Experimental signatures of interstitial electron density in transparent dense sodium

The transparent hP4 phase of dense sodium (Na), stable above 200 GPa, has been computed to be an electride in which valence electrons are localised on interstitial lattice sites within the structure. However, there is no experimental evidence for this interstitial electron localisation in Na, or indeed in other high-density electride phases. Using static compression and single-crystal X-ray diffraction techniques, we have grown and studied a single-crystal sample of Na in the hP4 phase at 223 GPa. Using atomic form factors for hP4-Na derived from quantum crystallography techniques, we present experimental results to support the electride nature of this phase.