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

More than thirty years ago, carbon nitrides featuring 3D frameworks of tetrahedral CN4 units were identified as one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond. Since then, no unambiguous experimental evidence of their existence has been delivered. Here, we report the high-pressure high-temperature synthesis of the long-sought-after covalent carbon nitrides, tI14-C3N4, hP126-C3N4, and tI24-CN2, in laser-heated diamond anvil cells. Their structures were solved and refined using synchrotron single-crystal X-ray diffraction. In these solids, carbon atoms, all sp3-hybridized, and nitrogen atoms are fully saturated, forming four and three covalent bonds, respectively, leading to three-dimensional arrangements of corner-sharing CN4 tetrahedra. These carbon nitrides are ultra-incompressible, with hP126-C3N4 and tI24-CN2 even rivalling diamond's incompressibility, and superhard. These novel compounds are recoverable to ambient conditions in crystalline form and chemically stable in air. Being wide-band gap semiconductors with intriguing features in their electronic structure, they are expected to exhibit multiple exceptional functionalities besides their mechanical properties, opening new perspectives for materials science.