Raman Spectroscopy
Figure 1: Pressure induced structural phase transition in cesium azide (CsN3) observed by Raman spectroscopy of crystal lattice excitations.
Raman spectroscopy is a powerful method to study the pressure effect on interatomic interactions in the materials, to monitor pressure induced phase transitions, and in combination with high-pressure synchrotron x-ray diffraction, to determine the structures of the new polymorph forms of matter. The Raman spectra can be used to characterize the elastic, vibrational, electronic, and magnetic subsystems through the observations of the corresponding elementary excitations. As the spectra of elemental excitation change with the application of pressure, the associated phenomena can be effectively studied by in situ Raman spectroscopy.
Crystal structures of PdSe2. The ambient pressure PdS2–type structure (a) can be described as an elongated pyrite structure, which transforms under pressure (> 6 GPa) to the pyrite-type structure (b). The change in the Raman spectra between 5 and 9.2 GPa (c) indicates the structural transition to pyrite-type structure. The mode assignment within the pyrite structure is shown on the spectrum at 9.2 GPa. Blue arrows schematically show the unusual evolution of the vibrational modes of the Se2 dumbbells with pressure. The pyrite phase of PdSe2 is stable up to 37 GPa, above which the Raman spectrum changes (the green lines for 38.5 GPa show the deconvolution of the observed spectrum into two broad peaks) indicating a structural transition to a new high pressure phase of PdSe2 referred to as γ-phase. [M.A.ElGhazali, P.G. Naumov, H. Mirhosseini, V. Süß, L. Müchler, W. Schnelle, C. Felser and S.A. Medvedev, “Pressure-induced superconductivity up to 13.1 K in the pyrite phase of palladium diselenidePdSe2”, Phys. Rev. B 96, 060509(R) (2017)]