The 6th International Conference on Matter and Radiation at Extremes (ICMRE2023)

238 / 2023-04-18 20:28:56

Pressure synthesis of the long-sought-after superhard and recoverable C3N4 and CN2 compounds

ultraincompressible,superhard,carbon nitrides,high pressure,single-crystal X-ray diffraction

High Pressure Physics and Materials Science > Hydrogen and high-energy density material-II

Carbon nitrides are one of the holy grails of materials science ever since the seminal paper of Liu and Cohen [1]. They predicted that a fully saturated polymeric C₃N₄ solid comprised of corner-sharing CN₄ units could be formed and would have exceptional mechanical properties; likely to have a hardness greater than diamond’s. In the last three decades, momentous efforts were devoted to the synthesis of such materials through a multitude of experimental approaches [2]. Yet, no credible and reproducible claim of such compounds was reported.



Here, we will present results that bring this quest to an end. Laser-heated diamond anvil cell experiments on carbon-nitrogen precursors were performed up to 137 GPa. Four carbon nitrides were synthesized, oP8-CN [3], tI14-C₃N₄, hP126-C₃N₄ and tI24-CN₂, and their crystal structure (Figure 1) was solved employing single-crystal X-ray diffraction. These solids form remarkable polymeric structures with fully saturated C and N atoms, producing either corner-sharing C(CN₃) or CN₄ tetrahedra.



Upon the samples’ decompression, all four compounds were found to be recoverable at ambient conditions—a feat never before accomplished for megabar-synthesized materials—and stable in air. As expected from their crystal chemistry, these C-N compounds are ultraincompressible, with an experimental bulk modulus ranging between 351 and 429 GPa. Ab-initio calculations revealed the solids’ superhardness, computed to be between 78.0 and 86.8 GPa based on a microscope hardness model—exceeding even that of c-BN (62.3 GPa) and closely approaching diamond’s (89.2 GPa). This is qualitatively supported by diamond anvil indentation experiments using the recovered materials. Further experiments and calculations suggest the multifunctional properties of these solids, featuring piezoelectricity, wide band gap, tunable photoluminescence and high energy density, underlining their attractivity.



[1] Liu, A. (1989). Science 245, 841-842.

[2] Kessler, F. K. (2017). Nat. Rev. Mater. 2, 17030.

[3] Stavrou, E. (2016). Chem. Mater. 28, 6925-6933.