Experimental Observation of a Calcium Silicon Double Carbonate

Abstract

Reactions between carbon dioxide (CO₂) and silica (SiO₂), as well as between carbonates and silicates, are central to understanding carbon behavior in planetary interiors and have important technological implications. Yet, only a few oxides are known in which carbon and silicon coexist within the same crystal lattice. These silicate–carbonates typically contain trigonal carbonate [CO₃] groups and tetrahedrally coordinated silicate [SiO₄] units. In the CaO–SiO₂–CO₂ system, no phase had previously been shown to contain both octahedrally coordinated silicon [SiO₆] and carbonate [CO₃] groups, nor had such a structure been theoretically predicted. Here, we report the synthesis of the calcium silicon double carbonate Ca₂Si(CO₃)₄, obtained by reacting tilleyite, Ca₅(Si₂O₇)(CO₃)₂, with CO₂ in diamond anvil cells at 39.5 GPa and 2600 K. The compound was subsequently temperature‐ and pressure‐quenched, remaining metastable at ambient conditions. Structural and spectroscopic characterization was performed using synchrotron single‐crystal X‐ray diffraction and Raman spectroscopy. The structure of Ca₂Si(CO₃)₄ is unique among silicate–carbonates, featuring sixfold‐coordinated Si sharing oxygen atoms with [CO₃] groups. Additionally, a novel calcium tetracarbonate, Ca₂(C₄O₁₀), containing tetrahedral [CO₄] units, was discovered. These findings reveal a new oxide chemistry under extreme conditions and open avenues for synthesizing metastable carbon‐bearing materials relevant to the deep carbon cycle.

High‐pressure and ‐temperature reactions between Ca₅(Si₂O₇)(CO₃)₂ tilleyite and CO₂ carbon dioxide yield the first experimental evidence of the double carbonate Ca₂Si(CO₃)₄ and a new Ca₂(C₄O₁₀) phase containing tetrahedral [CO₄] units. These findings reveal unexpected carbonate chemistry and highlight pathways for carbon incorporation under mantle conditions.

Introduction

Carbon dioxide (CO₂) is both highly abundant and of fundamental importance in the Universe. It plays a key role in planetary atmospheres, where it is a major constituent of Earth‐like planets, and is also present in solid icy phases on outer planets and asteroids.^{[ 1} , ^{2 ]} Under ambient conditions, CO₂ exists as a gas, and its increasing concentration in Earth's atmosphere poses a significant challenge to human societies and ecosystems.^{[ 3 ]} Upon compression, carbon dioxide first forms a series of molecular crystalline polymorphs in which the linear CO₂ configuration is preserved.^{[ 4} , ^{5 ]} At even higher pressures, CO₂ transforms into an extended covalent solid, in which carbon atoms adopt tetrahedral coordination, each bonded to four oxygen atoms.^{[ 6} , ⁷ , ⁸ , ^{9 ]}

Atmospheric CO₂ is consumed during the chemical weathering of calcium‐bearing silicate minerals, releasing calcium and bicarbonate ions that subsequently precipitate as calcium carbonates (CaCO₃), thereby forming a long‐term carbon sink.^{[ 2 ]} Surface carbonates, which contain trigonal [CO₃] groups, are transported into the Earth's mantle along with subducting oceanic lithosphere, exposing them to progressively higher pressures and temperatures.^{[ 1 ]} Under these conditions, carbonates could potentially react with surrounding silicate minerals to form high‐pressure carbonate‐silicate assemblages. However, CO₂‐ and SiO₂‐bearing compounds exhibit markedly different chemical behaviors at typical subduction P–T conditions, which hinders the formation of stable calcium silicate‐carbonate phases due either to the thermodynamic stability of separate carbonate and silicate phases or to kinetic limitations. Consequently, despite the coexistence of CO₂, carbonates, and silicates under extreme conditions, the formation of distinct calcium silicate‐carbonate compounds seems to be largely restricted to specific geological settings. To date, only three naturally occurring calcium silicate‐carbonate minerals—tilleyite, Ca₅(Si₂O₇)(CO₃)₂,^{[ 10} , ¹¹ , ^{12 ]} spurrite Ca₅(SiO₄)₂(CO₃)^{[ 11} , ^{13 ]} and galuskinite Ca₇(SiO₄)₃(CO₃)^{[ 14 ]}—have been documented, all forming in highly localized environments where silicate and carbonate components coexist in appropriate ratios under relatively low temperatures and pressures, with sufficient fluid mobility to enable ion exchange and crystallization. The three silicate‐carbonate minerals above feature carbonate [CO₃] groups and [SiO₄] tetrahedra, the latter either isolated or forming pyrosilicate anions. Because they do not share oxygen atoms, the carbonate and silicate components in these minerals are chemically distinct and structurally similar to the same moieties found in pure carbonates and pure silicates. A recent study reported the polymerization between carbonate and silicate groups in carbon‐bearing silica glasses above 8 GPa and discussed its implications for carbon‐bearing dense silicate melts.^{[ 15 ]}

Numerous groups have extensively searched for stable silicon carbonate phases across a range of P–T conditions.^{[ 16} , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² , ^{23 ]} Within the SiO₂–CO₂ system, only a disordered silicon carbonate compound has been experimentally observed, formed via the reaction of the oxides at approximately 20 GPa and 900 K.^{[ 16 ]} The structure of this phase was only partially characterized after temperature quenching, and its detailed atomic arrangement remains unknown, although it has been predicted by evolutionary algorithm calculations.^{[ 22} , ^{23 ]} In the CaO–SiO₂–CO₂ system, the formation of a dense post‐tilleyite structure has been reported, featuring a variety of cation sites capable of accommodating atoms of differing sizes.^{[ 12 ]} Post‐tilleyite is a silicate‐carbonate phase also comprising trigonal planar [CO₃] and tetrahedral [SiO₄] units. A recent preprint reports the synthesis of CaSi[C₂O₇], a high‐pressure high‐temperature phase (122(2) GPa, 2800(200 K) built from [CO₄] tetrahedra, which form [C₂O₇] units, and [SiO₆] octahedra.^{[ 24 ]} By contrast, the DFT‐predicted SiC₂O₆ phase contains distorted [SiO₆] octahedra alongside [CO₃] groups,^{[ 22} , ^{23 ]} but this structure has not yet been experimentally confirmed. Investigating the chemistry of these compositionally simple systems is of significant interest not only for materials science—owing to the potential formation of novel materials with unique physical properties—but also for Earth and planetary sciences, where such phases could represent accessory carbonates in the mantle.

Here, we present the results of high‐pressure single‐crystal X‐ray diffraction (SCXRD) experiments on quenched high‐temperature dense samples, combined with Raman spectroscopy measurements and density functional theory (DFT) calculations, which demonstrate the formation of a silicon carbonate phase, Ca₂Si(CO₃)₄. This new phase is unique in several aspects. First, it features silicon atoms in octahedral coordination, which, to the best of our knowledge, represents the first experimental observation of sixfold coordinated (octahedral [SiO₆] units) silicon in a carbonate, with the exception of the mineral thaumasite,^{[ 25} , ^{26 ]} Ca₃Si(OH)₆(CO₃)(SO₄)·12H₂O, which forms in aqueous environments and contains [Si(OH)₆] octahedra. Second, the structure of Ca₂Si(CO₃)₄ contains [CO₃] triangular groups, where the oxygen atoms are shared with the silica network, constituting the first crystalline example of oxygen‐sharing between carbonate and silicate groups observed. Importantly, Ca₂Si(CO₃)₄ can be quenched to ambient conditions, where it remains as a metastable phase. During these experiments, we also identified a novel dense polymorphic form of calcium carbonate, Ca₂(C₄O₁₀), in which tetrahedral [CO₄] units assemble into [C₄O₁₀]^{4 −} anions that are topologically analogous to P₄O₁₀ phosphorus oxide.

Results and Discussion

Phase Identification and Mapping

The pressure in the diamond anvil cell (DAC) was increased up to 39.5(4.0) GPa, at which point tilleyite appeared in its second, yet unresolved high‐pressure polymorph (HP2)^{[ 12 ]} and CO₂ was transformed to phase III (Figure 1a). Upon laser heating to 2600(260) K and subsequent temperature quench, the sample chamber was as shown in Figure 1b. Raman mapping of the heated crystal was then performed to determine the spatial distribution of phases and to identify possible chemical reaction products. Six distinct phases were detected in the Raman map (Figure 1c–h): two previously unknown phases (Figure 1c, d), tilleyite HP2 (Figure 1e), and CO₂‐II, III, and V (Figure 1f–h), with Raman peaks located approximately at 1214, 488, 1176, 388, 360, and 777 cm^{− 1}, respectively.

Figure 1.

a) Sample chamber of the DAC containing two tilleyite crystals, ruby as a pressure gauge, and CO₂‐III as the reactive medium at 39.5 GPa before laser heating. b) Sample chamber of the DAC at 39.5 GPa after laser heating. The red dashed box highlights the Raman mapping area of the heated crystal. c–h) 2D Raman maps after laser heating showing the phase distribution of: (c) Ca₂Si(CO₃)₄ (∼1214 cm^{− 1}), (d) Ca₂C₄O₁₀ – C2/c (∼488 cm^{− 1}), (e) tilleyite HP2 phase (∼1176 cm^{− 1}), (f) CO₂‐II (∼388 cm^{− 1}), (g) CO₂‐III (∼360 cm^{− 1}), and (h) CO₂‐V (∼777 cm^{− 1}) in the framed rectangular area depicted in (b).

After establishing the phase distribution, XRD mapping was carried out to locate the most suitable regions for probing the crystalline domains of the unknown phases. As discussed below, these correspond to (i) a new calcium silicon double carbonate (Ca₂Si(CO₃)₄) and (ii) a new polymorph of calcium tetracarbonate (Ca₂C₄O₁₀), featuring sp³ hybridized carbon. Structural refinement of the phases was subsequently undertaken in detail.

Crystal Structure of Ca₂Si(CO₃)₄

Although crystalline domains smaller than 1 × 1 µm² formed after laser heating, the structure was successfully solved by synchrotron single‐crystal X‐ray diffraction using multigrain data analysis techniques. At 39.5 GPa, the anhydrous dicalcium silicon tetracarbonate (Ca₂Si(CO₃)₄), crystallizes in the monoclinic space group P2₁/c. The refined lattice parameters at 39.5 GPa are: a = 4.1653(5) Å, b = 14.2462(16) Å, c = 5.2244(17) Å, β = 101.27(2)°, with a unit‐cell volume of V = 304.04(11) ų, Z = 2, and a calculated density of ρ = 3.804 g·cm^{− 3}. The crystal structure is shown in Figure 2. Details of the XRD data collection and refinement results are provided in Table 1. Experimental and calculated structural data at 39.5 GPa are provided in Tables S2–S4.

Figure 2.

Projections of the Ca₂Si(CO₃)₄ crystal structure at 39.5 GPa along the a) a‐axis, b) c‐axis, and c) c‐axis, highlighting the SiO₆–CO₃–SiO₆ linkages along the [100] direction. Calcium (Ca), silicon (Si), oxygen (O), and carbon (C) atoms are shown in light green, yellow, red, and gray, respectively.

Table 1.

Crystal data, data collection, and structure refinement parameters for Ca₂Si(CO₃)₄ obtained experimentally from single‐crystal X‐ray diffraction at 39.5 GPa and ambient pressure, together with crystallographic data calculated from DFT at 39.5 and 0 GPa.

	Experimental		DFT
Pressure (GPa)	39.5	10−4	39.5	0

Crystal data
CCDC number	2494285	2494286	−	−
Empirical formula	Ca2SiC4O12
Formula weight [g⋅mol−1]	348.29
Crystal system	Monoclinic
Space group (number)	P21/c (14)
a [Å]	4.1653(5)	4.3946(2)	4.13841	4.40532
b [Å]	14.2462(16)	15.5347(8)	14.17967	15.69894
c [Å]	5.2244(17)	5.8337(14)	5.16065	5.90113
β [°]	101.27(2)	104.155(12)	98.14	104.47
Volume [Å3]	304.04(11)	386.17(10)	299.99	394.37
Z	2
ρ calc [g.cm−3]	3.804	2.995	3.86	2.93
μ [mm−1]	0.217	0.171	−	−

Data collection
F(000)	348		−	−
Radiation	Synchrotron (λ = 0.2904)		−	−
2θ Range [°]	4.0704 to 28.006	3.906 to 27.994	−	−
Index Ranges	−6 ≤ h ≤ 6 −23 ≤ k ≤ 22 −6 ≤ l ≤ 5	−7 ≤ h ≤ 7 −25 ≤ k ≤ 23 −5 ≤ l ≤ 5	−	−
Reflections collected	1397	1831	−	−
Independent reflections	673	833	−	−

Crystal structure refinement
Data/restraints/parameters	673/0/88	833/0/88	−	−
Goodness‐of‐fit on F 2	1.152	1.006	−	−
Final R Indexes [I ≥ 2σ(I)]	R1  = 0.0453 wR2  = 0.1320	R1  = 0.0421 wR2  = 0.0982	−	−
Final R Indexes [All Data]	R1  = 0.0528 wR2  = 0.1362	R1  = 0.0598 wR2  = 0.1044	−	−
Δρ max/Δρ min [eÅ−3]	0.53/−0.94	0.61/−0.40	−	−

The structure of Ca₂Si(CO₃)₄ comprises chains of isolated [SiO₆] octahedra bridged by carbonate groups forming SiO₆–CO₃–SiO₆ linkages along the [100] direction. Two crystallographically independent and distorted carbonate groups, [C(1)O₃] and [C(2)O₃], are present. The [C(1)O₃] planar units share two of their oxygen atoms with adjacent [SiO₆] octahedra, establishing the connectivity that defines the chains, whereas the [C(2)O₃] units share only one corner. These carbonate groups are tilted by 26.4° and 23.5° relative to the ab plane, respectively. The coordination environment of the Ca atoms was evaluated using the atomic Voronoi–Dirichlet polyhedral analysis^{[ 27 ]} implemented in the ToposPro software.^{[ 28} , ^{29 ]} The coordination number was determined from the major faces of the Voronoi–Dirichlet polyhedra, which correspond to direct neighbors. Minor faces were neglected, as the present structural description is restricted to the first coordination sphere. The Ca atoms form double [CaO₁₀] bicapped square‐antiprismatic polyhedra arranged along the bc‐plane, with two oxygen atoms shared between adjacent [CaO₁₀] units. The [SiO₆] octahedra link to the [CaO₁₀] units through two shared edge oxygen atoms, thereby connecting neighboring double units within the bc‐plane.

The [CaO₁₀] unit exhibits an average Ca─O bond length of 2.368 Å, with eight individual bond lengths ranging from 2.163(3) to 2.357(4) Å and two bond lengths of 2.633(3) and 2.844(3) Å, a total polyhedral volume of 27.428 ų and an effective coordination number (ECoN)^{[ 30} , ^{31 ]} of 8.21. In contrast, the [SiO₆] octahedra display an average Si─O bond length of 1.719 Å, with individual bond lengths between 1.701(3) and 1.736(3) Å, a polyhedral volume of 6.757 ų and a distortion index of 0.007, which indicates a nearly ideal octahedral geometry The two types of carbonate groups, on the other hand, are highly irregular. In the [C(1)O₃] units, one C─O bond is short (1.223(5) Å) while the other two are longer (1.280(5) and 1.298(5) Å), whereas in the second type of carbonate units [C(2)O₃] two bonds are short (1.240(5) and 1.242(5) Å) and one is long (1.330(5) Å). Atoms‐in‐molecules delocalization indices,^{[ 32 ]} which effectively reflect bond orders, were calculated using maximally localized Wannier functions.^{[ 33 ]} The pattern of bond lengths in the two carbonates correlates with their respective C─O─Ca and C─O─Si connectivity and agrees with the calculated delocalization indices (DI) between the carbon and oxygen atoms. For C1, two bonds have higher DI than the third (approximately 0.95 and 0.75 at 39.5 GPa), corresponding to the bond whose oxygen is bonded to Si. For C2, one bond exhibits a large DI and the other two smaller values (approximately 1.05 and 0.82), consistent with a carbonate group sharing two vertices with adjacent Si atoms. The carbon atoms of the CO₃ groups connected to two Si atoms are slightly less positively charged, according to calculations (atomic charge of 2.19), than those connected to one Si atom (2.22).

Following structural characterization, the DAC was decompressed to ambient pressure to evaluate the stability of the newly synthesized high‐pressure compound. The double carbonate remains stable at ambient conditions, retaining its monoclinic crystal structure (space group P2₁/c) and confirming its recoverable nature. The refined lattice parameters at ambient pressure are: a = 4.3946(2) Å, b = 15.5347(8) Å, c = 5.8337(14) Å, β = 104.155(12)°, with a unit‐cell volume of V = 386.17(10) ų, Z = 2, and a calculated density of ρ = 2.995 g·cm^{− 3}. Table 1 summarizes the crystal data, data collection, and structure refinement parameters for Ca₂Si(CO₃)₄ obtained experimentally at high pressure and at ambient pressure, together with crystallographic data calculated from DFT, which show excellent agreement with the experimental results. Experimental and calculated structural data at room pressure are provided in Tables S5–S7.

The compressional behavior of the newly discovered double carbonate, Ca₂Si(CO₃)₄, was investigated to assess its compressibility and structural stability under pressure. Figure 3 illustrates the evolution of the unit‐cell volume as a function of pressure (a) and the pressure dependence of the lattice parameters (b). Since only 2 experimental data points are available, the bulk modulus and axial compressibility were primarily determined from DFT‐calculated data. A third‐order Birch–Murnaghan (BM3) equation of state was fitted to the DFT data using the EoSFit7‐GUI software.^{[ 34 ]} The fit yielded a zero‐pressure volume (V ₀) of 391.2(4) ų, a bulk modulus (K ₀) of 73.8(14) GPa, and a first pressure derivative (K′₀) of 5.32(10). Since the DFT‐calculated structural parameters showed excellent agreement with the experimental measurements, the bulk modulus was also estimated using the two experimental pressure points available. For this purpose, the zero‐pressure volume was fixed to the experimental value obtained under ambient conditions, and the first pressure derivative was constrained to the DFT‐derived value (K′₀ = 5.32), resulting in a bulk modulus of K₀ = 88(9) GPa. The bulk modulus of the double carbonate is consistent with previously reported values for silicate‐carbonate phases with tetrahedrally coordinated silicon, being 77(1) GPa for spurrite,^{[ 13 ]} 80(2) GPa for tilleyite,^{[ 12 ]} and 83(3) GPa for post‐tilleyite,^{[ 12 ]} demonstrating a strong correlation in compressibility behavior between the new calcium silicon carbonate and early known carbonate‐silicate phases. Moreover, the bulk modulus of this compound lies within the range typically observed for common carbonates and silicates, including calcite (K ₀ = 69(2) GPa),^{[ 35 ]} aragonite (K ₀ = 63(2) GPa),^{[ 36 ]} coesite (K ₀ = 94(1) GPa),^{[ 37 ]} and larnite (K ₀ = 103(2) GPa),^{[ 38 ]} The principal axes of compression and their corresponding compressibilities were determined using PASCal v2.2.0,^{[ 39 ]} based on the DFT‐optimized structural data. The approximate crystallographic directions of maximum, intermediate, and minimum compressibility were found to be $[\overline{1}02]$, [0 1 0], and [12,  0,  5], with linear compressibilities of κ ₁ = 3.65(3) × 10^{− 3} GPa^{− 1}, κ ₂ = 2.26(1) × 10^{− 3} GPa^{− 1}, and κ ₃ = 0.53(2) × 10^{− 3} GPa^{− 1}, respectively.

Figure 3.

a) Pressure dependence of the Ca₂Si(CO₃)₄ unit‐cell volume up to 50 GPa. DFT‐calculated data (black squares) and SCXRD experimental data (red circles) are shown, with solid lines representing 3^rd‐order Birch–Murnaghan fits. The inset shows the evolution of the β angle with pressure. b) Pressure dependence of the Ca₂Si(CO₃)₄ lattice parameters.

Raman spectroscopy provides additional insights into the bonding characteristics of Ca₂Si(CO₃)₄ under compression and after decompression. Group‐theoretical analysis based on the point group C _2h reveals a total of 114 vibrational modes at the Brillouin‐zone center (Γ), consisting of 111 optical and three acoustic modes. Among these, 54 optical modes are Raman‐active and are distributed according to Γ _Raman =  27A_g + 27B_g .^{[ 40} , ⁴¹ , ⁴² , ^{43 ]} Experimental and DFT‐calculated Raman spectra of Ca₂Si(CO₃)₄ at 39.5 GPa are shown in Figure S1. No pure Raman spectrum of the phase could be obtained, likely due to the limited spectrometer resolution, the submicrometric size of the crystalline domains, and partial spectral overlap with coexisting phases. In this context, several spurious peaks arising from coexisting phases were identified and labeled accordingly. From the experimental Raman spectra, 13 Raman‐active modes were observed at approximately 145, 216, 254, 305, 702, 813, 840, 892, 1214, 1247, 1481, 1706, and 1756 cm^{− 1}. Using Critic2^{[ 44} , ^{45 ]} and DFT calculations, these modes could be attributed to the characteristic vibrations of the new Ca₂Si(CO₃)₄ phase. The low‐frequency region (145–305 cm^{− 1}) corresponds to lattice modes involving Ca^{2 +} cations and the collective motion of [SiO₆] and [CO₃] units. The peak at 702 cm^{− 1} is assigned to the combined O─Si─O in‐plane bending (ν ₄) and O─C─O in‐plane bending (ν ₄) modes. The peaks between 813 and 892 cm^{− 1} correspond to O─Si─O symmetric stretching (ν₁) coupled with O─C─O out‐of‐plane bending (ν ₂). The peak at 1214 cm^{− 1} is attributed to O─Si─O asymmetric stretching (ν ₃), whereas the 1247 cm^{− 1} peak corresponds to O─C─O symmetric stretching (ν ₁). Finally, the high‐frequency region (1481–1756 cm^{− 1}) is dominated by O─C─O asymmetric stretching (ν ₃) vibrations.

Crystal Structure of Ca₂C₄O₁₀

Three types of carbonate groups exhibiting two distinct hybridization states are currently known. The first corresponds to the “conventional” carbonate group, consisting of nearly planar trigonal [CO₃]^{2 −} units with sp ²‐hybridized carbon atoms.^{[ 46} , ^{47 ]} Each group contains a central carbon atom bonded to three surrounding oxygen atoms, with typical C─O bond lengths of approximately 1.30 Å and O─C─O bond angles close to 120°. These sp ²‐hybridized carbonate groups constitute the fundamental building units of the most abundant and stable carbonates in the Earth's crust—such as magnesite (MgCO₃), calcite (CaCO₃), and dolomite (CaMg(CO₃)₂)—which together account for nearly 90% of the total carbonate content of this layer.^{[ 48} , ⁴⁹ , ⁵⁰ , ^{51 ]} The second and most recently discovered type corresponds to the pyrocarbonate group, which also exhibits sp² hybridization. It consists of [C₂O₅]^{2 −} units formed by the polymerization of two [CO₃]^{2 −} groups that share a common oxygen atom, resulting in a C─O─C linkage between adjacent carbonate units.^{[ 52} , ⁵³ , ^{54 ]} Pyrocarbonate compounds have been synthesized for the first time only recently.^{[ 55 ]} Since then, several pyrocarbonate salts have been successfully synthesized.^{[ 56} , ⁵⁷ , ⁵⁸ , ^{59 ]} The third type corresponds to the tetrahedrally coordinated carbonate group [CO₄]^{4 −}, which exhibits a tetrahedral geometry with sp³ ‐hybridized carbon atoms.^{[ 60} , ⁶¹ , ⁶² , ^{63 ]} These units are stable under pressures and temperatures corresponding to the Earth's transition zone and the uppermost lower mantle (>35 GPa and 2000–3000 K).^{[ 64} , ^{65 ]} Under ambient conditions, however, sp³ hybridization is energetically unfavorable due to the very small size of the C^{4 +} cation relative to O^{2 −}, which leads to excessive O─O steric repulsion.^{[ 66 ]} Under high‐pressure conditions, carbon stabilizes in the sp³‐hybridized state, adopting a coordination environment analogous to silicon in silicates. This increased coordination confers a volumetric advantage that offsets steric repulsion. Tetrahedrally coordinated carbonate groups may occur as isolated tetrahedra, dimers, rings, pyramidal clusters, or extended chains, with the tetracarbonate group represented by the pyramidal [C₄O₁₀]^{4 −} unit.^{[ 60} , ⁶¹ , ⁶⁵ , ⁶⁷ , ⁶⁸ , ⁶⁹ , ^{70 ]}

Calcium tetracarbonate (Ca₂C₄O₁₀) has previously been reported experimentally in the literature in two different space groups at 34(2) GPa: a tetragonal phase with space group $I\overline{4}2d$ and a cubic phase with space group $Fd\overline{3}m$.^{[ 64 ]} The tetragonal structure can be described as a slightly distorted derivative of the cubic one, with the relationship $a_{cubic}=\sqrt{2}{a}_{tetragonal}$. Density functional theory (DFT) calculations indicate that the tetragonal phase is energetically more favorable than the cubic form. In the present work, we identified Ca₂C₄O₁₀ crystallizing in the monoclinic space group C2/c. At 39.5 GPa, the refined lattice parameters are: a = 11.993(8) Å, b = 7.0949(9) Å, c = 12.316(5) Å, β = 109.07(7)°, with a unit‐cell volume of V = 990.4(9) ų, Z = 16, and a calculated density of ρ = 3.866 g·cm^{− 3}. The crystal structure is shown in Figure 4. It comprises three crystallographically independent calcium polyhedra: two [CaO₁₀] bicapped square antiprism and one distorted [CaO₁₂] icosahedron. These polyhedra are corner‐shared through oxygen atoms belonging to polymerized orthocarbonate [CO₄]^{4 −} groups, which form distorted pyramidal [C₄O₁₀]^{4 −} units with sp³‐hybridized carbon atoms.

Figure 4.

Projection of the Ca₂[C₄O₁₀] crystal structure along the b‐axis (left) at 39.5 GPa, and the tetrahedral [C₄O₁₀] unit (right). Calcium (Ca), oxygen (O), and carbon (C) atoms are shown in light green, red, and gray, respectively.

In projection onto the ac‐plane, the cationic framework forms a square‐grid topology, which can be compared to a tic‐tac‐toe pattern. In this analogy, the three‐by‐three grid (#) represents the cationic framework: Ca atoms occupy the grid intersections (forming the #), while the tetrahedral [C₄O₁₀] units are positioned at the “X” and “O” sites, completing the planar representation of the framework. Table 2 summarizes the crystal data, data collection, and structure refinement parameters for Ca₂C₄O₁₀ obtained experimentally at high pressure. Structural data, including atomic coordinates and isotropic and anisotropic displacement parameters, are provided in Tables S8 and S9.

Table 2.

Crystal data, data collection, and structure refinement parameters for Ca₂C₄O₁₀ were obtained experimentally from single‐crystal X‐ray diffraction (SCXRD) at 39.5 GPa.

	Experimental
Pressure (GPa)	39.5

Crystal data
CCDC number	2494194
Empirical formula	Ca2C4O10
Formula weight [g.mol−1]	288.20
Crystal system	Monoclinic
Space group (number)	C2/c (15)
a [Å]	11.993(8)
b [Å]	7.0949(9)
c [Å]	12.316(5)
β [°]	109.07(7)
Volume [Å3]	990.4(9)
Z	8
ρ calc [g.cm−3]	3.866
μ [mm−1]	0.233

Data collection
F(000)	1152
Radiation	Synchrotron (λ = 0.2904)
2θ range [°]	3.328 to 27.976
Index Ranges	−13 ≤ h ≤ 14 −11 ≤ k ≤ 11 −18 ≤ l ≤ 16
Reflections collected	2331
Independent reflections	948

Crystal structure refinement
Data/restraints/parameters	948/0/97
Goodness‐of‐fit on F 2	1.101
Final R Indexes [I ≥ 2σ(I)]	R 1 = 0.0633 wR 2 = 0.1624
Final R Indexes [All Data]	R 1 = 0.0969 wR 2 = 0.1772
Δρ max/Δρ min [eÅ−3]	0.99/−0.57

DFT‐relaxed structural optimization of the experimentally found C2/c phase results in a trigonal $R\overline{3}c$ crystal structure for the newly identified polymorph of calcium tetracarbonate. Phonon calculations performed in a 2 × 2 × 1 supercell at the DFT equilibrium structure corresponding to the closest point of the volume grid (39.9 GPa) yielded only positive harmonic frequencies, confirming that the $R\overline{3}c$ structure is dynamically stable. At 39.5 GPa, the DFT‐obtained lattice parameters are: a = 6.9210 Å and c = 34.2918 Å, with a unit‐cell volume of V = 1456.89 ų, Z = 12, and a calculated density of ρ = 3.94 g·cm^{− 3}. The detailed structural data at 39.5 GPa are provided in Table S10. The experimental data set could also be solved in this trigonal setting, yielding refined lattice parameters: a = 7.0222(9) Å and c = 34.873(3) Å, with a unit‐cell volume of V = 1489.3(4) ų, Z = 12, and a calculated density of ρ = 3.86 g·cm^{− 3}, but slightly worse figure of merit. Notably, the experimentally observed monoclinic C2/c cell displays a pseudo‐rhombohedral metric, as expected for a weakly distorted trigonal lattice. The detailed transformation relating the monoclinic and trigonal descriptions is discussed in the Supporting Information.

The $Fd\overline{3}m$ structure reported by König et al., ^{[ 64 ]} represents the aristotype from which all known CaC₂O₅ polymorphs can be derived. The previously reported $I\overline{4}2d$ phase^{[ 64 ]} and the C2/c and $R\overline{3}c$ structures discussed in this work arise from symmetry‐lowering distortions of this cubic parent structure while preserving the [C₄O₁₀]^{4 −} motif. The corresponding group–subgroup relationships are summarized in Figure S2 through a Bärnighausen tree. This analysis shows that the tetragonal polymorph arises directly from the cubic aristotype, whereas the trigonal $R\overline{3}c$ structure— and, subsequently, the monoclinic C2/c variant—derive from a distinct branch of the same cubic parent. Consequently, the polymorphs discussed here correspond to independent distortion pathways of the cubic aristotype.

Upon decompression, the phase was found to be unrecoverable at ambient conditions. Nevertheless, DFT calculations allow for the estimation of its elastic properties and the assessment of structural stability under pressure, in comparison with existing polymorphs and analogous M₂[C₄O₁₀] compounds (M = Ca, Fe, Cd). The evolution of the unit‐cell volume with pressure and the pressure dependence of the lattice parameters are shown in Figures S3 and S4, respectively. A third‐order Birch–Murnaghan fit yielded a zero‐pressure volume (V ₀) of 1210.53(18) ų, a bulk modulus (K ₀) of 104.2(3) GPa, and a first pressure derivative (K′₀) of 4.90(2). The principal crystallographic directions of compression were found to be [100], [010], and [103], with linear compressibilities of κ ₁ = 1.83(1) × 10^{− 3} GPa^{− 1}, κ ₂ = 1.83(1) × 10^{− 3} GPa^{− 1}, and κ ₃ = 1.81(2) × 10^{− 3} GPa^{− 1}, respectively. The nearly identical values indicate that the crystal exhibits essentially isotropic compressibility.

Table 3 summarizes the DFT equation‐of‐state (EoS) parameters for the M₂[C₄O₁₀] compounds (M = Ca, Cd, Fe). For completeness and future reference, a second‐order BM (BM2) fit for the present work is also included in the table. For monoclinic calcium tetracarbonate, the bulk modulus falls within the range of other sp³ calcium carbonates.^{[ 35} , ^{36 ]} Calcium orthocarbonate Ca₂CO₄, for instance, has a K ₀ value of 108(1) GPa.^{[ 68 ]} The tetragonal polymorph of calcium tetracarbonate, with K ₀ = 93.4(4) GPa, exhibits a bulk modulus approximately 10% lower than that of the monoclinic phase. This difference can reasonably be attributed to variations in cation coordination, from a 10‐fold coordination in the monoclinic structure to a 12‐fold coordination in the tetragonal polymorph. Cadmium tetracarbonate (Cd₂C₄O₁₀)^{[ 71 ]} and iron tetracarbonate (Fe₂C₄O₁₀),^{[ 72 ]} structural analogues of Ca₂C₄O₁₀ reported previously in the literature, display distinct compressibilities. Cd₂C₄O₁₀ exhibits a bulk modulus (K ₀ = 112.2(5) GPa) approximately 8% higher than monoclinic Ca₂C₄O₁₀, while Fe₂C₄O₁₀ presents the bulk modulus (K ₀ = 127(3) GPa) roughly 22% higher. These trends reinforce the general observation that the bulk moduli of carbonates containing [C₄O₁₀]^{4 −} units tend to increase with decreasing cation radius, which is directly related to the fact that the [CO₄] units are largely incompressible, and the bulk moduli mainly depend on the [MO_x] polyhedral compressibility.

Table 3.

Comparison of Birch–Murnaghan (BM) equation‐of‐state (EoS) parameters for M₂[C₄O₁₀] compounds (M = Ca, Cd, Fe), including space group symmetry, zero‐pressure volume (V₀), bulk modulus (K₀), and its first pressure derivative (K′₀). Table updated from Kovalev et al.^{[ 72 ]} Licensed under CC‐BY 4.0 (https://creativecommons.org/licenses/by/4.0/). [Correction added on January 28, 2026, after first online publication: Table 3 has been updated.]

Compound	Space group	Data	V0 (Å3)	K0 (GPa)	K’0	Ref.
Ca2C4O10	C2/c	DFT – BM2	1202.9(14)	120.2(11)	4.0	This work
		DFT – BM3	1210.53(18)	104.2(3)	4.90(2)
Ca2C4O10	$I\overline{4}2d$	DFT – BM2	611(3)	110(3)	4.0	[64]
		DFT – BM3	616.7(2)	93.4(4)	4.83(2)
Cd2C4O10	$Fd\overline{3}m$	DFT – BM2	1186.4(6)	132(3)	4.0	[71]
		DFT – BM3	1196.8(8)	112.2(5)	5.21(3)
Fe2C4O10	$Fd\overline{3}m$	DFT – BM2	1084(4)	152(3)	4.0	[72]
		DFT – BM3	1092(2)	127(3)	4.93(12)
		EXP – BM2	1059(17)	160(18)	4.0

Raman spectroscopy was performed for the new calcium tetracarbonate polymorph. Group‐theoretical analysis based on the point group C _2h reveals a total of 192 vibrational modes at the Brillouin‐zone center (Γ), consisting of 189 optical and three acoustic modes. Among these, 93 optical modes are Raman‐active and are distributed according to Γ _Raman =  46A_g + 47B_g .^{[ 40} , ⁴¹ , ⁴² , ^{43 ]} Experimental and DFT‐calculated Raman spectra of Ca₂C₄O₁₀ at 39.5 GPa are shown in Figure S5. No pure Raman spectrum of the phase could be obtained. Eight Raman‐active modes observed at approximately 450, 488, 593, 622, 745, 768, 1004, and 1025 cm^{− 1} were assigned to the CaC₂O₄ phase. The modes at 450, 488, 593, 745, and 768 cm^{− 1} correspond to O─C─O in‐plane bending (ν ₄), while those at 622, 1004, and 1025 cm^{− 1} correspond to O─C─O symmetric stretching (ν ₁). Four of the eight observed Raman modes (450, 622, 745, and 768 cm^{− 1}) show excellent agreement with previously reported modes for the tetragonal structure at 497, 631, 751, and 773 cm⁻¹,^{[ 64 ]} respectively. As noted above, the crystal structure of the new tetracarbonate is not recoverable at ambient conditions.

The crystal structures of calcium tetracarbonate and calcium silicon carbonate were successfully determined and described in detail. However, the HP2 phase of tilleyite, identified during Raman mapping, could not be observed in the SCXRD experiments and therefore remains structurally unresolved. The following section presents a DFT‐based thermodynamic and energetic analysis of the relevant systems, including carbon dioxide (CO₂), silica (SiO₂), calcium carbonate (CaCO₃), calcium oxide (CaO), and tilleyite, as well as the decomposition pathways of the newly observed calcium silicon carbonate phase.

Thermodynamic and Energetic Stability

To evaluate the thermodynamic stability of Ca₂Si(CO₃)₄ and Ca₂(C₄O₁₀) with respect to their possible decomposition into oxides, we calculated the equations of state (EoS) in the static approximation (no vibrations) for carbon dioxide (CO₂), silica (SiO₂), calcium carbonate (CaCO₃), calcium oxide (CaO), and tilleyite, along with the aforementioned compounds. For each system, the relevant experimentally observed crystal structures within the 0–45 GPa pressure range were considered. The enthalpy–pressure relations were constructed by identifying, at each pressure, the lowest‐enthalpy phase among the candidate structures. Additional phonon frequency calculations were carried out to incorporate the effects of temperature in the quasiharmonic approximation. The details for these calculations and results for the reference oxides and carbonates are provided in the Supporting Information.

The relative stabilities of tilleyite, Ca₂Si(CO₃)₄, Ca₂(C₄O₁₀), and the combinations of their corresponding oxides (CaO, CO₂, and CaCO₃) are shown in Figure 5. We must note that the data in Figure 5 informs only of thermodynamic stability, not the kinetics of the different transformations. Figures 5, S11, and S12 show the stabilities at ambient temperature, in the static approximation and at 2600 K, respectively. At low pressures, 5/2 Ca₂(C₄O₁₀) + 2 SiO₂ is unstable relative to 2 Ca₂Si(CO₃)₄ + CaO + 2 CO₂, and the latter is unstable with respect to decomposition into 5 CaCO₃ + 2 SiO₂ + 5 CO₂. At low temperature, the thermodynamic stability of these phases undergoes a marked change around 20 GPa, where a distinct variation in the slope of the Gibbs energy–pressure curves coincides with the appearance of phase V in the CO₂ phase diagram. Above this pressure, and at low temperature, the presence of free CO₂ becomes thermodynamically disfavored, and the stable phase is 5/2 Ca₂(C₄O₁₀) + 2 SiO₂ up to 45 GPa. The trend depicted in Figure 5 is consistent with previous theoretical predictions of the decomposition of Ca₂(C₄O₁₀)‐ $I\overline{4}2d$ below 20 GPa at ambient conditions obtained using DFT + QHA methods,^{[ 64 ]} as well as with experimental observations showing that this phase can be synthesized from CaCO₃ + CO₂ at pressures above 34 GPa but reverts to the starting materials below 18 GPa.^{[ 64 ]}

Figure 5.

DFT‐calculated Gibbs energy as a function of pressure at 300 K, illustrating the relative thermodynamic stability of tilleyite + CO₂ compared to Ca₂Si(CO₃)₄, Ca₂(C₄O₁₀), and the combinations of their corresponding oxides (CaO, CO₂, and CaCO₃). Color coding: solid blue line – tilleyite + 8 CO₂; orange line – 2 Ca₂Si(CO₃)₄ + CaO + 2 CO₂; green line – 5 CaO + 2 SiO₂ + 10 CO₂; red line – 5 CaCO₃ + 2 SiO₂ + 5 CO₂; purple line – 5/2 Ca₂(C₄O₁₀) + 2 SiO₂. The pressure range has been restricted to remove the zones where the quasiharmonic approximation behaves spuriously.

The thermodynamic stabilities in Figure 5 are also consistent with our experimental observations. A mixture of tilleyite and CO₂ was heated, acquiring enough energy to overcome the kinetic barriers to transition to more stable phases: Ca₂Si(CO₃)₄ and Ca₂C₄O₁₀. Upon pressure release, the transformation of the latter back to its component oxides occurred. At ambient conditions, the decomposition of Ca₂Si(CO₃)₄ is less favorable than that of Ca₂C₄O₁₀ and likely kinetically hindered, hence its metastability.

Conclusion

High pressure modifies the electron density of compounds by redistributing it within and between atoms, leading to changes in bonding, orbital hybridization, and maximizing the packing efficiency of the structure. Novel phases form to minimize enthalpy, a process that typically leads to an increase in atomic coordination. High temperatures, on the other hand, are crucial to overcoming kinetic barriers that often hinder structural transformations. At ambient pressure, silicates are usually formed by either isolated or condensed [SiO₄] tetrahedra. Known silicate‐carbonate minerals consist of distinct and separate [SiO₄] and [CO₃] units with no oxygen sharing between them.

The present work investigates the chemistry of the CaO–SiO₂–CO₂ system under extreme pressure and temperature conditions using a combination of Raman spectroscopy, single‐crystal X‐ray diffraction, and DFT calculations. The results demonstrate that a crystalline calcium silicon carbonate with the formula Ca₂Si(CO₃)₄ is obtained from the reaction of naturally‐occurring silicate‐carbonate mineral tilleyite (Ca₅(Si₂O₇)(CO₃)₂) with CO₂ at approximately 39.5 GPa and 2600 K, and that this novel phase can be recovered down to ambient conditions as a metastable solid. The structure of this new phase is unique: it is an oxide with sixfold‐coordinated silicon, and the triangular carbonate groups share oxygen atoms with the silica network. The octahedral coordination of Si by O is favored by the oxygen coordination to the relatively more electronegative C atoms, which draw electrons out of the Si─O bonds, lengthening them, and decreasing O─O repulsion.^{[ 73 ]} The formation of this calcium silicon carbonate also reflects a balance between the respective stabilities of three‐ and fourfold coordination in carbon under high pressure, as evidenced by the simultaneous synthesis of a novel monoclinic polymorph of calcium tetracarbonate, Ca₂(C₄O₁₀), with sp ³‐hybridized carbon atoms, while exceeding the pressure required for sixfold coordination of silicon.

The structure observed in this work is an example of a new oxide chemistry in which silicates and carbonates interact, producing a unique, light, carbon‐rich oxide that remains metastable at ambient conditions. At yet higher pressure, the formation of a novel phase with coexisting 4‐coordinated carbon and 6‐coordinated silicon has been reported.^{[ 24 ]}

Supporting Information

The authors have cited additional references within the Supporting Information.^{[ 74} , ⁷⁵ , ⁷⁶ , ⁷⁷ , ⁷⁸ , ⁷⁹ , ⁸⁰ , ⁸¹ , ⁸² , ⁸³ , ⁸⁴ , ⁸⁵ , ⁸⁶ , ⁸⁷ , ⁸⁸ , ⁸⁹ , ⁹⁰ , ⁹¹ , ⁹² , ⁹³ , ⁹⁴ , ⁹⁵ , ⁹⁶ , ⁹⁷ , ⁹⁸ , ⁹⁹ , ¹⁰⁰ , ¹⁰¹ , ¹⁰² , ¹⁰³ , ¹⁰⁴ , ¹⁰⁵ , ¹⁰⁶ , ¹⁰⁷ , ¹⁰⁸ , ¹⁰⁹ , ¹¹⁰ , ¹¹¹ , ¹¹² , ¹¹³ , ¹¹⁴ , ¹¹⁵ , ¹¹⁶ , ¹¹⁷ , ¹¹⁸ , ¹¹⁹ , ^{120 ]} The supplementary material provides detailed experimental and computational methods. Further information on the single‐crystal structure solution and DFT‐based calculations is also included. Experimental structural data have been deposited with the Cambridge Crystallographic Data Centre (CCDC).^{[ 121 ]}

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

Supporting Information

Supporting Information

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.