Transparent dense sodium (0911.3190v1)

Abstract: Under pressure, metals exhibit increasingly shorter interatomic distances. Intuitively, this response is expected to be accompanied by an increase in the widths of the valence and conduction bands and hence a more pronounced free-electron-like behaviour. But at the densities that can now be achieved experimentally, compression can be so substantial that core electrons overlap. This effect dramatically alters electronic properties from those typically associated with simple free-electron metals such as lithium and sodium, leading in turn to structurally complex phases and superconductivity with a high critical temperature. But the most intriguing prediction - that the seemingly simple metals Li and Na will transform under pressure into insulating states, owing to pairing of alkali atoms - has yet to be experimentally confirmed. Here we report experimental observations of a pressure-induced transformation of Na into an optically transparent phase at 200 GPa (corresponding to 5.0-fold compression). Experimental and computational data identify the new phase as a wide bandgap dielectric with a six-coordinated, highly distorted double-hexagonal close-packed structure. We attribute the emergence of this dense insulating state not to atom pairing, but to p-d hybridizations of valence electrons and their repulsion by core electrons into the lattice interstices. We expect that such insulating states may also form in other elements and compounds when compression is sufficiently strong that atomic cores start to overlap strongly.

• The paper reveals that sodium becomes a wide-bandgap insulator in an unexpected transparent phase above 200 GPa.
• The paper employs diamond-anvil cells, Raman spectroscopy, and XRD to trace structural transitions and confirm the novel Na-hP4 phase.
• The paper uses advanced GW calculations to show a bandgap increase from 1.3 eV to 6.5 eV as pressure escalates, indicating potential new material applications.

Insights into High-Pressure Phases of Sodium: Optical Transparency and Insulating State

This examination explores the rich phase behavior of sodium (Na) under extreme pressures as revealed by a detailed paper combining experimental observations and theoretical predictions. The research focuses particularly on the transition of Na to an optically transparent and insulating state, challenging preconceived notions about the metallic behavior of alkali metals under compression.

The work addresses the intriguing electronic transformations sodium undergoes under pressures around 200 GPa. Such high pressures were achieved using diamond-anvil cells, with precision measurements of pressure and structural changes facilitated by Raman spectroscopy and X-ray diffraction (XRD). The central finding of this paper is the unexpected emergence of sodium as a wide-bandgap insulator in a yet unexplored Na-hP4 structural phase, characterized by its transparency to visible light and significant bandgap energy as confirmed by spectroscopic analysis.

Structural Transformations and Stability Analysis

Sodium exhibits notable structural transitions under compression: from body-centered cubic (b.c.c.) to face-centered cubic (f.c.c.) at 65 GPa, and subsequently to the cI16 phase at 103 GPa. Beyond these pressures, particularly above 150 GPa, alternative phases such as oP8 (Pnma) and tI19 were recorded, highlighting sodium's rich polymorphism. These phases are associated with diminishing reflectance and evolving Raman spectral features, suggesting complex phase transitions. At pressures surpassing 200 GPa, the appearance of a transparent phase aligns with the novel Na-hP4 structure, which is typified by an extensive bandgap around 1.3 eV.

Structural searches and theoretical models authenticated these phases, with evolutionary algorithms indicating a competitive enthalpy landscape primarily concerning the oP8 and tI19 phases within the specified pressure range. These findings are compelling for the high-pressure physics community, as they suggest phases not previously anticipated by simplistic models of metallic behavior.

Electronic Properties and Insulating Phase

The distinctive attributes of the Na-hP4 phase arise from the p-d hybridization of valence electrons under high pressure, which leads to their localization in interstitial spaces rather than associating closely with ionic cores. The analysis deploys GW calculations known for their fidelity in bandgap prediction, confirming a gap increment from 1.3 eV at 200 GPa to a remarkable 6.5 eV at 600 GPa. Such quantitative predictions underscore a highly insulating state and underpin its optical transparency.

This unusual electronic configuration and the resultant insulating properties parallel trends found in electrides, where localized electron density functions akin to ionic anions. These phenomena illustrate electronic localization's impact on modifying the perceived straightforward metallic nature of compressed sodium.

Implications and Future Directions

The exploratory outcomes prompt a reevaluation of simple metals' behavior under extreme conditions, suggesting potential new materials with insulating properties emergent from dense metallic states. These implications extend to understanding planetary and stellar core materials where similar conditions may prevail.

This paper systematically advances our understanding of metallic phases under extreme pressures, providing ample ground for further paper on alkali metals and related systems. Future research may investigate the impact of temperature variations on these transitions, and explore broader implications in materials science, particularly concerning conductivity, superconductivity, and potential applications in microelectronics. This paper is a critical step in reconciling theoretical predictions with experimental realities, fostering a deeper appreciation for the structural and electronic complexity presented by alkali metals under pressure.