Spectroscopic evidence for broken-symmetry transitions in dense lithium up to megabar pressures

We report optical spectroscopic studies of Li-7 and Li-6 at variable temperatures to 123 GPa. The optical reflectivity decreases above 40 GPa (25 GPa at room temperature). The Raman spectra reveal a broad mid-frequency band that we assign as a maximum in the phonon density of the states. Interpretation is consistent with the theoretical calculations for the observed crystal structures. The observed changes in Raman spectra and optical properties correlate with those reported previously from x-ray and superconductivity studies and attributed to structural transitions. Above 60 GPa new Raman bands appear that we ascribe to a theoretically predicted symmetry breaking transition associated with changes in bonding.