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Edited by Ho-Kwang Mao, Center for High Pressure Science and Technology Advanced Research, Shanghai, China, and approved September 17, 2019 (received for review April 5, 2019)
Significance
Oxygen diatomic molecules have lone-pair electrons and magnetic moments. A high-pressure phase called epsilon oxygen is considered stable in a wide pressure range. This material exhibits the transition to metal at ∼100 GPa (1,000,000× atmospheric pressure). The change in the electronic structure involved in the transition under pressure is difficult to measure using conventional methods. In this study, the electronic structures of oxygen have been successfully measured with oxygen K-edge X-ray Raman scattering spectroscopy. We found a change in the spectra related to the metallization of oxygen. Another change in the electronic structure was also observed at ∼40 GPa. This is likely related to the semimetallic transition.
Abstract
Electronic structures of dense solid oxygen have been investigated up to 140 GPa with oxygen K-edge X-ray Raman scattering spectroscopy with the help of ab initio calculations based on density functional theory with semilocal metageneralized gradient approximation and nonlocal van der Waals density functionals. The present study demonstrates that the transition energies (Pi*, Sigma*, and the continuum) increase with compression, and the slopes of the pressure dependences then change at 94 GPa. The change in the slopes indicates that the electronic structure changes at the metallic transition. The change in the Pi* and Sigma* bands implies metallic characteristics of dense solid oxygen not only in the crystal a–b plane but also parallel to the c axis. The pressure evolution of the spectra also changes at ∼40 GPa. The experimental results are qualitatively reproduced in the calculations, indicating that dense solid oxygen transforms from insulator to metal via the semimetallic transition.
- dense solid oxygen
- electronic structure
- insulator–metal transition
- X-ray Raman scattering
- DFT calculation
Footnotes
- ↵1To whom correspondence may be addressed. Email: fukuih{at}sci.u-hyogo.ac.jp.
Author contributions: H.F. designed research; H.F., L.T.A., M.W., N. Hiraoka, T. Iitaka, N. Hirao, and Y.A. performed research; T. Irifune contributed new reagents/analytic tools; H.F., L.T.A., and M.W. analyzed data; and H.F., L.T.A., N. Hiraoka, and T. Iitaka wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1905771116/-/DCSupplemental.
Published under the PNAS license.
Electronic structure of dense solid oxygen from insulator to metal investigated with X-ray Raman scattering
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