Björn Winkler

Methanetetrol, C(OH)4 (orthocarbonic acid), is an elusive tethrahydroxymethane that, due to its rapid dissociation under ambient conditions, has never been synthesized in bulk. A recent study detected the formation of C(OH)4 molecules in the gas phase. Here, we show that C(OH)4 can be synthesized in a diamond anvil cell through the reaction between H2O and CO2 at 45(3) GPa and 50(3) GPa and temperatures <1200(200) K. The crystal structure of C(OH)4 was determined by single-crystal X-ray diffraction and corroborated by density-functional theory calculations together with Raman spectroscopy. It is composed of isolated [C(OH)4]-molecules linked by strong hydrogen bonds. Under high-pressure conditions, orthocarbonic and carbonic acids are denser than coexisting water ice phases, allowing them to segregate gravitationally and form stable reservoirs of oxidized carbon. These phases incorporate large amounts of carbon into their crystal structures as part of [CO4]4–- or [CO3]2–-building blocks. Because the amount of carbon that can be stored as CO2 in clathrates or filled-ices is significantly smaller, orthocarbonic and carbonic acids may provide a host mechanism for sequestering large carbon inventories in the icy mantles of water-rich exoplanets.

The inorganic pyrocarbonate Na2[C2O5] was synthesized at 41(3) GPa and 1000(300) K using a laser-heated diamond anvil cell. The compound was obtained by heating a mixture of Na2[CO3] and CO2 under high-pressure conditions. Its crystal structure was determined by single crystal synchrotron X-ray diffraction. This high pressure polymorph of Na2[C2O5] crystallizes in the space group Fdd2 with Z = 48 and an extremely anisotropic unit cell shape. The lattice parameters at 41(3) GPa are a = 50.466(7) Å, b = 23.681(6) Å, and c = 2.7212(4) Å, corresponding to a unit cell volume of 3252.1(10) Å3. The two [CO3]2−-groups which form the [C2O5]2−-groups are slightly tilted with respect to each other. Density functional theory based calculations confirmed the structure and supported the interpretation of the Raman spectra. Na2[C2O5]−Fdd2 is a high pressure polymorph of the previously reported Na2[C2O5]−P21.

The presence of H2O and CO2 on the icy moons in our solar system and in potentially habitable water worlds makes the understanding of phase stabilities and structure–property relations of phases in the H2O–CO2 system highly relevant for astrophysical research. Here we show that single crystals of carbonic acid monohydrate can be synthesized in a laser-heated diamond anvil cell at low pressures ( ≈3 GPa) either by reacting H2O with CO2 or by decompressing H2CO3 to this pressure. The crystal structure of H2CO3·H2O was obtained using synchrotron single crystal X-ray diffraction. At 3 GPa, H2CO3·H2O crystallizes in the orthorhombic space group Pca21 with Z = 8 formula units per unit cell. Density functional theory (DFT)-based calculations satisfactorily reproduce the experimental Raman spectrum, thus corroborating the structural model derived from the diffraction data. DFT enthalpy calculations are also consistent with the formation of H2CO3·H2O by decompression of H2CO3, with a concomitant release of CO2. Carbonic acid and its monohydrate store far more CO2 than filled-ices or clathrates, and hence they provide a mechanism for sequestering large amounts of carbon in icy moons in our solar system or in the icy mantles of water-rich exoplanets.

Two anhydrous tellurium carbonates, Te[CO3]O and Te4O7[CO3], have been synthesized in laser-heated diamond anvil cells by a reaction of TeO2 with CO2. Te[CO3]O was obtained at higher pressures (40(2) GPa), while Te4O7[CO3] was synthesized at moderate pressures (20(2) GPa). The crystal structures of both compounds were determined from synchrotron single crystal X-ray diffraction data and confirmed by density functional theory-based calculations. For Te[CO3]O the experimental Raman data were accurately reproduced by a Raman spectrum derived from the calculations. Both carbonates belong to the family of sp2-carbonates and are characterized by the presence of trigonal-planar [CO3]2–-groups. While the crystal structure of Te[CO3]O is rather simple, Te4O7[CO3] is a layered compound with layers of [CO3]2–-groups alternating with complex Te–O-layers. Our DFT-based calculations show that stereochemically active lone electron pairs are present in both crystal structures at elevated pressures. The successful synthesis of Te[CO3]O is a significant enlargement of the crystal chemistry of carbonates, as it demonstrates that anhydrous carbonates of chalcogenide group elements hosting no further cations can be synthesized. Excluding the noble gases, the chalcogenide group was the last main group of the periodic table where such carbonates were not known up to now.

The transport of carbon into the deep Earth is governed by the stability and properties of carbon-bearing phases. However, despite extensive research efforts, it is still an open question whether there are high-pressure minerals that can incorporate both silicon and carbon simultaneously. Multiple theoretical studies suggest that Si─C─O compounds could be stabilized at high pressures, but so far, no reliable experimental evidence for their presence has been presented. Here, we demonstrate that at 40(2) gigapascals and ≈ 1800(200) kelvin, CO ₂ reacts with silicic acid or cristobalite and forms the anhydrous silicon carbonate Si[CO ₃ ] ₂ . The structure consists of [SiO ₆ ] octahedra coordinated by six [CO ₃ ] ²⁻ groups. Similar groups occur in many ambient and high-pressure carbonates, suggesting that mixed carbonate-silicate phases may be stable at midmantle pressures. Silicon carbonate decomposes upon decompression at pressures of <6 gigapascals but could act as a potential host for carbon in Earth’s lower mantle.

Temperature dependent diffraction measurements of small (≈60 μm) single crystals of tellurium were carried out between 26 K and 300 K using the recently upgraded high photon energy (101.5 keV) beamline P21.1 at PETRA III (DESY). Small crystals can be efficiently investigated at that beamline as 2D aluminum X-ray lenses for focusing of the X-ray beam were integrated into the beam. A helium open-flow cryostreamer provides low temperatures. The diffractometer setup was calibrated using standard ruby spheres from NIST (SRM 1990). The data confirmed results of earlier dilatometer-based thermal expansion measurements for Te, and in addition allows to derive the structural changes during cooling. The large number of reflection intensities collected (≈10.000 for Al2O3, ≈40.000 for Te) allows to benchmark a recent very efficient implementation of structure-factor calculations based on widely used pseudopotential/plane wave DFT calculations. The DFT calculations reproduce the experimentally determined structure factors and atomic displacement parameters very well, and also numerous other physical properties, such as the elastic stiffness tensor, and, to a lesser extent, the SHG tensor. The combination of high photon energy diffraction and DFT model calculations therefore provides a comprehensive summary of structure-property relations of tellurium.

The synthesis of carbonates with novel types of anions is important for geoscience, chemistry, and materials science. Herein, we present the first inorganic tricarbonate salt, K₂C₃O₇, discovered in laser-heated diamond anvil cells at 55(3) and 45(2) GPa. The crystal structure of K₂C₃O₇ was determined in situ by using synchrotron single-crystal X-ray diffraction from a polycrystalline sample. It features nonplanar [C₃O₇]^2- anions, which consist of three corner-sharing planar CO₃ groups rotated relative to one another. This anion extends the homologous series of sp²-carbonates: [CO₃]^2-─[C₂O₅]^2-─[C₃O₇]^2-. Raman spectroscopy establishes the characteristic vibrational fingerprint of the [C₃O₇]^2- anion. Density functional theory (DFT) calculations corroborate the experimental results and suggest a thermodynamic stability of K₂C₃O₇ between 10 and 55 GPa. DFT calculations predict a phase transition between 80 and 90 GPa associated with polymerization of the [C₃O₇]^2- groups, accompanied by a change in the coordination polyhedra of two carbon atoms from triangles to tetrahedra. These results imply that other sp²-and mixed sp²/sp³-carbonates might be stabilized at a high pressure.

Hydrogen-free melaminate salts M₃(C₃N₆) (M = Cd, Ca) were synthesized in laser-heated diamond anvil cells at 34-48 GPa and 2000-2500 K. Cd₃(C₃N₆) was synthesized via a direct reaction between the elements, while Ca₃(C₃N₆) was obtained following a rational chemical design approach from calcium carbodiimide, Ca(NCN), which served as a single-source precursor. Both compounds contain the fully deprotonated melaminate anion (C₃N₆)^6-, representing a fundamental milestone in nitridocarbonate chemistry. The crystal structures of M₃(C₃N₆) were solved and refined using synchrotron single-crystal X-ray diffraction data and were fully corroborated by density functional theory calculations. Cd₃(C₃N₆) crystallizes in the acentric R3c and Ca₃(C₃N₆) in the centrosymmetric R3̅c space groups, and both compounds are recoverable to ambient conditions. Extending this design principle, our calculations indicate that Zn and Pb melaminates are thermodynamically accessible under similar conditions, highlighting the general stability of hydrogen-free nitridocarbonates of selected divalent metals.