Sr[CO]O Antiperovskite with Tetrahedrally Coordinated sp-Hybridized Carbon and OSr Octahedra.

We have synthesized the orthocarbonate Sr[CO]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 [CO] groups at ambient conditions. Single-crystal diffraction shows the presence of [CO] groups, i.e., sp-hybridized carbon tetrahedrally coordinated by covalently bound oxygen atoms. The [CO] tetrahedra are located in a cage formed by corner-sharing OSr 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 (4/). Upon pressure release, there is a phase transition with a symmetry lowering to an orthorhombic phase (), where the octahedra tilt and deform slightly.