Indian Institute of Science Education and Research, Kolkata 16th Aug. – 13th Sept. 2010

1 | Experimental Report submitted by Harsh Purwar

Raman Spectroscopy of Lead Calcium Titanate sample under High Pressure

Harsh Purwar (07MS – 76) 4th Year, Integrated M.S. (Physics)

Indian Institute of Science Education and Research, Kolkata

Abstract Raman spectra were exploited to study some of the physical properties of Lead Calcium Titanate (PbCaTiO3) under high pressures (of the order of GPa) with the help of a Diamond Anvil Cell, Metal Gasket and Pressure Transmitting Medium (Ethanol methanol mixture). Ruby crystals were used for pressure calibration. Results were deduced from the Raman shift of some of the prominent peaks at various pressures.

Theoretical Background DIAMOND ANVIL CELL:

A diamond anvil cell (DAC) is a hand – held device used in scientific experiments. It allows compressing a small (sub-millimeter sized) piece of material to extreme pressures, which can exceed 3,00,000 atmospheres (30 GPa). The device has been used to recreate the pressure existing deep inside planets, creating materials and phases not observed under normal conditions. Notable examples include the non – molecular ice, polymeric nitrogen and MgSiO3 perovskite, thought to be the major component of the Earth's mantle. A DAC consists of two opposing diamonds with a sample compressed between the culets. Pressure may be monitored using a reference material whose behavior under pressure is known. Common pressure standards include ruby fluorescence, and various structurally simple metals, such as copper or platinum. The uniaxial pressure supplied by the DAC may be transformed into uniform hydrostatic pressure using a pressure transmitting medium, such as argon, xenon, hydrogen, helium, paraffin oil or a mixture of methanol and ethanol. The pressure-transmitting medium is enclosed by a gasket and the two diamond anvils. The sample can be viewed through the diamonds and illuminated by X-rays and visible light. In this way, X-ray diffraction and fluorescence; optical absorption and photoluminescence; Mossbauer, Raman and Brillouin scattering; positron annihilation and other signals can be measured from materials under high pressure. Magnetic and microwave field can be applied externally to the cell allowing nuclear magnetic resonance, electron paramagnetic resonance and other magnetic measurements. Attaching electrodes to the sample allows electrical and magneto-electrical measurements as well as heating up the sample to a few thousand degrees. Much higher temperatures (up to 7000 K) can be achieved with laser-induced heating and cooling down to milli-K has been demonstrated.

PRESSURE CALIBRATION TECHNIQUE (RUBY FLUORESCENCE) Ruby fluorescence technique is indirect pressure measurement technique. A small ( ) crystal of ruby has to be introduced with sample. The ruby fluorescence has been excited with laser which gives sharp lines at . The line shifts with pressure almost linearly which has been calibrated with standard substances so that we can get a good pressure calibration.

Indian Institute of Science Education and Research, Kolkata 16th Aug. – 13th Sept. 2010

2 | Experimental Report submitted by Harsh Purwar

RAMAN SPECTROSCOPY: Raman spectroscopy (named after Dr. C.V. Raman) is a spectroscopic technique used to study vibrational, rotational, and other low – frequency modes in a system. It relies on inelastic scattering of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system. Typically, a sample is illuminated with a laser beam. Light from the illuminated spot is collected with a lens and sent through a mono-chromator. Wavelengths close to the laser line, due to elastic Rayleigh scattering, are filtered out while the rest of the collected light is dispersed onto a detector. Modern instrumentation employs notch or edge filters for laser rejection and spectrographs (either axial transmissive (AT), Czerny-Turner (CT) monochromator) or FT (Fourier transform spectroscopy based), and CCD detectors. Basic Theory: The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud and the bonds of that molecule. For the spontaneous Raman effect, which is a form of scattering, a photon excites the molecule from the ground state to a virtual energy state. When the molecule relaxes it emits a photon and it returns to a different rotational or vibrational state. The difference in energy between the original state and this new state leads to a shift in the emitted photon's frequency away from the excitation wavelength. The Raman effect, which is a light scattering phenomenon, should not be confused with absorption (as with fluorescence) where the molecule is excited to a discrete (not virtual) energy level.

Figure 1: Energy level diagram showing the states involved in Raman signal.

The line thickness is roughly proportional to the signal strength from the different transitions.

If the final vibrational state of the molecule is more energetic than the initial state, then the emitted photon will be shifted to a lower frequency in order for the total energy of the system to remain balanced. This shift in frequency is designated as a Stokes shift. If the final vibrational state is less energetic than the initial state, then the emitted photon will be shifted to a higher frequency, and this is designated as an Anti-Stokes shift. As mentioned earlier Raman scattering is an example of inelastic scattering because of the energy transfer between the photons and the molecules during their interaction. A change in the molecular polarization potential — or amount of deformation of the electron cloud — with respect to the vibrational coordinate is required for a molecule to exhibit a Raman effect.

Indian Institute of Science Education and Research, Kolkata 16th Aug. – 13th Sept. 2010

3 | Experimental Report submitted by Harsh Purwar

The amount of the polarizability change will determine the Raman scattering intensity. The pattern of shifted frequencies is determined by the rotational and vibrational states of the sample. Raman Spectra: Raman spectra are typically expressed in wave numbers, which have units of inverse length. In order to convert between spectral wavelength and wave numbers of shift in the Raman spectrum, the following formula may be used:

(

)

where is the Raman shift expressed in wave number, is the excitation wavelength, and is the Raman spectrum wavelength. Most commonly, the unit chosen for expressing wave number in Raman spectrum is inverse centimeters ( ). Since wavelength is often expressed in units of nanometers (nm), the formula above can scale for this units conversion explicitly, giving

( ) (

( )

( ))

( )

( )

Experimental Setup The experimental setup mainly consists of a Diamond Anvil Cell (DAC) as shown in Figure 2.

Figure 2: Arrangement of Diamonds and Metal Gasket in DAC.

Following protocol was followed in order to build up the main experimental setup.

The diamond anvil cell with the two 16 faced diamonds fixed on the two parts of the DAC were ready.

The visible surface of both the diamonds was cleaned under the magnifying lens (microscope) using acetone and fibreless tissue paper with the help of forceps.

The metal gasket was also wiped off using acetone and tissue paper.

It was then placed on the visible surface of the lower diamond. Putty was used to temporarily fix the gasket on its surface.

The cell was then closed and a small pressure was applied by tightening the screws so as to make an impression of the diamond on the two surfaces of the gasket. The cell was then opened and the gasket was taken out.

A small hole of about (B in Figure 4) was drilled at the approximate center of the impression made by the diamond faces on the metal gasket using the clean needles.

Indian Institute of Science Education and Research, Kolkata 16th Aug. – 13th Sept. 2010

4 | Experimental Report submitted by Harsh Purwar

Figure 3: Metal Gasket with loaded sample.

The scrap (left over) after drilling was cleaned using a wooden toothpick and the gasket was heated under flame to burn the fibers of the toothpick.

The gasket was then replaced on the lower diamond after cleaning the diamond surface as well as the gasket using tissue and acetone. Again putty was used for fixation.

Small amount of the sample was filled inside the hole using a needle under the microscope and traces of the ruby crystal were also added for pressure calibration.

Finally the pressure transmitting medium i.e. methanol and ethanol mixture in 4:1 ratio was filled inside the hole using a syringe.

The DAC was then closed by tightening the two screws on the upper half of the cell carefully moving them equally simultaneously.

Measurement Following protocol was followed for pressure calibration and taking Raman spectra of the sample. Wavelength of LASER used: 488 nm at mW power.

The DAC with sample, Ruby crystals and PTM was aligned under the microscope to get an approximate idea of the sample inside the gasket hole.

For pressure calibration LASER beam was focused on the ruby crystal by seeing the spectra for a peak close to .

An appropriate filter, grating (600 g/mm) was chosen for quick pressure measurement. After fixing the position 1800 g/mm grating was selected and the spectra was re-recorded.

After ruby spectrum was recorded, Raman spectrum was taken for each pressure. The position of the DAC may be changed if required to focus beam on to the sample properly and not on ruby crystals.

Indian Institute of Science Education and Research, Kolkata 16th Aug. – 13th Sept. 2010

5 | Experimental Report submitted by Harsh Purwar

Figure 4: DAC and Metal Gasket.

Results / Observations Following are the Raman spectra, intensity versus shift in wavenumber at various pressures.

@ 0 GPa

Indian Institute of Science Education and Research, Kolkata 16th Aug. – 13th Sept. 2010

6 | Experimental Report submitted by Harsh Purwar

@ 1.65 GPa

@ 6.95 GPa

Indian Institute of Science Education and Research, Kolkata 16th Aug. – 13th Sept. 2010

7 | Experimental Report submitted by Harsh Purwar

@ 11.19 GPa

In Lead Calcium Titanate sample as can be observed in the graphs above, the first peak around 70 – 90 cm-1 does not diminish or broadens as we move from low pressure to high pressures. Rest all the peaks almost vanish at high pressures. This is to say that at high pressure we are observing an average effect. On comparing the results with the theory it is suggested that the first peak corresponds to the Lead – Lead (Pb-Pb) vibrations of E(TO)1 vibrational mode. While other peaks correspond to the TiO-

3 octahedral units.

Conclusion From this data we can infer that TiO3 octahedral units have stronger bonds so their peaks have higher values. When the crystal is at normal pressure it is almost tetragonal and so we get many peaks. As we increase the pressure tetragonal structure is converted to cubic structure, so our large peaked intensity goes down. Low peaks remain almost unchanged which are due to Pb – Pb vibrations. In our crystal lattice we have Ca also (act like Pb), which gives rise to long range effects. This can be interpreted as if the crystal now has two tetragonal structures for Pb – Pb vibrations and it corresponds to the peaks at around 81.32 cm-1 which are almost unchanged at high pressures.

Sources of Error The two faces of the diamonds may not be exactly parallel due to which there would be a pressure

gradient instead of homogeneous pressure acting on the sample.

Make sure that the hole drilled in the gasket is almost at the center; otherwise the gasket will not stably sit on the diamond surface and might even break in between the experiment due to non-uniform pressure.

The diamond surfaces, gasket etc. must be cleaned thoroughly so as there is no chance of the contamination of the sample finally resulting into uninterruptable results.

The ruby spectra for calibration and Raman spectra should be measured very carefully as there are a lot of fluctuations as the sample is even slightly displaced.

Indian Institute of Science Education and Research, Kolkata 16th Aug. – 13th Sept. 2010

8 | Experimental Report submitted by Harsh Purwar

Works Cited 1. Diamond anvil cell and high-pressure physical investigations. Jayaraman, A. 1, s.l. : The American Physical Society, Jan 1983, Reviews of Modern Physics, Vol. 55, pp. 65-108. 2. High Pressure Calibration. Decker, D. L., et al., et al. 3, Utah : s.n., 1972, Journal of Physical Chemical Reference Data, Vol. 1, pp. 1-79. 3. Lattice Dynamics of crystal with tetragonal BaTiO3 structure. Freire, J. D. and Katiyar, R. S. 4, s.l. : The American Physical Society, Feb 1988, Physical Review B, Vol. 37, pp. 2074-2085. 4. Ultrahigh pressures. Jayaraman, A. s.l. : American Institute of Physics, Feb 1986, pp. 1013-1031. 5. Raman Spectroscopy. wikipedia.org. [Online] http://en.wikipedia.org/wiki/Raman_spectroscopy. 6. Diamond Anvil Cell. wikipedia.org. [Online] http://en.wikipedia.org/wiki/Diamond_anvil_cell.