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) A, 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.