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.