Since the seminal paper of Liu and Cohen (Science 245, 1989), the synthesis of novel carbon nitrides has been the focus of intense research. Liu and Cohen predicted that a fully saturated polymeric C₃N₄ solid comprised of corner-sharing CN₄ units could be formed and might have exceptional mechanical properties, including a hardness approaching or surpassing that of diamond. In the last three decades, the synthesis of such materials was targeted through a multitude of experimental approaches, including chemical and physical vapor depositions, solvothermal methods, and static and dynamic high-pressure methods. Yet, no credible and reproducible claim of such material was reported.

Using extreme conditions, a CSEC-led international team with collaborators from the University of Bayreuth and Linköping University subjected carbon-nitrogen precursors to pressures between 70 and 135 gigapascals and temperatures exceeding 2000 K. Characterization at the ESRF, APS and DESY synchrotrons with single-crystal X-ray diffraction revealed the synthesis of three carbon nitrides—tI14-C₃N₄, hP126-C₃N₄, and tI24-CN₂—all three featuring the necessary building blocks for ultra-incompressibility and superhardness.

These compounds were determined to exhibit experimental bulk moduli (a measure of incompressibility) between 365 and 419 GPa, surpassing that of cubic boron nitride. According to complementary calculations, their hardness approaches that of diamond. They were found to possess additional interesting properties including photoluminescence, high energy density, piezoelectricity, and non-linear optical characteristics. Recoverable to ambient conditions, these ultraincompressible carbon nitrides are hypothesized to present vast potential for applications, which will need to be further explored. The research, published in Advanced Materials, finally brings to an end a 30-year long quest for the first fully saturated polymeric C₃N₄ solids featuring CN4 species, and opens up new research avenues.

D. Laniel et al., Synthesis of Ultra-Incompressible and Recoverable Carbon Nitrides Featuring CN₄ Tetrahedra, Advanced Materials 36, 2308030 (2024).