell. In this present work PerkinElmer Lamda Instrument was used to record the UV-Visible spectrum Fourier Transform Infrared (FTIR) Spectral Analysis Infrared spectra result from transitions between quantized vibrational energy states.(griffiths.p et., 2007) Infrared spectroscopy is non destructive technique for materials analysis and used in the laboratory for over seventy years. Infrared absorption spectroscopy is the study of interaction of infrared radiation with matter as a function of photon frequency. Fourier Transform Infrared Spectroscopy (FTIR) provides specific information about the vibration and rotation of the chemical bonding and molecular structures, making it useful for analyzing organic materials and certain inorganic materials. An infrared spectrum represents a fingerprint of a sample with absorption peaks which correspond to the frequencies of vibrations between the bonds of the atoms making up the material. Because each different material is a unique combination of atoms, no two compounds produce the exact same infrared spectrum. Therefore, infrared spectroscopy can result in a positive identification (qualitative analysis) of every different kind of material. In addition, the size of the peaks in the spectrum is a direct indication of the amount of material present. With modern software algorithms, infrared is an excellent tool for quantitative analysis. Molecular vibrations that oscillate with the same frequency absorb IR light. The frequency of vibration and the probability of absorption are influenced by intra and intermolecular effects. Thus, information about structure and environment can be deduced from the spectral parameters, bandwidth and absorption coefficient. There are no rigid rules for interpreting the vibration spectrum. The spectrum must be adequately resolved and adequate intensity. The spectrum should be that of a reasonable pure compound. The spectrometer should be calibrated. The method of sample handling must be specified. If a solvent is employed, the solvent, concentration and the cell thickness should be indicated. 52

14 A precise treatment of the vibrations of a complex molecule is not feasible. Thus, the spectrum must be interpreted from empirical comparison of spectra and extrapolation of studies of similar molecules. Many of group frequencies of organic compounds vary over a wide range. Because the bands arise from complex interacting vibrations within the molecule, absorption bands may, however represent predominantly a single vibration mode. Important details of structure may be revealed by the exact position of an absorption band within a narrow region. Shifts in absorption position and changes in band contours, accompanying changes in molecular environment, may also suggest important structural details. The FTIR instrument consists of Nernst glower as source, an interferometer chamber comprising of KBr beam splitters followed by a sample chamber and detector.entire region of 4000cm cm-1 is covered by this instrument. The spectrometer works under purged conditions. Solids samples are dispersed in KBr or Polyethylene pellets depending on the region of interest. This instrument has a typical resolution of 4cm-1. Signal averaging. Signal enhancement, Base line Correction and other spectral manipulations are possible. The photograph of FTIR instrument is shown in figure 3.12 and Schematic diagram of a FTIR Spectrometer is shown in figure

15 Figure 3.12 Photograph of FTIR spectrometer 54

16 Figure 3.13.Schematic diagram of a FTIR Spectrometer Powder X-Ray Diffraction (XRD) studies In a crystalline solid, the constituent particles (atoms, ions or molecules) are arranged in a regular order. An interaction of a particular crystalline solid with X-rays helps in investigating its actual structure. Crystals are found to act as diffraction gratings for X-rays and this indicates that the constituent particles in the crystals are arranged in planes at close distances in repeating patterns. The phenomenon of diffraction of X-rays by crystals was studied by (W. L. Bragg and W. H. Bragg.,1913) 2dsinθ=nλ 55

17 Bragg's equation can be used to calculate the distance between repeating planes of the particles in a crystal. Similarly, if interplanar distances are given, the corresponding wavelengths of the incident beam of X-ray can be calculated. In case of fine particles, with reduction in the size of the particles, the XRD lines get broadened, which indicates clearly that particle size has been reduced. A typical powder XRD instrument consist of four main components such as X-ray source, specimen stage, receiving optics and X-ray detector is shown in Figure 3.14.The source and detector with its associated optics lie on the circumference of focusing circle and the sample stage at the centre of the circle. The angle between the plane of the specimen and the X-ray source is θ (Bragg s angle) and the angle between the projection of X-ray and the detector is 2θ. For the XRD analysis, fine powder samples can be mounted on the sample holder and the powder was assumed to consist of randomly oriented crystallites. When a beam of X-ray is incident on the sample, X-rays are scattered by each atom in the sample. If the scattered beams are in phase, these interfere constructively and one gets the intensity maximum at that particular angle. The atomic planes from where the X-rays are scattered are referred to as reflecting planes. After recording the X-ray diffraction pattern, first step involves the indexing of XRD peaks. The indexing means assigning the correct Miller indices to each peak of the diffraction pattern.the microstructural parameters grain size (d) and dislocation density (ρ) values are calculated using Scherer s formula for different 2θ values. Grain size ( Dislocation density ( Where λ is the wavelength of X-ray diffraction β is the full width at half maximum θ is the glancing angle. 56

18 Figure Photograph of X-ray Diffractometer Single Crystal X-ray Diffraction Studies Single-crystal X-ray Diffraction is a non-destructive analytical technique which provides detailed information about the internal lattice of crystalline substances, including unit cell dimensions, bond-lengths, bond-angles, and details of site-ordering. Directly related is singlecrystal refinement, where the data generated from the X-ray analysis is interpreted and refined to obtain the crystal structure. X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce 57

19 monochromatic radiation, collimated to concentrate, and focused towards the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law (nλ=2d sinθ). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By changing the geometry of the incident rays, the orientation of the centered crystal and the detector, all possible diffraction directions of the lattice should be attained. X-ray diffractometers consist of three basic elements, an X-ray tube, a sample holder, and an Xray detector. X-rays are generated in a cathode ray tube by heating a filament to produce electrons, accelerating the electrons toward a target by applying a voltage, and impact of the electrons with the target material. When electrons have sufficient energy to dislodge inner shell electrons of the target material, characteristic X-ray spectra are produced. These spectra consist of several components, the most common being K α and Kβ. Kα consists, in part, of Kα1 and Kα2. Kα1 has a slightly shorter wavelength and twice the intensity as Kα2. The specific wavelengths are characteristic of the target material. Filtering, by foils or crystal monochrometers, is required to produce monochromatic X-rays needed for diffraction. Kα1and Kα2 is sufficiently close in wavelength such that a weighted average of the two is used. Molybdenum is the most common target material for single-crystal diffraction, with MoKα radiation = Å. These X-rays are collimated and directed onto the sample. When the geometry of the incident X-rays impinging the sample satisfies the Bragg Equation, constructive interference occurs. Single-crystal diffractometers use either 3- or 4-circle goniometers. These circles refer to the four angles (2θ, χ, φ, and Ω) that define the relationship between the crystal lattice, the incident ray and detector. Samples are mounted on thin glass fibers which are attached to brass pins and mounted onto goniometer heads. Adjustment of the X, Y and Z orthogonal directions allows centering of the crystal within the X-ray beam. X-rays leave the collimator and are directed at the crystal. Rays are either transmitted through the crystal, reflected off the surface, or diffracted by the crystal lattice. A beam stop is located directly opposite the collimator to block transmitted rays and prevent burn-out of the detector. Reflected rays are not picked up by the detector due to the angles involved. Diffracted rays at the correct orientation for the configuration are then collected by the detector (A. Putnis, 1992). 58

20 Vicker s microhardness studies: Mechanical strength of the materials plays a key role in device fabrication. It is a measure of the resistance, the lattice offers to local deformation (B.W.Mott, 1956). As the hardness properties are basically related to the crystal structure of the material and the bond strength, micro hardness studies has been applied to understand the plasticity of the crystals. Hardness tests are commonly carried out to determine the mechanical strength of materials and it correlates with other mechanical properties like elastic constants and yield stress (D.Taber. 1951).Hardness measurements can be defined as macro,micro and nano according to the forces applied and displacement obtained(j.b.pethica and D.Taber, 1979). Microhardness of a crystal is its ability to resist indentation.the well-polished crystal was mounted on the platform of the microhardness tester and the loads of different magnitudes was applied over a fixed interval of time. The indentation time was fixed as 10 sec. The indentation hardness is measured as the ratio of applied load to the surface area of the indentation. For each load, several indentations were made and the average value of the diagonal length (d) was used to calcultate the microhardness. Vickers microhardness number was evaluated using the expression, Hv = P/d2 kg/mm2.where P is the applied load, d is the diagonal length of the indentation impression and is a constant of a geometrical factor for the diamond pyramid. The data obtained for P and d can be analyzed by Meyers equation=kdn where k and n are constants for the material. The constant k is usually referred to as the standard hardness. Meyer index (n) which represents work hardening capacity of the material can be determined by plotting log P vs log d. The dependence of Vickers hardness number on applied load shows different behaviour on different materials (J.N.Sherwood.J.N,1998,I.V.Kity,2001 and T.M.Onitch,1947).In certain cases, micro hardness increases with applied load and reaches a constant value at higher loads(m.hanneman,1941,c.hays,et al 1973 and Martin Britto,et al 2007.) We can apply large amount of load to the crystals surface. Micro hardness is found to decreases with increasing load and attains a constant value after a particular load for certain crystals (W.A.Wooster,1953) and schematic figure as shown in figure According to Onitsch and Hanneman, n should lie between 1 and 1.6 for hard materials and above 1.6 for softer ones.elastic Stiffness constant (C 11) of a material can be calculated from the Wooster s empirical relation as 59

21 C11=Hv7/4 If the stiffness constant C11 is high, it reveals force are quite strong Kurtz and Perry powder technique The nonlinear property of crystal was studied by employing Kurtz and Perry powder technique (S.K.Kurtz, and T.T.Perry,1968 ).Nonlinear optics (NLO) is attracting increasing attention due to its wide applications in the area of laser technology, optical communication and data storage technology. The development of highly efficient nonlinear optical crystals is of great importance to extend the frequency range provided by normal laser source into the ultraviolet (UV) and infrared (IR) regions. Second harmonic generation helps in extending the range of laser wavelengths into blue and UV part of the spectrum (Subramayam Brij lal and M.N.Avadhanulu, 2004).In this technique, the grown sample was powdered into fine microcrystalline samples and then densely packed between two transparent glass slides. A Qswitched mode Nd:YAG laser operated at the fundamental wavelength 1064 nm with 8 ns pulse width and 10 Hz pulse rate was used as the radiation source. The experimental set-up of second harmonic generation was reported elsewhere. The laser beam was allowed to pass through the sample cell. The output beam from the sample was filtered by an IR detector and then identified by a photomultiplier tube. A colour filter was used to absorb IR radiation and transmit that of second harmonic radiation of wavelength 532 nm. The final output was displayed on the oscilloscope. The frequency conversion efficiency of the crystal was confirmed by the emission of green radiation from the sample Thermo Gravimetric Analysis (TGA) Thermo gravimetric analysis instruments are routinely used in all phases of research, quality control and production operations.thermo gravimetric Analysis (TGA) continuously monitors the weight of a sample during isothermal or dynamic temperature scans over the range upto 500 C in an air, nitrogen, oxygen, or specialty atmosphere. Its principal uses include measurement of a material's thermal stability and composition. TGA provides quantitative measurement of mass change in materials associated with transition and thermal degradation. TGA records change in mass from dehydration, decomposition, and oxidation of a sample with time and temperature. Characteristic thermo-gravimetric curves are given for specific materials and chemical 60

22 compounds due to unique sequence from physicochemical reactions occurring over specific temperature ranges and heating rates. These unique characteristics are related to the molecular structure of the sample. Differential Thermal Analysis (DTA) Differential thermal analysis (DTA) is a thermo analytic technique. The method has been used predominantly for the determination of inorganic systems. In DTA, the material under study and an inert reference (which does not go through phase transition in the temperature range of interest) are heated (or cooled) under identical conditions, while recording any temperature difference between sample and reference. This differential temperature is then plotted against time, or against temperature (DTA curve or thermo gram). Changes in the sample, either exothermic or endothermic, can be detected relative to the inert reference. Thus, a DTA curve provides data on the transformations that have occurred, such as glass transitions, crystallization, melting and sublimation (M.E.Brown,2001).Differential temperatures can arise between two inert samples when their response to the applied heat-treatment may not identical. So DTA can also be used to study thermal properties and phase changes which do not lead to a change in enthalpy. TG Analysis can be carried out by raising the temperature gradually and plotting weight (percentage) against temperature. The temperature in many testing methods routinely reaches 500 C or greater, but the oven is so greatly insulated that an operator would not be aware of any change in temperature even if standing directly in front of the device. After the data are obtained, curve smoothing and other operations may be done such as to find the exact points of inflection. Simultaneous TGA-DTA/DSC measures both heat flow and weight changes (TGA) in a material as a function of temperature or time in a controlled atmosphere. Simultaneous measurement of these two material properties not only improves productivity but also simplifies interpretation of the results. The complementary information obtained allows differentiation between endothermic and exothermic events which have no associated weight loss (e.g., melting and crystallization) and those which involve a weight loss (e.g., degradation) Dielectric Studies 61

23 The term dielectric analysis (DEA) refers to a group of techniques that measure changes in different physical properties of a polar material, such as polarization, permittivity, and conductivity, with temperature or frequency. The reorientation of dipoles and the translational diffusion of charged particles in an oscillating electric field provide the basis of the analysis based on alternating-current (AC) dielectric methods, which principally involve measurements of the complex permittivity (ε*) in the frequency or time domain and at constant or varying temperature. The corresponding changes in the dielectric constant and polarizability of the materials are quite large and are easily detected during phase transitions (e.g., the glass transition, melting, or crystallization) and secondary transitions. Crystal with high transparency and large defect free area was selected and used for the AC electrical conductivity measurements. The Extended portions of the crystals were removed completely and the opposite faces was polished and coated with electronic grade silver paste to obtain a good Ohmic contact. The AC electrical conductivity measurements was carried out along C-direction for the grown crystals using the conventional two-probe technique using a megohm meter at various temperature ranging from 40 C to 80 C(Freeda and Mahadevan 2000,Deepa et al 2002,Anne Assencia and Mahadevan 2005).The Dimensions of the crystal was measured using a travelling microscope. The A.C. Conductivity calculated using the relation of the crystal was. The capacitance and dielectric loss factor (tanδ) measurements was carried out using HIOKI 3532 LCR HITESTER instruments with a constant frequency of 1kHz at different temperatures ranging from 40 C to 80 C and is shown in figure 3.15 (Neelakanta Pillai and Mahadevan 2007).The observation was made while cooling the sample. The samples was prepared and annealed in a way similar to that followed for the resistance measurements. The dielectric Ɛr was calculated using the relation Where is the vacuum dielectric constant ( = F/m), ω is the angular frequency (ω=2лf), C is the capacitance, d is the thickness of the crystal and A is the area of the crystal. 62

24 Figure HIOKI 3532 LCR HITESTER Instrument Vibrating Sample Magnetometer (VSM) A vibrating sample magnetometer or VSM is a scientific instrument that measures magnetic properties, invented in 1955 by Simon Foner at Lincoln Laboratory, Simon (1959). A 63

25 sample is placed inside a uniform magnetic field to magnetize the sample. The sample is then physically vibrated sinusoidally, typically through the use of a piezoelectric material. Commercial systems use linear actuators of some form, and historically the development of these systems was done using modified audio speakers, though this approach was dropped due to the interference through the in-phase magnetic noise produced, as the magnetic flux through a nearby pickup coil varies sinusoidally. The induced voltage in the pickup coil is proportional to the sample's magnetic moment, but does not depend on the strength of the applied magnetic field. In a typical setup, the induced voltage is measured through the use of a lock-in amplifier using the piezoelectric signal as its reference signal. By measuring in the field of an external electromagnet, it is possible to obtain the hysteresis curve of a material. The vibrating sample magnetometer measures the magnetization of a small sample of magnetic material placed in an external magnetizing field by converting the dipole field of the sample into an ac electrical signal (D.O.Smith, 1956). 64