52

This chapter aims to describe the experimental techniques that are used to

characterize the present investigation. A brief description of the glass

preparation procedure is given in general. The detailed description with referring

to a particular system is given in the respective chapters. A general sketch of an

EPR spectrometer and the detailed description of its components and working

principle are given. The description of a JASCO V-670 UV-VIS-NIR

spectrophotometer, FT-IR and XRD is also included in the last part of this

chapter.

2.1 Glass preparation by melt-quenching method

The oldest established method of producing an amorphous solid is to cool

the molten form of the material quickly. The distinguishing feature of the melt-

quenching process of producing amorphous material is that the amorphous solid

is formed by the continuous hardening (i.e. increase in viscosity) of the melt. In

the present study all the glass systems were prepared by the melt-quenching

technique and the method of preparation is explained as follows.

The starting materials used for the preparation of the present glass

systems are of high purity. The chemical compositions of the different dopant

concentrations are tabulated.

10 SrO:(30-x) ZnO:60 B2O3: (x) T.M

(x) concentrations

V2O5 → 0 0.1 0.3 0.5 0.7 0.9

Cr2O3 → 0 0.1 0.3 0.5 0.7 0.9

MnO → 0 0.1 0.3 0.5 0.7 0.9

CuO → 0 0.1 0.3 0.5 0.7 ----

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53

All the chemicals were weighed accurately using an electrical balance,

grounded to fine powder and mixed thoroughly. The batches were melted in

silica crucibles by placing them in an electrical furnace at required temperature

(1150 0C) and a photograph showing the set of an electrical furnace is shown in

Fig. 2.1. The melts were then poured on a polished brass plate and pressed

quickly with another plate. The glasses thus obtained were transparent. The

colour of the glass samples depends upon the nature of the dopants.

Fig.2.1: Photograph of an electrical furnace

2.2 EPR spectrometer

When a material containing electron magnetic dipole is placed in a static

magnetic field and subjected to electromagnetic radiation, absorption attributable

to magnetic dipole transitions occurs at one or more characteristic frequencies in

the microwave region of the electromagnetic spectrum.

54

There are several commercially available EPR spectrometers operating at

different frequencies produced by different companies. These spectrometers

operate at a fixed microwave frequency and scan the EPR spectra by linear

variation of the magnetic field. The majority of EPR spectrometers are designed

for a frequency around 9.205 GHz (X - band).

The essential features of any EPR spectrometer are [1],

(i) A source of microwave radiation of constant frequency and variable

amplitude.

(ii) A means of applying the microwave power to the paramagnetic sample.

(iii) A means of measuring the power absorbed from the microwave field and

(iv) A homogeneous but variable magnetic field.

In the present work, Bruker EMX spectrometer operating at X-band

(9.205 GHz) microwave frequency was used. The block diagram of the EPR

spectrometer is shown in Fig. 2.2. The construction of Bruker EMX EPR

spectrometer is shown in Fig.2.3.

Fig.2.2: The block diagram of Electron Paramagnetic Resonance spectrometer.

55

Fig.2.3: The construction of Bruker EMX EPR spectrometer.

The main parts of the EPR spectrometer can be explained as follows.

(i) Source

Most of the EPR spectrometers are operated by the Klystron oscillator

and some are operated by Gunn diode oscillator. In Bruker spectrometer, the

microwave generator is Gunn diode oscillator, which is operated in the X-band

region at power variable from 0.1 to 200 mW. Power may be withdrawn from

the oscillator through a waveguide by a loop of wires, which couples with the

oscillating magnetic field and sets up a corresponding field in the waveguide. A

waveguide consists of hollow rectangular (copper or brass) tubing, 2.210 cm,

with silver or gold plating inside to produce a highly conducting, flat surface.

Reflection of microwave power back into the oscillator is prevented by an

isolator � a strip of ferrite material, which passes microwave in one direction

56

only. The oscillation frequency in mechanically varied by varying the cavity

resonant frequency. The automatic frequency control (AFC) can be used to

measure the stability of the frequency so that the oscillation frequency of Gunn

diode oscillator matches the resonance frequency of the sample cavity. The

microwaves that are generated are allowed to pass through the reference line and

signal line and divided by means of the directional coupler. The signal line is

attenuated to the required power and finally enters the cavity resonator. When

the cavity resonator coupling is adjusted for critical coupling, there are no

reflected waves from the cavity resonator. When EPR is excited, microwaves

from the cavity resonator are reflected and enter the balance mixer that made up

from the magic T and crystal mount. The waves are then detected and amplified

by the pre-amplifier. In this case, the reference line is adjusted to the same phase

as the signal by the phase shifter. Then it passes through the delay line and the

optimum bias power is applied to the detection diode.

(ii) Sample cavity

The sample contained in a cylindrical quartz tube is held in a cavity

between the poles of the magnet. A standing wave is set up in the reflection

cavity, the standing wave is composed of both magnetic and electric fields at

right angles to each other. To minimize any influence of a high dielectric

constant when such material is the sample, the sample tube is located in the

cavity in a position of maximum rf magnetic field.

Bruker EMX spectrometer is equipped with TE011 cylindrical cavity

resonator with an ultraviolet ray irradiation aperture, a cooling device, a 100 kHz

modulation coil, bayonet connectors for connecting the variable temperature

attachments, a nitrogen gas inlet port and so on.

57

(iii) Microwave bridge

The microwave system to be operated as a balanced bridge with all the

advantages of null methods in electrical circuits. The microwave bridge will not

allow microwave power to pass in a straight line from one arm to the opposite

arm.

A set of coils mounted on the walls of the sample cavity and fed by a

0.1MHz sweep generator provides modulation of the dc magnetic field at the

sample position.

(iv) Magnets

The homogeneous magnetic field is applied to the sample, which is

placed in between the two magnetic poles. For Bruker spectrometer, the

magnetic field varies from 0 to 6000 gauss. The magnetic field can be swept in

different rates from 360mm/0.5min to 360mm/128min. The Hall element

supplies AC voltage to the magnetic field control unit that is proportional to the

field so that linear field sweep can be carried out. Excitation power supply

supplies a highly stabilized excitation current to the electromagnet.

(v) Detectors

All modern spectrometers use semiconducting crystal diode rectifiers as

the basis of the detecting system. This converts microwave power into direct

current output.

58

(vi) Oscilloscope and Pen recorder

After detection of the signal, the signal is amplified. But the amplified

signal contains a lot of noise. The reduction of noise is achieved by sending the

signal through phase sensitive detector.

Finally, the signal from phase sensitive detector and sweep unit is

recorded by the DYT type recorder, which records the EPR signal on a chart of

width 250 360 mm. The oscilloscope screen (133 mm) provides a facility for

mode check before recording the sample.

2.3 Measurement of EPR

The sample crystal is mounted at the end of the quartz sample holder with

quickfix and is placed in the sample cavity with the help of goniometer. If the

sample is in the powder form, 100 mg-powder sample is taken in the quartz tube.

The sample is subjected to microwave magnetic field of constant

frequency, which is perpendicular to H. The magnitude of H is changed by

varying the electromagnet excitation current and when the resonance condition is

fulfilled, part of microwave energy is absorbed into the sample, as a result, the

cavity resonator Q value changes. This Q variation is detected, amplified and

recorded.

When the magnetic field is varied while the frequency is kept constant, an

absorption signal is observed. In addition, an alternating magnetic field having

the same direction as H and amplitude smaller than the absorption signal width

is applied. If now, H is varied, the detector output at each point on the absorption

59

signal will form a sinusoidal wave having the same period as the gradient of the

absorption line. This sinusoidal wave is amplified by a selective amplifier.

The EPR spectra can be recorded either by first differential curve or

second differential curve of the absorption signal. The instrument has a very

high sensitivity (21010 - 31011 spins/gauss), a high resolution (1105) and

frequency stability of 110-6. The high Q cavity and hot carrier diode enables to

accomplish high sensitivity in EPR measurements.

An X-Y recorder is used to record the spectrum either by choosing the

first or second differential curve of absorption signal. The field range can be

chosen in different steps and the central field on the chart can be fixed at any

desired value. The temperature dependence of EPR spectra was recorded

between liquid nitrogen and room temperature using JES - VCT - 2AX variable

temperature accessory and liquid nitrogen dewar insert. Using this arrangement,

EPR spectra can be recorded at any desired temperature between 103 and 443K.

A temperature stability of 1K was easily obtained by waiting for half an hour

before recording each spectrum. EPR spectra at different temperatures,

particularly at low temperatures are useful because they provide a great deal of

information about the spin system, apart from the temperature dependence of the

g-factor and hyperfine interaction. For low temperature measurements, dry

nitrogen gas is flushed through the cavity to avoid condensation.

The polycrystalline Diphenyl Picryl Hydrazyl (DPPH) with a �g� value

2.0036 0.0002 is used as a standard field marker. In the EPR spectrometer, in

practice, adjusting frequency () is more difficult than varying the magnetic field

H. Hence the frequency () is kept constant and the magnetic field H is varied.

60

2.3 Optical absorption studies

From the absorption spectrum, one can get the information about the

electric dipole and magnetic dipole transitions. These transitions can be assigned

by observed band positions. By assigning the crystalfield transitions, crystalfield

parameters can be calculated. The calculations of crystalfield parameters are

different for different impurity ions.

Optical absorption spectra of samples studied in the present work were

recorded using double beam JASCO UV/VIS/NIR V-670 spectrophotometer.

The block diagram of the JASCO UV/VIS/NIR V-670 spectrophotometer [2] is

shown in Fig. 2.4 and a photograph showing the set of JASCO UV/VIS/NIR

V-670 spectrophotometer is shown in Fig. 2.5.

Fig.2.4: The optical spectrophotometer (Model: JASCO V670).

61

Fig. 2.5: Photograph of JASCO V670 UV-VIS-NIR spectrophotometer

The above model measures the absorption spectra at a wavelength of 190

to 2500 nm at room temperature. A deuterium discharge tube (190 to 350 nm) is

used for use in UV region and a tungsten iodine lamp (340 to 2500 nm) is used

for use in the VIS/NIR region as light source. The light from the light source is

converged and enters the monochromator. It is dispered by the grating in the

monochromator and the light passing through the exit slit is monochromated.

This light is split into two light paths by a sector mirror, one indicated on the

sample to be measured and the other on the reference sample such as solvent or

other. The light that has passed through the sample or reference sample is

incident on the photomultiplier tube or PbS photoconducting cell and converted

into an electrical signal and after being synchronously rectified, is converted into

a digital form and enters the personal computer. The signal processed by the

personal computer is displayed on the output device as spectrum. Light source

changeover, wavelength drive, slit drive, filter drive, etc., are controlled by the

personal computer.

62

In the present work, the optical absorption spectra of the samples were

recorded in absorbance mode.

2.5 Infrared spectrophotometer

The Infrared spectra of the samples studied in the present work were

recorded on JASCO FT-IR 5300 Spectrophotometer at room temperature.

The infrared region is divided into three segments based on instrumental

capabilities. The optical arrangement for a filter-grating spectrophotometer [3] is

shown in Fig.2.6. Different radiation sources, optical systems and detectors are

needed for different regions. The standard infrared spectrophotometer is a filter-

grating or prism-grating instrument covering the range from 4000 to 650 cm-1

(2.5to 15.4m). Grating instruments offer high resolution that permits separation

of closely spaced absorption bands, accurate measurements of band positions

and intensities and high scanning speeds for a given resolution and noise level.

The block diagram of the infrared spectrophotometer is shown in Fig.2.7.

Fig.2.6: The optical arrangement of infrared spectrophotometer.

63

Fig.2.7: The block diagram of the infrared spectrophotometer.

(i) Radiation sources

In the region beyond 5000 cm-1(2.0 m), blackbody sources without

envelopes commonly are used. The same spectral characteristics cited for the

tungsten incandescent lamp apply to these as well. Unfortunately, the emission

maximum lies in the near-infrared.

A hotter and therefore brighter, source is the Nernst glower, which has an

operating temperature as high as 1500C. Nernst glowers are constructed from a

fused mixture of oxides of zirconium, yttrium and thorium, molded in the form

of hollow rods are cemented to short ceramic tubes to facilitate mounting, short

platinum leads provide power connections.

The Glowbar, a rod of silicon carbide 6-8 mm in diameter and 50 mm in

length, possesses characteristics intermediate between heated wire coils and

Nernst glower. It is self-starting and has an operating temperature near 1300C.

64

(ii) Detectors

At the short-wavelength end, below about 1.2 m, the preferred detection

methods are the same as those used for visible and ultraviolet radiation. The

detectors used at longer wavelength can be classified into two groups.

(a) thermal detectors (b) photon detectors.

(a) Thermal detectors

The active element in any thermal detectors is as small as possible to

maximize its temperature change for any level of infrared radiant energy.

Thermal detectors are usable over a wide range of wavelengths, which includes

both visible and infrared radiation and they operate at room temperature. Their

main disadvantages are slow response time (milliseconds) and lower sensitivity

relative to other types of detectors.

(b) Photon detectors

The more sensitive infrared detectors rely on a quantum interaction

between the incident photons and a semiconductor- the result producing

electrons and holes. This is the internal photoeffect. A sufficiently energetic

photon that strikes an electron in the detectors can raise that electron from a

nonconducting state into a conducting state. The excitation of electrons requires

a definite minimum energy in the photon. The detectors thus exhibit a sharp

cutoff towards the far-infrared.

Most infrared spectrophotometers are double-beam instruments in which

two equivalent beams of radiant energy are taken from the source. By means of a

combined rotating mirror and light interrupter, the source is flicked alternately

65

between the reference and sample paths. In the optical-null system, the detector

responds only when the intensity of the two beams is unequal. Any imbalance is

corrected for by a light attenuator (an optical wedge or shutter comb) moving in

or out of the reference beam to restore balance. The recording pen is coupled to

the light attenuator.

Fourier transform IR spectrophotometer uses Michelson interferometer to

produce an interferogram. The interferogram is related to the IR spectrum by a

mathematical operation known as Fourier Transformation [4] as shown in

Fig.2.8.

Fig.2.8: Schematic diagram of Fourier Transform Infrared spectrophotometer.

Instead of using a monochromator, the infrared radiation, after passage

through a sample, can be analyzed by means of a scanning Michelson

interferometer. This consists of a moving mirror, a fixed mirror and a beam

splitter, radiation from the beam splitter, half of the beam passing to mirror and

half-reflected to the mirror. After reflection, the two beams recombine at the

beam splitter and for any particular wavelength, constructively or destructively

66

interfere depending on the difference in optical paths between the two atoms of

the interferometer.

Both the sampling rate and mirror velocity is controlled by a reference

signal from detector produced by modulation of the beam from the helium-neon

laser. The resulting signal is known as an interferogram and contains all the

information required to reconstruct the spectrum via a mathematical process

known as Fourier Transformation.

The Advantages of FT-IR over Dispersive IR is

* Higher signal to noise ratio.

* Greater throughput energy.

* For a given S/N ratio, FT instrument takes much shorter time.

* Time taken for producing quality spectrum after averaging several

scans is 15-30 seconds without hard copy.

* It has the wave number accuracy of 0.01 cm-1.

The XRD spectrum has been recorded at the sophisticated laboratories at

University of Hyderabad (UOH), Hyderabad-INDIA.

67

2.6. References

[1] Manual of Bruker Electron Spin Resonance instrument, Bruker Ltd.,

Germany, 1999

[2] Manual of JASCO UV-VIS-NIR V-670 Optical Spectrophotometer,

Spectroscopic Instruments Ltd., Japan, 1993

[3] H.H. Willard , L.L.Jr. Merritt, J.A. Dean , F.A. Settle., Instrumental

Methods of analysis, 6th edition, CBS Publishers and Distributors, Delhi.

1986

[4] R. Srinivasan, In Workshop on Materials and Characterization, Lecture

notes, CECRI, Karaikudi, Tamilnadu, India, Ed. By M.J. Chockalingam,

M. Jayachandran , K.R. Murali, 1998, p.45