New experimental developments in Raman spectroscopy at high pressures and temperatures on crystalline and amorphous phases

Abstract

Recent developments in Raman spectroscopy at high pressure, high temperature and combined high pressure and variable temperature are presented. The instrumental and technical aspects of Raman spectroscopy, and coupling 'of diamond anvil cells and miniature furnaces to Raman microspectrometers are discussed. Several pitfalls are discussed: (1) the generation of shear stresses on the sample during room temperature compression and their effects on phase transitions; (2) thermal radiation during hightemperature measurements; (3) thermal pressure in laser heated diamond anvil cells. The application of in-situ Raman spectroscopy to the study of phase transitions occurring at extreme pressures is illustrated & the new recently discovered phase transformations in the SiOz system: quartz I + quartz I1 or pressure-induced amorphization of quartz. Finally, it is shown how vibrational mode anharmonicity can be obtained from the pressure-and temperature-induced shifts of Raman modes. Accurate determination of these shifts in conjunction with well-defined equation of states permits a separation of the temperature dependence of a phonon frequency into its two components: the implicit volume-driven (quasi-harmonic) contribution produced by thermal expansion and the explicit phonon-excitation (intrinsic anharmonicity) contribution or phonon-phonon interaction. This last point is illustrated by the example of the hard Raman modes of MgzSiO4-forsterite and Si02-quartz.

A High-Pressure Raman Study of Mixed Perovskites BaCexZr1−xO3 (0≤x≤1)

“…However, we cannot exclude that the bands' broadness, in addition to poorly resolved components, can also be due to a partial amorphization of the sample. This amorphization phenomenon has been observed on other perovskites (19). After the pressure release, spectra are similar to the initial ones for all compositions, which indicates the reversibility of the transitions.…”

In Situ Ultraviolet Raman Study on the Phase Transition of Hafnia up to 2085 K

“…Such high-temperature measurements have been performed using a pulsed-excitation method 1,2 and a micro-Raman technique. 3,4 However, these techniques require a quite high energy density (typically 80 -5000 kW in 5-10 ns for the pulsed-excitation method), which would cause serious damage to the sample. We have demonstrated that Raman scattering using ultraviolet (UV) excitation is well suited for in situ investigation of materials at high temperatures up to 1773 K ( Fig.…”