ABSTRACT
Optical, vibrational spectroscopies, e.g. Raman scattering and Infrared (IR) absorption spectroscopy, have been major tools, through the years, for investigating the physical and chemical properties of molecular materials under high pressures, along with X-ray diffraction (XRD). While the direct outcome of XRD techniques is the microscopic static structure, Raman and IR spectroscopies probe molecular dynamics, and are the leading techniques in providing direct information on the inter-atomic(molecular) interactions, which drive molecular motions. On the other hand, these interactions depend on the interatomic distances, and therefore vibrational spectroscopies also provide an indirect probe of the microscopic structure of materials, which can constrain the assignment of XRD patterns. More crucially, in some cases where the application of XRD is extremely challenging, such as those involving very thin samples of liquid and amorphous materials made of low Z elements confined in the diamond anvil cell (DAC), vibrational spectroscopy has provided a unique tool to unveil the unknown structure of new materials (see the case of amorphous, non molecular CO2 in section 5). Also, Raman and IR spectroscopies are commonly used in the investigation of phase diagrams of materials, since the intensity, frequency and linewidth of the vibrational peaks are extremely sensitive to the fluid-solid and solid-solid phase transitions. Indeed, optical spectroscopy techniques coupled to low, room and high temperatures DACs allow one, at the state of the art, to investigate the pressure-temperature (P-
T) phase diagram of molecular systems in the 0.1-300 GPa and 4-2500 K P-T range.
