# High temperature decomposition of polymeric carbon monoxide at pressures up to 120 GPa

Demetrio Scelta 🌐 ; Matteo Ceppatelli 🌐 ; Roberto Bini 🌐 ; Anna Pakhomova 🌐 ; Gaston Garbarino 🌐 ;

Mohamed Mezouar 🌐 ; Mario Santoro 🌐

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J. Chem. Phys. 159, 084501 (2023)

https://doi.org/10.1063/5.0157907

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# High temperature decomposition of polymeric carbon monoxide at pressures up to 120 GPa

Cite as: J. Chem. Phys. 159, 084501 (2023); doi: 10.1063/5.0157907

Submitted: 12 May 2023 • Accepted: 1 August 2023 •

Published Online: 23 August 2023

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Demetrio Scelta,<fnref n="1" />,<fnref n="2" />,ᵃ ID Matteo Ceppatelli,<fnref n="1" />,<fnref n="2" /> ID Roberto Bini,<fnref n="1" />,<fnref n="2" />,<fnref n="3" /> ID Anna Pakhomova,<fnref n="4" /> ID

Gaston Garbarino,<fnref n="4" /> ID Mohamed Mezouar,<fnref n="4" /> ID and Mario Santoro<fnref n="1" />,<fnref n="5" />,ᵇ ID

## AFFILIATIONS

## ABSTRACT

While polymeric carbon monoxide (pCO) has been experimentally found to remain amorphous and undecomposed at room temperature up to 50 GPa, the question of whether crystalline counterparts of it can be obtained naturally raises. From different computational studies, it can be inferred that either the crystallization of amorphous pCO (a-pCO) or its decomposition into a mixture of CₓOᵧ suboxides (x > y) or carbon and CO₂ may occur. In this study, we report experimental investigations of the high temperature (700–4000 K) transformation of a-pCO in the 47–120 GPa pressure range, conducted by x-ray diffraction in laser heated diamond anvil cells. Our results show the formation of no crystalline phases other than CO₂ phase V, thus indicating the decomposition of the pristine a-pCO into CO₂ and, likely, a mixture of amorphous CₓOᵧ suboxides and amorphous carbon hardly detectable at extreme conditions. These results support the theoretical picture of the pCO decomposition. We also show that the pressure-temperature kinetic border for this decomposition is very steep, thus indicating a strongly pressure-dependent kinetic barrier.

© 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0157907

Simple, covalently bonded molecular systems with multiple bonds are well known to polymerize into disordered or amorphous forms when compressed at room or nearly room temperature above several GPa/tens of GPa.<fnref n="1" />⁻⁹ While these forms provide a variety of novel, dense, often strongly metastable states, the structural description of which is always challenging, the search for their crystalline counterparts is also of great interest.

Particularly, the search for these crystalline solids is of fundamental relevance for determining the thermodynamic as opposed to kinetic phase diagrams of the corresponding systems. The main methodology for crystallizing the amorphous forms here in play is high temperature annealing at high pressures, often to temperatures in excess of 1000 K, which in turn could also lead to the decomposition of some of the polymeric materials. Undecomposed

crystallization is nowadays well assessed to occur in a few cases, such as polymeric CO₂ and polymeric nitrogen,¹⁰⁻¹⁶ whereas the fate of other systems in this respect is largely unknown. Polymeric carbon monoxide (pCO) is a unique material synthesized from molecular CO at pressures of a few GPa, the structure of which is a matter of continuous debate. The high pressure polymerization of CO and the recoverability at ambient conditions of the solid products were reported and investigated via a variety of experimental techniques such as optical spectroscopy and microscopy, x-ray diffraction, and electron microscopy, along with standard chemical analysis methods.¹⁷⁻²⁴ In many of these studies, the interpretation of the results is made difficult by the strong sensitivity of both molecular and extended CO to laser and x-ray radiation damage and by the high hygroscopicity of the recovered products. On the other

<footnote n="1">¹European Laboratory for Nonlinear Spectroscopy, LENS, Via Nello Carrara 1, 50019 Sesto Fiorentino (FI), Italy</footnote>

<footnote n="2">²Consiglio Nazionale delle Ricerche–Istituto di Chimica dei Composti OrganoMetallici, CNR-ICCOM, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy</footnote>

<footnote n="3">³Dipartimento di Chimica “Ugo Schiff,” Università di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy</footnote>

<footnote n="4">⁴European Synchrotron Radiation Facility, ESRF, 71 Avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9, France</footnote>

<footnote n="5">⁵Consiglio Nazionale delle Ricerche–Istituto Nazionale di Ottica, CNR-INO, Via Nello Carrara 1, 50019 Sesto Fiorentino (FI), Italy</footnote>

<footnote>ᵃ) Author to whom correspondence should be addressed: scelta@lens.unifi.it</footnote>

<footnote>ᵇ) Electronic mail: santoro@lens.unifi.it</footnote>
hand, in *situ* IR spectroscopy measurements have been showing that, below room T, compressed CO at several GPa transforms into an extended amorphous material (a-pCO) with a 1:1 CO composition stoichiometry, formed by polycarbonyl linear chains containing a large number of anhydride groups.⁸ In this material, whose disorder likely reflects the head-to-tail disorder in the pristine molecular solid, a major amount of C is in sp² hybridization, while oxygen forms single and double bonds with carbon.⁸ For T > 300 K, polymerization occurs simultaneously with decomposition, and its extent increases with temperature, as shown by the formation of CO₂. On the side of *ab initio* computer simulations, after pioneering predictions about the high pressure polymerization of CO into a-pCO forms,²⁵ more recent studies have focused on the lowest energy structures of CO.²⁶,²⁷ At all the investigated pressures (0–140 GPa), crystalline pCO phases resulted in be more stable than molecular CO.²⁷ Among these phases, the local atomic coordination was shown to increase from 3 to 4 for carbon and from mixed 1 and 2 to fully 2 for oxygen at pressures above 12 GPa.²⁷ Recent IR absorption and x-ray diffraction (XRD) measurements on a-pCO, conducted at room temperature up to 56 GPa, have confirmed the predicted increase in the local atomic coordination above 12 GPa.⁹ Although the predicted polymeric crystalline phases have never been experimentally observed so far, most likely for kinetic reasons, the experimentally synthesized a-pCO represents a valid proxy for their local structure. After that, the question naturally arises whether the crystallization of a-pCO can be pursued experimentally by suitably overcoming the kinetic barriers here in play. It was this question that prompted the present work. In this respect, high temperature annealing of a-pCO by laser heating in diamond anvil cells (DACs) appeared to be a proper strategy. Yet, high temperature annealing could rather end up resulting in the decomposition of polymeric CO into more stable forms, a possibility that was entirely neglected in the previous computer simulation studies²⁶,²⁷ and that needs to be experimentally checked. In fact, while the decomposition of a-pCO under high temperatures was found to occur at pressures of several GPa,⁸,²³ the fate of decomposition vs transformation into crystalline pCO at much higher pressures, such as those approaching 100 GPa, has so far remained completely unexplored. More recent *ab initio* studies have been predicting that crystalline pCO is indeed considerably less stable, at least at 0 K, than mixtures of crystalline phases of CₓOᵧ suboxides (x > y) and of CO₂ with total CO composition stoichiometry over the large 0–160 GPa pressure range.²⁸ A more recent simulation study of the relative stability of pCO and mixtures of suboxides and CO₂ found similar results.²⁹ Actually, theoretical predictions indicate that the thermodynamically favored products of the decomposition of CO should be CO₂ and pure carbon.²⁸ In the following paragraphs, after a brief description of the experimental methods, we will report a synchrotron powder XRD investigation on the high temperature annealing of a-pCO in laser heated DACs at pressures between 47 and 120 GPa. Strong attenuation of the x-ray beam was ensured to avoid any radiation damage to the samples. Our main outcome is that a-pCO underwent decomposition at temperatures in excess of 1000 K, requiring higher temperatures with increasing pressure, into the well-known crystalline phase V of CO₂¹⁰⁻¹³ and what should be amorphous and hence hard to detect CₓOᵧ (x > y) suboxides and pure amorphous carbon, thus confirming the decomposition scenario inferred from the most recent *ab initio* computer simulations.²⁸,²⁹

Pure carbon monoxide (CO, Sapio, purity ≥ 99.99%) was cryo-loaded, in its liquid phase, into both four pins and piston-cylinder custom designed high-temperature membrane Diamond Anvil Cells (mDACs)³⁰ at several GPa. Liquid CO was condensed at 77 K and about 1 bar into a liquid-nitrogen cooled, sealed, and pre-evacuated vessel that also contained the mDAC, the load of which was controlled remotely through the membrane. The procedure guaranteed the high purity of the sample. The cells were equipped with type Ia and IIa synthetic diamonds with culet diameters ranging from 250 µm (flat) to 150/300 µm (beveled). The loaded samples were sideways contained using rhenium gaskets, and the sample chambers were drilled via spark erosion with an initial diameter and thickness of about 100 and 30 µm (for 250 µm flat culets) and of 50 and 20 µm (for 150/300 beveled µm culets), respectively. The rhenium gaskets were gilded with sputtered gold powder to avoid any catalytic effect due to the gasket material, while at the same time allowing the use of gold as an additional pressure marker following the gold equation of state;³¹ we used the rhenium equation of state as well,³² and, for the 250 µm culets DAC, we also added a small ruby chip as a pressure sensor.³³ The ruby fluorescence was measured using a very efficient micro-Raman set-up,³⁴ with laser (λ = 660 nm) intensity as low as a few hundred W/cm² and an acquisition time of a few seconds. We previously verified, by IR spectroscopy and visual observations, that these values for the laser irradiation parameters were low enough to avoid any photo-chemical effects.⁹ The samples were directly heated by CO₂ laser (λ = 10.6 µm) heating in DACs by exploiting the resonance of this laser with the strong medium infrared absorption of the material.⁹ The laser maximum power and beam spot diameters were 200 W and 20 µm, respectively. The laser spot was spanned through the entire sample area, with a total heating time of a few minutes for each laser heating run. Diamond culets were not coated by any insulating materials in order to avoid side reactions with CO at high temperatures. Sample temperatures in excess of 1000 K were measured by optical pyrometry, as reported elsewhere.¹³ For one of the samples (sample A, see the following), temperatures below 1000 K were estimated from a here determined temperature vs laser power (LP, expressed in Watt) calibration curve: T(K) = 12.2 × LP + 300. Synchrotron angle-dispersive powder XRD measurements in DACs were performed at the ESRF (λ = 0.3738 Å, beamline ID27), using the large area EIGER2 X CdTe 9M flat panel detector from DECTRIS and a micrometer-sized x-ray spot. The x-ray flux was attenuated down to several units in 10¹⁰ photons/s in order to avoid radiation damage to the samples. The diffractometer was

<figure><img src="image_9.png" /><figcaption>**FIG. 1.** Pictures of the pristine a-pCO samples as for the laser heating experiment: (a) 30 GPa; (b) 35.5 GPa; and (c) 27 GPa. Photographs were acquired using transmitted (a, b, and c) and both transmitted and reflected (b and c) light.</figcaption></figure>
calibrated using a CeO₂ powder standard, and the 2D diffraction patterns were analyzed and integrated using Dioptas software.³⁵

Three pristine CO samples (A, B, and C) were loaded into DACs as described in the experimental section. The pressure was increased well above the polymerization threshold at room temperature,⁹ where a-pCO was extensively investigated.⁹ Figure 1 shows pictures of the a-pCO samples at 30 GPa (a), 35.5 GPa (b), and 27 GPa (c), all exhibiting the typical yellow-brownish color previously observed.⁹ The three samples were then compressed to 52, 47.5, and 102 GPa, respectively, and at these pressures, laser heatings were subsequently started. Detailed XRD mappings (10 × 10 grids, 8 µm spacing) were performed before and after each laser heating run to monitor the spatial distribution of potential Bragg peaks across the sample. Figure 2 reports selected, azimuthally averaged XRD patterns measured at room temperature on the starting sample A and on the quenched samples after successive laser heating runs performed at increasing laser powers. We also report selected portions of the 2D XRD patterns corresponding to the various transformation stages of a-pCO. The pressures indicated here and in the following Figs. 3 and 4 are those measured after quenching, and the maximum temperature achieved in each laser heating run is also reported.

In the initial XRD pattern (black trace in Fig. 2), the weak, diffuse, liquid-like peak of a-pCO at about 3.2 Å⁻¹, superimposed on the strong Compton scattering background from diamonds (not subtracted here and thereafter), can be barely detected, in

agreement with previous observations.⁹ After the first laser heating run (red trace in Fig. 2) at 790 K, we did not observe any evidence of crystallization. On the other hand, after heating at 1280 K, we observed two weak and broad Bragg peaks positioned at about 2.09 and 3.31 Å⁻¹, which can be confidently assigned to the (101) and

<figure><img src="image_10.png" /><figcaption>FIG. 3. Left panel: Selected azimuthally averaged XRD patterns of pristine a-pCO (sample B) acquired at room temperature before laser heating (bottom black trace) and of the quenched sample after successive laser heatings at increasing temperature (red and green traces) and upon decompression at room temperature. The Miller indices for the Bragg peaks of CO₂-V are shown. All the patterns have been vertically translated for the sake of clarity. Right panel: portions of selected 2D XRD patterns at the given annealing temperatures for the points exhibiting the best crystalline quality for CO₂-V: (a) 69 GPa and 3280 K, and (b) 64.3 GPa and room temperature.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>FIG. 2. Left panel: Selected azimuthally averaged XRD patterns of pristine a-pCO (sample A) acquired at room temperature before laser heating (bottom black line) and of the quenched sample after successive laser heatings at the indicated increasing temperatures. Different colors label different annealing temperatures. For some temperatures, the patterns acquired at different points on an extended XRD map are reported. The Miller indices for the Bragg peaks of CO₂-V are also shown. All the patterns have been vertically translated for the sake of clarity. Right panel: portions of selected 2D XRD patterns at the given annealing temperatures for the points exhibiting the best crystalline quality of CO₂-V: (a) 1280 K, (b) 2000 K, and (c) 2500 K.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>FIG. 4. Left panel: Selected azimuthally averaged XRD patterns of pristine a-pCO (sample C) acquired at room temperature before (black and blue) and of the quenched sample after laser heating (red, green, cyan, and magenta). The Miller indices for the Bragg peaks of CO₂-V are shown. All the patterns have been vertically translated for the sake of clarity. Right panel: portion of the selected 2D XRD pattern (a) at the given annealing temperature (77 GPa and 4000 K).</figcaption></figure>
(112) Bragg peaks of the well-known CO₂-V, the thermodynamically stable phase of CO₂ in this pressure range.¹⁰⁻¹² The appearance of CO₂ is unquestionable evidence that polymeric CO has decomposed. After further heatings at 2000 and 2500 K, we observed a clear and progressive increase in both the integrated and the maximum intensities of the Bragg peaks from CO₂-V and also a substantial peak narrowing, although to a much lesser extent at some points as resulting from an XRD mapping of the sample area. This increase attests to an increased quantity and crystalline quality of the obtained CO₂-V phase. The Debye-Scherrer rings in the 2D XRD patterns are correspondingly more intense, although somewhat spotty. The full width at half-maximum (fwhm) of the Bragg peaks provides an estimate of the spatial correlation length *l* (*l* ~ 2π/fwhm) of the crystalline CO₂ phase, a parameter that qualifies its crystalline quality. From the (101) and (112) peaks, we found, for the best patterns, *l* values equal to 78, 210, and 940 Å after the three successive laser heatings where CO₂-V was observed to form, showing that the crystalline quality was increasing by more than one order of magnitude. Importantly, here and in the following, no Bragg peaks were observed other than those of CO₂-V. In particular, no signatures of pure carbon crystalline phases were found in any case. On the other hand, in order to maintain the stoichiometry of the total composition, suboxide phases should also have formed, probably amorphous and, consequently, difficult to detect. The formation of pure amorphous carbon is also possible.

**Figure 3** shows selected XRD patterns measured on sample B at room temperature before and after laser heating, together with portions of the 2D XRD patterns. Upon successive laser heatings, we found the initial pressure to be increased to 75 GPa, as indicated. In this case, the CO₂-V Bragg peaks, attesting to the decomposition of a-pCO, were observed at two successive annealing temperatures of 3280 and 3600 K. The Debye-Scherrer rings in the reported 2D XRD patterns are rather uniform, indicating a randomly oriented powder. The CO₂-V phase obtained in sample B after these laser heatings has been further decompressed at room temperature down to ambient pressure. The obtained unit cell volumes are consistent with those obtained from the known decompression curves of CO₂ phase V.¹¹⁻¹³ In sample B, the spatial correlation lengths *l* of crystalline CO₂ after the two successive thermal annealings are equal to 105 and 126 Å, respectively, showing a worse crystalline quality than in sample A after the last heating, even though the annealing temperatures were much higher. This observation likely shows that the relevant kinetic barrier for the decomposition of a-pCO is significantly higher at the higher pressures where sample B was annealed.

Finally, Fig. 4 shows selected 1D and 2D XRD patterns acquired at room temperature on sample C before and after laser heating runs. Here, under much more extreme pressure conditions (102 and 120 GPa) than those adopted previously, the sample-laser coupling was found to be less efficient, and we could only achieve a maximum temperature of 2600 K at full laser power. Phase CO₂-V was not formed at these conditions, thus suggesting a strongly increased metastability of a-pCO in the megabar range. We then decompressed the sample down to 86 GPa and laser heated it again. Indeed, these milder pressure conditions allowed for better laser-sample coupling, resulting in higher temperatures (3700–4000 K). Under these conditions, we observed again the formation of CO₂-V, although to a much lower extent with respect to samples A and B, as

attested from the XRD patterns. In sample C, the spatial correlation length *l* is as low as 62 Å, suggesting a lower crystalline quality than in samples A and B, thus confirming that the kinetic barriers for the a-pCO decomposition should be higher, likely because of the higher pressures involved.

The results of the three heating runs are summarized in the kinetic phase diagram in Fig. 5, together with the density functional theory (DFT) predicted low pressure phase boundary and melting line for CO₂-V.⁵⁶,³⁷ Circles, squares, and diamonds refer, respectively, to the experimental data from samples A, B, and C and represent the P-T points where the initial a-pCO has been annealed along the successive laser heatings as reported in Figs. 2–4. The pressure value for these points is the average pressure between those measured before and after the laser heating, which in turn corresponds to the extremes of the horizontal bars. Vertical error bars reflect the differences among a few successive temperature measurements performed while holding the laser power constant. Full/open symbols, respectively, indicate the points where CO₂-V was/was not observed. Open triangles refer to undecomposed a-pCO compressed at room temperature up to about 50 GPa.⁹ Based on the present and the literature sets of data, an approximate kinetic boundary for the decomposition of polymeric CO can be traced in the two colored subsections of the phase diagram: cyan and light cyan for undecomposed and decomposed polymeric CO, respectively. Significantly, the slope of the kinetic boundary is highly positive, thus attesting

<figure><img src="image_13.png" /><figcaption>**FIG. 5.** Kinetic phase diagram of polymeric CO as determined in this work. Circles, squares, and diamonds refer, respectively, to the experimental data from samples A, B, and C and represent the P–T points at which pristine a-pCO has been annealed. Full/open symbols indicate points where the formation of CO₂-V was/was not observed. Open triangles refer to undecomposed a-pCO compressed at room temperature up to about 50 GPa.⁹ The cyan sector highlights the P-T domain within which it has been deduced that amorphous polymeric CO remains metastable, whereas the light cyan sector indicates the range of P–T conditions at which it is expected to decompose. The approximate kinetic boundary separating the two sectors was constrained by the current P–T points (full and open symbols) and also by passing through 300 K and 6 GPa, as determined from previous decomposition data.⁸ Dotted–dashed (dashed) line: low pressure phase boundary (melting line) of the thermodynamically stable CO₂-V.³⁶,³⁷</figcaption></figure>
that the relevant decomposition kinetic barrier strongly increases with pressure.

In this work, we have investigated the transformations of amorphous polymeric carbon monoxide at temperatures as high as 4000 K and pressures in the 47–120 GPa range by XRD in laser heated DACs. Our data unambiguously show the decomposition of a-pCO through the formation of CO₂-V above a kinetic boundary characterized by high temperature values and a steep, positive dT/dP slope, which in turn attests to a high and strongly pressure dependent kinetic barrier for the decomposition. This barrier makes a-pCO strongly metastable and most likely reflects the head-to-tail disorder of the molecular CO crystal from which a-pCO originates. No other crystalline phases were observed along with CO₂-V, particularly no pure carbon crystalline phases (graphite or diamond). As a consequence, in order to retain the total CO 1:1 composition stoichiometry, the formation of a mixture of likely amorphous CₓOᵧ suboxides (x > y) and pure amorphous carbon, simultaneously with CO₂, must be hypothesized. These amorphous forms are hardly detectable through XRD at extreme pressures. It is now interesting to compare the high pressure behavior of CO and CO₂. Indeed, they differ substantially. Both CO and CO₂ polymerize into amorphous forms at high pressures, while their local structures modify upon increasing pressure toward fourfold and twofold coordination for carbon and oxygen, respectively.<fnref n="1" />,<fnref n="9" /> On the other hand, while it is largely reported that amorphous polymeric CO₂ crystallizes into the undecomposed, thermodynamically stable phase V of CO₂ once temperature annealed,<fnref n="10" />⁻³⁶,³⁸,³⁹ our results show that amorphous polymeric CO rather decomposes. These findings on CO support recent *ab initio* studies, predicting crystalline pCO to be considerably less stable than mixtures of crystalline phases of CₓOᵧ suboxides (x > y) and of CO₂ with total CO composition stoichiometry²⁸,²⁹ and also predicting that the thermodynamically favored products of the decomposition of CO should be CO₂ and pure carbon.²⁸ Accordingly, while the decomposition of a-pCO is hindered by a strong kinetic barrier, once this barrier is overcome by a sufficiently large energy injection, it ultimately occurs as dictated by thermodynamics. On the other hand, the predicted suboxide crystals remain to be experimentally unveiled.

We thank the European Laboratory for Nonlinear Spectroscopy (LENS) for hosting part of the research, the Fondazione Cassa di Risparmio di Firenze for the support through the grant SALUS, and the ESRF synchrotron for providing access to the ID27 beamline and for financial support under Proposal No. CH-6151. We thank Irina Snigirev for her support with the gold coating of the gaskets. M.C. would like to acknowledge the projects HP-PHOTOCHEM (Fondazione Cassa di Risparmio di Firenze) and “GreenPhos-alta pressione” (CNR).

## AUTHOR DECLARATIONS

### Conflict of Interest

The authors have no conflicts to disclose.

### Author Contributions

**Demetrio Scelta:** Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Writing –

original draft (lead); Writing – review & editing (lead). **Matteo Ceppatelli:** Conceptualization (equal); Investigation (equal); Writing – original draft (supporting); Writing – review & editing (supporting). **Roberto Bini:** Conceptualization (equal); Investigation (equal); Writing – original draft (supporting); Writing – review & editing (supporting). **Anna Pakhomova:** Investigation (equal). **Gaston Garbarino:** Investigation (equal). **Mohamed Mezouar:** Investigation (equal). **Mario Santoro:** Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Supervision (equal); Writing – original draft (lead); Writing – review & editing (lead).

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## REFERENCES

<footnote n="1">¹M. Santoro, F. Gorelli, R. Bini, G. Ruocco, S. Scandolo, and W. Crichton, “Amorphous silica-like carbon dioxide,” *Nature* **441**, 857–860 (2006).</footnote>

<footnote n="2">²A. Goncharov, E. Gregoryanz, H.-K. Mao, Z. Liu, and R. Hemley, “Optical evidence for a nonmolecular phase of nitrogen above 150 GPa,” *Phys. Rev. Lett.* **85**, 1262–1265 (2000).</footnote>

<footnote n="3">³E. Gregoryanz, A. F. Goncharov, R. J. Hemley, and H.-K. Mao, “High-pressure amorphous nitrogen,” *Phys. Rev. B* **64**, 052103 (2001).</footnote>

<footnote n="4">⁴H. Zhang, O. Tóth, X.-D. Liu, R. Bini, E. Gregoryanz, P. Dalladay-Simpson, S. de Panfilis, M. Santoro, F. Gorelli, and R. Martoňák, “Pressure-induced amorphization and existence of molecular and polymeric amorphous forms in dense SO₂,” *Proc. Natl. Acad. Sci. U. S. A.* **117**, 8736–8742 (2020).</footnote>

<footnote n="5">⁵J. Yan, O. Tóth, W. Xu, X.-D. Liu, E. Gregoryanz, P. Dalladay-Simpson, Z. Qi, S. Xie, F. Gorelli, R. Martoňák, and M. Santoro, “High-pressure structural evolution of disordered polymeric CS₂,” *J. Phys. Chem. Lett.* **12**, 7229–7235 (2021).</footnote>

<footnote n="6">⁶L. Ciabini, M. Santoro, F. A. Gorelli, R. Bini, V. Schettino, and S. Raugei, “Triggering dynamics of the high-pressure benzene amorphization,” *Nat. Mater.* **6**, 39–43 (2006).</footnote>

<footnote n="7">⁷M. Ceppatelli, M. Santoro, R. Bini, and V. Schettino, “Fourier transform infrared study of the pressure and laser induced polymerization of solid acetylene,” *J. Chem. Phys.* **113**, 5991–6000 (2000).</footnote>

<footnote n="8">⁸M. Ceppatelli, A. Serdyukov, R. Bini, and H. Jodl, “Pressure induced reactivity of solid CO by FTIR studies,” *J. Phys. Chem. B* **113**, 6652–6660 (2009).</footnote>

<footnote n="9">⁹M. Santoro, R. Bini, M. Ceppatelli, G. Garbarino, F. A. Gorelli, M. Hanfland, and D. Scelta, “High pressure structural changes in amorphous polymeric carbon monoxide by combined infrared spectroscopy and X-ray diffraction,” *J. Phys. Chem. C* **126**, 11840–11845 (2022).</footnote>

<footnote n="10">¹⁰M. Santoro, F. Gorelli, R. Bini, J. Haines, O. Cambon, C. Levelut, J. Montoya, and S. Scandolo, “Partially collapsed cristobalite structure in the non molecular phase V in CO₂,” *Proc. Natl. Acad. Sci. U. S. A.* **109**, 5176–5179 (2012).</footnote>

<footnote n="11">¹¹F. Datchi, B. Mallick, A. Salamat, and S. Ninet, “Structure of polymeric carbon dioxide CO₂-V,” *Phys. Rev. Lett.* **108**, 125701 (2012).</footnote>

<footnote n="12">¹²K. F. Dziubek, M. Ende, D. Scelta, R. Bini, M. Mezouar, G. Garbarino, and R. Miletich, “Crystalline polymeric carbon dioxide stable at megabar pressures,” *Nat. Commun.* **9**, 3148 (2018).</footnote>

<footnote n="13">¹³D. Scelta, K. F. Dziubek, M. Ende, R. Miletich, M. Mezouar, G. Garbarino, and R. Bini, “Extending the stability field of polymeric carbon dioxide phase V beyond the Earth’s geotherm,” *Phys. Rev. Lett.* **126**, 065701 (2021).</footnote>

<footnote n="14">¹⁴M. Eremets, A. Gavriliuk, I. Trojan, D. Dzivenko, and R. Boehler, “Single-bonded cubic form of nitrogen,” *Nat. Mater.* **3**, 558–563 (2004).</footnote>

<footnote n="15">¹⁵D. Tomasino, M. Kim, J. Smith, and C.-S. Yoo, “Pressure-induced symmetry-lowering transition in dense nitrogen to layered polymeric nitrogen (LP-N) with colossal Raman intensity,” *Phys. Rev. Lett.* **113**, 205502 (2014).</footnote>
<footnote n="16"><sup>16</sup>D. Laniel, G. Geneste, G. Weck, M. Mezouar, and P. Loubeyre, "Hexagonal layered polymeric nitrogen phase synthesized near 250 GPa," Phys. Rev. Lett. **122**, 066001 (2019).</footnote>

<footnote n="17"><sup>17</sup>D. T. Cromer, D. Schiferl, R. LeSar, and R. L. Mills, "Room-temperature structure of carbon monoxide at 2.7 and 3.6 GPa," Acta Crystallogr. C39, 1146-1150 (1983).</footnote>

<footnote n="18"><sup>18</sup>A. Katz, D. Schiferl, and R. Mills, "New phases and chemical reactions in solid CO under pressure," J. Phys. Chem. **88**, 3176-3179 (1984).</footnote>

<footnote n="19"><sup>19</sup>R. Mills, B. Olinger, and D. Cromer, "Structures and phase diagrams of N<sub>2</sub> and CO to 13 GPa by x-ray diffraction," J. Chem. Phys. **84**, 2837-2845 (1985).</footnote>

<footnote n="20"><sup>20</sup>M. Lipp, W. Evans, V. Garcia-Baonza, and H. Lorenzana, "Carbon monoxide: Spectroscopic characterization of the high-pressure polymerized phase," J. Low Temp. Phys. **111**, 247-256 (1998).</footnote>

<footnote n="21"><sup>21</sup>M. Lipp, W. Evans, B. Baer, and C.-S. Yoo, "High-energy-density extended CO solid," Nat. Mater. **4**, 211-215 (2005).</footnote>

<footnote n="22"><sup>22</sup>W. Evans, M. Lipp, C.-S. Yoo, H. Cynn, J. Herberg, R. Maxwell, and M. Nicol, "Pressure-induced polymerization of carbon monoxide: Disproportionation and synthesis of an energetic lactonic polymer," Chem. Mater. **18**, 2520-2531 (2006).</footnote>

<footnote n="23"><sup>23</sup>N. Rademacher, L. Bayarjargal, W. Morgenroth, B. Winkler, J. Ciezak-Jenkins, I. Batyrev, and V. Milman, "The local atomic structures of liquid CO at 3.6 GPa and polymerized CO at 0 to 30 GPa from high-pressure pair distribution function analysis," Chem. - Eur. J. **20**, 11531-11539 (2014).</footnote>

<footnote n="24"><sup>24</sup>Y.-J. Ryu, M. Kim, J. Lim, R. Dias, D. Klug, and C.-S. Yoo, "Dense carbon monoxide to 160 GPa: Stepwise polymerization to two-dimensional layered solid," J. Phys. Chem. C **120**, 27548-27554 (2016).</footnote>

<footnote n="25"><sup>25</sup>S. Bernard, G. Chiarotti, S. Scandolo, and E. Tosatti, "Decomposition and polymerization of solid carbon monoxide under pressure," Phys. Rev. Lett. **81**, 2092-2095 (1998).</footnote>

<footnote n="26"><sup>26</sup>J. Sun, D. Klug, C. Pickard, and R. Needs, "Controlling the bonding and band gaps of solid carbon monoxide with pressure," Phys. Rev. Lett. **106**, 145502 (2011).</footnote>

<footnote n="27"><sup>27</sup>K. Xia, J. Sun, C. Pickard, D. Klug, and R. Needs, "Ground state structure of high-energy-density polymeric carbon monoxide," Phys. Rev. B **95**, 144102 (2017).</footnote>

<footnote n="28"><sup>28</sup>C. Sun, W. Guo, J. Zhu, X. Li, and Y. Yao, "High-energy-density polymeric carbon oxide: Layered CₓOᵧ solids under pressure," Phys. Rev. B **104**, 094102 (2021).</footnote>

<footnote n="29"><sup>29</sup>X. Huang, F. Jiao, C. Zhang, Y. Chen, Y. Xie, and W. Xie, "Investigation of polymeric CO synthesized at high pressure and its stability under ambient conditions: A first-principles study," J. Phys. Chem. C **126**, 19571-19579 (2022).</footnote>

<footnote n="30"><sup>30</sup>M. Santoro, A. Hajeb, and F. Gorelli, "Resistively heated, high pressure, membrane and screw driven diamond anvil cell," High Pressure Res. **40**, 379-391 (2020).</footnote>

<footnote n="31"><sup>31</sup>D. L. Heinz and R. Jeanloz, "The equation of state of the gold calibration standard," J. Appl. Phys. **55**, 885-893 (1984).</footnote>

<footnote n="32"><sup>32</sup>S. Anzellini, A. Dewaele, F. Occelli, P. Loubeyre, and M. Mezouar, "Equation of state of rhenium and application for ultra high pressure calibration," J. Appl. Phys. **115**, 043511 (2014).</footnote>

<footnote n="33"><sup>33</sup>H. Mao, P. Bell, J. Shaner, and D. Steinberg, "Specific volume measurements of Cu, Mo, Pd, and Ag and calibration of the ruby R<sub>1</sub> fluorescence pressure gauge from 0.06 to 1 Mbar," J. Appl. Phys. **49**, 3276-3283 (1978).</footnote>

<footnote n="34"><sup>34</sup>M. Ceppatelli, F. Gorelli, J. Haines, M. Santoro, and R. Bini, "Probing high-pressure reactions in heterogeneous materials by Raman spectroscopy," Z. Kristallogr. - Cryst. Mater. **229**, 83-91 (2014).</footnote>

<footnote n="35"><sup>35</sup>C. Prescher and V. B. Prakapenka, "DIOPTAS: A program for reduction of two-dimensional X-ray diffraction data and data exploration," High Pressure Res. **35**, 223-230 (2015).</footnote>

<footnote n="36"><sup>36</sup>B. H. Cogollo-Olivo, S. Biswas, S. Scandolo, and J. A. Montoya, "Ab initio determination of the phase diagram of CO₂ at high pressures and temperatures," Phys. Rev. Lett. **124**, 095701 (2020).</footnote>

<footnote n="37"><sup>37</sup>A. Teweldeberhan, B. Boates, and S. Bonev, "CO₂ in the mantle: Melting and solid-solid phase boundaries," Earth Planet. Sci. Lett. **373**, 228-232 (2013).</footnote>

<footnote n="38"><sup>38</sup>M. Santoro, F. A. Gorelli, K. Dziubek, D. Scelta, and R. Bini, "Structural and chemical modifications of carbon dioxide on transport to the deep earth," in Carbon in Earth's Interior (American Geophysical Union (AGU), 2020), Chap. 6, pp. 55-65.</footnote>

<footnote n="39"><sup>39</sup>B. Boates, A. M. Teweldeberhan, and S. A. Bonev, "Stability of dense liquid carbon dioxide," Proc. Natl. Acad. Sci. U. S. A. **109**, 14808-14812 (2012).</footnote>