Coupling of multi-LO phonons to crystal-field excitations inUO2 studied by Raman spectroscopy

Abstract

Electronic Raman spectra of UO2 were measured under high hydrostatic pressure up to 21.5 GPa at room temperature.Resonant coupling was found between four (instead of the expected two) intermultiplet crystal-field (CF) excitations(γ1–γ4 rangesfrom 4130 to 4580 cm−1) and nLO phonons (n = 1–4, ωLO≈578 cm−1). The additional two CF excitations concurrently emerge under high pressureand are characterized by significantly higher pressure coefficients. Anincrease in pressure and excitation energy beyond the onset of absorption at∼2 eV enhances the resonant coupling, similar to that previously found for multi-LOphonon Raman scattering. The pressure coefficients of the CF–multi-LO coupledmode frequencies fit well with the sum of the pressure coefficients of their CF andnLO constituents. The very good correlation between the positions of theCF+nLO Raman bands and previously measured optical absorption bands points to the significantcontribution of LO phonon-assisted CF electronic transitions to the absorptionspectrum. These findings are attributed to the strong interactions between electronsand LO phonons, which play an important role in the electronic properties ofUO2.