High pressure raman scattering studies on Mid-chain hydrocarbons, perfluorocarbons and temperature dependent brillouin scattering studies on ABO4 tungstates and AgGas2

Abstract

The present thesis uses two optical probes, namely, Raman and Brillouin spectroscopy to study the physical properties of material. In order to understand the systeM.S. we have used the two thermodynamic quantities, pressure and temperature. There is a lot of interest in the study of mid chain alkanes and perfluroalkanes in their condensed phase. This information is readily usable when one makes hybrid molecules out of these as well as when one tries to understand the interactions of these molecules in various environment. Tungstate with scheelite structure has been the main focus of recent spectroscopic studies at low temperature and high pressure, because of the identification of several phase transitions in these compounds. It might help to determine the factors involved in structure stability. Further the influence of cation on the crystal structure can help in modeling the effect of impurities on the physical properties. The I-III-VI2 Chalcopyrite Structure compounds have quite similar physical properties as the binary II-VI analogs with the cubic (zinc blende) structure. The temperature induced changes in optical properties of these materials will be useful for designing optical devices with these compounds. Chapter 1 describes the relevant introduction for the present work. Chapter 2 describes the designing of low cost Raman spectrometers for high pressure studies and experimental details about low temperature Brillouin spectroscopy. Chapter 3 describes the pressure induced phase transition of mid chain alkanes in the condensed phase. These alkanes undergo order-disorder transition accompanied with dampening of methyl end group. Compared to n-heptane and n-hexane, the behavior of n-pentane with pressure is different due to its small size and crystal structure. The solid-solid transition pressure is inversely proportional to the chain-length. Chapter 4 describes the observation of liquid-solid transition as well as solid-solid transitions in the condensed phase of mid-chain perfluorocarbons. Both perfluorohexane and perfluoroheptane undergoes two types of solid-solid transition 1.conformational disorder transition due to reversal of the helical pitch and 2.disorder-order transition due to all-trans conformation. Chapter 5 describes the temperature dependent structural transition of PbWO4 and BaWO4. The sound velocity and elastic behavior of these compounds will be useful in designing acousto- optic devices. Chapter 6 describes the temperature induced elastic moduli behavior of AgGaS2. The changes in elastic behavior have been coupled with change in refractive index of the material. Beyond particular temperature, there is a transition from semi-conductor to semi-insulator.