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More than thirty years ago, carbon nitrides featuring 3D frameworks oftetrahedral CN4 units were identified as one of the great aspirations ofmaterials science, expected to have a hardness greater than or comparable todiamond. Since then, no unambiguous experimental evidence of their existencehas been delivered. Here, we report the high-pressure high-temperaturesynthesis of the long-sought-after covalent carbon nitrides, tI14-C3N4,hP126-C3N4, and tI24-CN2, in laser-heated diamond anvil cells. Their structureswere solved and refined using synchrotron single-crystal X-ray diffraction. Inthese solids, carbon atoms, all sp3-hybridized, and nitrogen atoms are fullysaturated, forming four and three covalent bonds, respectively, leading tothree-dimensional arrangements of corner-sharing CN4 tetrahedra. These carbonnitrides are ultra-incompressible, with hP126-C3N4 and tI24-CN2 even rivallingdiamond's incompressibility, and superhard. These novel compounds arerecoverable to ambient conditions in crystalline form and chemically stable inair. Being wide-band gap semiconductors with intriguing features in theirelectronic structure, they are expected to exhibit multiple exceptionalfunctionalities besides their mechanical properties, opening new perspectivesfor materials science.

Synthesis of Ultra-Incompressible Carbon Nitrides Featuring Three-Dimensional Frameworks of CN4 Tetrahedra Recoverable at Ambient Conditions