Carbonates containing CO4 groups as building blocks have recently been discovered. A new orthocarbonate, Sr2CO4 is synthesized at 92 GPa and at a temperature of 2500 K. Its crystal structure was determined by in situ synchrotron single-crystal X-ray diffraction, selecting a grain from a polycrystalline sample. Strontium orthocarbonate crystallizes in the orthorhombic crystal system (space group Pnma) with CO4, SrO9 and SrO11 polyhedra as the main building blocks. It is isostructural to Ca2CO4. DFT calculations reproduce the experimental findings very well and have, therefore, been used to predict the equation of state, Raman and IR spectra, and to assist in the discussion of bonding in this compound.

Dominique Laniel, Jannes Binck, Bjoern Winkler, Sebastian Vogel, Timofey Fedotenko, Stella Chariton, Vitali Prakapenka, Victor Milman, Wolfgang Schnick, Leonid Dubrovinsky, Natalia Dubrovinskaia: Synthesis, crystal structure and structure–property relations of strontium orthocarbonate, Sr2CO4. Acta Crystallographica Section B 77, 131-137 (2021) DOI: 10.1107/S2052520620016650. abstract

Isosurface of the electron density difference. The isosurface is plotted for a value of 0.2 e Å−3 and shows those regions in which the electron density, after reaching self-consistency, is larger than the electron density obtained by overlapping electron densities of non-interacting atoms. Clearly, there is charge accumulation halfway along each of the four C—O vectors, which is indicative of the formation of covalent bonds.