Structural complexity in ramp-compressed sodium to 480 GPa

Abstract

The properties of all materials at one atmosphere of pressure are controlled by the configurations of their valence electrons. At extreme pressures, neighboring atoms approach so close that core-electron orbitals overlap, and theory predicts the emergence of unusual quantum behavior. We ramp-compress monovalent elemental sodium, a prototypical metal at ambient conditions, to nearly 500 GPa (5 million atmospheres). The 7-fold increase of density brings the interatomic distance to 1.74 Å well within the initial 2.03 Å of the Na+ ionic diameter, and squeezes the valence electrons into the interstitial voids suggesting the formation of an electride phase. The laser-driven compression results in pressure-driven melting and recrystallization in a billionth of a second. In situ x-ray diffraction reveals a series of unexpected phase transitions upon recrystallization, and optical reflectivity measurements show a precipitous decrease throughout the liquid and solid phases, where the liquid is predicted to have electronic localization. These data reveal the presence of a rich, temperature-driven polymorphism where core electron overlap is thought to stabilize the formation of peculiar electride states.

Topological Electride Phase of Sodium at High Pressures and Temperatures

“…In addition, first principles molecular dynamics simulations at temperatures typical of those accessed using ramp-compression experiments (1500 K at 260 GPa, and 2000 K at 315 GPa, see Section S7), revealed that the (refined) atomic configurations in P 6 3 /m Na, including the soon-tobe-discussed honeycomb channels, are thermally stable. At 400 GPa Na hP 4 possessed the lowest Gibbs free energy to at least 3500 K (Figure S7), in agreement with experimental diffraction data that could be attributed to this phase at 409 ± 15 GPa [7].…”

“…However, at the high temperatures consistent with those attained in rampcompression experiments, the P 6 3 /m phase was computed to possess the lowest Gibbs free energy between 250 GPa (above 710 K) and 350 GPa (above 1900 K). Above 400 GPa, Na hP 4 again became preferred, in agreement with experimental observations [7]. At 200 GPa (350 GPa) the newly discovered P 6 3 /m structure was computed to be more stable than Na hP 4 above 150 K (1900 K) (Figure 1(b)).…”

“…In contrast, laser-driven ramp-compression experiments of Na to nearly 500 GPa and ∼3000 K hint of temperaturedependent structural complexity at these conditions [7]. In this experiment the thermodynamic pathway led to pressure-driven melting and recrystallization on nanosecond timescales.…”

“…Compression to ∼120 GPa results in an astounding ∼700 K decrease in the melting point [3], whose minimum is associated with rich structural diversity [4,5]. Near 200 GPa and room temperature an insulating hP 4 phase has been made, [6] persisting upon ramp-compression to nearly 500 GPa [7].…”

“…Therefore, further studies are essential to uncover the unknown Na phases created in Ref. [7] during ramp-compression of Na at intermediate pressures.…”