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.alkali metals ͉ crystal structure ͉ high pressure A t ambient pressure the alkali metals are simple metals; i.e., they can be described as nearly-free-electron metals that are characterized by a weak interaction between their single valence electron and the atomic core (1, 2). At ambient conditions the alkali metals all crystallize in the close-packed body-centered cubic (bcc) structure. However, under sufficient compression, they undergo a series of structural phase transitions. At pressures ranging from 2.2 GPa in cesium to 65 GPa in sodium, they transform from the bcc to the face-centered-cubic ( fcc) structure (3-5). Further compression leads to the formation of a wide variety of lower-symmetry and often very complex crystal structures (6, 7), which range from distorted variants of the bcc structure in lithium and sodium (5,8,9) to an incommensurate composite or ''host-guest'' crystal structure in rubidium (10, 11). The discovery of a whole series of these symmetry-lowering transitions over the last decade-not only in the alkali metals, but also in various other elements (6, 7, 9)-had been unexpected because it often involves a reduction in coordination number, which is opposite to the usual trend for pressureinduced phase transitions. The experimental discoveries have been complemented by computational studies, but the physical mechanisms that lead to the formation of the complex phases are not yet fully understood, and their physical properties not yet known in detail.Because of the low x-ray scattering power and relatively high transition pressures of sodium, it is only recently that it has become possible to investigate its high-pressure behavior. A number of high-pressure phases have been identified above 100 GPa (ref. 12 and M. Hanfland, K. Syassen, N. E. Christensen, and D. L. Novikov, unpublished data-see ref. 5), and the melting line was discovered to be very unusual: it first rises close to 1,000 K at Ϸ30 GPa and then falls to room temperature (Ϸ300 K) at 118 GPa (12) (see Fig. 1). Previous x-ray diffraction studies have shown that, with increasing pressure at room temperature, sodium transforms first from the bcc phase to fcc at 65 GP...

The alkali metals: 200 years of surprises

“…Between 2002 and 2008, a number of phase transitions of sodium at pressures up to 160 GPa were observed [42][43][44]. In 2009, in three letters to Nature in the same issue [45][46][47], it was shown that both lithium and sodium lose their metallic character under pressure.…”

Microscopic Origins of the Anomalous Melting Behavior of Sodium under High Pressure

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Recent experiments have shown that sodium, a prototype simple metal at ambient conditions, exhibits unexpected complexity under high pressure [1][2][3][4] . One of the most puzzling phenomena in the behaviour of dense sodium is the pressureinduced drop in its melting temperature, which extends from 1000 K at ∼30 GPa to as low as room temperature at ∼120 GPa 1 .

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Electronic dynamics and plasmons of sodium under compression

“…From 1 bar to 200 GPa, its volume reduces by fivefold, and its Wigner-Seitz radii, r s ¼ ½3∕ð4πn e Þ 1∕3 , reduces from 3.93 to 2.33, where n e is the conduction electron density (6, 7). During such a large range of electronic densification, Na exhibits numerous surprising behaviors, including melting at 118 GPa and 300 K (3), transitions to nine high-pressure phases including some truly exotic, complex crystal structures with as many as 512 atoms per unit cell (4,8,9), and eventually to an optically transparent, insulating solid (6, 10). These structural and optical studies indirectly revealed the unique physics of FE under compression (2).…”