Effect of pressure on the resonant multiphonon Raman scattering in UO2

The phonon Raman scattering in UO2 was investigated under pressures up to 29 GPa with the excitation energy range of E-i=1.16-2.41 eV. At ambient pressure up to the sixth order polarized multi LO-phonon bands are detected with a resonant profile that follows the UO2 absorption (threshold of similar to 2.0 eV). The 1150 cm(-1) band is reassigned to the 2 LO band rather than to the Gamma(5)-Gamma(3) crystal field electronic transition. The resonance profile of the 1-6 LO intensities is likely to be attributed to a forbidden Frohlich LO(Gamma) scattering. However, identification with this form of scattering cannot explain the different polarization sensitivity between the even (strongly polarized) and odd (weakly polarized) bands. Hence, the significant contribution of nearly degenerate disorder induced phonons, extending over the Brillouin zone from Gamma to L boundaries, is suggested. Three factors are mainly responsible for the pressure dependent behavior of the 1 LO and 2 LO bands: (i) A redshift in the resonance onset, which is related to the decrease in the band gap energy, (ii) an increase in the relative intensities of the bands, that is due to the increase in electron-phonon interactions, and (iii) a decrease in the incident and scattered light penetration depth, that is due to the increased absorption. An interplay between the first two (increasing) and the third (decreasing) is dictating the intensity of the LO bands.