People Grow Crystals For Two Main Reasons

Published Date: 02 Nov 2017

2.1 Introduction

It is necessary to have a good insight into various experimental techniques for carrying out a fruitful research work. For making the best use of available technique to study various properties of the material, a good working knowledge of the instrument used is essential.

People grow crystals for two main reasons that are

To understand how crystal grow.

To know scientific and technological utility of grown crystals.

For either of these purposes one must evaluate the quality of grown crystals. Thus the crystal grower is directly involved with the assessment of grown crystal. The assessment of the physical and chemical properties of material is called as characterization.

In United States National Academy of Science and National Academy of Engineering committee of material advisory board has defined characterization as "Characterization describes those features of composition and structures (including defects) of material that are significant for a particular preparation, study of properties or use and suffice for the reproduction of the crystal".

A crystal is fully characterized when one know the identity, concentration and positions of all its constituent atoms. Thus, no crystal is fully characterized, but many crystals have been characterized to a point where relationship between their properties and the location and concentration of their constituent atoms could be made.

Characterization is not the measurement user oriented properties alone, e.g. in electronic science, measurement of conductivity and mobility of a semiconductor. Single crystals do not characterize but one has to determine number of atoms and their location with appropriate accuracy. All other properties including conductivity and mobility are determined. Thus solid state physicist or device engineer is often busy in measuring properties of interest for physical theory or device performance, but it remains for crystal grower to collect these properties, supplement these measurements with analytical, chemical, optical and other physical instrumental measurement to deduce things relevant to the identity and position of the atoms in crystal to characterize it. Most important of all, it is crystal grower’s responsibility to relate the results of studies to growth mechanism to the growth process in way enabling him to alter and control the process so as to control the characteristic, perfection of grown crystal in a desired ways. In the preliminary stages of investigating a new system, particularly in solution and vapour growth studies, crystal grower must separate crystal with small dimensions, crystallites and polycrystalline masses.

Phase identity and knowledge of phase homogeneity are the first requirements.

As we know X-Ray diffraction is the classic method for phase identification.

In many cases characterization should begin with microscopic examination. A single crystal will often be revealed by habit, some index of refraction, bier fringes or colour to be heterogeneous.

When powder diffraction and optical techniques do not result then in identification, one must turn to chemical analysis, EDAX, TGA, DTA and DSC work is often used.

In the present work, grown crystals were characterized by following methods:-

X-ray diffraction (XRD)

Fourier Transform Infrared Spectroscopy (FT-IR)

Thermal analysis

Thermal Gravimetric Analysis (TGA)

Differential Thermal Analysis (DTA)

iii) Differential Scanning Calorimetry (DSC)

Chemical Analysis

i) Gravimetric

ii) Volumetric

5) Energy Dispersive Analysis by X-ray (EDAX)

6) Scanning Electron Microscope (SEM)

2.2 Analytical methods employed

2.2.1 X-ray Diffraction (XRD)

X-ray diffractometry is based on the scattering of x-ray by crystal. By using this method one can identify the crystal structure of various solid compounds.

According to authors [53-56], X-ray diffraction method is specially used for the investigation of the internal structures; which consist of a regular arrangement of atoms which follow certain laws of symmetry. The internal structure of crystal can be described as follows.

The first stage is the determination of the crystal system and the lattice parameters. Crystal can be classified into seven crystal system based on the shape of the unit cell.

The next stage is the determination of the symmetry. The atomic arrangements in the crystal displays a number of symmetry elements present in the crystal which are

i) Mirror planes,

ii) Rotation axis,

iii) Rotation inversion axis and

iv) Centre of inversions.

The combination of these symmetry elements defines a "point group". The combinations of all symmetry elements are associated with symmetry the space group which can determined.

There are 230 possible space groups for each crystal system, the detailed description of all 230–spaces groups with complete diagrams were reported [57]. It also provides much other useful information which is given in International tables for crystallography.

The X-rays are diffracted by the crystals; the basic law of X-ray diffraction is the Bragg’s law,

2 d sin = n

Where d is the inter planar spacing,

 is the Bragg angle

 is the wavelength of diffracted ray

n is order of the spectrum, n = 1, 2, 3…

Using these,‘d’ values are calculated. Unknown substances are identified by comparing their diffraction pattern with the standard ones to find the one that matches [57].

Various methods are used to study diffraction of X-ray by crystal –

Laue photographic method

Bragg X-ray spectrometer method

Rotating crystal method

Powder method.

In present work powder method have been used to determine unit cell

parameters and its structure.

A narrow beam of X-ray radiation, through a source is incident on crystal powder placed on small circular plate, the diffracted X-ray from the crystal powder is collected on a photographic detector and diffraction pattern is recorded.

The radiation emitted by X-ray tube is characteristic of the target material. For diffraction, three components of the emitted radiations are used. These are K1, K2 and K.

Certain elements if present in the crystal may absorb specific wavelength from the incident radiation therefore a suitable target must be chosen so that the radiations must not be absorbed by the elements. Mostly for K1, K2 and K radiation, copper is used as a target material and nickel as a filter.

Qualitative Analysis

Any one X-ray powder diffraction pattern is characterized by a set of peak position 2θ and a set of relative peak intensities I/Io. But the angular position of the peak depends on the wavelength used, and a more fundamental quantity is the spacing, ‘d’ of the lattice planes forming each diffraction peak. From 1941 to 1969, the American Society for Testing and Materials (ASTM) published these in the form of 3 × 5 inch files cards. This activity has been carried out by the Joint Committee on Powder Diffraction Standards (JCPDS). The substances includes are elements, alloys, inorganic compounds, minerals, organic compounds and organometallic compounds.

Identification of the unknown begins with making its diffraction pattern. The pattern is recorded with a diffractometer. After the pattern of unknown is prepared, the plane spacing ‘d’ corresponding to each line is calculated or obtained from tables with ‘d’ as a function of 2θ and the normalized intensity I/Io are tabulated in increasing order of intensity. The unknown can be identified by the following procedure:

Locate the proper d1 (highest intensity d-value) group in the numerical search manual.

Read down the second column of d values to find the closest match to d2 (next higher intensity d-value).

After the closest match has been found for d1, d2 and d3 compare their relative intensities with the tabulated values.

When good agreement has been found for the lines listed in the search manual, locate the proper data card in the file and compare the‘d’ and I/Io values of all the observed lines with those tabulated. When full agreement is obtained then identification is completed.

In the present work, X-ray diffraction of Strontium Oxalate crystals were recorded at Department of Physical Sciences, North Maharashtra University, Jalgaon, (M. S.) on "Miniflex Rigaku" X-ray diffractometer as shown in fig.2.1.

2.2.2 Fourier Transform Infrared Spectroscopy (FT-IR)

Infrared radiations refer to that part of the electromagnetic spectrum between the visible and microwave regions. These frequencies occur in the range 4000 cm-1 to 400 cm-1. Recently there has been increasing interest in near infrared region 14290 cm-1 to 4000 cm-1 and the far infrared region 700 cm-1 to 200 cm-1.

Infrared spectroscopy has wide application in chemical factories; it gives information about the presence of specific functional groups in the molecule, IR used for chemical identification, quantitative analysis and structural chemical substances as compared to other absorption spectroscopy. IR spectra give information about the structural formula, the absence of gel inclusion in the crystal and the identity of the crystal reported [58-61].

IR spectroscopy involves two kinds of fundamental vibrations for molecules, stretching and bending.

Stretching: In this case the distance between the two atoms increases or decreases but the atoms remain in the same band axis. Stretching vibrations are found to occur in the order of band strength.

Bending: In this case, the position of the atoms changes relative to the original band axis. Bending vibrations generally requires less energy and occur at longer wavelengths.

Band intensities in IR spectrum may be expressed either as Transmittance (T) or Absorbance (A).

Transmittance: It is defined as the ratio of radiant power transmitted by a sample to the radiant power incident on the sample.

Absorbance: It is defined as the logarithm to the base 10, of the reciprocal of the transmittance.

A= log10 (1/ T)

In the present work, FT-IR spectra of Strontium Oxalate were recorded at AISSMS College of Pharmacy, Pune on JASCO instrument model 460 plus as shown in fig. 2.2 (a) and at University Department of Chemical Technology, North Maharashtra University, Jalgaon, (M. S.) on SHIMADZU FT-IR 8400 spectrophotometer as shown in fig. 2.2 (b).

The sample was prepared in the form of pellets. The instrument was first set for 100 % transmittance. The spectrum was scanned by placing the sample pellet in the sample beam in the range 400-4000 cm-1. The key components of Fourier transform system are the source, the interferometer and detector. The interferometer provides a means for the spectrometer to measure all optical frequencies simultaneously. The interferometer modulates the intensity of individual frequencies of radiation before the detector picks up the signal. The product of an interferometer scan is called an interferogram, a plot of intensity versus mirror position. The interferogram is a summation of all the wavelengths emitted by the sample; for all practical purposes it cannot be interpreted in its original form.

Using a mathematical process called Fourier Transformation; the system computer converts the interferogram into a spectrum. The spectrum shows the emission at all the frequencies measured and thus can be used to identify the sample. FT-IR spectrometers record the interaction of IR radiation with experimental samples, measuring the frequencies at which the sample absorbs the radiation and intensities of the absorptions. Determining these frequencies allows identification of sample’s chemical makeup, since chemical functional groups are known to absorb light at specific frequencies. FT-IR spectroscopy is exceptionally suitable for obtaining spectra in energy limited situations and conditions under which conventional dispersive instruments fail to produce the desired spectra. The use of FT-IR in research, analytical and quality control laboratories has brought new and extended capabilities to all users [62, 63].

2.2.3 Thermal Analysis

In thermal analysis, a physical property of a substance is measured as a function of temperature, while the substance is subjected to a control temperature programmed [64]. A complete modern thermal analysis instrument measures temperature of transitions, weight loses in materials, energies of transitions. Dimensional changes, modulus, viscoelastic properties.

This technique includes following methods of analysis:

Thermo Gravimetric Analysis (TGA)

Differential Thermal Analysis (DTA)

Differential Scanning Calorimetry (DSC)

2.2.3.1 Thermo- Gravimetric Analysis (TGA)

TGA provides a quantitative measurement of any weight change associated with a transition. TGA can be directly recorded the loss in weight with time or temperature due to dehydration or decomposition. The basic principle involved in this technique is that the sample is continuously weighed as it is heated to very high temperature. The sample is placed in small crucible attached to a balance. Therefore, the change in weight of the sample due to dissociation, decomposition or dehydration, as the case may be, is directly recorded as a function of temperature. The usual working range of temperature is from ambient to 1000oC.

2.2.3.2 Differential Thermal Analysis (DTA)

The temperature of a sample and a thermal inert reference material are measured as a function of temperature (Usually sample temperature). Any transition, which the sample undergoes, will result in liberation or absorption of energy by the sample with a corresponding deviation of its temperature from that of the reference. The differential temperature (T) versus the programmed temperature (T) at which the whole system is being changed tells the analyst the temperature of transitions and whether the transition is exothermic or endothermic.

2.2.3.3 Differential Scanning Calorimetry (DSC)

In addition to the determination of heats and temperatures of physical and

chemical transitions, DSC is also useful in finding out calorimetric purity and second order transitions. In this technique, aluminium pan is used to accommodate the weighted sample powder. The sample is heated at the rate of 5oC/min. up to 600oC.Here change in energy (in mW) is recorded as a function of temperature, which provides either exothermic or endothermic peaks at the time of dehydration or decomposition reaction. Probably the commonest use of DSC curve is in fingerprinting in which simple or complex materials can be compared for identification using measurements of peak positions, sizes or shapes.

These methods are widely used in chemical analysis and for obtaining thermodynamic and kinetic data. Numbers of reviews are available on application of thermodynamic methods [65-68].

In the present work TGA, DTA and DSC of grown crystal was carried out at and National Chemical Laboratory, Pune, (M. S.). For TGA, DTA study Diamond TG/ DTA Perkin Elmer instrument was used as shown in fig. 2.3(a) and 2.3(b) and DSC on Perkin Elmer instrument, Pyris 6 DSC as shown in fig. 2.3(c). Also TGA and DSC of grown crystal was carried out at University Department of Chemical Technology, North Maharashtra University, Jalgaon, (M. S.) on SHIMADZU DSC 600, Japan as shown in fig. 2.3(d).

2.2.4 Chemical Analysis

In a modern industrialized society, most manufacturing industries rely upon both qualitative and quantitative chemical analysis to ensure that the raw materials used to meet certain specification; they also check the quality of the product. For this purpose analytical methods must be employed.

For the quantitative analysis of the substance gravimetric and volumetric methods are useful [69, 70].

a) In gravimetric analysis, the substance to be determined is converted into an insoluble precipitate, which is collected and weighted.

b) In volumetric analysis, the substance to be determined is allowed to react with an appropriate reagent added as a standard solution and the volume of solution needed for complete reaction is determined.

In the present work chemical analysis a) and b) of grown crystal was carried out at Department of Chemistry, Shri Shivaji Vidya Prasarak Santha’s Bapusaheb Shivajirao Deore College of Engineering, Dhule, (M. S.).

2.2.5 Energy Dispersive Analysis by X-ray (EDAX)

The percentage of cations and anions present in the crystal was estimated by EDAX. This technique used for identify the elemental composition of the specimen. The principle underlying EDAX is the same as that of electron probes microanalysis. During EDAX analysis the specimen bombarded with an electron beam inside the scanning electron microscope. The innermost and outermost shell energy identified using emitting X-rays and the energy of these X-rays is strictly related to the atomic number of the elements excited. Therefore their detection forms the basis of elemental analysis in the electron microscope and EDAX spectrum normally displays peaks. The higher peak in a spectrum is more concentrated element in the specimen [55, 65].

In EDAX, X-rays are absorbed in semiconductor atom creating a photon (photoelectron) leaving the atom in ionized state, which causes emission of Auger electron or X-rays. It is again reabsorbed with emission of another photoelectron and subsequently other Auger electron. The energetic electron scattered inelastically and charge carriers drifts in a time of 100 ns. The collected charge gives the value, which is proportional to the energy of original photo.

The EDAX consists of several components such as detector, pre and main amplifier, wave shaping circuits, monitors, multichannel analyzer or computer X-ray analyzer for spectrum analysis, display and manipulations. The resolution and count rate of EDAX system depends upon measurement of time for each pulse. The maximum input count rate is greater than or equal to 3 KHz. The data is qualitatively analyzed. In this analysis the first step is the extraction of characteristics X-ray from EDAX spectrum. Ideally a flat mechanically polished surface with topography reduced below 50 nm is required.

In the present work EDAX of grown crystal was carried out at Sophisticated Instrumentation Centre for Applied Research and Testing (SICART), Sardar Patel Centre for Science and Technology, Aanand, Gujarat, India.

2.2.6 Scanning Electron Microscope (SEM)

Scanning electron microscope is one of the most versatile instruments available for the examination and analysis of the micro structural characteristics of solid objects. The primary reason for the scanning electron’s usefulness is the high resolution that can be obtained when bulk objects are examined; values as good as 1 nm (10 Ao) are now quoted for commercial instruments. This resolution approaches the bonding distance of atoms and results in photographs with useful detail at magnification in excess of 100,000 X. Another important feature of the scanning electron microscope is the three dimensional appearance of specimen image. This three dimensional appearance is a direct result of the large depth of focus [71]. The area to be examined is irradiated with a finely focused electron beam, which may be static or swept in a raster across the surface of the specimen. The types of signals, which are produced when the focused electron beam impinges on a specimen surface include,

Low energy secondary electrons, which display the surface topography of the specimen.

High energy back scattered electrons, which produces images of chemical inhomogenities within the specimen.

Characteristics X- rays.

Photons of various energies.

They are obtained from specific emission volumes within the specimen and are used to measure many characteristics of the specimen like composition, surface topography, crystallography, magnetic or electric character, etc. [72].

The schematic diagram of scanning electron microscope is shown in fig. 2.4. A very fine beam of electrons obtained from the source (electron gun) A after passing through the lens system (magnetic coils) falls on the specimen B which is placed on cellulose thin film held on sample holder. The beam of electrons falling on the specimen is scanned over the entire specimen by scanning coils incorporated with third condenser lens in the lens system. Low energy secondary electrons are emitted at the point of impact from the surface of the specimen. The secondary electrons are then collected by a collector C, which is essentially a scintillator or photomultiplier tube system. The scintillator crystal, which is at positive potential, converts each electron striking it into a tiny flash of light, which generates a small current in the photomultiplier tube. The signal from the detector is fed to the electronic circuit, which controls the display cathode ray tube so that the resulting image is an exact representation of the surface of the specimen. An electron probe spot is scanned over the surface of the specimen. The secondary electrons emitted by the specimen produce a flash of light, which enters the photomultiplier tube. The output of the tube is amplified and made to modulate the brightness of bright spot, which moves synchronously with the probe spot inside a cathode ray display tube. A direct picture of the surface of specimen is thus produced on the screen.

In the present work, SEM studies are done by using latest computerized scanning electron microscope Quanta 200 3D at National Chemical Laboratory, Pune, (M. S.).