| dc.description.abstract | In conclusion, we have observed pressure induced polymerization in the case of C but not in C . This calls for detailed theoretical analysis, incorporating the changes in electronic structure with the application of pressure to explore the formation of polymerized structures and analyse their stability, as has been carried out for C [6]. It would also be interesting to look for polymerization in alkali doped C .
In conclusion, we have observed a drastic change in the appearance of the A_g(2) mode and a steep increase in the relative intensities of the H_g modes compared with solid C . We have also observed that at low temperatures, the relative intensities of the 1457 and 1465 cm ¹ modes become completely reversed compared with room temperature. The observed increase in linewidths for the various modes is much smaller than the linewidths observed in A C as well as the calculated linewidths based on electron–phonon coupling. This difference may arise from the structural anisotropy in the band structure of C –TDAE. Our experiments suggest a need for careful X ray and neutron diffraction measurements at low temperatures and theoretical calculations of the Raman intensities.
There are similarities and significant differences between the pressure dependence of the line shape parameters in the coupled phonon model and the corresponding temperature dependence [6] across the tetragonal to cubic transition. In both the temperature and pressure experiments, increases as the transition is approached. In temperature experiments, shows a slight increase and shows a slight decrease monotonically with increasing temperature, whereas the pressure dependence is non monotonic. These differences may arise because the effects of pressure occur solely through volume changes, whereas temperature manifests through the combined effect of volume change and phonon population. In this sense, pressure has advantages over temperature in studying the effect of the ferroelectric to paraelectric phase transition on the coupling of phonon states.
It may be noted that the Raman spectra in the pressure induced paraelectric cubic phase (Fig. 7.2) are very similar to the spectra of the temperature induced cubic phase at ambient pressure shown in Ref. 5 at 134 °C. Hence, the conclusions about the existence of a soft mode, in relation to treating the paraelectric phase as a disordered version of the tetragonal phase, should be similar in both cases-temperature or pressure induced paraelectric phases. In temperature induced phase transitions, different experiments such as neutron scattering, infrared and hyper Raman studies point to the existence of a heavily overdamped soft mode of symmetry F u(TO), although its contribution to the Curie–Weiss law of the static dielectric constant is still controversial.
In conclusion, the coupled phonon model has been quantitatively used to determine the pressure dependence of the A (TO) phonon mode frequencies, linewidths, transition amplitudes and coupling parameters. These parameters show interesting behaviour, especially the drastic change in across the phase transition, as observed in temperature experiments. It will be desirable to perform first principles lattice dynamic calculations and molecular dynamic simulations as a function of pressure, similar to those used to study temperature induced phase transitions [3], to understand the coupling of phonons as brought out by our study. | |
