Evidence for a low-compressibility carbon nitride polymorph elaborated at ambient pressure and mild temperature

Superhard materials like diamond are essential for abrasive or cutting tool applications. In this way, carbon nitrides are of relevant interest because they are expected to exhibit exceptional mechanical properties, high values of bulk modulus being predicted. A smart and simple method was used to synthesize carbon nitrides and allowed elaborating a low-compressibility polymorph. The processing consists in the decomposition of commercial thiosemicarbazide (H 2 NC(S)N 2 H 3) powder at ambient pressure and 600°C under nitrogen flow. Besides the presence of a graphitic C 3 N 4 phase, a nanocrystalline material is obtained and is identified as a cubic carbon nitride with a cell parameter of 3.163Å. Its impressive bulk modulus of 355GPa propels it as a challenger for cubic boron nitride among low-compressibility materials. Thus, we prove that, in accordance with previous numerous theoretical predictions, crystalline carbon nitrides exhibit exceptional compressibility behaviour. Moreover, our synthesis strategy evidences that, despite usual admitted considerations, severe and expensive pressure and temperature conditions are not mandatory to elaborate low-compressibility covalent materials, which could give new insights for their development.