  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.
