Carbon tetrachloride under extreme conditions Michael Pravica, Daniel Sneed, Yonggang Wang, Quinlan Smith, and Garimella Subrahmanyam Citation: The Journal of Chemical Physics 140, 194503 (2014); doi: 10.1063/1.4876220 View online: http://dx.doi.org/10.1063/1.4876220 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-pressure single-crystal elasticity study of CO2 across phase I-III transition Appl. Phys. Lett. 104, 141901 (2014); 10.1063/1.4870526 Structural and vibrational properties of phenanthrene under pressure J. Chem. Phys. 139, 104302 (2013); 10.1063/1.4820359 Vibrational and structural properties of tetramethyltin under pressure J. Chem. Phys. 138, 024307 (2013); 10.1063/1.4774022 Lattice phonons of solid phases ( α , β , δ , ε ) of carbon monoxide by optical studies Low Temp. Phys. 36, 424 (2010); 10.1063/1.3432267 A transformation of carbon dioxide to nonmolecular solid at room temperature and high pressure J. Appl. Phys. 102, 053501 (2007); 10.1063/1.2775539
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THE JOURNAL OF CHEMICAL PHYSICS 140, 194503 (2014)
Carbon tetrachloride under extreme conditions Michael Pravica,1,a) Daniel Sneed,1 Yonggang Wang,1 Quinlan Smith,1 and Garimella Subrahmanyam2 1
High Pressure Science and Engineering Center (HiPSEC) and Department of Physics, University of Nevada Las Vegas (UNLV), Las Vegas, Nevada 89154-4002, USA 2 Canadian Light Source, Department of Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada
(Received 27 February 2014; accepted 1 May 2014; published online 20 May 2014) We report on three experiments on carbon tetrachloride subjected to extreme conditions. In the first experiment, Raman spectra of CCl4 were acquired up to 28 GPa. Evidence was observed for at least two new phases of CCl4 above 14 GPa (phase VI) and above 22 GPa (phase VII). Decompression of the sample showed no evidence of pressure-induced decomposition. In the second experiment, a synchrotron x-ray diffraction study was performed up to 30 GPa verifying phase V and potential phases above 14 (VI) and 22 GPa (VII), respectively. In the third study, we examined irradiated CCl4 using synchrotron infrared spectroscopy to reduce fluorescent contamination. Some sort of carbon allotrope appears as a byproduct suggesting the following reaction with hard x-rays: CCl4 + hν → C + 2Cl2 . © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4876220] INTRODUCTION
In a recent study of carbon tetrachloride (CCl4 ) irradiated with hard x-rays,1 we observed molecular decomposition which released molecular chlorine (Cl2 ), and that this chlorine flows (slowly) even at high pressures of 5 GPa.1 Though the production of Cl2 via x-rays was easily detected using Raman spectroscopy, we were unable to determine the other products of the reaction via the same probing method due to sample fluorescence in the presence of 514.5 nm light. This is a typical problem that we have encountered in prior high pressure studies due to highly damaged and stressed samples that are subjected to extreme conditions of pressure and ionizing radiation.2, 3 Additionally, as the application of pressure to CCl4 is an important degree of freedom that enables us to confine our samples and the reaction products inside the diamond anvil cell,4, 5 control the x-ray induced phase-dependent decomposition rate,6 and even potentially control the diffusion rate of the molecular product,7 we sought to further investigate the behavior of this fundamental molecule with pressure. At ambient temperature, CCl4 exhibits four8 (possibly 9, 10 five ) known phases (I, II, III, IV, and V) with pressure. The liquid solidifies into rhombohedral Phase I near 0.13 GPa.9 Between 0.4 and 0.7 GPa the material exists as Phase II which is in the C2/c monoclinic symmetry.8 Above this pressure, the materials transform into Phase III, which is also monoclinic, P21 /c, and Z = 4 symmetry.8, 11 Between 3 and 7 GPa (or above 2.3 GPa10 ), the compound adopts cubic symmetry (Phase IV)8 and above 7 GPa, a new phase has been reported9, 10 that exists at least up to 13 GPa. No other phase transitions beyond this were reported up to 22 GPa.8 As there appears to be some controversy pertaining to the exact number of phase transitions that occur up to 22 GPa, we undertook separate Raman spectroscopic and x-ray diffraction (XRD) a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0021-9606/2014/140(19)/194503/5/$30.00
studies with the aim of better-determining the phase/pressure behavior of this important compound with pressure coupled with our infrared study to garner further information on the post reaction products of irradiated CCl4 . EXPERIMENTAL
We used a symmetric-style Diamond Anvil Cell (DAC) with 250 μm thick stainless steel gaskets to pressurize and confine CCl4 . The diamonds used each had a culet diameter of ∼300 μm and were low fluorescence type II quality. The sample-confining gasket was preindented to ∼50 μm thickness and a sample hole of diameter ∼100 μm was drilled via electric discharge machining. Liquid CCl4 (Burdick & Jackson spectrophotometer grade >99%) was loaded via a syringe into the gasket hole along with one ruby (for pressure measurement) and then the assembly was closed to seal the sample. No pressure-transmitting medium was used in our experiments and all were performed at room temperature. The first experiment was conducted within the Pravica group Raman facility at University of Nevada Las Vegas (UNLV). The laser power source consisted of a Spectra R Ar ion laser tuned to the 514.5 nm laser line set at Physics R spec100 mW (20 mW at the sample). A Jobin Yvon U1000 trometer dispersed the scattered light which was then stored R detector. The Ravia an ISA Instruments Spectrum One man spectrometer instrumental resolution was ∼1 cm−1 . The R Rayleigh scattered light was filtered using a Kaiser optics 514.5 nm holographic notch filter. Acquisitions typically required 60 s to complete. The second experiment was performed at the 16 BM-D beamline at the Advanced Photon Source (APS). Fresh CCl4 was loaded into a symmetric-style (the same one used in the Raman experiment) and pressurized. White x-rays were filtered to 24.350 keV and used to irradiate the sample. Scattered R image plate detector. x-rays were detected using a Mar 345
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This energy was chosen to minimize damage to CCl4 1 based on our earlier experiments. For our third experiment, we utilized the same sample that was irradiated in our earlier study1 (pressurized to about 5 GPa) and performed synchrotron infrared (IR) experiments at the Canadian Light Source (using the 01B1-1 mid-IR beamline) to better-ascertain the decomposition products beyond molecular chlorine contained in that sample. We have observed in prior studies that the exciting IR does not excite fluorescence in the sample and is a far-superior method to acquiring vibrational mode behavior in organic and stressed/damaged samples at high pressure.2 Mid-IR spectra were collected with a Bruker Vertex 77 v/S, Hyperion 3000 IR microscope. A liquid nitrogen cooled MCT detector was used. The mid-IR microscope system typically focuses the IR beam to 20 μm which is then spatially filtered using a 20 μm diameter circular aperture. A background spectrum was acquired by releasing the original irradiated/damaged CCl4 sample between the diamonds and then subtracted from the spectrum acquired with the pressurized sample. RAMAN RESULTS
We first present the results of our Raman experiment during compression in Fig. 1. The pressure dependence of the observed and fitted spectral modes is plotted in Fig. 2. We have also included selected ranges of pressure in Figs. 3–5 to highlight changes in the spectra that yield supporting evidence for the proposed phase IV → V (Fig. 3), phase V → VI (Fig. 4), and phase VI → VII (Fig. 5) transitions. Due to the complexities associated with rapidly sealing and pressurizing a high vapor pressure liquid that rapidly evaporates, we loaded our sample at 1.3 GPa, which implies that it commenced in Phase III.10 The ν 2,s (E) vibration8 near 220 cm−1 (i.e., C-Cl symmetric stretching mode) at ambient pressure has split as expected8, 9 at high pressure. The ν 4,s (F2 ) vibration (i.e., C-Cl asymmetric bending mode) near 325 cm−1 appears at high pressure except at a higher wavenumber value as do the ν 1,s (A1 ) modes near 460 cm−1 (i.e., C-Cl symmetric stretching mode) and the ν 1 and ν 1+4 modes near 760 cm−1
FIG. 1. Stacked Raman spectra of CCl4 at all of the studied pressures in compression.
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FIG. 2. Stacked plots displaying the pressure-dependent behavior of the observed Raman modes of CCl4 .
(Fermi resonance doublet)9 suggesting that our sample comprised solely CCl4 . All modes consistently shift toward higher frequencies up to the highest pressure studied (28 GPa) indicating a stiffening of molecular bonds. The fits were perR . The errors of the fits are within the formed using PeakFit size of the dots used to make the plots. Comparing the spectra at 1.3 GPa and 4.3 GPa, a number of changes occur in the Raman spectra which include a splitting of the line near 225 cm−1 , the appearance of a new peak near 240 cm−1 , and major changes in the broader spectral features in the 750–800 cm−1 range. These changes were observed previously and assigned to the phase III→VI transition.8, 9 Near 7.4 GPa, a new peak emerges near 320 cm−1 (just to the left of the 325 cm−1 peak), which may suggest a phase transition (IV → V) as observed in Ref. 9 (see Figs. 1 and 3). Slope discontinuities in dω/dP were observed near 7 GPa in all of the lines with the exception of the aforementioned new peak near 320 cm−1 (see Fig. 3) lending further credence to the IV→V phase transition. Between
FIG. 3. Selected stacked Raman spectra of CCl4 within the 1.3–11.2 GPa region to highlight evidence for the possible phase IV → V transition just above 7 GPa.
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FIG. 4. Selected stacked Raman spectra of CCl4 within the 11.2–19.1 GPa region to highlight evidence for the possible phase V → VI transition just above 14 GPa. In particular, a new line emerges near 360 cm−1 .
7–14 GPa, no further phase transition was observed,8, 9 which agrees with our data. Above 14 GPa, however, a new peak emerges near 370 cm−1 commencing near 14.8 GPa (see Fig. 4). The data from Ref. 9 only go up to 13 GPa but Ref. 8 displays data from another high pressure polarized Raman study up to 22 GPa which displays some potential slope discontinuities in this pressure range (see Fig. 2 of Ref. 8). This study, however, does not mention any phases beyond Phase IV. In our study, above 21.9 GPa, yet another peak appears near 400 cm−1 (see Fig. 5). Near 22 GPa, the slope discontinuities are more pronounced and suggest a sluggish phase transition – possibly a mixture – which has been observed in other covalentlybonded systems such as cyclohexane.12 After reaching the terminal pressure (28 GPa), the sample pressure was reduced to 3.9 GPa, and the spectral frequencies appeared to return to the earlier values near this pressure (see Fig. 6). We found no evidence of laser-induced damage of our sample during the course of our studies.
FIG. 5. Selected stacked Raman spectra of CCl4 within the 19.1–28.2 GPa region to highlight evidence for the possible phase VI → VII transition near 21.9 GPa.
J. Chem. Phys. 140, 194503 (2014)
FIG. 6. Stacked Raman spectra acquired from CCl4 at high pressure. The first two lowest spectra at 4.3 GPa and 28.2 GPa, respectively, were recorded during sample compression. The remaining spectra were acquired in decompression of the sample and have a * labeling them as such.
XRD RESULTS
We used the 16 BM-D white x-ray beamline for our xray diffraction experiments. The flux irradiating our sample was far lower than at the 16 BM-B beamline (where we performed our earlier CCl4 decomposition experiments) due to the use of a monochromator1 that filters selected energies. As a result, no damage was evident during the course of our studies. The goal for the XRD study was to confirm any phases observed in the Raman study. We present our results in Fig. 7. The sample was pressurized up to 31.7 GPa and then depressurized to near ambient to verify if any of the sample suffered any irreversible changes with pressure cycling.
FIG. 7. Stacked x-ray diffraction curves of CCl4 at various pressures illus´ Two regions of interest trating some of the phase changes (λ = 0.509175 Å). showing changes of the XRD patterns are highlighted.
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FIG. 8. LeBail fitting of the decompressed XRD pattern of CCl4 at 0.58 GPa (top pattern in Fig. 7) illustrating the reversibility of pressurization up to 31.7 GPa. Miller indices are included.
Between 1 GPa and 4.35 GPa, a phase transition appears which would likely be a transition from phase III to phase IV.8, 9 From 4.35 GPa to 7.64 GPa, evidence of another phase transition (which we ascribe to IV→V) manifests (particularly when examining the XRD pattern lines above 14◦ ) which complements our Raman data. No significant changes (beyond the compression of unit cell parameters and thus shifting of the XRD lines toward higher angle) appear until 14.4 GPa where new peaks emerge near 5◦ , 12.5◦ , and 15.5◦ which suggests a phase transition somewhere around 14 GPa. Again, this corroborates our Raman data for a potential (V→VI) phase transition. Finally, at 19.2 GPa and above, most lines within 12.5◦ up to 16◦ largely disappear except for the appearance of two lines near 16o lending credence to the phase VI→VII transition. The lines within 5.5◦ –7◦ also largely disappear. A broader peak also appears near 16.5◦ at 25 GPa which shifts rightward in the 31.7 GPa pattern suggesting that the proposed phase transition is complex and probably incomplete at 19 GPa, and that the sample comprises mixed phases in the 19–25 GPa pressure range which agrees with our observed Raman slope discontinuities. Upon pressure release near 0.6 GPa, the XRD pattern was successfully fit to the structure of phase I suggesting that no molecular damage occurred in the sample and that the pressure cycling was reversible with perhaps some hysteresis.10, 12 The LeBail refinement to this decompressed pattern is displayed in Figure 8.
Material (G-NIM)14 or Single Walled Carbon Nano Tubes (SWNT)15 which have modes near this frequency. The slight contamination evident in the central pattern labeled “unirradiated region” is likely due to the large spot size of the exciting infrared radiation. Bear in mind that the sample is pressurized to 5 GPa thus the peak should be slightly higher in energy than the peak would be at ambient pressure. The virgin CCl4 spectrum does not contain this peak. Thus, based on this evidence, we propose the following reaction: CCl4 + hν → C + 2Cl2 . It should be noted that carbon particles were observed in shocked CCl4 and C3 O2 .16
IR RESULTS
Figure 9 displays the results of our IR spectroscopic interrogation of the irradiated sample from Ref. 1, both in the region where Cl2 was detected (top curve) via Raman spectroscopy1 and in regions away from the irradiated and consolidated region (middle curve), along with virgin CCl4 (bottom curve) confined by a IR-transmitting microscope slide. We do not anticipate observing Cl2 as it has no electric dipole moment.13 A large peak near 2750 cm−1 is immediately evident which we suspect is some sort of C-C vibration (given the high frequency of the vibration) and thus a carbon-based compound such as Graphene Nanoparticle Ionic
FIG. 9. Infrared data comparing of virgin CCl4 (bottom plot), irradiated CCl4 in a region of the sample that was not irradiated (middle plot), and in the region of irradiation in the same sample (top plot). The bottom spectrum was taken with a virgin sample of CCl4 placed on an IR-transmitting microscope slide. The top two spectra were recorded confined in a DAC at around 5 GPa but at two different locations within the sample. A peak near 2750 cm−1 is particularly prominent in the irradiated region (top plot) and absent in the virgin sample of CCl4 that suggests some sort of carbon allotrope – possibly some graphene precursor (G-NIM)12 or SWNT13 – produced via irradiation.
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CONCLUSION
We have performed three studies of an important and fundamental solvent CCl4 to ascertain the viability of this molecular system for use in useful hard x-ray induced chemistry. We have confirmed the presence of phase V near 7 GPa. We have observed evidence for two new phase transitions of CCl4 : phase VI near 14 GPa and phase VII above 22 GPa. The samples appear to survive pressure cycling up to 26 GPa and 32 GPa, respectively. In the third study, we have also garnered IR data for the other product(s) from the x-ray induced decomposition of CCl4 which appears to be some form of pure carbon with little or no introduction of heat. This may offer a unique means to produce carbon allotropes under extreme conditions. ACKNOWLEDGMENTS
We thank Melanie White, Ferenc Borondics, and Xia Liu for aid. We acknowledge support from the Department of Energy National Nuclear Security Administration under Award No. DE-NA0000979. We also acknowledge support from the DOE Cooperative Agreement No. DE-FC08-01NV14049 with the University of Nevada, Las Vegas. One of us (D.S.) acknowledges partial support from the UNLV NSF REU program, Award No. 1005247. Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357.
J. Chem. Phys. 140, 194503 (2014)
A portion of the research described in this paper was performed at the Mid-IR beamline of the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. 1 M.
Pravica, D. Sneed, Q. Smith, and L. Bai, Chem. Phys. Lett. 590, 74–76 (2013). 2 M. Pravica, B. Yulga, S. Tkachev, and Z. Liu, J. Phys. Chem. A 113(32), 9133–9137 (2009). 3 M. Pravica, Y. Liu, and L. Bai, Chem. Phys. Lett. 555, 115–118 (2013). 4 M. Pravica, L. Bai, C. Park, Y. Liu, M. Galley, J. Robinson, and N. Bhattacharya, Rev. Sci. Instrum. 82, 106102 (2011). 5 M. Pravica, L. Bai, C. Park, Y. Liu, M. Galley, J. Robinson, and N. Bhattacharya, Rev. Sci. Instrum. 83, 036102 (2012). 6 M. Pravica, L. Bai, and N. Bhattacharya, J. Appl. Cryst. 45, 48–52 (2012). 7 M. Pravica, D. Popov, S. Sinogeikin, D. Sneed, G. Guardala, and Q. Smith, Appl. Phys. Lett. 103, 224103 (2013). 8 Y. Chen, S. Sun, Y. Wu, S. Gao, Z. Li, S. Ouyang, D. Li, Z. Men, M. Zhou, and C. Sun, Phys. Stat. Solidi 249(11), 2113–2117 (2012). 9 T.-C. Liu, M. Zhou, S.-Q. Gao, Z.-W. Li, Z.-L. Li, P. Zhang, L. Li, T.-Q. Lv, and D.-P. Xu, Chin. Phys. Lett. 26(7), 070701 (2009). 10 H. Kawamura, Y. Yamamoto, N. Matsui, I. Nakahata, M. Kobayashi, and Y. Akahama, Rev. High Press. Sci. Tech. 7, 805–807 (1998). 11 G. Piermarini and A. Braun, J. Chem. Phys. 58(5), 1974–1982 (1973). 12 M. Pravica, YR Shen, Z. Quine, E. Romano, and D. Hartnett, J. Phys. Chem. B 111, 4103–4108 (2007). 13 P. Larkin, IR and Raman Spectroscopy (Elsevier, 2011). 14 Z. Tang, L. Zhang, C. Zeng, T. Lina, and B. Guo, Soft Matter 8, 9214–9220 (2012). 15 U. J. Kim, C. A. Furtado, X. Liu, G. Chen, and P. C. Eklund, J. Amer. Chem. Soc. 127, 15437–15445 (2005). 16 R. Starke, B. Kock, P. Roth, A. Eremin, E. Gurentsovb, V. Shumova, and V. Ziborov, Combust. Flam. 135, 77–85 (2003).
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THE JOURNAL OF CHEMICAL PHYSICS 140, 194503 (2014)
Carbon tetrachloride under extreme conditions Michael Pravica,1,a) Daniel Sneed,1 Yonggang Wang,1 Quinlan Smith,1 and Garimella Subrahmanyam2 1
High Pressure Science and Engineering Center (HiPSEC) and Department of Physics, University of Nevada Las Vegas (UNLV), Las Vegas, Nevada 89154-4002, USA 2 Canadian Light Source, Department of Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada
(Received 27 February 2014; accepted 1 May 2014; published online 20 May 2014) We report on three experiments on carbon tetrachloride subjected to extreme conditions. In the first experiment, Raman spectra of CCl4 were acquired up to 28 GPa. Evidence was observed for at least two new phases of CCl4 above 14 GPa (phase VI) and above 22 GPa (phase VII). Decompression of the sample showed no evidence of pressure-induced decomposition. In the second experiment, a synchrotron x-ray diffraction study was performed up to 30 GPa verifying phase V and potential phases above 14 (VI) and 22 GPa (VII), respectively. In the third study, we examined irradiated CCl4 using synchrotron infrared spectroscopy to reduce fluorescent contamination. Some sort of carbon allotrope appears as a byproduct suggesting the following reaction with hard x-rays: CCl4 + hν → C + 2Cl2 . © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4876220] INTRODUCTION
In a recent study of carbon tetrachloride (CCl4 ) irradiated with hard x-rays,1 we observed molecular decomposition which released molecular chlorine (Cl2 ), and that this chlorine flows (slowly) even at high pressures of 5 GPa.1 Though the production of Cl2 via x-rays was easily detected using Raman spectroscopy, we were unable to determine the other products of the reaction via the same probing method due to sample fluorescence in the presence of 514.5 nm light. This is a typical problem that we have encountered in prior high pressure studies due to highly damaged and stressed samples that are subjected to extreme conditions of pressure and ionizing radiation.2, 3 Additionally, as the application of pressure to CCl4 is an important degree of freedom that enables us to confine our samples and the reaction products inside the diamond anvil cell,4, 5 control the x-ray induced phase-dependent decomposition rate,6 and even potentially control the diffusion rate of the molecular product,7 we sought to further investigate the behavior of this fundamental molecule with pressure. At ambient temperature, CCl4 exhibits four8 (possibly 9, 10 five ) known phases (I, II, III, IV, and V) with pressure. The liquid solidifies into rhombohedral Phase I near 0.13 GPa.9 Between 0.4 and 0.7 GPa the material exists as Phase II which is in the C2/c monoclinic symmetry.8 Above this pressure, the materials transform into Phase III, which is also monoclinic, P21 /c, and Z = 4 symmetry.8, 11 Between 3 and 7 GPa (or above 2.3 GPa10 ), the compound adopts cubic symmetry (Phase IV)8 and above 7 GPa, a new phase has been reported9, 10 that exists at least up to 13 GPa. No other phase transitions beyond this were reported up to 22 GPa.8 As there appears to be some controversy pertaining to the exact number of phase transitions that occur up to 22 GPa, we undertook separate Raman spectroscopic and x-ray diffraction (XRD) a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0021-9606/2014/140(19)/194503/5/$30.00
studies with the aim of better-determining the phase/pressure behavior of this important compound with pressure coupled with our infrared study to garner further information on the post reaction products of irradiated CCl4 . EXPERIMENTAL
We used a symmetric-style Diamond Anvil Cell (DAC) with 250 μm thick stainless steel gaskets to pressurize and confine CCl4 . The diamonds used each had a culet diameter of ∼300 μm and were low fluorescence type II quality. The sample-confining gasket was preindented to ∼50 μm thickness and a sample hole of diameter ∼100 μm was drilled via electric discharge machining. Liquid CCl4 (Burdick & Jackson spectrophotometer grade >99%) was loaded via a syringe into the gasket hole along with one ruby (for pressure measurement) and then the assembly was closed to seal the sample. No pressure-transmitting medium was used in our experiments and all were performed at room temperature. The first experiment was conducted within the Pravica group Raman facility at University of Nevada Las Vegas (UNLV). The laser power source consisted of a Spectra R Ar ion laser tuned to the 514.5 nm laser line set at Physics R spec100 mW (20 mW at the sample). A Jobin Yvon U1000 trometer dispersed the scattered light which was then stored R detector. The Ravia an ISA Instruments Spectrum One man spectrometer instrumental resolution was ∼1 cm−1 . The R Rayleigh scattered light was filtered using a Kaiser optics 514.5 nm holographic notch filter. Acquisitions typically required 60 s to complete. The second experiment was performed at the 16 BM-D beamline at the Advanced Photon Source (APS). Fresh CCl4 was loaded into a symmetric-style (the same one used in the Raman experiment) and pressurized. White x-rays were filtered to 24.350 keV and used to irradiate the sample. Scattered R image plate detector. x-rays were detected using a Mar 345
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This energy was chosen to minimize damage to CCl4 1 based on our earlier experiments. For our third experiment, we utilized the same sample that was irradiated in our earlier study1 (pressurized to about 5 GPa) and performed synchrotron infrared (IR) experiments at the Canadian Light Source (using the 01B1-1 mid-IR beamline) to better-ascertain the decomposition products beyond molecular chlorine contained in that sample. We have observed in prior studies that the exciting IR does not excite fluorescence in the sample and is a far-superior method to acquiring vibrational mode behavior in organic and stressed/damaged samples at high pressure.2 Mid-IR spectra were collected with a Bruker Vertex 77 v/S, Hyperion 3000 IR microscope. A liquid nitrogen cooled MCT detector was used. The mid-IR microscope system typically focuses the IR beam to 20 μm which is then spatially filtered using a 20 μm diameter circular aperture. A background spectrum was acquired by releasing the original irradiated/damaged CCl4 sample between the diamonds and then subtracted from the spectrum acquired with the pressurized sample. RAMAN RESULTS
We first present the results of our Raman experiment during compression in Fig. 1. The pressure dependence of the observed and fitted spectral modes is plotted in Fig. 2. We have also included selected ranges of pressure in Figs. 3–5 to highlight changes in the spectra that yield supporting evidence for the proposed phase IV → V (Fig. 3), phase V → VI (Fig. 4), and phase VI → VII (Fig. 5) transitions. Due to the complexities associated with rapidly sealing and pressurizing a high vapor pressure liquid that rapidly evaporates, we loaded our sample at 1.3 GPa, which implies that it commenced in Phase III.10 The ν 2,s (E) vibration8 near 220 cm−1 (i.e., C-Cl symmetric stretching mode) at ambient pressure has split as expected8, 9 at high pressure. The ν 4,s (F2 ) vibration (i.e., C-Cl asymmetric bending mode) near 325 cm−1 appears at high pressure except at a higher wavenumber value as do the ν 1,s (A1 ) modes near 460 cm−1 (i.e., C-Cl symmetric stretching mode) and the ν 1 and ν 1+4 modes near 760 cm−1
FIG. 1. Stacked Raman spectra of CCl4 at all of the studied pressures in compression.
J. Chem. Phys. 140, 194503 (2014)
FIG. 2. Stacked plots displaying the pressure-dependent behavior of the observed Raman modes of CCl4 .
(Fermi resonance doublet)9 suggesting that our sample comprised solely CCl4 . All modes consistently shift toward higher frequencies up to the highest pressure studied (28 GPa) indicating a stiffening of molecular bonds. The fits were perR . The errors of the fits are within the formed using PeakFit size of the dots used to make the plots. Comparing the spectra at 1.3 GPa and 4.3 GPa, a number of changes occur in the Raman spectra which include a splitting of the line near 225 cm−1 , the appearance of a new peak near 240 cm−1 , and major changes in the broader spectral features in the 750–800 cm−1 range. These changes were observed previously and assigned to the phase III→VI transition.8, 9 Near 7.4 GPa, a new peak emerges near 320 cm−1 (just to the left of the 325 cm−1 peak), which may suggest a phase transition (IV → V) as observed in Ref. 9 (see Figs. 1 and 3). Slope discontinuities in dω/dP were observed near 7 GPa in all of the lines with the exception of the aforementioned new peak near 320 cm−1 (see Fig. 3) lending further credence to the IV→V phase transition. Between
FIG. 3. Selected stacked Raman spectra of CCl4 within the 1.3–11.2 GPa region to highlight evidence for the possible phase IV → V transition just above 7 GPa.
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FIG. 4. Selected stacked Raman spectra of CCl4 within the 11.2–19.1 GPa region to highlight evidence for the possible phase V → VI transition just above 14 GPa. In particular, a new line emerges near 360 cm−1 .
7–14 GPa, no further phase transition was observed,8, 9 which agrees with our data. Above 14 GPa, however, a new peak emerges near 370 cm−1 commencing near 14.8 GPa (see Fig. 4). The data from Ref. 9 only go up to 13 GPa but Ref. 8 displays data from another high pressure polarized Raman study up to 22 GPa which displays some potential slope discontinuities in this pressure range (see Fig. 2 of Ref. 8). This study, however, does not mention any phases beyond Phase IV. In our study, above 21.9 GPa, yet another peak appears near 400 cm−1 (see Fig. 5). Near 22 GPa, the slope discontinuities are more pronounced and suggest a sluggish phase transition – possibly a mixture – which has been observed in other covalentlybonded systems such as cyclohexane.12 After reaching the terminal pressure (28 GPa), the sample pressure was reduced to 3.9 GPa, and the spectral frequencies appeared to return to the earlier values near this pressure (see Fig. 6). We found no evidence of laser-induced damage of our sample during the course of our studies.
FIG. 5. Selected stacked Raman spectra of CCl4 within the 19.1–28.2 GPa region to highlight evidence for the possible phase VI → VII transition near 21.9 GPa.
J. Chem. Phys. 140, 194503 (2014)
FIG. 6. Stacked Raman spectra acquired from CCl4 at high pressure. The first two lowest spectra at 4.3 GPa and 28.2 GPa, respectively, were recorded during sample compression. The remaining spectra were acquired in decompression of the sample and have a * labeling them as such.
XRD RESULTS
We used the 16 BM-D white x-ray beamline for our xray diffraction experiments. The flux irradiating our sample was far lower than at the 16 BM-B beamline (where we performed our earlier CCl4 decomposition experiments) due to the use of a monochromator1 that filters selected energies. As a result, no damage was evident during the course of our studies. The goal for the XRD study was to confirm any phases observed in the Raman study. We present our results in Fig. 7. The sample was pressurized up to 31.7 GPa and then depressurized to near ambient to verify if any of the sample suffered any irreversible changes with pressure cycling.
FIG. 7. Stacked x-ray diffraction curves of CCl4 at various pressures illus´ Two regions of interest trating some of the phase changes (λ = 0.509175 Å). showing changes of the XRD patterns are highlighted.
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FIG. 8. LeBail fitting of the decompressed XRD pattern of CCl4 at 0.58 GPa (top pattern in Fig. 7) illustrating the reversibility of pressurization up to 31.7 GPa. Miller indices are included.
Between 1 GPa and 4.35 GPa, a phase transition appears which would likely be a transition from phase III to phase IV.8, 9 From 4.35 GPa to 7.64 GPa, evidence of another phase transition (which we ascribe to IV→V) manifests (particularly when examining the XRD pattern lines above 14◦ ) which complements our Raman data. No significant changes (beyond the compression of unit cell parameters and thus shifting of the XRD lines toward higher angle) appear until 14.4 GPa where new peaks emerge near 5◦ , 12.5◦ , and 15.5◦ which suggests a phase transition somewhere around 14 GPa. Again, this corroborates our Raman data for a potential (V→VI) phase transition. Finally, at 19.2 GPa and above, most lines within 12.5◦ up to 16◦ largely disappear except for the appearance of two lines near 16o lending credence to the phase VI→VII transition. The lines within 5.5◦ –7◦ also largely disappear. A broader peak also appears near 16.5◦ at 25 GPa which shifts rightward in the 31.7 GPa pattern suggesting that the proposed phase transition is complex and probably incomplete at 19 GPa, and that the sample comprises mixed phases in the 19–25 GPa pressure range which agrees with our observed Raman slope discontinuities. Upon pressure release near 0.6 GPa, the XRD pattern was successfully fit to the structure of phase I suggesting that no molecular damage occurred in the sample and that the pressure cycling was reversible with perhaps some hysteresis.10, 12 The LeBail refinement to this decompressed pattern is displayed in Figure 8.
Material (G-NIM)14 or Single Walled Carbon Nano Tubes (SWNT)15 which have modes near this frequency. The slight contamination evident in the central pattern labeled “unirradiated region” is likely due to the large spot size of the exciting infrared radiation. Bear in mind that the sample is pressurized to 5 GPa thus the peak should be slightly higher in energy than the peak would be at ambient pressure. The virgin CCl4 spectrum does not contain this peak. Thus, based on this evidence, we propose the following reaction: CCl4 + hν → C + 2Cl2 . It should be noted that carbon particles were observed in shocked CCl4 and C3 O2 .16
IR RESULTS
Figure 9 displays the results of our IR spectroscopic interrogation of the irradiated sample from Ref. 1, both in the region where Cl2 was detected (top curve) via Raman spectroscopy1 and in regions away from the irradiated and consolidated region (middle curve), along with virgin CCl4 (bottom curve) confined by a IR-transmitting microscope slide. We do not anticipate observing Cl2 as it has no electric dipole moment.13 A large peak near 2750 cm−1 is immediately evident which we suspect is some sort of C-C vibration (given the high frequency of the vibration) and thus a carbon-based compound such as Graphene Nanoparticle Ionic
FIG. 9. Infrared data comparing of virgin CCl4 (bottom plot), irradiated CCl4 in a region of the sample that was not irradiated (middle plot), and in the region of irradiation in the same sample (top plot). The bottom spectrum was taken with a virgin sample of CCl4 placed on an IR-transmitting microscope slide. The top two spectra were recorded confined in a DAC at around 5 GPa but at two different locations within the sample. A peak near 2750 cm−1 is particularly prominent in the irradiated region (top plot) and absent in the virgin sample of CCl4 that suggests some sort of carbon allotrope – possibly some graphene precursor (G-NIM)12 or SWNT13 – produced via irradiation.
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CONCLUSION
We have performed three studies of an important and fundamental solvent CCl4 to ascertain the viability of this molecular system for use in useful hard x-ray induced chemistry. We have confirmed the presence of phase V near 7 GPa. We have observed evidence for two new phase transitions of CCl4 : phase VI near 14 GPa and phase VII above 22 GPa. The samples appear to survive pressure cycling up to 26 GPa and 32 GPa, respectively. In the third study, we have also garnered IR data for the other product(s) from the x-ray induced decomposition of CCl4 which appears to be some form of pure carbon with little or no introduction of heat. This may offer a unique means to produce carbon allotropes under extreme conditions. ACKNOWLEDGMENTS
We thank Melanie White, Ferenc Borondics, and Xia Liu for aid. We acknowledge support from the Department of Energy National Nuclear Security Administration under Award No. DE-NA0000979. We also acknowledge support from the DOE Cooperative Agreement No. DE-FC08-01NV14049 with the University of Nevada, Las Vegas. One of us (D.S.) acknowledges partial support from the UNLV NSF REU program, Award No. 1005247. Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357.
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A portion of the research described in this paper was performed at the Mid-IR beamline of the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. 1 M.
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