Single-crystal studies of incommensurate Na to 1.5 Mbar

Synchrotron single-crystal diffraction data show that sodium transforms from the oP8 structure to an incommensurate host-guest composite structure above 125 GPa in which the guest component is nearly ``melted.'' At 147 GPa, the correlation length between the guest chains is only 28(3)0.3em0ex, or approximately six times the chain spacing. This configuration shows a wide range of stability from above 125 GPa to at least 155 GPa and 550 K. The transition to this phase is accompanied by a marked decrease in optical reflectivity. Electronic band-structure calculations are used to interpret this change.