Optical and electronic properties of dense sodium

High-resolution Raman spectroscopy of dense sodium reveals additonal low-energy lattice excitations in the three high-pressure phases cI 16, oP 8, and tI 19 from 108 to 178 GPa. The Raman-active lattice modes indicate profound changes of the chemical bonding in Na upon its transformation to an insulating state (hP 4 phase) at pressures above 178( 2) GPa. The calculated values of the valence electron localization maxima versus pressure provide insights into the experimentally observed solid-solid phase transitions and anomalous melting behavior of sodium.