Lkhamsuren Bayarjargal

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.

ZUSAMMENFASSUNG: Die Reaktion von KReO ₄ mit B ₂ O ₃ in einer Schmelze aus Re ₂ O ₇ führt zu der Bildung eines farblosen, kristallinen Pulvers von K[B(ReO ₄ ) ₄ ]. Die durch Einkristalldiffraktometrie bestimmte Kristallstruktur ist nicht‐zentrosymmetrisch (Raumgruppe I ) und weist das bis dato unbekannte [B(ReO ₄ ) ₄ ] ⁻ ‐Anion auf. Letzteres ist das erste Exemplar eines monomeren Anions mit Bor als Zentralatom, welches über B‐O‐Re‐Bindungen zu vier Perrhenat‐Anionen koordiniert. Die Verbindung wurde zusätzlich via Ramanspektroskopie untersucht und die jeweiligen Schwingungsmodi den beobachteten Banden mittels DFT‐Rechnungen zugeordnet. Des Weiteren konnte durch Frequenzverdopplung (second harmonic generation; SHG) das Fehlen eines Inversionszentrums nachgewiesen werden.

The reaction of KReO ₄ with B ₂ O ₃ in molten Re ₂ O ₇ results in the formation of a colorless crystalline powder of K[B(ReO ₄ ) ₄ ]. The single‐crystal structure determination reveals a non‐centrosymmetric structure (space group I ), containing the hitherto unknown [B(ReO ₄ ) ₄ ] ⁻ anion. This anion presents the first example of a monomeric anion with boron as a central atom coordinated by four perrhenate anions forming B─O─Re bonds. The compound has been further characterized by Raman spectroscopy, with additionally DFT calculations to assign the vibration modes. Moreover, the absence of inversion symmetry has been proven by second harmonic generation (SHG) measurements.

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.

H ₂ CO ₃ - Pnma is a low temperature carbonic acid polymorph, which can occur at the same pressure (∼9 GPa) than the high-temperature phase H ₂ CO ₃ - P 2 ₁ / n . The crystal structure of H ₂ CO ₃ - Pnma was refined from synchrotron single crystal diffraction data.

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.

In this joint experimental/computational work, we study the ambient-conditions structure of bismutite (BiO)2(CO3) and its evolution under high pressure by means of a combination of in situ synchrotron X-ray diffraction (XRD) measurements, Raman spectroscopy, and density-functional theory (DFT) calculations. Using single-crystal XRD, (BiO)2(CO3) is determined to be an orthorhombic Cmcm phase with disorder arising from carbonate rotations at ambient-conditions and DFT confirms the stability of the structure. In situ high-pressure synchrotron powder XRD measurements showed that the Cmcm ambient-pressure structure adequately accounts for the observed diffraction patterns up to 17.7 GPa. The compressibility was determined, obtaining an experimental bulk modulus of 67.2(11) GPa and significant anisotropy in the axial compressibilities: the a and c axes, parallel to the (BiO)22+ layers, are less compressible than the b direction perpendicular to those layers, which we justify on the basis of the bismuth lone electron pair compressibility and the slight reorientation of the carbonate groups. Raman spectroscopy measurements at ambient conditions and under pressure were used to characterize the vibrational properties of this carbonate, unvealing a differentiated behavior of the nonequivalent carbonate units.

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.