Polymerization of Carbon Dioxide: A Chemistry View of Molecular-to

Nov 15, 2011 - ... phase transformations under high pressure and large shear. Mahdi Javanbakht , Valery I. Levitas. Physical Review B 2016 94 (21), ...
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Polymerization of Carbon Dioxide: A Chemistry View of Molecular-toNonmolecular Phase Transitions Amartya Sengupta,† Minseob Kim, and Choong-Shik Yoo* Institute of Shock Physics and Department of Chemistry, Washington State University, Pullman, Washington 99164, United States

John S. Tse Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N SE2, Canada ABSTRACT: Under high pressure, simple molecular solids transform into nonmolecular (extended) solids as compression energies approach the energies of strong covalent bonds in constituent chemical species. Unlike molecular and extended phase transitions, these exhibit path dependent phases, phase boundaries, phase metastabilities, and structural distortions that lead to large uncertainties in both experimental and theoretical phase diagrams. Here we present experimental and theoretical evidence that carbon dioxide polymerizes to extended phase V at 20 GPa, indicating a substantially lower equilibrium phase boundary than previously suggested. Clearly, these results indicate extended structures are inherently more stable above 20 GPa and the presence of a strong activation barrier hindering the polymerization in the intermediate pressure region between 20 and 40 GPa. Further, the present results advocate a chemistry view of molecular to nonmolecular phase transitions governed by constraints to kinetics and local energy minima that go beyond thermodynamics and are analogous to the graphite diamond transition.

1. INTRODUCTION Over the past several years, new materials and novel phenomena have been discovered and predicted at high pressures and temperatures. Many of these phenomena are fundamental chemistry problems,1,2 reflecting how chemical bonds break and form, how atoms and molecules organize over short and long ranges, and how kinetics and thermodynamics govern materials stability. It is common to observe the transformation of molecular solids into more compact structures with itinerant electrons (such as metallic and nonmetallic extended phases3 8). Such nonmolecular extended solids, particularly those composed of low Z molecules, constitute a new class of high-energy-density solids. These new solids store a large sum of chemical bond energy in their three-dimensional network structure (approximately several eV/bond).6,7,9 The large cohesive energy of singly bonded or sp3 hybridized electrons gives rise to an extremely stiff lattice9,10 and novel electronic and optical properties.3 7 Importantly, nonmolecular solids with monolithic network structures, held together by strong covalent bonds, have high kinetic barriers against reversal, offering opportunities to recover these novel materials at ambient conditions. Broadly speaking, molecular-to-nonmolecular transitions occur due to electron delocalization manifested as a rapid increase in electron kinetic energy at high density. The detailed mechanisms, however, are more complex and the transitions often exhibit path-dependent phase boundaries, phase metastabilities, and structural distortions.11,12 As a result, the equilibrium phase boundary is difficult to precisely locate (experimentally or theoretically) and the r 2011 American Chemical Society

results are often controversial, as in recent studies of carbon dioxide.13 Current debate on the carbon dioxide phase diagram centers around the existence of intermediate phases, or an intermediate pressure range, where intermolecular interactions between nearby carbon dioxide molecules are characteristically different from those of molecular (i.e., quadruploar) and extended (i.e., covalent bonds above 40 GPa) phases (Figure 1).14 16 The resolution of this debate not only is central to understanding the carbon dioxide phase diagram but also provides key insight into the molecular-to-nonmolecular phase transition mechanism. This knowledge is applicable to many other molecular systems including N2, CO, O2, H2O, and N2O. Does the transition (or more broadly the electron delocalization) occur abruptly in a single step,15,16 or continuously via intermediate phases?14,17 The latter model intrinsically advocates an energy landscape model and suggests the presence of many local energy minima along various transition pathways, thus posing theoretical and computational challenges. The stability (or metastability) of such phases would be controlled by constraints to kinetics and phase

Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: May 10, 2011 Revised: October 27, 2011 Published: November 15, 2011 2061

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Figure 1. A phase/chemical diagram of carbon dioxide, highlighting the current debate on the presence of intermediate phases between 20 and 40 GPa in blue and red lines. Also, note several outstanding issues regarding the exact location of the phase II IV boundary and the crystal structures of phases IV, V, and VI. A potential liquid liquid transition is also hypothesized, while the melt line of carbon dioxide was undetermined above the VII IV liquid triple point.

metastabilities that are well beyond thermodynamics and poses a challenge in determining the exact phase boundary.18 To overcome this challenge, we investigated the molecular to nonmolecular transition of carbon dioxide using metallic catalysts such as Pt, Ti, and Si. Catalysts lower the activation barrier and help establish the exact location of the equilibrium phase boundary, as demonstrated in the graphite diamond phase transition.19 Our results indicate that carbon dioxide transforms to extended phase V at 20 GPa in the presence of Ti, exploiting a substantially lower transition threshold than previous results at 40 GPa.3,10

2. EXPERIMENTAL AND THEORETICAL METHODS Because most carbon dioxide phases are metastable over a large pressure temperature range (well beyond their stability fields), to evaluate the phase stability and boundary, it is important to maintain a consistent P T path in the experiments. Therefore, in this study, we used membrane diamond anvil cells to provide constant loading force during heating. The experiments were typically performed along either isotherms or isobars. The occurrence of phase transitions can easily be observed from the characteristic Raman spectra and visual appearances (or crystal morphologies). CO2 samples were loaded in Re gaskets from a liquid by condensing CO2 gas to 238 K and 15 atm. Type IA diamond anvils were used with a culet size of 0.3 mm. A few micrometer sized ruby chips were scattered inside the cell for in situ pressure measurements. A resistive heater (Chromalox) wrapped around the cell and a K-type thermocouple mounted on the back of diamond provided external heating. During heating, the Raman spectra were collected as close as every 10 K and every few gigapascals. We used a home-built confocal micro-Raman system using an Ar+ laser and a laser heating system using a single mode Yb fiber coupled diode-pumped infrared laser with output power 100 W optimized at 1064 nm (YLM-100-SM-CS from IPG photonics). CO2 samples were heated indirectly via thin (∼10 μm) metal foils of Pt, Ti, and Si, which also served as catalysts.

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The temperatures were measured across the heating area with a resolution of 10 μm. We measured a large temperature gradient (∼100 K/μm) across the laser heating spot. The Raman data were collected along the existing temperature gradient of the quenched sample. For the X-ray diffraction experiments, we use a microdiffraction beamline (16IDB) of HPCAT/APS using microfocused (10 μm full width at the 95% intensity) monochromatic X-ray (λ = 0.3682 Å) and a high-resolution image plate detector (MAR3450). We used Fit2D to integrate the 2D diffraction images to the 1D angleresolved X-ray diffraction (ARXD) patterns and the GSAS to analyze the ARXD data using Le Bail intensity fitting. All calculations were performed with the VASP pseduopotential planewave code.20 Efficiency of ab initio total energy calculations for metals and semiconductors using a plane-wave basis set.20 The project augmented potentials21 constructed with the PBE generalized gradient approximation22 for C and O atoms were used. Tight convergence criteria of the force tolerances from 0.0001 to 0.0004 eV/Å were used in the geometry optimization calculations. A very stringent criterion is necessary in order to obtain reliable atomic positions and lattice parameters as illustrated in Supplementary B. It was found that for some cases in the molecular phases, initial geometry obtained from a lower energy cutoff (e.g., 400 eV) and a smaller k-point set may deviate significantly from the converged results. This becomes particularly important at high pressure (>40 GPa) where the potential energy surface for the CO2 molecule is very shallow and great care must be exercised to ensure the optimal structure is obtained.

3. EXPERIMENTAL RESULTS In this study, we performed two sets of experiments utilizing two different initial phases of CO2 (III and IV) with several catalysts of Pt, Si, and Ti. The results are summarized in Figure 2. For the first set of experiments, we investigated the catalytic effect on phase III (Figure 2a). Freshly loaded CO2 samples were compressed to phase III at several pressures and then laserheated to 1000 2000 K using three catalytic metal foil heat absorbers. Polymerization to phase V was evident in all cases based on its characteristic Raman C O C bending mode at ∼800 cm 1. Note, however, the subtle differences in transition threshold pressures (Figure 2a) and in the samples’ spectral characteristics. With Pt, the transition occurs at 40 GPa as observed previously.3 But the transition occurs at ∼35 GPa with Si and ∼32 GPa with Ti. The similar atomic size of Ti with carbon likely accounts for the greater catalytic effect of Ti. When heated with Si, the laser-heated area became transparent and the 800 cm 1 band spit into two. We attribute these changes to melting of the Si foil during laser heating, which can lead to formation of a CO2 and Si alloy. This conclusion is further supported by the observation of carbon species after laser heating, as confirmed by a broad Raman band at 1600 cm 1 for CdC stretching. In the second series of experiments, we examined the catalytic effect on phase IV (Figure 2b). Phase IV was produced by compressing freshly loaded CO2 samples to phase III at ∼22 GPa, followed by Ohmic heating to phase II at 470 K and then to phase IV at ∼580 K. The presence of phase IV was confirmed by characteristic Raman peaks,14 which, as expected, are considerably broadened at high temperature. These peaks, however, became well resolved as the temperature was reduced slowly to room temperature. During this cooling process, we observed a 2062

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Figure 2. (Left) Raman spectra after laser heating CO2 III with different heat absorbers (or catalysts) of Pt, Si, and Ti, showing their respective transition pressures at 40, 35, and 32 GPa. (Right) Raman spectra after laser heating CO2 IV with Pt, Si, and Ti, at 20 GPa, signifying the transition of phase VI in Ti to phase V at 20 GPa. The Raman spectrum of phase V at 20 GPa downloaded after the synthesis at 40 GPa with Pt, for comparison. The second harmonic behavior of the sample (inset) also supports the presence of phase V.

Figure 3. (Left) The intensity of 527 nm or the second harmonic of the IR laser through phase V synthesized in Ti at 20 GPa, showing strong SHG behavior over a large pressure range of 10 120 GPa. The inset shows the microphotograph of the sample at the maximum SHG intensity at around 45 GPa, for comparison with that at 20 GPa in Figure 2 inset. (Right) The Raman spectra of the sample across the laser heated area, showing the temperature dependent transformation. The center of heating spot (bottom) is seen to have CO2 phases of IV and V together with a significant amount of TiO2, whereas the edge and outside are mostly CO2 phases. The temperature was ∼2000 K at the center and 1600 K at the edge.

pressure drop of about 10%, and a single phase of phase IV was stabilized at ∼20 GPa and ambient temperature. The phase IV samples, with Pt, Si, and Ti, were then laser-heated to 1000 2000 K. We found phase V only in samples with Ti heated to ∼1600 ((50) K. The quenched phase V exhibits a Raman peak at ∼730 cm 1. This is closely compared to the

735 740 cm 1 peak at 20 GPa of pure phase V (produced at 40 GPa). Furthermore, it exhibits a strong second harmonic generation (SHG) behavior between 10 and 120 GPa (see Figure 2b inset and Figure 3a). Laser-heating phase IV with a Ti catalyst typically yields a mixed phase of CO2 V, IV, ω-Ti,23 and baddeleyite-type 2063

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spectrum. Interestingly, phase IV at 23 GPa fits only with the Pbcn structure with a = 3.987(1) Å, b = 6.053(1) Å, c = 4.404(2) Å, V = 106.3(1) Å3, and 2.750 g/cm3 with reduced χ2 = 0.11. The relatively large distortion (∼10%) in the ac-basal plane does not permit fitting this phase with the tetragonal P41212 structure. Note that the b/a (or b/c) ratio of ∼1.5 is somewhat larger than that of pure CO2 IV of ∼1.4 at the same pressure and indicates an even larger disparity in the nonbonded C 3 3 3 O distance and a larger C O C bending angle. Similarly, the diffraction pattern of CO2 V in the mixed sample fits better with a hexagonal tridymite structure of P63/ mmc (a = 5.715(1) Å, c = 9.116(2) Å)27 than the orthogonal P212121 cell previously used for pure CO2 V.10 Nevertheless, the density of F = 3.401 g/cm3 with Z = 12 is reasonably well matched to that of pure V (F = 3.320 g/cm3), and the c/a ratio of 1.60 with that of SiO2 β-tridymite, 1.63.27 A slightly larger density of phase V can easily be attributed to a small (0.001 42.895 3.4351 3.4345 3.9129 39.703 3.4570 3.4571 3.9338 34.879 3.4868 3.4924 3.9789

Pnnm

>0.024 P42/mnm 3.4264

3.8845

0.001

P42/mnm 3.4348

3.9129

0.001 0.001

P42/mnm 3.4570 3.9338 Pnnm 3.4868 3.4929 3.9789

>0.006 P42/mnm 3.4896 30.492 3.5229 3.5174 4.0287

0.001

Pnnm

0.001

Pnnm

4.0686

23.445 3.5851 3.5932 4.1100

0.001

20.507 3.6291 3.6168 4.1524

>0.009 P42/mnm 3.5982 4.1100 0.001 Pnnm 3.6291 3.5992 4.1100

3.5981 3.5992 4.1100

>0.013 P42/mnm 3.6230 17.900 3.6581 3.6552 4.1950

0.001

Pnnm

All tolerance values are in Å.

4.1100

3.6581 3.6552 4.1950

>0.003 P42/mnm 3.6567 a

4.0287

3.5563 3.5336 4.0686

>0.003 P42/mnm 3.5550 Pnnm

3.9789

3.5229 3.5174 4.0287

>0.006 P42/mnm 3.5202 26.792 3.5563 3.5536 4.0686

3.8504

3.4381 3.4146 3.8845

4.1950

consider the experimentally suggested structure of an idealized α-tridymite (P212121(2)),10 the theoretically predicted structures of α-cristobalite (P41 21 2), 4,13 and a new tridymite (P2 1 2 1 2 1 (1)) found in the present constant-pressure molecular dynamics simulation by heating phase III (Cmca) to 1000 K above 50 GPa. Calculated enthalpies of the structures are compared in Figure 5a. It is surprising and significant that even at the relatively low 23 GPa, the nonmolecular 3D extended structures are more stable than the molecular structures and this relative stability increases with increasing pressure. Among the 3D extended structures studied here, the previously predicted P41212 phase is indeed most stable at high pressure. However, the energetic differences among the two P212121 structures and the P41212 diminish at lower pressures. Below 23 GPa, the molecular phases become more stable than the extended phases. All molecular phases are energetically very competitive. Another important finding of the present calculations is the stability of bent CO2 molecules in the Pbcn structure; this was not indicated in previous calculations.13,16 The calculations show the O C O valence angle distorted to 178.6° at 17.5 GPa reaching 177.7° at 46 GPa (Figure 5a inset). Note that the bending of CO2 is an inevitable precursor to the formation of a polymeric structure. The calculated equations of state of all considered structures are compared with the experimental results in Figure 5b. The agreement is only qualitative. The calculated specific volumes for molecular phases are very close. The predicted volume for II (P42/mnm) is in good agreement with experiments above 20 30 GPa. For phase III (Cmca), except at low pressures below 23 GPa, the theoretical volume is 10% smaller than the observed. The enthalpies for phases II and III are very competitive between

Figure 5. (a) The enthalpies of various carbon dioxide phases as a function of pressure, showing the stability of the 3D extended structures above 23 GPa. In the inset, the calculated bending angle of phase IV (Pbcn) as a function of pressure, showing a linear increase of bending angle with pressure. (b) The calculated specific volumes of various CO2 phases (lines with the symbols) as a function of pressure, presented in comparison with the experimental values (lines without the symbols). 2065

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The Journal of Physical Chemistry C 20 and 50 GPa with the former being slightly lower. The predicted specific volumes of the extended P212121(1) and P412121 phases are obviously too small (density too high) with respect to the experimental result of phase V. In comparison, the calculated volumes of the P212121(2), albeit still low, but compare better with the experimental values. The large difference between the calculated and observed volume is puzzling, because it exceeds the accepted regime of accuracy for the DFT method. The discrepancy is highly unlikely due to the basis set and the pseudopotentials employed. The trend presented here is consistent with previous calculations using different pseudopotentials and computer codes. To ensure the quality and the appropriateness of the computational parameters used in the present study, the EOS for the Pa3, Cmca and P42/mnm molecular phases were computed employing the same PAW pseuodpotentials. The results are in quantitative agreement with those reported in ref 16. We speculate that the difference may arise from the fact that the experimental structures were obtained under nonthermodynamic conditions either from the quenched phases V and II from high temperatures or from metastable phase III.14 16 Potential anharmonic effects, thermal expansions, and structural distortions have not been taken into account by the theoretical calculations. If this is indeed the cause, results from earlier calculations may also be viewed with caution, since the results may be dependent on the trajectory chosen12 and the observed structures need not be thermodynamically most stable.11

5. CONCLUDING REMARKS Clearly, the present experimental and theoretical results indicate that above ∼20 23 GPa, nonmolecular 3D extended structures are thermodynamically more stable than molecular phases. The fact that such structures were not observed in the experiments without the presence of a catalyst, unless the pressure exceeds 40 GPa at 1000 K, indicates kinetic control of the observed structural transformation from the molecular phase. The activation barrier required to break the CdO π bond is too large to be overcome simply from the gain in the PΔV work by external compression. The presence of this large kinetic barrier, on the other hand, results in intermediate phases appearing in the intermediate pressure range of 20 and 40 GPa prior to their eventual transformations to nonmolecular phases. The metastabilities, path dependent boundaries, large strains, and lattice distortions observed in these intermediate phases are rare for molecular phases, underscoring the presence of relatively strong intermolecular interactions or intermolecular bonds. Finally, the present chemistry-oriented view of the molecular to nonmolecular phase transition in carbon dioxide may offer new insights into other molecular systems such as N2, CO, and carbon. For example, CO and N2 behave similarly below 5 GPa where they remain as molecular phases.25 Yet, at higher pressures, δ-CO polymerizes at 5 GPa and room temperature,7,29 whereas δ-N2 undergoes a series of structural transitions (or distortions) to intermediate phases (ε, ζ, η, ι, and θ),30 which eventually polymerize to cg-N only above 110 GPa and 2000 K.5,31 Clearly, this highlights the chemical difference between these two isoelectronic systems at high pressure. The graphite-todiamond transition involves a strong modification in chemical bonds from sp2 to sp3 and accompanies with a high-energy barrier. As a result, despite a small energy difference of 0.02 eV per atom, the graphite-to-diamond transition has never been

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observed at its equilibrium pressure of ∼5 GPa without use of catalysis or seed.19 The transition is typically observed at substantially higher pressures (for example, 100 GPa under shock32), and the graphite instead becomes structural disorder above 5 GPa.33

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses †

Department of Geosciences, Princeton University, Princeton, NJ 08544.

’ ACKNOWLEDGMENT The present study has been supported by NSF(DMR0854618), DARPA(W911NF-10-1-0081), and DTRA (HDTRA109-1-0041). The X-ray work was done using the microdiffraction beamline (16IDB) at the High Pressure Collaborating Access Team’s (HPCAT) of the Advanced Photon Source. We appreciate Dr. Y. Meng for technical support at the beamline. Use of the HPCAT facility was supported by DOE-BES, DOE-NNSA (CDAC, LLNL, UNLV), NSF, DOD-TACOM, and the W. M. Keck Foundation. ’ REFERENCES (1) Hemley, R. J.; Ashcroft, N. W. Phys. Today 1998, 51, 26–32. (2) Grochala, W.; Hoffmann, R.; Feng, J.; Ashcroft, N. W. Angew. Chem., Int. Ed. 2007, 46, 3620–3642. (3) Iota, V.; Yoo, C. S.; Cynn, H. Science 1999, 283, 1510–1513. (4) Serra, S.; Corazon, C.; Chiarotti, G. L.; Scandolo, S.; Tossatti, E. Science 1999, 284, 788–790. (5) Eremets, M. I.; Gavriliuk, A. G.; Trojan, I. A.; Dzivenko, D. A.; Boehler, R. Nat. Mater. 2004, 3, 558–563. (6) Mailhiot, C.; Yang, L. H.; McMahan, A. K. Phys. Rev. B 1992, 46, 14419–14435. (7) Lipp, M. J.; Evans, W. J.; Baer, B. J.; Yoo, C. S. Nat. Mater. 2005, 4, 211–215. (8) Bernard, S.; Chiarotti, G. L.; Scandolo, S.; Tosatti, E. Phys. Rev. Lett. 1998, 81, 2092–2095. (9) Cohen, M. L. Phys. Rev. B 1985, 32, 7988–7991. (10) Yoo, C. S.; Cynn, H.; Gygi, F.; Galli, G.; Iota, V.; Nicol, M. F.; Carlson, S.; Hausermann, D.; Mailhiot, C. Phys. Rev. Lett. 1999, 83, 5527–5530. (11) Sengupta, A.; Yoo, C. S. Phys. Rev. B 2010, 82, 012105. (12) Sun, J.; Klug, D. D.; Martonak, R.; Montoya, J. A.; Lee, M. S.; Scandolo, S.; Tosatti, E. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 6077– 6081. (13) The current controversies are centered on (i) phase boundaries (see Figure 1), (ii) the presence of intermediate phases (Figure 1), (iii) bent CO2 structures in phase IV (Yoo, C. S.; Iota, V.; Cynn, H. Phys. Rev. Lett. 2001, 86, 444–447. Gorelli, F. A.; Giordano, V. M.; Salvi, P. R.; Bini, R. Phys. Rev. Lett. 2004, 93, 205503. (iv) structure of phase VI (Iota, V.; Yoo, C. S.; Klepeis, J.-H.; Jenei, Z.; Evans, W.; Cynn, H. Nat. Mater. 2007, 6, 34–38. )and a-carbonia (Santoro, M.; Gorelli, F. A.; Bini, R.; Ruocco, G.; Scandolo, S.; Crichton, W. A. Nature 2006, 441, 857–860. Montoya, J. A.; Rousseau, R.; Santoro, M.; Gorelli, F.; Scandolo, S. Phys. Rev. Lett. 2008, 100, 163002. )that are different from theory ref 12; (v) structure of phase V between experiments (ref 10) and theoretical works (Dong, J.; Tomfohr, J. K.; Sankey, O. F. Phys. Rev. B 2000, 61, 5967– 5971. Lee, M. S.; Montoya, J. A.; Scandolo, S. Phys. Rev. B 2009, 79, 144102. 2066

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