Orthocarbonates of Ca, Sr, and Ba—The Appearance of sp³-Hybridized Carbon at a Low Pressure of 5 GPa and Dynamic Stability at Ambient Pressure

Abstract

Orthocarbonates are a newly discovered class of compounds that are stable at high pressures. The presence of sp3-hybridized carbon, having structural similarity to orthosilicates, and their potential participation in the global planetary carbon cycle have triggered intensive theoretical and experimental investigations into these compounds. Here, based on the density functional theory and crystal structure prediction calculations, we predict new stable crystal structures of the orthocarbonates Sr3CO5-Cmcm, Sr3CO5-I4/mcm, Ba2CO4-Pnma, and Ba3CO5-I4/mcm. Summarizing the obtained data, we show that orthocarbonates of alkaline-earth metals are isotypic to ambient-pressure orthosilicates with only rare exceptions. The lower-pressure stability limit for Ba-orthocarbonates is around 5 GPa. However, the stability limit increases with decreasing cation radius and reaches 13 GPa for Ca-orthocarbonates. Based on the calculations of Gibbs free energies with the quasi-harmonic approximation, the reaction 2M2CO4 = M3CO5 + MCO3 (M = Sr and Ba) is established. At 20 GPa, this reaction is realized at temperatures above 1080 K for Sr2CO4 and above 740 K for Ba2CO4, and the Clapeyron slope is positive in both cases. The obtained P–T diagrams for SrCO3 and BaCO3 show that equilibrium between the structures of aragonite and postaragonite is observed at 15–17 GPa for SrCO3 and 5–7 GPa for BaCO3. The transition pressure is almost independent of temperature. No other more favorable structures than postaragonite have been found for these compounds in the considered pressure range, up to 200 GPa. Thus, in contrast to CaCO3 and MgCO3, the transition from sp2 to sp3 hybridization is not realized for these compounds. Two of the found structures, Sr2CO4-Pnma and Sr3CO5-Cmcm, are dynamically stable at ambient pressure. This indicates the possibility of recovering the crystals from a high-pressure environment and conducting further laboratory investigation.

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

“…The anisotropic displacement parameters of the symmetrically independent strontium as well as of the oxygen atoms were constrained to be identical. 25 We suggest that this reaction occurs in our experiments during the second stage of heating.…”

“…28 In addition to the established orthoanions, it was recently predicted and experimentally shown that carbonates with [CO 4 ] 4− groups may even be recovered at ambient conditions. 21,25 Further, stable structures with stoichiometries M 2 [CO 4 ] and M 3 [CO 4 ]O (M = Mg, Ca, Sr, and Ba) have been predicted by DFT-based calculations. 22−25 Here, we report the successful outcome of the synthesis of one of these structures using M = Sr.…”

“…Our diffraction data and the Raman spectra are inconsistent with the predicted metastable low-pressure ]O with space group symmetry Cmcm. 22,25 The results of the structure refinements are shown in Figure 4 and Tables S1 and S2. The structure is best described as a framework antiperovskite structure, where the OSr 6 octahedra are linked by corner-sharing Sr 2+ , and the [CO 4 ] 4− group replaces the A cation in a prototypical ABX 3 perovskite.…”

“…Tetrahedral coordination implies the presence of C-sp 3 hybrid orbitals. Initially, such sp 3 carbonates were obtained as high-pressure polymorphs of conventional carbonates at very high pressures ( p > 70 GPa) and high temperatures (a few thousand Kelvin) and were thought to exist only at these extreme conditions. − This situation changed when, instead of transforming carbonates by pressure, novel sp 3 carbonates were formed by reactions. − Guided by density functional theory (DFT)-based predictions, − alkaline-earth oxides were reacted with the corresponding carbonates at high temperatures and Sr 2 [CO 4 ]- Pnma and isostructural Ca 2 [CO 4 ]- Pnma were formed in a laser-heated diamond anvil cell (LH-DAC) at ≈20 GPa. − …”

“…We employed synchrotron powder X-ray diffraction to characterize the temperature-quenched samples. We analyzed the powder X-ray diffraction data recorded at 20(1) GPa and found that the diffraction data could be well fitted with tetragonal Sr 3 [CO 4 ]­O, which had recently been predicted and is thought to be isostructural to other predicted carbonates, Ca 3 [CO 4 ]O and Ba 3 [CO 4 ]­O. , The Rietveld refinement (Figure S2) of Sr 3 [CO 4 ]O was carried out in the tetragonal space group I 4/ mcm with Z = 4 ( wR = 3.9%). At 20(1) GPa, we obtained the lattice parameters of a = 6.562(1) Å and c = 9.751(3) Å, which are in good agreement with the values obtained by DFT calculations (Table S1).…”