Extending carbon chemistry at high-pressure by synthesis of CaC2 and Ca3C7 with deprotonated polyacene- and para-poly(indenoindene)-like nanoribbons

Metal carbides are known to contain small carbon units similar to those found in the molecules of methane, acetylene, and allene. However, for numerous binary systems ab initio calculations predict the formation of unusual metal carbides with exotic polycarbon units, [C6] rings, and graphitic carbon sheets at high pressure (HP). Here we report the synthesis and structural characterization of a HP-CaC2 polymorph and a Ca3C7 compound featuring deprotonated polyacene-like and para-poly(indenoindene)-like nanoribbons, respectively. We also demonstrate that carbides with infinite chains of fused [C6] rings can exist even at conditions of deep planetary interiors ( ~ 140 GPa and ~3300 K). Hydrolysis of high-pressure carbides may provide a possible abiotic route to polycyclic aromatic hydrocarbons in Universe.

dumbbells (e.g., in YC 2 , LaC 2 , La 2 C 3 ≡ La 4 (C 2 ) 3 , CeC 2 , TbC 2 , YbC 2 , and LuC 2 ) [2][3][4] , isolated carbon atoms and (almost) linear C 3 units (in Me 4 C 7 with Me = Y, Ho, Er, Tm, Lu) 5,6 , or, most remarkably, C 1 -C 3 units in Sc 3 C 4 and Ln 3 C 4 (Ln = Ho-Lu) 8 .However, due to the metallic bonding character, the assignment of ionic charges is not as straightforward as for the salt-like carbides mentioned above.At high pressure, for the binary systems Mg-C 9 , Ca-C 10 , Y-C 11 , La-C 12 , and Li-C 13 ab initio structure search predicts the formation of unusual metal carbides with exotic [C 4 ] and [C 5 ] units, [C 6 ] rings, and graphitic carbon sheets, as well as a number of structural transitions so that studying them under compression might enable to explore novel catenations of carbon atoms.
Calcium carbide has been the focus of high-pressure research during recent years.CaC 2 is of particular interest, as it is characterized by a rich polymorphism.Three polymorphs of CaC 2 (CaC 2 -I, CaC 2 -II, CaC 2 -III) are known at ambient conditions 14 .Their structures feature [C 2 ] 2− dumbbells common for acetylides.It was proposed that at relatively low pressures, these [C 2 ] 2− units start to polymerize, and at about 7 GPa, CaC 2 transforms into a metallic phase (space group Cmcm) with 1D zig-zag polymeric carbon chains, as established using Raman spectroscopy and X-ray diffraction (XRD) 15 .(Note that in ref. 15  at ~10 GPa, the signal of the C≡C stretching vibration is still dominating, and the quality of the resulting diffraction patterns is too low for an unambiguous Rietveld fit).Theory predicts further structural transformations in CaC 2 upon pressure increase coupled with the progressive polymerization of carbon atoms.According to calculations by Li et al. 16 , above 20 GPa, the structure of CaC 2 should adopt the P 1 symmetry and contain infinite carbon strips built of interconnected pairs of fused five-membered rings.This phase was supposed to be thermodynamically stable up to 37 GPa 16 .Above this pressure, according to the calculation, metallic P 1 -CaC 2 should transform into metallic Immm-CaC 2 13,16   , in which carbon atoms are polymerized to form infinite quasi-1D ribbons built of fused six-membered rings.The latter prediction is of particular interest, as the suggested carbon catenation in Immm-CaC 2 13,16 resembles deprotonated polyacene-like nanoribbons.
Here, we report the results of high-pressure high-temperature (HPHT) studies of CaC 2 in laser-heated diamond anvil cells (LHDACs) performed using synchrotron single-crystal X-ray diffraction (SCXRD) up to ~150 GPa at temperatures up to ~3000 K.The predicted Immm-CaC 2 13,16 high-pressure polymorph of CaC 2 (HP-CaC 2 ) containing deprotonated polyacene nanoribbons was synthesized and fully characterized.A novel Ca 3 C 7 compound, never anticipated or observed before, with an orthorhombic crystal structure (space group Pnma) featuring infinite, fully deprotonated para-poly-indenoindene (p-PInIn)-like chains was discovered, and its structure was solved and refined using SCXRD.We demonstrate that compounds with fused sixmembered (reminiscent in shape of benzene) carbon rings can be synthesized above 100 GPa.We demonstrated that on decompression in the presence of water, HP-CaC 2 forms polycyclic aromatic hydrocarbons (PAHs).

Results and discussion
Several diamond anvil cells with different samples of calcium carbide were prepared as described in "Methods".Studies cover the pressure range up to ~150 GPa.A summary of all experiments presented in this work, including pressure-temperature (P-T) conditions of the synthesis and the unit cell parameters of the observed phases, is given in Supplementary Table S1.
Upon compression to the target pressures, we performed Raman spectroscopy measurements of the starting material (Supplementary Fig. 1).In agreement with previous reports 14,16 at pressures below ~5 GPa we observed spectral signatures of co-existing tetragonal CaC 2 -I and monoclinic CaC 2 -II phases; above ~10 GPa only CaC 2 -I was observed, and at ~25 GPa the Raman spectrum became featureless (suggesting amorphization of CaC 2 as proposed in ref. 14).Microphotographs taken during compression reveal changes in the sample's visual appearance (Supplementary Fig. 1): the crystal, which was initially transparent (3.3 GPa), became translucent at ~10 GPa and then opaque above ~25 GPa.Such changes can be attributed to the progressive band gap closure in CaC 2 upon compression that agrees with resistivity measurements performed by Zheng et al. 17 .
The systematic analysis of the XRD data obtained from different DACs after laser heating at different pressures (Supplementary Table 1) allowed identifying diamond (as a product of paraffin oil decomposition), CaO (in some cells, due to possible contamination of the commercial calcium carbide with the oxide or Ca(OH) 2 ), and two new phases: a high-pressure polymorph of CaC 2 (called here HP-CaC 2 ) and a new Ca-C compound Ca 3 C 7 .After laser heating, the sample remains dark and non-transparent (Supplementary Fig. 9).The results of different experiments are mutually consistent and reproducible.The structures of the new solids were solved and refined using the best quality SCXRD datasets, obtained at 44(1) GPa and 147(2) GPa for HP-CaC 2 and at 38(1) GPa for Ca 3 C 7 (see Supplementary Tables 2, 3, and 5).They are described in detail below.
Theoretical calculations perfectly reproduce experimental structural data (Supplementary Tables 2 and 3).The pressure dependences of the unit cell parameters and volumes of novel calcium carbides were derived from DFT calculations (Fig. 3).The results were described by the Birch-Murnaghan equation of state (EoS) with the parameters V 0 = 1 27.17(2)Å 3 , K 0 = 144.5(2)GPa, and K´= 3.92(1) for HP-CaC 2 and V 0 = 432.0(2)Å 3 , K 0 = 125(1) GPa, and K´= 4 (fixed) for Ca 3 C 7 .As seen in Fig. 3, both compounds experience an anisotropic contraction upon compression, with the most incompressible directions being along the polyacene-like nanoribbons in HP-CaC 2 (a-axis) and deprotonated p-PInIn chains in Ca 3 C 7 (b-axis).The unit cell parameters derived from several independent SCXRD experiments conducted at various pressures (Supplementary Table 1) show a good agreement with the calculated EoSes (Fig. 3).
Harmonic phonon dispersion calculations reveal that the novel calcium carbides are dynamically stable at their synthesis pressures (Fig. 4).The calculated electron density of states shows that HP-CaC 2 is metallic and the main contributions at the Fermi level come from the Ca 3d and C 2p states (Fig. 4).The computed charge density was analyzed in terms of the electron localization function (ELF), which revealed a strong covalent bonding between carbon atoms within deprotonated polyacene nanoribbons, and ionic bonds between Ca1 and C2 atoms (Fig. 1).Bader charge analysis at 40 GPa for Ca1, C1, and C2 atoms yields values of 1.191, −0.378, and −0.813 (Supplementary Table 4), respectively.
To explore the thermodynamic stability of HP-CaC 2 and Ca 3 C 7 in comparison to other calcium carbides, a convex hull was constructed for the binary Сa-C system at a pressure of ~40 GPa considering the predicted phases 16 .Both phases observed in the present study lie on the convex hull and therefore are (theoretically) thermodynamically stable at 40 GPa (Fig. 5).Notably, HP-CaC 2 also remains thermodynamically stable at 140 GPa (Supplementary Fig. 6).Although an attempt to recover HP-CaC 2 and Ca 3 C 7 at ambient conditions was unsuccessful, according to our calculations, the compounds should be dynamically stable even at atmospheric pressure (Supplementary Fig. 7).
Polycyclic aromatic hydrocarbons (PAHs) consisting of a finite number of fused aromatic rings give rise to various classes of polymeric compounds, if a PAH is extended into a polymer.For example, a particular class of fully conjugated polymers is represented by an extension of indenofluorene into a polymer, that is poly(indenoindene) (PInIn) 22 , where the alternation of fiveand sixmembered rings gives rise to a ladder structure.Although its targeted synthesis in bulk has remained elusive so far, very recently, onsurface synthesis of para-type oligo(indenoindene) (p-OInIn) was   reported to be successful through a sequence of thermally activated reactions from an organic precursor 18 .In the present work, we have demonstrated the inorganic synthesis of the Ca 3 C 7 compound in bulk with the structure featuring deprotonated para-poly-indenoindenelike chains.The HP-CaC 2 synthesized in this work also gives a unique example of polyacene-like nanoribbon moieties previously unknown among any organic or metal-organic compounds.Moreover, the synthesis of single crystals of this material with fused carbon rings similar to those in benzene at pressures above 140 GPa demonstrates that, eventually, organic chemistry may be extended to truly extreme conditions.
The fact that carbides (and eventually other classes of chemical compounds) with deprotonated polyacene-and para-poly(indenoindene)-like nanoribbons can exist (and even form) at conditions of planets' interiors may have important implications for geosciences and astrobiology.Indeed, PAHs were detected at many locations in the Universe [23][24][25][26] , but their origin (as well as a possible role in the emergence of life) remains controversial.The hydrolysis of carbides in planets' interiors, on their surfaces or atmospheres may be a source of hydrocarbons.Hydrolysis products of carbides as complex as HP-CaC 2 or Ca 3 C 7 we discovered may contain PAHs.In order to test this hypothesis, a mixture of CaC 2 -I and Ca(OH) 2 was loaded into a DAC chamber, compressed to ~40 GPa, and the whole sample was doublesided laser-heated between 2500 K and 3000 K. On decompression at ambient temperature, featureless Raman spectra were observed down to ~1.8 GPa (Supplementary Fig. 8).At this pressure, the DAC was warmed to 85 °C (above the melting point of water) using a hot plate for 72 h.Several of the Raman modes we observed (Supplementary Fig. 8) belong to C-H vibrations of saturated and aromatic hydrocarbons.Moreover, at some spots within the sample, the Raman spectra closely resemble those of pyrene (C 16 H 10 ) (Supplementary Fig. 8), confirming a possible abiotic route to aromatic compounds through the hydrolysis of high-pressure carbides within planetary interiors.

Sample preparation
Calcium carbide (CaC 2 ) in the form of pieces of technical grade was purchased from Sigma-Aldrich.Single crystals with an average size of ~0.05 × 0.05 × 0.01 mm 3 were preselected using a three-circle Bruker diffractometer (SMART APEX CCD detector, Ag-Kα radiation, wavelength λ = 0.5594 Å) installed in the Bayerisches Geoinstitut (BGI, Bayreuth, Germany).All manipulations with the crystals were conducted in paraffin oil to avoid their contamination by air  and/or moisture.Although XRD showed a phase purity of the selected samples (no other phases except CaC 2 -I 13 were detected), we cannot exclude that they might contain some very small, undetectable amount of calcium oxide CaO or calcium hydroxide Ca(OH) 2 .In order to exclude the existence of CaO and Ca(OH) 2 completely, we did additional experiments with a pre-synthesized CaC 2 sample of high purity.It was prepared from distilled calcium (70.6 mg, 1.76 mmol, 1 eq.) and a slight surplus of graphite (44.4 mg, 3.70 mmol, 2.1 eq.) heated at 800 °C in high vacuum for 24 h prior to the syntheses.Calcium and graphite were sealed in a purified tantalum ampoule (~0.7 bar argon pressure) and sealed under vacuum in a second ampoule made of quartz to avoid oxidation of the inner tantalum ampoule.These ampoules were heated at 1200 °C for 14 h.The purity of the products was checked by powder X-ray diffraction.It revealed that a mixture of CaC 2 -I, CaC 2 -II, and CaC 2 -III had formed.No reflections indicating CaO or Ca(OH) 2 impurities were found.All handling of these air and moisture-sensitive samples was carried out in inert atmospheres (argon), and they were loaded without a pressure-transmitting medium.The preselected single crystals of commercial CaC 2 were loaded into BX90 diamond anvil cells 27 (DAC) equipped with pairs of Boehler-Almax diamond anvils with culet diameters of 250 µm or 120 µm.The gasket was made of chemically pure Re-foil indented to ~25 μm thickness with a hole in the center of the indent of ~110 or ~60 μm in diameter.Paraffin oil was used as a pressure-transmitting medium.The in-house laser heating was performed in a continuous mode using a double-sided laserheating setup at the BGI 28 .The NIR laser beam (λ = 1070 nm) was focused to a ~5 µm spot, with the temperature determined according to the gray body approximation of Planck's law (Supplementary Fig. 10).As a rule, a sample easily absorbs laser light and its heating requires relatively low power (~5 to ~15 W from each side, depending on pressure).Heating duration varies from ~15 to ~60 s.Two or three spots were heated at each sample, and transformed materials were found in all heated spots.The pressure was determined using the Raman signal from the diamond anvils 29 and additionally monitored by the equation of states (EOS) of Re gasket 30 .

Synchrotron X-ray diffraction studies
High-pressure SCXRD experiments were conducted on the Extreme Conditions Beamline P02.2 at PETRA III, Hamburg, Germany (λ = 0.2885 Å, beam size ~1.5 × 1.7 μm 2 at FWHM, PerkinElmer XRD1621 detector), on the 13-IDD beamline at the Advanced Photon Source (APS), Chicago, USA (λ = 0.2952 Å, beam size ~3 × 3 μm 2 , Pilatus CdTe 1 M detector), and on the Material Science beamline ID11, ESRF, Grenoble, France (λ = 0.2952 Å, beam size ~0.5 × 0.5 μm 2 , Eiger 4 M CdTe detector).Prior to the collection of the SCXRD datasets, a 2D X-ray mapping was performed over the heated area in order to find the best spots for data acquisition.During the SCXRD measurements, the DACs were rotated about the vertical ω-axis in a range of ±38°, and the diffraction images were recorded with an angular step of Δω = 0.5°.

XRD data processing
The SCXRD data analysis, including the indexing of diffraction peaks, data integration, frame scaling, and absorption correction, was performed using CrysAlisPro software package 31 .A crystal of orthoenstatite (Mg 1.93 ,Fe 0.06 )(Si 1.93 ,Al 0.06 )O 6 (space group Pbca, a = 8.8117(2) Å, b = 5.18320(10) Å, c = 18.2391(3)Å) was used as a calibration standard for refinement of the instrumental parameters of the diffractometer (the sample-to-detector distance, the detector's origin, offsets of the goniometer angles and rotations of the X-ray beam and the detector around the instrument axis).The structures of novel phases were solved with the SHELXT 32 structure solution program using intrinsic phasing and refined with the Jana2006 program 33 .Crystal structure visualization was made with the VESTA software 34 .

Raman spectroscopy
The spectra were collected using a LabRam system equipped with a He:Ne laser source (excitation wavelength of 632 nm) and a DilorXY system equipped with an Ar laser source (excitation wavelength of 514 nm).Laser power in the range of 5-15 mW with the He:Ne laser and up to 100 mW for the Ar laser were applied.Raman spectra were collected in the region 1100-2000 cm −1 by means of 5 accumulations for 30 s each.The frequency resolution was 2 cm −1 .

DFT calculations
The first-principles calculations were done using the framework of density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) 35 .To expand the electronic wave function in plane waves, we used the Projector-Augmented-Wave (PAW) method 36 .The Generalized Gradient Approximation (GGA) function was used for calculating the exchange-correlation energies, as proposed by Perdew-Burke-Ernzerhof (PBE) 37 .The PAW potentials with the following valence configurations of 3s3p4s for Ca and 2s2p for C were used.Convergence tests with a threshold of 1 meV per atom in energy and 1 meV/Å per atom for forces led to a Monkhorst-Pack 38 k-point grid of 16 × 6 × 6 for CaC 2 and 9 × 5 × 5 for Ca 3 C 7 and an energy cutoff for the plane wave expansion of 900 eV.The phonon frequencies and phonon band structure calculations were performed in the harmonic approximation with the help of PHONOPY software using the finite displacement method 39 for 3 × 3 × 3 (CaC 2 ) and 3 × 2 × 2 (Ca 3 C 7 ) supercells with respectively adjusted k-points.For Brillouin zone integrations, meshes of 10 × 10 × 10 (CaC 2 ) and 10 × 6 × 6 (Ca 3 C 7 ) k-points were used within the tetrahedron method.The charge densities obtained from VASP calculations were used for Bader's charge analysis in order to obtain the total charges associated with each atom and the zero flux surfaces defining the Bader volumes.Equation of state (EoS) and static enthalpy 40 calculations were performed via variable-cell structural relaxations between 0-100 GPa for Ca 3 C 7 and 0-150 GPa for HP-CaC 2 .In our calculations, temperature, configurational entropy, and the entropy contribution due to lattice vibrations were neglected.

Fig. 1 |
Fig. 1 | Crystal structure of HP-CaC 2 at 44(1) GPa. a A ball and stick model with the unit cell outlined; calcium atoms are shown as white spheres, and carbon atoms as red and blue balls for two distinct crystallographic positions, C1 (Wyckoff site 4 g) and C2 (4 h), respectively.b The geometry of a single deprotonated polyacene nanoribbon; the C-C distances and C-C-C angles are labeled.Cross-sections of the calculated electron localization function (ELF) are shown in the planes perpendicular (c) and parallel (d) to the polyacene nanoribbons.

Fig. 3 |
Fig.3| Pressure dependence of the lattice parameters and the unit cell volumes for the two Ca-C compounds.a HP-CaC 2 ; b Ca 3 C 7 .Solid symbols are experimental data points; dashed curves represent theoretical data; the black dashed lines are the fit of the calculated PV data using the third and second-order Birch-Murnaghan equations of state with the following parameters: V 0 = 127.17(2)Å 3 , K 0 = 144.5(2)GPa, and K´= 3.92 for HP-CaC 2 ; and V 0 = 432.0(2)Å 3 , K 0 = 125(1) GPa, K´= 4 (fixed) for Ca 3 C 7 .

Fig. 2 |
Fig. 2 | Crystal structure of Ca 3 C 7 at 38(1) GPa. a A projection of the Ca 3 C 7 structure along the a-axis, emphasizing 2D chains of carbon atoms aligned along the b-axis.Calcium atoms are shown as white spheres, and carbon atoms as red and blue balls for the two distinct crystallographic positions C1 (4c) and C2 (8d), respectively.Carbon atoms, named C3 (8d) and C4 (8d), are shown as gray balls.b The geometry of a single deprotonated para-poly(indenoindene) (p-PInIn) chain with the C-C distances and C-C-C angles labeled.c, d Cross-sections of the calculated electron localization function (ELF) are shown in the two different planes containing p-PInIn chains.

Fig. 5 |
Fig. 5 | The calculated convex hull for the Ca-C binary system with predicted and observed calcium carbides at 40 GPa.The phases HP-CaC 2 and Ca 3 C 7 lie on the convex hull (black dashed line) and are, thus, thermodynamically stable.The phases Ca 2 C 3 (P 1), Ca 2 C 3 (C2/c) and CaC 2 (P6/mmm) are off the convex hull and are, therefore, unstable.