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Electronic Structure of Carbon Dioxide Under Pressure and Insights Into the Molecular-To-Nonmolecular Transition

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Electronic Structure of Carbon Dioxide Under Pressure and Insights Into the Molecular-To-Nonmolecular Transition Electronic structure of carbon dioxide under pressure and insights into the molecular-to-nonmolecular transition Sean R. Shieha,1, Ignace Jarrigeb, Min Wuc,d, Nozomu Hiraokae, John S. Tsed, Zhongying Mia,2, Linada Kacia, Jian-Zhong Jiangc, and Yong Q. Caib aDepartments of Earth Sciences, and Physics and Astronomy, University of Western Ontario, London, ON, Canada N6A 5B7; bPhoton Sciences, Brookhaven National Laboratory, Upton, NY 11973; cInternational Center for New Structured Materials and Laboratory of New Structured Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China; dDepartment of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2; and eNational Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan Edited* by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved October 8, 2013 (received for review March 23, 2013) Knowledge of the high-pressure behavior of carbon dioxide (CO2), to nonmolecular phase transformation ranging from 48 to 65 GPa an important planetary material found in Venus, Earth, and Mars, (17, 22, 23). is vital to the study of the evolution and dynamics of the planetary The discrepancies between different experiments and ambi- interiors as well as to the fundamental understanding of the C–O guities in the transition pressures in solid CO2 motivate a thorough bonding and interaction between the molecules. Recent studies investigation of the pressure dependence of the local structural have revealed a number of crystalline polymorphs (CO2-I to -VII) and electronic environment. Here we report a survey of the evo- – and an amorphous phase under high pressure temperature condi- lution of the chemical bonding of CO2 under pressure, using X-ray tions. Nevertheless, the reported phase stability field and transition Raman scattering (XRS)—a nonresonant, hard X-ray photon-in pressures at room temperature are poorly defined, especially for photon-out technique at both the carbon (C) and oxygen (O) K the amorphous phase. Here we shed light on the successive pres- edges. XRS provides a convenient way to measure absorption sure-induced local structural changes and the molecular-to-nonmo- spectra of low Z materials in high-pressure diamond anvil cell. lecular transition of CO2 at room temperature by performing an in From analysis of the spectral features, insights into the effects of EARTH, ATMOSPHERIC, situ study of the local electronic structure using X-ray Raman scat- pressure on the unoccupied π* and σ* orbitals of C and O aided AND PLANETARY SCIENCES tering, aided by first-principle exciton calculations. We show that with first-principles calculations have been obtained. The present the transition from CO2-I to CO2-III was initiated at around 7.4 GPa, results provide a framework to further the understanding of the and completed at about 17 GPa. The present study also shows that local mechanism of polymerization of CO2 under pressure. at ∼37 GPa, molecular CO starts to polymerize to an extended 2 Results and Discussion structure with fourfold coordinated carbon and minor CO3 and CO-like species. The observed pressure is more than 10 GPa below The O K-edge XRS spectrum obtained for the molecular phase previously reported. The disappearance of the minority species at CO2-I at 2 GPa (Fig. 1A) is characterized by an intense π* res- 63(±3) GPa suggests that a previously unknown phase transition onance peak at 534.6 eV and very weak σ* features located within the nonmolecular phase of CO2 has occurred. between 540 and 550 eV. In comparison, the spectrum of gas- eous CO2 shows a single, relatively stronger π* peak and a σ* mineral physics | diamond anvil cell | inelastic x-ray scattering peak of moderate intensity (24). The C K-edge XRS spectrum of CO2-I shares similarities with the O K-edge spectrum with an π olecular compounds such as N and H O have been known intense * resonance peak at 291 eV and a very broad and weak 2 2 σ B Mto acquire a nonmolecular structure under compression * peak at about 313.2 eV (Fig. 1 ). The sensitivity of the XRS and ultimately transform into highly disordered and/or amor- spectrum to local chemical bonding is exhibited by the stark con- – trast in the spectral features for the transitions to π*attheCK edge phous phases (1 4). Other solids, such as group IV oxides SiO2 2 between CO2 (sp hybridization), and graphite (sp hybridization) (5) and GeO2 (6), are prone to amorphize under pressure despite the covalent framework structure. However, it was not until re- fi cently that CO2, both a molecular compound and group IV oxide, Signi cance was also reported to form several polymorphs and a pressure- induced amorphous phase (7–23). Previous experimental evi- In this paper, both oxygen and carbon K-edge spectra of CO2 dence on the formation of nonmolecular phases of N2 and CO2 polymorphs at high pressure and room temperature were was mainly based on the loss of optical vibrons (1) and/or the reported. The electronic structure of CO2 shows remarkable emergence of a broad IR or Raman band in the stretching mode change from molecular to nonmolecular transition. Further- region. Unfortunately, these new features are often very weak and more, a 532-eV feature was observed only at a limited pressure accurate measurement was hindered by significant noises (17, 22, range from ∼37 to 53 GPa, suggesting the presence of a tran- 23), making it difficult to extract useful information on the local sient species in the nonmolecular phase. coordination in the amorphous phase. This difficulty is illustrated by studies of the high-pressure high-temperature amorphous a- Author contributions: S.R.S. and Y.Q.C. designed research; S.R.S., I.J., N.H., Z.M., and L.K. performed research; S.R.S., I.J., M.W., N.H., J.S.T., Z.M., L.K., and J.-Z.J. analyzed data; CO2 phase in which the polymeric local structure was claimed to M.W. contributed new reagents/analytic tools; and S.R.S., I.J., J.S.T., and Y.Q.C. wrote be a mixture of tetrahedral and octahedral coordinated carbon the paper. based on Raman spectroscopy (17), but to a mixture of tetrahedral The authors declare no conflict of interest. and threefold coordinated polymeric CO2 from a combined theo- *This Direct Submission article had a prearranged editor. retical and infrared spectroscopy study (20). Moreover, knowledge 1To whom correspondence should be addressed. E-mail: [email protected]. of the phase diagram and kinetics for the various phase tran- 2Present address: Center for High Pressure Science and Technology Advanced Research, sitions in solid CO2 are obscured by significant disparities in the Pudong, Shanghai 201203, China. estimated phase transition pressures from different studies and This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. techniques. An example is the pressures reported for the molecular 1073/pnas.1305116110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1305116110 PNAS Early Edition | 1of5 Downloaded by guest on September 28, 2021 ABOxygen -edge P (GPa) Carbon -edge 63(3) 53(3) P (GPa) 37 53(3) 30 37 Intensity (a.u.) Intensity (a.u.) 10.7 2.0 21 0.0001 2.0 (in vacuum) 0.0001(in vacuum) 530 540 550 560 280 290 300 310 320 E-E0(eV) E-E0 (eV) CD Oxygen -edge Carbon -edge 30 GPa 21 GPa 37 GPa 37 GPa 53(3) Gpa 53(3) GPa 63(3) GPa Intensity (a.u.) Intensity (a.u.) 528 532 536 540 544 288 292 296 300 E-E0 (eV) E-E0(eV) Fig. 1. Representative X-ray Raman spectra. (A)Oxygenand(B)carbonK edge of CO2 at pressures up to 63(±3) GPa. The ambient pressure spectra of oxygen and carbon K edge were obtained at low temperature in vacuum condition. Evolution of the π*andσ* peaks between CO2-III and the amorphous phase, showing (C) at the O K edge the decrease of π*fromCO2-III at 534.6 eV and the appearance and then weakening of π*ofthei-CO2 phase at about 532 eV, and (D)attheCK edge the decrease of the π* and increase of the σ* peaks between CO2-III and the amorphous phase. The pressure is labeled along with the spectra. and diamond (sp3 hybridization) (Fig. 2) in which graphite shows transition pressure range. The lower onset pressure of the tran- both π*andσ* features whereas there is only a broad σ* band in sition observed here can be attributed to the local character of the spectrum of diamond. XRS, which is sensitive to the first coordination shells of the excited K fi In solid CO2 the O -edge spectral pro le is observed to atom in contrast to X-ray diffraction or Raman and infrared undergo gradual changes upon compression at pressure above spectroscopy. We note that pressure gradient measurements on 7.4 GPa (Fig. 3A). The two σ* peaks at 542.3 and 546.7 eV the sample detected no gradient up to 10 GPa and only a small become more distinct but the intensities remain rather weak. At pressure gradient (0.017 GPa/μm) from 10 to 17 GPa (25). about 10.7 GPa, the σ*peakoftheOK edge at 542.3 eV has Upon further compression to about 37 GPa, we observe dra- disappeared and concomitantly a new peak starts to emerge at matic changes in both O and C K-edge XRS patterns (Fig. 1 A– σ 537.4 eV. At the same time, the * peak at 546.7 eV becomes D). Note that the pressure distribution within the CO sample ∼ 2 stronger with the intensity increases up to 17 GPa.
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