Structure of sodium above 100 GPa by single-crystal x-ray diffraction

Abstract

At pressures above a megabar (100 GPa), sodium crystallizes in a number of complex crystal structures with unusually low melting temperatures, reaching as low as 300 K at 118 GPa. We have utilized this unique behavior at extreme pressures to grow a single crystal of sodium at 108 GPa, and have investigated the complex crystal structure at this pressure using high-intensity x-rays from the new Diamond synchrotron source, in combination with a pressure cell with wide angular apertures. We confirm that, at 108 GPa, sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. The results demonstrate the extension of single-crystal structure refinement beyond 100 GPa and raise the prospect of successfully determining the structures of yet more complex phases reported in sodium and other elements at extreme pressures.

Fig. 1.

The phase diagram of Na to 140 GPa (adapted from ref. 12), showing the stability fields of the bcc, fcc, and cI16 phases, and the remarkable decrease in the melting temperature observed above 30 GPa.

Fig. 2.

The cI16 crystal structure. It can be regarded as a distorted 2 × 2 × 2 superstructure of the bcc structure. One of the eight distorted-bcc subunits is highlighted, and the local coordination is indicated for two further subunits. The atoms occupy the 16c Wyckoff site, (x, x, x), of space group I4̄3d and positions are shown for x = 0.044.

Fig. 3.

One quadrant of a composite image of five superimposed Mar345 images selected from the overall 14° scan range to show representative data. Eight observed reflections are marked. As discussed in the text, the (204) and (222) reflections are absent. Weak intensity just to the left of the (103) reflection arises from a second crystal; reflections from this are ≈10 times weaker than from the main crystal. The powder diffraction rings are from the rhenium gasket and from some tantalum included with the sample as a pressure marker. One part of the image has been enhanced to reveal the (112) reflection on one of the rhenium diffraction rings. Other small diffraction spots in the image are of a different appearance from the sample reflections, and are not from the sample.