Synthesis, crystal structure and structure–property relations of strontium orthocarbonate, Sr2CO4

Synthesis of LaCN3, TbCN3, CeCN5, and TbCN5 Polycarbonitrides at Megabar Pressures

Inorganic ternary metal-C-N compounds with covalently bonded C-N anions encompass important classes of solids such as cyanides and carbodiimides, well known at ambient conditions and composed of [CN]- and [CN2]2- anions, as well as the high-pressure formed guanidinates featuring [CN3]5- anion. At still higher pressures, carbon is expected to be 4-fold coordinated by nitrogen atoms, but hitherto, such CN4-built anions are missing. In this study, four polycarbonitride compounds (LaCN3, TbCN3, CeCN5, and TbCN5) are synthesized in laser-heated diamond anvil cells at pressures between 90 and 111 GPa. Synchrotron single-crystal X-ray diffraction (SCXRD) reveals that their crystal structures are built of a previously unobserved anionic single-bonded carbon-nitrogen three-dimensional (3D) framework consisting of CN4 tetrahedra connected via di- or oligo-nitrogen linkers. A crystal-chemical analysis demonstrates that these polycarbonitride compounds have similarities to lanthanide silicon phosphides. Decompression experiments reveal the existence of LaCN3 and CeCN5 compounds over a very large pressure range. Density functional theory (DFT) supports these discoveries and provides further insight into the stability and physical properties of the synthesized compounds.

Pb2[C2O6]-P3̄m1: new insights into the high-pressure behavior of carbonates

In the present study, based on density functional theory and crystal structure prediction approaches, we found a new high-pressure structure of lead carbonate, named Pb2[C2O6]-P3̄m1. This structure differs significantly from previously known modifications of lead carbonate. The Pb2[C2O6]-P3̄m1 structure is characterized by the presence of ethane-like [C2O6] groups, which can also be classified as orthooxalate groups. This structure is most energetically favorable at pressures above 92 GPa at low temperatures, while Pmmn (post-aragonite structure) is most favorable below this pressure. As temperature increases to 2000 K, the pressure required for the Pmmn → P3̄m1 phase transition increases to 100 GPa. The high-pressure modification Pb2[C2O6]-P3̄m1 retains its stability at least up to 200 GPa. In addition, the Raman spectrum of the newly discovered modification was calculated, which may be useful for subsequent identification of this phase in high-pressure experiments. At 100 GPa, the most intense band located at 1148 cm-1 corresponds to the symmetric stretching mode of the C-C bond in the [C2O6] orthooxalate groups. The second and third most intense modes appear at 1021 and 726 cm-1, correspondingly.

Study of high-pressure thermophysical properties of orthocarbonate Sr3CO5 using deep learning molecular dynamics simulations

The exploration of the physical attributes of the recently discovered orthocarbonate Sr3CO5 is significant for comprehending the carbon cycle and storage mechanisms within the Earth's interior. In this study, first-principles calculations are initially used to examine the structural phase transitions of Sr3CO5 polymorphs within the range of lower mantle pressures. The results suggest that Sr3CO5 with the Cmcm phase exhibits a minimal enthalpy between 8.3 and 30.3 GPa. As the pressure exceeds 30.3 GPa, the Cmcm phase undergoes a transition to the I4/mcm phase, while the experimentally observed Pnma phase remains metastable under our studied pressure. Furthermore, the structural data of SrO, SrCO3, and Sr3CO5 polymorphs are utilized to develop a deep learning potential model suitable for the Sr-C-O system, and the pressure-volume relationship and elastic constants calculated using the potential model are in line with the available results. Subsequently, the elastic properties of Cmcm and I4/mcm phases in Sr3CO5 at high temperature and pressure are calculated using the molecular dynamics method. The results indicate that the I4/mcm phase exhibits higher temperature sensitivity in terms of elastic moduli and wave velocities compared to the Cmcm phase. Finally, the thermodynamic properties of the Cmcm and I4/mcm phases are predicted in the range of 0-2000 K and 10-120 GPa, revealing that the heat capacity and bulk thermal expansion coefficient of both phases increase with temperature, with the constant volume heat capacity gradually approaching the Dulong-Petit limit as the temperature rises.

Crystal structures and P-T phase diagrams of SrC 2 O 5 and BaC 2 O 5

In this study, we present the results of a search for new stable structures of SrC2 O5 and BaC2 O5 in the pressure range of 0-100 GPa based on the density functional theory and crystal structure prediction approaches. We have shown that the recently synthesized pyrocarbonate structure SrC2 O5 - P 2 1 / c is thermodynamically stable for both SrC2 O5 and BaC2 O5 . Thus, SrC2 O5 - P 2 1 / c is stable relative to decomposition reaction above 10 GPa, while the lower-pressure stability limit for BaC2 O5 - P 2 1 / c is 5 GPa, which is the lowest value for the formation of pyrocarbonates. For SrC2 O5 , the following polymorphic transitions were found with increasing pressure: P 2 1 / c → F d d 2 at 40 GPa and 1000 K, F d d 2 → C 2 at 90 GPa and 1000 K. SrC2 O5 - F d d 2 and SrC2 O5 - C 2 are characterized by the framework and layered structures of [CO4 ]4 - tetrahedra, respectively. For BaC2 O5 , with increasing pressure, decomposition of BaC2 O5 - P 2 1 / c into BaCO3 and CO2 is observed at 34 GPa without any polymorphic transitions.

Stabilization Of The CN3 5- Anion In Recoverable High-pressure Ln3 O2 (CN3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) Oxoguanidinates

A series of isostructural Ln3 O2 (CN3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high-pressure (25-54 GPa) high-temperature (2000-3000 K) conditions in laser-heated diamond anvil cells. The crystal structure of this novel class of compounds was determined via synchrotron single-crystal X-ray diffraction (SCXRD) as well as corroborated by X-ray absorption near edge structure (XANES) measurements and density functional theory (DFT) calculations. The Ln3 O2 (CN3 ) solids are composed of the hitherto unknown CN3 5- guanidinate anion-deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln3 O2 (CN3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) compounds are found to be dictated by the lanthanide contraction phenomenon. Decompression experiments show that Ln3 O2 (CN3 ) compounds are recoverable to ambient conditions. The stabilization of the CN3 5- guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry.

High-pressure transformations of CaC2O5 - a full structural trend from double [CO3] triangles through the isolated group of [CO4] tetrahedra to framework and layered structures

Over the past few years, the concept of carbonates, as the salts of MCO₃ or composition with [CO₃] triangles in the crystal structures, was sufficiently extended. In addition to carbonates, crystal structures with stoichiometry M₃CO₅, M₂CO₄ and MC₂O₅ were predicted and successfully synthesized. In the present study, based on density functional theory and crystal structure prediction algorithms, we found a novel structure of CaC₂O₅, namely Ca-pyrocarbonate with monoclinic symmetry Cc, which is one of the possible agents of the global carbon cycle. This structure is characterized by the isolated [C₂O₅] groups consisting of two [CO₃] triangles connected through a common oxygen atom. The thermodynamic stability field of Ca-pyrocarbonate with respect to the decomposition reaction into calcium carbonate and carbon dioxide begins at a pressure of 10 GPa. As the pressure increases to 21 GPa, the structure of Ca-pyrocarbonate transforms into the recently synthesized tetragonal modification I4̄2d, in the structure of which carbon is in the sp³-hybridized state and [CO₄] tetrahedra form isolated pyramidal [C₄O₁₀] anionic groups. At 59 GPa in the temperature range of 0-2500 K, CaC₂O₅-I4̄2d undergoes a phase transition to CaC₂O₅-Fdd2, with the framework structure of [CO₄] tetrahedra. On further compression to about 80 GPa, the framework structure transforms into layered ones, C2 and Pc. In addition, we estimated the thermodynamic stability of CaC₂O₅ with respect to the minerals of the Earth's mantle. We found that CaC₂O₅ can coexist with bridgmanite up to pressures of 54 GPa at 300 K, where it reacts with the formation of a Ca-perovskite, magnesite, and solid CO₂-V.

Synthesis and Structure of Pb[C2O5]: An Inorganic Pyrocarbonate Salt

We have synthesized Pb[C₂O₅], an inorganic pyrocarbonate salt, in a laser-heated diamond anvil cell (LH-DAC) at 30 GPa by heating a Pb[CO₃] + CO₂ mixture to ≈2000(200) K. Inorganic pyrocarbonates contain isolated [C₂O₅]^2- groups without functional groups attached. The [C₂O₅]^2- groups consist of two oxygen-sharing [CO₃]^3- groups. Pb[C₂O₅] was characterized by synchrotron-based single-crystal structure refinement, Raman spectroscopy, and density functional theory calculations. Pb[C₂O₅] is isostructural to Sr[C₂O₅] and crystallizes in the monoclinic space group P2₁/c with Z = 4. The synthesis of Pb[C₂O₅] demonstrates that, just like in other carbonates, cation substitution is possible and that therefore inorganic pyrocarbonates are a novel family of carbonates, in addition to the established sp² and sp³ carbonates.

Sr[C2O5] is an Inorganic Pyrocarbonate Salt with [C2O5]2- Complex Anions

The synthesis of a novel type of carbonate, namely of the inorganic pyrocarbonate salt Sr[C₂O₅], which contains isolated [C₂O₅]^2--groups, significantly extends the crystal chemistry of inorganic carbonates beyond the established sp²- and sp³-carbonates. We synthesized Sr[C₂O₅] in a laser-heated diamond anvil cell by reacting Sr[CO₃] with CO₂. By single crystal synchrotron diffraction, Raman spectroscopy, and density functional theory (DFT) calculations, we show that it is a pyrocarbonate salt. Sr[C₂O₅] is the first member of a novel family of inorganic carbonates. We predict, based on DFT calculations, that further inorganic pyrocarbonates can be obtained and that these will be relevant to geoscience and may provide a better understanding of reactions converting CO₂ into useful inorganic compounds.

Sr3[CO4]O Antiperovskite with Tetrahedrally Coordinated sp3-Hybridized Carbon and OSr6 Octahedra

We have synthesized the orthocarbonate Sr₃[CO₄]O in a laser-heated diamond anvil cell at 20 and 30 GPa by heating to ≈3000 (300) K. Afterward, we recovered the orthocarbonate with [CO₄]^4- groups at ambient conditions. Single-crystal diffraction shows the presence of [CO₄]^4- groups, i.e., sp³-hybridized carbon tetrahedrally coordinated by covalently bound oxygen atoms. The [CO₄]^4- tetrahedra are located in a cage formed by corner-sharing OSr₆ octahedra, i.e., octahedra with oxygen as a central ion, forming an antiperovskite-type structure. At high pressures, the octahedra are nearly ideal and slightly rotated. The high-pressure phase is tetragonal (I4/mcm). Upon pressure release, there is a phase transition with a symmetry lowering to an orthorhombic phase (Pnma), where the octahedra tilt and deform slightly.

Tetrahedrally Coordinated sp3-Hybridized Carbon in Sr2CO4 Orthocarbonate at Ambient Conditions

We have synthesized the orthocarbonate Sr₂CO₄, in which carbon is tetrahedrally coordinated by four oxygen atoms, at moderately high pressures [20(1) GPa] and high temperatures (≈3500 K) in a diamond anvil cell by reacting a SrCO₃ single crystal with SrO powder. We show by synchrotron powder X-ray diffraction, Raman spectroscopy, and density functional thoery calculations that this phase, and hence sp³-hybridized carbon in a CO₄^4- group, can be recovered at ambient conditions. The C-O bond distances are all of similar lengths [≈1.41(1) Å], and the O-C-O angles deviate from the ideal tetrahedral angle by a few degrees only.