High Pressure Stabilizes the Elusive Hexacoordinated Carbon

Unusually high coordination number for elements has been of great interest in chemistry. Herein, a new class of hexacoordinated carbon compounds with stoichiometry MCF 6 (M = Ca, Sr) at 400 and 500 GPa and K 2 CF 6 at 500 GPa is found using crystal structure predictions assisted by first‐principles calculations. . At high pressure, MCF 6 (M = Ca, Sr) adopts a trigonal structure ( R ) that is isomorphous with several other species with M II A IV F 6 (A = Si, Ge, and Sn) while K 2 CF 6 adopts a trigonal structure ( P m 1) akin to M 2 I A IV F 6 . Both the species are thermodynamically more stable than the bicomponent MF 2 /KF…CF 4 mixture at high‐pressure region. These solid‐state structures are dynamically stable over a wide range of pressure. For all cases, the main building block of the solid‐state structures is the [CF 6 ] 2− unit in which carbon attains a perfectly octahedral coordination of the six fluoride anions. The present study establishes new fluorine‐rich carbon compounds and predicts a new class of perfectly hexacoordinated carbon species. For the first time, the stabilization of [CF 6 ] 2− in the solid‐state is reported.