Copolymerization of CO and N2 to Extended CON2 Framework Solid

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials 2

Copolymerization of CO and N to Extended CON Framework Solid at High Pressures 2

Choong-Shik Yoo, Minseob Kim, Jinhyuk Lim, Young-Jay Ryu, and Iskander G Batyrev J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03415 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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The Journal of Physical Chemistry

Copolymerization of CO and N2 to Extended CON2 Framework Solid at High Pressures

Choong-Shik Yoo1*, Minseob Kim1, Jinhyuk Lim1, Young Jay Ryu1#, and Iskander G. Batyrev2 1. Department of Chemistry and Institute for Shock Physics, Washington State University, Pullman, Washington 99164 2. Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5069 * Corresponding author: [email protected] # Current address: GSECARS, University of Chicago, Chicago, IL 60637

ABSTRACT Synthesis of novel extended forms of nitrogen and nitrogen-rich materials has been a topic of interest in development of high energy density materials. Here, we present the formation of high-density (3.983 g/cm3) copolymer CON2, formed in crystalline form by laser heating of CON2 mixtures above 1700 K and 45 GPa – a substantially lower pressure-temperature condition than those required for converting pure nitrogen (above 110 GPa and 2000 K). It can be made even at lower pressures ~20 GPa at ambient temperature for amorphous solid. According to the refined structure, the crystalline polymer is made of nitrogen hybridized, eight membered rings of singly bonded CON2 in a three-dimensional framework structure in the space group of P43, as one of the previously predicted structures. However, unlike the predicted structures, the present P43 solid converts back to ε-N2-like and δ-N2-like molecular phases as pressure unloads to 20 and 10 GPa, respectively.

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INTRODUCTION Nitrogen-rich extended solids contain large chemical energy because of high stability of nitrogen molecules at ambient conditions. For example, cubic gauche nitrogen (cg-N)1, a polymeric form of nitrogen molecules made of three-fold coordinated nitrogen single bonds in a threedimensional (3D) network, predicted to contain large chemical energies (~33 kJ/cm3)2 - three times that of HMX (11 kJ/cm3)3, one of the most powerful explosives used today. Under a right condition, cg-N can exothermically disintegrate or depolymerize to stable nitrogen, rapidly releasing a large sum of energy. The specific impulse of cg-N is estimated to be about 400 s,4 ideally suited for novel environment-friendly propellants that can be used in oxygen deficient environments, for example. Therefore, the synthesis and recovery of novel extended forms of nitrogen molecules and nitrogen-rich materials have been a topic of current research interest to development of high energy density materials. The transitions to single bonded nitrogen polymers such as cg-N1 and more recently discovered layered polymeric nitrogen (LP-N)5, however, require not only the formidably high pressure and temperature (PT) above 110 GPa and 2000 K, but these polymers also become unstable below 40-60 GPa upon pressure unloading. Therefore, recent research efforts have emphasized the synthesis of similar nitrogen polymers, using various forms of nitrogen-rich compounds as precursors such as metal- and molecular- azides at high pressures6-8, as well as polynitrogen ions and azole-based salts at ambient pressure9-10. The concept here is to use a relatively high-density nitrogen-rich compound that is already in an intermediate regime between low-density molecular nitrogen (< 1.5 g/cm3) and high-density extended nitrogen polymer (~ 3.5 to 4.5 g/cm3).

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In this study, we consider an alternate concept using chemical mixtures of nitrogen for synthesis of high energy-density nitrogen-rich solids. This concept stems from the presence of internal chemical pressure in mixtures11-13, which can lower the required external pressure to form high-density nitrogen-rich extended solids. A good candidate for such an application is an isoelectronic mixture of CO and N2. CO and N2 are isoelectronic diatomic systems with similar melting temperatures, phase transitions, and crystal structures at low pressures below ~4-5 GPa14. However, their chemical behaviors are quite contrast at higher pressures. For example, cubic δ-CO (Fm3m) chemically transforms into a highly colored polymer (poly-CO) above 5 GPa15-17, whereas δ-N2 (Fm3m, isostructural with δ-CO) undergoes a series of structural distortions to ε-N2, ζ-N2 and η-N218, and polymerizes into singly bonded cg-N or LP-N only above 110 GPa and 2000 K1,5. Therefore, it is considered that the diverse chemical behavior between CO and N2 arises from a small dipole on CO molecules, yet collectively large in dense solid CO at high pressures. CO-N2 mixtures, on the other hand, are highly miscible below 3-4 GPa, providing a way of tuning the magnitude of collective dipoles and presumably chemical reactivity of the mixture. Recent theory has shown that in CO-N2 mixtures CO catalyzes the molecular dissociation of N2, resulting in 1D copolymers below 18 GPa and 3D networks of CON in Pbam and Fdd2 at 18 GPa19. The Pbam structure becomes most stable above 52 GPa. A more recent calculation20, however, has predicted yet another P43 structure of N2CO to be most stable above 35.6 GPa. These calculations have predicted high energy content in these materials, ranging 2.2 kJ/g for the Pbam- CON structure to 4.6 kJ/g for the P43 - N2CO structure. Moreover, the calculations suggest that the predicted polymers are dynamically stable at the ambient pressure, which may

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offer an opportunity to develop novel high energy density solids. In contrast, the experimental efforts to synthesize nitrogen-rich polymers have primarily been for the photochemical reactions in 5-10% CO doped in N2 at high pressures21,22. As a result, there has been no experimental evidence for the predicted, thermo-mechanical (or pressure-induced) reactions between CO and N2 structures to occur at high pressures. In this paper, we present the formation of high-density (3.983 g/cm3) copolymer of CON2, formed by laser heating of CO-N2 mixtures above 1400 K and 40 GPa – a substantially lower PT condition than that of pure nitrogen polymers (above 110 GPa and 2000 K)1,5. This polymer is made of singly bonded four-fold coordinated carbon, three-fold nitrogen and two-fold oxygen in a three-dimensional (3D) framework structure (in the space group of P43), as predicted in first-principles calculations19,20. However, unlike the calculation, the P43 phase converts back to ε-N2-like and δ-N2-like molecular phases as pressure unloads below 20 GPa.

EXPERIMENTAL AND THEORETICAL METHODS High-pressure CO and N2 (99.9% purity) gas mixtures were loaded in diamond anvil cells (DACs) at a total gas pressure of ~2000 atmospheres with a desired composition, using a custom-designed high-pressure gas loader (designed at Washington State University by C. S. Yoo). The sample pressure was determined by the Ruby luminescence23. Raman spectra were occasionally acquired, using a low laser-power (less than 10 mW) to minimize unwanted photochemical reaction at low pressures. The mixture samples, however, become none photoreactive above 20 GPa, where laser-heating experiments were performed using 1070 nm IPG Photonics ytterbium fiber laser. X-ray diffraction experiments were conducted at the (16IDB) of

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the High Pressure Collaborative Access Team (HPCAT) at the Advanced Photon Source (APS), using focused (~ 0.01 mm diameter) monochromatic X-rays (λ = 0.4062 Å or 0.3554 Å). Inelastic X-ray Raman scattering (XRS) experiments were performed at the 16IDD at the HPCAT using monochromatic X-rays (E = 9.903 keV, ~0.03 x 0.06 mm2 at FWHM). The samples were prepared in an x-ray translucent Be gasket. A bent Si (555) single crystal analyzer (100 mm in diameter) were vertically mounted on a 870-mm Rowland circle to refocus inelastically scattered x-ray photons onto a Si detector (Amp Tek) at a scattering angle of 25 degrees to the focusing polycapillary in a nearly back scattering geometry (Bragg angle of 87.15 degrees)24. The overall system provides an energy resolution of ~1eV. The “ab initio” evolutionary simulation for prediction of crystal structure was used for search of stable structures at a given pressure. Both variable- and fixed-composition options of the USPEX25 were used in the calculations. Unit cell containing 16 atoms was corresponding to generation size and the initial number of structures was 4 times larger than the generation size. The number of evolutionary generations was 150, quite large for this type of calculations. New structures are created by combination of the variational operators (i) heredity operator, (ii) transmutation operator, (iii) soft mutations operator; and (iv) randomly generated structures for each generation.25 Density functional theory (DFT) calculations in the evolutionary calculations were performed using plane waves VASP code26 with PBE27 projector augmented waves with ultra-soft part and 340 eV of cut-off for wave functions, ~30 irreducible k-points and vdW-DF2 dispersion correction.28 Minimum enthalpy structures were recalculated using norm-conserving pseudopotentials29 with 750 eV energy cut-off, self-consistent wave function optimization energy convergence of 10-6 eV/atom and 16-96 k-points in irreducible Brillion-zone. Full geometry optimization at high pressure was performed using DFT plane waves code CASTEP

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[30] so that forces were converged to 0.001 eV/Ǻ. Phonon dispersion curves were calculated using linear response and variational density functional perturbation theory (DFPT) as implemented in CASTEP.30 Calculations of band structure were performed using screened exchange Heyd−Scuseria−Ernzerhof functional HSE06.31

RESULTS The experiments were performed at four compositions of CO-N2 mixtures (1:9, 3:7, 5:5, and 7:3 volume ratios of CO:N2). Figure 1 shows a series of microphotographs (a) and Raman spectra (b) of 1:1 CO:N2 mixtures at high pressures, showing a typical pressure-induced transformation to a reddish-brown amorphous solid. CO and N2 are completely miscible and form transparent homogeneous mixture below 4 GPa, where CO and N2 crystallize into isostructural δ (Fm3m)phase. The δ-CO phase (for example, one shown at 4.7 GPa in Fig. 4b) is highly photoactive and polymerizes to dark polymer upon the illumination of laser lights17, as evident from a small dark spot at 6.3 GPa in Fig. 1a, whereas the entire sample area becomes dark brown at 8-15 GPa. Upon further increasing pressure, the sample becomes more homogeneous and translucent, especially above 40-45 GPa. Raman spectra of the sample (Fig. 1b) are characteristic to homogeneous mixtures of βCO and β-N2 at 2 GPa and of δ-CO and δ-N2 above 4.7 GPa. The β- and δ- structures are evident, respectively, from the singlet arising from spherically disordered, hexagonal closed packed structure (P63/mmc)32 and the doublet arising from the two atomic sites of Pm3m with the occupancy ratio of 3:133. Also shown at 4.7 GPa is a broad peak centered at ~1600 cm-1, most likely of C=O/C=N vibrons17,34, which indicates the photo-induced polymerization of δ-CO as

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seen in Fig. 1a. The pressure-induced polymerization, however, is most indicative from the formation of dark-brown products above 8.2 GPa, which accompany the significant spectral change. That is the disappearance of δ-CO and δ-N2 vibrons, as well as the 1600 cm-1 band. This clearly indicates a copolymerization of N2 and CO and/or an incorporation of N2 and CO into the photo-induced polymer, converting C≡O/C≡N and/or C=O/C=N into C-O/C-N bonds, respectively. Above 20 GPa, the sample shows no vibrational Raman feature, indicating the formation of highly amorphous solid. Note that all amorphous phases of both nitrogen (both ηand reddish- phases)35,36 and CO (polymeric CO-II and III)17 exhibit no characteristic Raman peaks. Similar pressure-induced changes in visual appearance and Raman spectra were observed in other N2-rich (1:9) and CO-rich (7:3) CO:N2 mixtures (Figs. S1 and S2). Considering the large kinetic barrier associated with the formation of extended nitrogen phases, we have performed laser-heating experiments on CO-N2 mixtures and investigated its structural changes using synchrotron X-ray diffraction. The results are summarized in Figs. 2 and 3, indicating the transformation of amorphous solids to crystalline, extended CON2 copolymers. Upon laser heating of 7:3 CO:N2 mixtures to 1725 K at 46 GPa (Fig. 2a), amorphous CO-N2 solid transforms into a crystalline solid with a series of sharp diffraction lines that cannot be explained in terms of δ- or ε- nitrogen. Instead, the diffraction pattern can be described reasonably well in terms of a mixture model of ε-N2 and P43 solids. The presence of excess ε-N2 is evident after the laser heating (Fig. S3). Laser-heating at low pressures below 40 GPa, however, depolymerizes (or decompose) the amorphous polymer to δ-like N2 and presumably CO (Fig. S4).

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The proposed P43 structure was determined using a full-pattern Rietveld refinement method37. Initial structure and atomic coordinates were adopted from theoretically predicted model. Because of the limited number of observed peaks and the peak intensity contributed from co-existed ε-N2 phase, the P43 structure was refined independently while ε-N2 phase was fixed. Scale factor, zero-shift, vibrational parameters, peak profile coefficients were refined. The (121) diffraction plane was applied as a preferred orientation. While refining atomic coordinates, soft constraints were initially placed on the interatomic bonds between C, N and O. This procedure results in a reasonable fit but one at 2θ=12 degree. The 12o peak is from a spurious singlecrystal-like diffraction spot, likely coming from a minor byproduct or, even, a defected CON2 structure especially along the (211) plane. The overall fitness of the diffraction pattern was, nevertheless, reasonably good with the reduced-χ2 of 1.636 and the cell parameters of a = b = 4.947 (2) Å and c = 3.818 (3) Å with density = 3.983 g/cm3. The refined atomic positions are: O(4a) (0.606, 0.036, 0.2820), C(4a) (0.106, 0.410, 0.891), N1(4a) (0.016, 0.842, 0.431), and N2(4a) (0.548, 0.352, 0.670). Upon refined atomic positions, four fold coordinated O2-C-N2 polyhedral units with nitrogen bridged eight membered-ring compose three-dimensional network polymer (see Fig. 3b). Averaged C-O, C-N and N-N bond distances are 1.36 Å, 1.32 Å and 1.46 Å, respectively. Figure 3 shows the X-ray diffraction patterns of 1:1 CO:N2 obtained before and after laser heating at 60 GPa and during the pressure unloading to 1.5 GPa. The measured diffraction pattern after laser heating, again, shows the formation of P43 phase that was observed in 7:3 CO:N2 mixture and, upon pressure unloading, its transformation to ε-N2-like phase at 30 GPa and further to δ-N2-like phase below ~15 GPa. Considering large lattice strains, disorders and heterogeneity of the sample, the refinements of the X-ray data (Fig. 3b) are not perfect but

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reasonable to be described in terms of the P43 (red bars and labels), ε-N2-like (green)38 and δ-N2like (yellow)34 structures. The resulting structures of these phases are summarized in Table S1. Figure 4 plots the pressure-volume compression curves of extended (red circles) and molecular (green triangles and yellow squares) CNO phases in comparison with those of cg-N (blue line), LP-N (brown), δ-N2 (yellow), and ε-N2 (green). Note that the transition from ε-like molecular phase to extended P43 solid accompany a huge increase in density by ~49 % from 2.680 g/cm3 to 3.983 g/cm3 at 44 GPa (see Table S1). The density of P43 phase is similar to that of cg-N,1,36 but smaller than LP-N.5 In order to understand the nature of chemical bonding in CO-N2 mixtures, we have also measured inelastic X-ray Raman scatterings (IXRS) of CO-N2 mixtures above 40 GPa, where they form amorphous extended solids and become photo-chemically inactive. The measured IXRS spectra (Fig. 5) at the K-edges of carbon (a), nitrogen (b) and oxygen (c) consist of a series of absorption bands arising from the transition from singly bonded C-N/C-O, N-N and N-O bonds, respectively. The absence of noticeable absorption feature expected from lower energy K→ π* transitions, on the other hand, underscores the fact that there are no longer significant sp2 hybridized C=C, N=O or O=O bonds, noting that strong peak at ~400 eV in Fig. 3b is coming from excess nitrogen molecules39. Therefore, this result unambiguously suggests that the recovered polymers are mainly consisted of singly bonded CON2 framework structure. To calculate thermodynamically stability of the structures, we have investigated a pseudo binary convex hull diagram of CO-N2 mixtures using minimum enthalpy structures of polymericCO in I212121 and N in P41212.40,41 For both structures, we have recalculated their enthalpies at 30 GPa using norm-conserving projected augmented waves42, the same as for the predicted

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calculations of CO-N2 mixtures. The resulting binary hulls (Fig. 6) suggests that both the present P43 structure of 1:1 CO:N2 mixture and three previously predicted structures (polymeric Pbam, framework Pbam and stacked Fdd2)19 of 2:1 mixtures are stable with respect to the separate CO and N2 phases. Phonon calculations of structure performed at 30 GPa yield no imaginary frequencies, suggesting that it is dynamically stable structure (see Fig. S5). The band gap of the P43 structure is calculated to be 3.726 eV using PBE-GGA27, or more realistically 5.430 eV using hybrid functional HSE06.30

DISCUSSION The present study presents an experimental evidence for the predicted high-energy density CON2 polymer. The refined structure suggests an extended framework structure of CON2 in the space group P43, made of nitrogen bridged, eight-membered C-N rings. Therefore, it clearly represents a copolymer of CO and N2, formed above 40 GPa and 1700 K. The copolymerization is evident even at ambient temperature above 20 GPa, but only to form an amorphous solid similar to those extended polymeric products of CO and N2 formed at relatively low temperatures (below 700 K). Note that these pressure-temperature conditions (above 20 GPa to amorphous solid and above 40 and 1700 K to crystalline solid) are substantially lower than those required to form extended nitrogen phases, either crystalline cg-N and LP-N (above 110-150 GPa and 2000 K)1,5 or noncrystalline η- and reddish-phases (above 100 GPa and room to ~700 K)35,36. This in turn underscores the role of CO promoting the N2 polymerization process. In fact, there is a very little composition dependence; the polymerization occurs even in 1:9 CO:N2 mixture at ~20 GPa. Note that pure CO transforms to polymeric CO phase II in a similar 3D

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framework structure of all single bonds. The thermal stability of the amorphous solid, however, is not strong, as such, laser-heating it below 40 GPa decomposes, rather than crystallizes it to the P43 phase. It is interesting to note that there is no apparent vibrational band even for crystalline P43 phase in experimental spectra (Figs. S1 and S2). This is despite the predicted phonons (Fig. S5) and vibrational collective modes (Fig. S6)43; the main peaks of calculated Raman spectra at 555, 950, 1177, and 1250 cm-1 correspond mainly to C-O stretching and rocking, scissoring on N atoms, rocking of N-C bonds, and stretching of N-C bond, respectively. A plausible explanation for this is the local “atomic” disorder between CO and N2 (or between C/O sites and N1/N2 sites). Such a disorder would have a minimal effect on the diffraction pattern, as the X-ray scattering cross sections are essentially the same for isoelectronic CO and N2. However, it would have a large effect on the vibrational spectra, destroying coherent Raman scatterings. This is because the random structural distortion would split fundamental vibrational modes of crystalline solids into a greater number of near ground state modes, resulting in poorly resolved wavy features in similar intensities and, eventually, washing out all spectral details (see Figs. S7 and S8). The disappearance of sharp Raman features in amorphous solids has been observed previously in several CO2 phases including CO2-IV, CO2-V, CO2-VI, and coesite-like c-CO2.44,45

It is important to recognize that the laser-heating experiments are performed in highly heterogeneous samples in terms of the composition, because of large thermal diffusion at high pressures. In this regard, it is not surprising to find the polymerization with a little composition dependence. On the other hand, this is sharply contrast to the strong composition dependence seen in the predicted structures, not only in the binary systems of CO and N2,19,20 but also in the

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ternary systems of C, O, and N.46 The predicted systems in the latter study46 are C2N2O in Cmc21 found in the ternary hull above 10 GPa and other metastable structures of (CO)2N and (CO)3N2 at 10-100 GPa. In fact, the present calculation (Fig. 6) also found the similar composition dependence; that is, the P43 structure is thermodynamically stable compared with the Pbam and Fdd2 at the 1:1 stoichiometry, but the Fdd2 is the most stable at the 2:1 compared with P43 and Pbam. However, the differences between these structures are within ~10 meV, and the relative stability is only marginal that can be changed depending on the temperatures and kinetics of synthesis. We attribute this apparent difference between the experiments and theories is due to the kinetics; that is, the transformation to the P43 requires a considerably lower activation barrier than those of the Pbam, Fdd2 and Cmc21. The lower kinetic barrier in the P43 stems from the following facts: Firstly, the nature of copolymerization maintaining its molecular units and stoichiometry, which requires no bond breaking and restructuring in dense solid mixtures.47 In this regard, the P43 structure (Fig. 2b) can be considered as being made of two hybridized N2 molecules in four-membered N2-dimer ring adjoining with two hybridized CO molecules in fourmembered CO-dimer ring.

Secondly, the role of carbon monoxide in promoting the

polymerization of nitrogen. The polymerization process (Fig. 1a) initiates from the formation of dark products with conjugated carbonyl groups (based on the 1600 cm-1 in Fig. 1b), which provide the active reaction sites for N2 molecules to co-polymerize. Note that the polymerization of pure N2 requires a large kinetic barrier, as previously observed in both cg-N1,36 and LP-N5. Thirdly, the concerted reaction mechanism to form CO-N2 copolymer, which should involve a substantially lower kinetic barrier than the reconstructive mechanism to form non-stoichiometric cross products of stacked Fdd2 and framework Pbam. The presence of organized dipoles and

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active reaction sites in solid CO can minimize the specific volume of the transition states toward the copolymerization with nitrogen and, thereby, lower the reaction barrier especially in dense solid mixtures.47 Therefore, it is likely that the lower kinetic barrier is responsible to which the P43 phase was synthesized regardless the initial composition of CO-N2 mixtures. In summary, the present study presents the evidence of amorphous and crystalline CON2 copolymers in singly bonded 3D network structure. The density of this polymer is 3.983 g/cm3, similar to that of cg-N in high energy density, but can be made at substantially lower pressures (at ~20 GPa at ambient temperature for amorphous solid and at ~40 GPa and 1700 K for crystalline form). Yet, these polymers are not stable below 20 GPa, it requires further step of stabilization for development of high energy density solids for ambient uses48.

ACKNOWLEDGEMENTS This work has been done in support of the NSF-DMR (Grant No. 1701360), ARO (W911NF-17-1-0468), DARPA (W31P4Q-12-1-0009), DOE-NNSA (DE-NA0003342), and ADD (U14358RF). The X-ray works were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. We thank Dr. Ross Hrubiak for his assistance with the laser-heating/X-ray diffraction experiment at the 16IDB and Dr. Yuming Xiao and Dr. Paul Chow for their assistance with the IXRS experiment at the 16IDD. HPCAT operations are supported by DOE-NNSA (DE-NA0001974) and with partial instrumentation funding by the NSF. We also thank to Dr. Chris Pickard for scientific discussions.

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ASSOCIATED CONTENT Supporting Information Supporting information is available for one supplementary table and eight supplementary figures and captions. This material is available free of charge via the Internet at https://urldefense.proofpoint.com/v2/url?u=http3A__pubs.acs.org&d=DwIFaQ&c=C3yme8gMkxg_ihJNXS06ZyWk4EJm8LdrrvxQbJe7sw&r=DpgZH45bhs-8Q-6FGPq00A&m=AV5_trxn3NjTmYzo9kNPEDDacA6c25VgLTQRoxtudA&s=GCM0i9BFc0JVNud_e94K5b31FTYY5yArPDv16jiJD ps&e=

AUTHOR INFORMATION Corresponding author: C. S. Yoo, e-mail: [email protected] Notes: The authors declare no competing financial interest.

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FIGURE CAPTIONS Figure 1 (a) Microphotographs of CO-N2 (1:1 in a volume fraction) mixtures to 65 GPa, showing the pressure-induced polymerization from a transparent mixture to initially heterogeneous a dark-brown solid at ~8-15 GPa and then a more homogeneous light-brown solid above 20 GPa. (b) Raman spectra of 1:1 CO-N2 mixtures at several selected pressures, showing the disappearance of both CO and N2 and the emergence of a new broad vibron centered at ~1600 cm-1 above 4.7 GPa, and the disappearance of CO and N2 vibrons as well as the 1600 cm-1 band above 8.2 GPa. No vibrational Raman features are apparent above 20 GPa.

Figure 2

(a) Background subtracted (x symbols) and refined (red) diffraction patterns of

measured X-ray diffraction patterns of 7:3 CO:N2 at 46 GPa. The inset shows the as-measured diffraction patterns before and after laser heating. The small vertical bars mark the calculated (hkl) reflection positions of ε-N2-like (blue) and P43 (red) structures. X-ray wavelength was used 0.4062 Å. (b) The crystal structure of P43 solid, showing nitrogen bridged, eight membered ring of four-fold coordinated carbon atoms (brown), three-fold nitrogen (grey), and two-fold oxygen (red) in a three-dimensional framework structure.

Figure 3 (a) X-ray diffraction patterns of 1:1 CO:N2 obtained before and after laser-heating (LH) at 60 GPa and, then, during the pressure unloading to 1.5 GPa, showing the formation of P43 phase upon laser-heating and its transformation to ε-N2-like and δ-N2-like phases upon pressure unloading. (b) The measured (x symbols), refined (black lines) and difference (blue lines) diffraction patterns of 1:1 CO:N2 at several selected pressures from (a). The refined

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structures and the calculated (hkl) reflections are based on the P43 (red bars and labels), ε-N2-like (green) and δ-N2-like (yellow) structures. X-ray wavelength was used 0.3554 Å.

Figure 4 The pressure-volume compression curves of extended (red circles) and molecular (green triangles and yellow squares) CNO phases in comparison with those of cg-N (blue line), LP-N (brown), δ-N2 (yellow), and ε-N2 (green). The inset shows the crystal structure of P43 solid, consisting of four-fold coordinated carbon atoms (brown), three-fold nitrogen (grey), and twofold oxygen (red) in a three-dimensional framework structure.

Figure 5 Inelastic X-ray Raman spectra of CNO polymer taken near the K-absorption edges of carbon (a), nitrogen (b), and oxygen (c) at several high pressures, indicating its single bonded framework structure. The π* absorption band in (b) is from excess N2 molecules.

Figure 6 A convex-hull diagram of CO-N2 mixtures, comparing the stability of 1:1 CO:N2 in P43 with three structures of 2:1, in polymeric and framework of Pbam and stacking Fdd2 structures, predicted previously. These structures are illustrated in the inset models. The calculated enthalpies are relative to the most stable phases of carbon monoxide (I212121) and nitrogen (P41212) structures, predicted previously.

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TOC Graphic

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Figure 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(a)

(b)

3.0

6.3

8.2

9.5

11

15

20

25

30

35

40

45

50

55

60

65


Figure 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(a)

46 GPa


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(b) P43

b

c


a

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Figure 3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(a)

(b)

after LH

before LH

60.0 GPa



Figure 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

δ-like CO/N2

ε-like CO/N2 δ-N2 ε-N2 cg-N

P43 CON2

LP-N


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Figure 5

(a)

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(b)

6$-./01/$

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Figure 6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

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Pbam (F)

Pbam (P)

Pbam (P,F) Fdd2

P43 P43


Fdd2

Copolymerization of CO and N2 to Extended CON2 Framework Solid

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