Raman spectra of a high-pressure iodine single crystal

Raman spectra have been collected in iodine single crystal at room temperature in the pressure range 1--14 GPa using a micro-Raman apparatus. The pressure at the sample was generated by means of a diamond anvil cell. Single crystals were grown in situ over the inner diamond surface. A careful analysis of the frequencies and intensities of internal (stretching) and external (libration) modes as a function of the pressure is reported. In particular, analysis of the relative intensities of the Raman modes confirms a theoretical prediction about the pressure evolution of the Ag stretching mode eigenvector and the huge intensity increase of the external modes with respect to the internal ones is well consistent with the predicted increased role of the charge-transfer interaction when approaching the metallization transition pressure. At pressures higher than 10 GPa, spectral features were observed simultaneously with the appearance of two peaks forbidden in the adopted sample geometry. The experimental results obtained are eventually discussed in comparison with recent experimental and theoretical works.