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

We have synthesized the orthocarbonate Sr 3 [CO 4 ]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 4 ] 4– groups at ambient conditions. Single-crystal diffraction shows the presence of [CO 4 ] 4– groups, i.e., sp 3 -hybridized carbon tetrahedrally coordinated by covalently bound oxygen atoms. The [CO 4 ] 4– tetrahedra are located in a cage formed by corner-sharing OSr 6 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 ( I 4/ 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.