Dense Carbon Monoxide to 160 GPa: Stepwise Polymerization to Two

Nov 14, 2016 - Carbon monoxide (CO) is the first molecular system found to transform into a nonmolecular “polymeric” solid above 5.5 GPa, yet been...
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Dense Carbon Monoxide to 160 GPa: Stepwise Polymerization to Two-Dimensional Layered Solid Young-Jay Ryu,† Minseob Kim,† Jinhyuk Lim,† Ranga Dias,†,‡ Dennis Klug,§ and Choong-Shik Yoo*,† †

Department of Chemistry, Materials Science and Engineering, and Institute of Shock Physics, Washington State University, Pullman, Washington 99164, United States § Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, K1A 0R6, Canada ABSTRACT: Carbon monoxide (CO) is the first molecular system found to transform into a nonmolecular “polymeric” solid above 5.5 GPa, yet been studied beyond 10 GPa. Here, we show a series of pressure-induced phase transformations in CO to 160 GPa: from a molecular solid to a highly colored, low-density polymeric phase I to translucent, high-density phase II to transparent, layered phase III. The properties of these phases are consistent with those expected from recently predicted 1D P21/m, 3D I212121, and 2D Cmcm structures, respectively. Thus, the present results advocate a stepwise polymerization of CO triple bonds to ultimately a 2D singly bonded layer structure with an enhanced ionic character.

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INTRODUCTION Under high pressure, simple (or low Z) molecular solids transform into nonmolecular (extended) solids as their compression energies approach that of strong covalent bonds in constituent chemical species.1,2 The pressure-induced molecular-to-nonmolecular transitions occur as a result of electron delocalization at high density due to a rapid increase in electron kinetic energy. As such, they often lead to dense “fully saturated” covalent solids (or wide bandgap insulators) in extended 3D networks of corner (or edge)-sharing polyhedrals.3,4 The pressure-induced broadening of electronic bands, on the other hand, may lead to an insulator−metal transition, providing a competing mechanism.5,6 Carbon monoxide is one of the first molecular systems found to transform into a highly disordered and colored (brown-dark red) nonmolecular “polymeric” solid above 5.5 GPa that contains high-energy density,7,8 as predicted in its isoelectronic system of polymeric nitrogen.9 Yet, the structure of polymeric carbon monoxide (pCO) and its pressure-induced changes above 5−7 GPa are not well understood. This is largely due to an unstable nature of pCO; it is highly photosensitive, strongly hygroscopic, and chemically unstable at ambient conditions.10 The structure of pCO, on the other hand, was theoretically suggested to be a 3D network structure with five-membered lactonic rings.11 Yet, the predicted bandgap (2.1 eV) and high density (2.7 g/cm3) of this structure appear to be inconsistent with the observed dark color and estimated low density of recovered pCO (1.68 g/cm3).8 Recently, several additional forms of extended carbon monoxide have also been suggested to be stable at various pressures, including a 1D chain polymer (P21/m), a 3D network polymer (I212121) consistent with the earlier predicted structure, and a 2D layer polymer (Cmcm).12 © XXXX American Chemical Society

As such, it underscores an interesting structure-bonding relationship of CO in molecular phase, CO in the 1D chain, and C−O in the 3D network and 2D layer structures. These predictions, however, have yet been confirmed experimentally. To validate those predicted structures,11,12 we have investigated the pressure-induced transformations in carbon monoxide to 160 GPa and found a series of transformations to various polymeric solids: initially to phase I at 5.5 GPa, then, to phase II at ∼7.5 GPa and phase III at ∼50 GPa, showing characteristic structural, optical, and electrical properties analogous to those of the predicted structure,12 as well as the present structure optimization, electronic band structures, and frequency-dependent linear optical dielectric tensors employing the ABINIT code.13

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EXPERIMENTAL AND THEORETICAL METHODS High pressure CO gas (99.999% pure at ∼2000 atm) was loaded into membrane-diamond anvil cell (DAC) using a custom-built, high-pressure gas loader at Washington State University. A micron-sized ruby chip was loaded into the cell for pressure measurements. The pressure of samples was determined by the Ruby R-line luminescence,14 using a minimum laser-power to minimize the photochemical damage of the sample. The pressure above 80 GPa was determined based on diamond Raman edge.15 For synthesis of a microscopic amount of polymeric CO, we used both a Bridgeman anvil cell in combination of a 100 ton Received: September 18, 2016 Revised: November 7, 2016 Published: November 14, 2016 A

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Figure 1. Visual appearance of carbon monoxide at high pressures taken during pressure uploading (a) and downloading (b), showing the pressureinduced polymerization to black solid phase I at 5.5 GPa and its phase transitions to translucent phase II at ∼7 GPa and transparent phase III above ∼50 GPa. The sample remains highly transparent to 160 GPa−the maximum pressure studied. The numbers on the images represent the pressures in GPa.

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press8 and a transparent large anvil press (TLAP) in combination of Paris-Edinburgh (PE) hydraulic press.16 The former utilizes liquid CO loaded at liquid N2 temperature to produce a 5−12 mg quantity of samples, whereas the latter utilizes high-pressure gas CO loaded using a high-pressure gas loader to produce a 1−3 mg quantity of samples. We found no difference of the products synthesized in the two methods. The recovered sample was characterized by a scanning electron microscope (SEM, FEI Quanta 200F) and a transmission electron microscope (TEM, FEI Tecnai G20 T-20 Twin) at the Washington State University’s Franceschi Electron Microscopy and Image Center. The X-ray diffraction data at high pressures above 50 GPa was collected using the microdiffraction beamline (16IDB) at the HPCAT/APS with the X-ray wavelength, 0.40663 Å or E = 30.5 keV. The resulted diffraction data are typical of amorphous solids, showing no apparent diffraction lines or spots. Below this pressure polymeric CO is photochemically active, and no X-ray or laser spectroscopic investigation is possible without photochemical decomposition. The theoretical calculations of structures, enthalpies, and band structures were performed using density functional perturbation theory and the ABINIT code.17 Troullier-Martins norm-conserving pseudopotentials17,18 with a plane-wave cutoff energy of 60 hartree were used for the Cmcm structure. Large numbers of k-points were sampled to obtain an energy convergence of better than 10−6 eV and force convergence of better than 2 × 10−4 eV/Å.

RESULTS Figure 1 shows the pressure-induced chemical and phase transformations in carbon monoxide to 85 GPa, accompanying dramatic changes in the visual appearance during (a) pressureloading and (b) unloading. These changes indicate that carbon monoxide transforms to a black polymer above 5.5 GPa as previously reported.7,8 The polymerization is completed at around 6.3 GPa as the entire sample chamber becomes opaque. This polymeric phase I, however, is only stable within a small pressure range and transforms into a translucent polymer (phase II) at pressures between ∼7 and 10 GPa. Above 15 GPa the color of phase II slowly darkens to dark red at ∼20−40 GPa and, then, further transforms into a transparent solid (phase III) over the pressure range of 50−70 GPa. This phase III remains highly transparent to 160 GPa−the maximum pressure studied. The polymerization in carbon monoxide can also be induced photochemically at lower pressures, as apparent from small black dots at 3.2 GPa in Figure 1 produced after a weak laser illumination. This photochemically polymerized area (call it I′) expands as pressure increases and, interestingly, remains black to 15−50 GPa, where all other thermally polymerized area becomes translucent phase II. Nevertheless, it eventually becomes transparent, as phase III emerges above 50−70 GPa. In retrospect, it elucidates the presence of different, photochemical, and thermomechanical pathways to phase III. Upon pressure unloading, the sample remains highly transparent and textureless to 60 GPa, then, abruptly with an B

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Figure 2. Pressure-induced Raman spectra change of carbon monoxide, taken in the spectral regions of (a) 1400−1600 cm−1 for νS(CC)/νS(C O), (b) 500−800 cm−1 for νb(O−C−O), and (c) 2100−2200 cm−1 for νS(CO). The most characteristic signature for polymerization is an emergence of new peaks at 1600 cm−1 for νS(CC/CO) and 650 cm−1 for νb(O−C−O), both of which significantly reduce in its intensity above 20 GPa and nearly disappears above 50 GPa. The feature near 1400 cm−1 in (a) is the Raman of diamond anvils.

Figure 3. (a) Integrated intensity of transmission light through polymeric CO as a function of pressure, showing the onset of polymerization. (b) Images of recovered products from 8 (A), 9 (B), and 10 (C) GPa from large volume cells, showing the variation of phases I and II in physical appearance and texture. (c) Images of recovered products from 30 (D) and 85 (E) GPa.

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It is challenging to determine the crystal structure of polymeric CO phases at high pressures. This stems from Xray induced decomposition (especially of phases I and II below 50 GPa) as well as highly disordered nature of all extended CO polymorphs including phase III. Furthermore, the recovered polymorphs I and II are also disordered and undergo chemical decomposition as observed in many previous studies.7,8,10,11 Nevertheless, the phases II and III recovered from higher pressures (above 30 GPa) is relatively stable over extended pressure regions, although they too transforms back to phase Ilike black solids at least in parts. Nevertheless, these products exhibit the evidence of layered structure, as illustrated in Figure 4. Transmission electron microscope (TEM) images of the recovered phase III from 67 GPa shown in Figure 4a show small (less than 50 nm) crystallites embedded (or structurally frozen) in a largely disordered lattice. The crystallites exhibit

audible crack sound develops small brownish particles and grain boundaries upon further lowering the pressure. The crack sound is not from the failure of diamond, but from the sample -- probably associated with its energetic transformation to phase II. Nevertheless, the vast of the sample area remains highly transparent down to 30−25 GPa (mostly phase III), below which it develops further particles and grain structures (phase II). The sample becomes completely opaque rather abruptly within a small pressure range between 7 and 5 GPa, signifying the transformation to phase I or II. These pressure-unloading behaviors, however, strongly depend on a wide range of experimental parameters such as the maximum pressure and unloading pressure step. Nevertheless, the recovered sample generally appears glossy and dark reddish in color at ambient condition. The sample remains dry (unlike phase I), but photoreactive, at ambient conditions. Figure 2 plots the pressure-induced Raman changes of CO in different spectral ranges. The polymerization of CO is evident from the disappearance of νS(CO) at 2150 cm−1 above 5.5 GPa and the emergence of two broad Raman bands of pCO-I centered at 1600 and 1850 cm−1 (noted with asterisks) above 4.2 GPa (Figure 2a). These peaks are most likely associated with the stretching modes of νS(CC) and νS(CO) in highly conjugated 2D layers. A small peak also appears at ∼650 cm−1 signifying the bending mode of νb(O−C−O) (Figure 2b). The intensities of these two new peaks gradually decrease above 20 GPa and nearly disappear above 50 GPa. Note that the initial polymerization occurs from δ-CO as evident from its distinctive doublet feature of νS(CO) (Figure 2c). Upon recovery, the 1600 cm−1 peak appears below 5.1 GPa, although the incipient of this peak can be seen at considerably reduced intensity at around 7 GPa (not shown). The onset pressures of the transitions can be found by plotting the integrated intensity of optical transmission lights through the sample as a function of pressure (Figure 3a); molecular δ-CO to opaque polymeric pCO-I at 5.5 GPa, pCO-I to translucent pCO-II at 7.8 GPa and to transparent pCO-III above 50 GPa. Note that the transparency of pCO-II decreases with increasing pressures, indicating the pressure-inducing band gap closing. Then, it transforms to transparent pCO-III. A few-mg quantities of polymeric CO products (mostly phases I and II) were synthesized between 5 and 12 GPa using a large volume press8,16 and recovered to ambient conditions at a various stage of polymerization with a wide range of stability, color, and morphology, as shown in Figure 3b. The polymers recovered at ∼5−6 GPa (i.e., phase I) rapidly decomposes or sublimes, leaving behind small spherical particles; whereas, those recovered above 7−12 GPa (i.e., phase II) shows reddishbrown products, which remains stable and nonhygroscopic. The density of recovered phase II is estimated in the range of 2.4−2.7 g/cm3. This estimation is based on the recovered mass and the initial sample volume, which should be considered as a lower bound, considering an unaccounted amount lost during the recovery. Yet, this density is substantially larger than the previous estimation of 1.68 g/cm3,8 but is well in the range of that theoretically predicted, 2.7 g/cm3.11 Therefore, it is likely that the previously recovered CO polymers were mostly phase I or mixtures of phases I and II, as those were recovered from the first formed black polymer. This, in turn, advocates for the theoretically suggested 3D network structure11,12 to be more closely related to that of transparent, high-density phase II. The phases II and III recovered from higher pressures (from DAC) exhibit more glossy appearance, as shown in Figure 3c.

Figure 4. (a) TEM image of polymeric CO recovered from 67 GPa, well in the stability field of phase III. It shows two groups of nmlamellar layers, each intersecting at roughly 60 degree with the interlayer distances of 7 and 0.7 nm. Note that the layer structure is expected from the theoretically predicted 2D Cmcm structure.12 (b) Calculated lattice parameters based on the predicted Cmcm structure as a function of pressure, showing the rapidly increase b-axis below a few GPa. D

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The Journal of Physical Chemistry C evidence for distinctive nm-lamellar layers, each intersecting at roughly 120 degree with the interlayer distances of 7 and 0.7 nm. Therefore, these TEM images seem to suggest that the structure of phase III is similar to the theoretically predicted 2D layered Cmcm structure.12 In fact, the observed interlayer distance of 0.7 nm is roughly consistent with the interlayer spacing of the Cmcm structure along the half of the b-axis, which changes rapidly with pressure (Figure 4b). The calculated lattice parameters of the Cmcm structure are a = 2.262 Å, b = 11.982 Å, c = 2.642 Å, and ρ = 2.50 g/cm3 (Z = 4) at 0.15 GPa. Note that the b-axis expands from 9.8 Å at 2.0 GPa to 12.0 Å at 0.15 GPa, reflecting a rapid change in the interlayer interaction to that of weak van der Waals. In fact, it is important note that there is a large variation in the interlayer distance observed in recovered products. This variation in turn indicates the metastability of phase III at low pressures below ∼5 GPa. We attribute the larger spacing 7 nm in Figure 4a to the size of crystal domains, which are stacked up approximately 120 degree offset in the ac-plane. The Cmcm layered structure was predicted to be the most stable layered structure at pressures above ∼40 GPa in agreement with the experimental finding, as shown in Figure 5a. The calculated enthalpy predicts the structural phase transition from a 1D linear chain CO polymer, −(CO)− C(O)− in P21/m to a 3D network polymer with lactone moieties −O−(CO)− in I212121 at ∼3 GPa and then to a fully saturated (or all single-bonded) 2D layered polymer in Cmcm at ∼40 GPa. The calculated densities of these phases (Figure 5b) are 22.9 Å/cm3 (or 1.90 g/cm3) for P21/m, 17.0 Å/ cm3 (or 2.64 g/cm3) for I212121, and 14.5 Å/cm3 (or 3.10 g/ cm 3 ) for Cmcm, in reasonable agreement with those experimentally observed. The calculated electronic band structure of the Cmcm structure shows that it has a band structure with a very small overlap between the valence band and the conduction band suggesting a weak semimetallic character, where the top of the valence band at the Y point in the Brillouin zone rises above that of the conduction band at the Γ point (Figure 6). However, the sample remains highly transparent to 140 GPa (Figure 7) with the measured resistance value greater than 200 MΩ, the limit of our measurements. Nevertheless, this apparent difference between experiment and theory can be explained in terms of a highly anisotropic layer structure of the Cmcm phase, analogous to that of graphite giving rise to a semimetal character within layers (or the ab-plane) but an insulating character along the c-axis, or may simply be due to the underestimation of band gaps in solids that occurs with density functional theory.

Figure 5. (a) Difference in enthalpy of 3D (I212121) and 2D (Cmcm) structures with respect to that of 1D (P21/m), showing the transformations of 1D polymer to 3D at ∼3 GPa and to 2D at ∼40 GPa. The inset shows the low-pressure region using the structures in ref 12. (b) Calculated unit cell volumes of CO phases as a function pressures. The calculated densities at ambient conditions are in good agreements with those estimated from recovered products.

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DISCUSSION The present results suggest the presence of three polymeric phases: (i) highly colored relatively low-density phase I (or I′) in the predicted linear chain structure12 or other previously suggested 2D conjugated ladder structures,8,10 (ii) translucent high-density phase II in the 3D network structure, and (iii) transparent phase III in the 2D layer structure that becomes metallic above 150 GPa. The presence of three polymorphs of polymeric CO then reconciles the discrepancies between the previously reported experimental and theoretical densities, structures, and band gap energies.8,11 Clearly, the proposed 3D (I212121) and 2D (Cmcm) structures10 are more consistent with the observed properties of phases II and III, respectively. The estimated density 2.4−2.7 g/cm3 of the recovered products

Figure 6. Calculated electronic band structure of the Cmcm structure of CO at 147 GPa, showing the semimetallic character where the top of the valence band at the Y point in the Brillouin zone rises above that of the conduction band at the Γ point. E

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Figure 7. Microphotograph images of CO samples for electric resistance measurements using a four-probe method, showing the sample remains highly transparent to 140−160 (the maximum pressure studied) GPa with the resistance value exceeding 200 MΩ, the limit of our measurements. The numbers are sample pressures in GPa, and the four electric probes are made of platinum (about 5 μm thin). The gasket hole is about 100 μm, insulated by dense Al2O3 (less than 1 μm powders precompressed to ∼20 GPa) in the region of initially transparent but becomes gradually black as pressure increases above 10 GPa. The sample is at the center (∼75−40 μm diameter), which undergoes the change from transparent molecular CO at 1.5 GPa, opaque pCO-I at 6.3 GPa, transparent to translucent pCO-II between 10 and 50 GPa, and transparent pCO-III at 69 GPa and above.

from 9 to 12 GPa agrees well with the calculated one, 2.7 g/cm3 in ref 11 and 3.2 g/cm3 in ref 12. The predicted band gap (ΔEg = ∼1.9 eV11 or 4.4 eV12) is also consistent with the observed transparency of phase II. The above-described transformations in dense carbon monoxide underscore an interesting structure−bond relationship and a stepwise polymerization from CO in molecular phase, to highly conjugated, unsaturated CO in the 1D chain or 2D conjugated ladder (phase I or I′), and to mostly saturated C−O in the 3D network (phase II) and fully saturated 2D lamellar layer structures (phase III). The stepwise polymerization is consistent with the sequence of theoretically predicted transitions,12 as well as the recently discovered 3D cg-N to 2D LP-N transformation.19 Therefore, these results advocate the pressure-induced ionization as the electrostatic stabilization (or structural packing) energy overcomes the electron delocalization (or chemical bonding) energy at very high density states of both CO and N2. The similar layered structure observed in dense CO and N2 is a deja vu of the similar behavior of these two isoelectronic molecular systems below 5 GPa20 and, in retrospect, it undersores the difference between CO and N2 in the intermediate pressure range. The presence of small dipole in CO molecules collectively induces a difference pathway; that is, the observed stepwise polymerization to a singly bonded layered CO polymer. In contrast, molecular δ-N2 phase undergoes a series of structural distortions to 110 GPa21 where it eventually polymerizes to cg-N and LP-N at high temperatures. Hence, this comparison notes on the chemistry (or kinetics) aspect of the pressure-induced molecular-tononmolecular transition in this intermediate pressure range, as also evident from photochemically produced phase I′ transforming to phase III bypassing phase II in CO. In summary, we have studied carbon monoxide to 160 GPa, more than 20-fold increase from the past studies, which makes it possible to compare its behavior with other densely packed molecular systems such as nitrogen. The results have shown

that CO undergoes structural transitions with pressure from a highly colored, low-density phase I to translucent, high-density phase II to transparent, indirect-gap semimetallic phase III above 100 GPa. A previously predicted Cmcm layered 2D structure yields properties strongly consistent with the experimental observations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (509) 335−2712. ORCID

Choong-Shik Yoo: 0000-0002-2664-0730 Present Address ‡

Lyman Laboratory of Physics, Harvard University, Cambridge, MA 02138. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present study was supported by the DARPA (W31P4Q12-1-0009 and HR0011-14-C-0035). A part of this work was also supported by NSF-DMR (Grant No. 1203834) and DTRA (HDTRA1-12-01-0020). We are greatly thankful to Dr. Judah Goldwasser and Dr. John Paschkewitz for their enthusiastic encouragement and support. During the review process, there has been a new theoretical study by Kang Xia et al.,22 predicting that Pna21 and Cc structures are energetically more favorable below 10 GPa. This result, however, does not alter any conclusion made in the present paper.

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