High-Pressure Synthesis of oP28-C₃N₄ Recoverable to Ambient Conditions

The thermodynamic parameter pressure is ideal for producing novel ultraincompressible and superhard materials as it promotes the formation of polymeric frameworks and higher atomic coordination. In this regard, carbon and nitrogen are particularly attractive elements as they can produce extended arrangements of strong covalent bonds. In this study, a previously unobserved C₃N₄ polymorph, denoted as oP28-C₃N₄ (Pnnm, #58), is synthesized at pressures between 73 and 104 GPa in laser-heated diamond anvil cells and found recoverable to ambient conditions and stable in air. The crystal structure of oP28-C₃N₄, comprised of corner-sharing CN₄ tetrahedra, is solved and refined using synchrotron single-crystal X-ray diffraction. With a bulk modulus of 334(3) GPa deduced from experimental data, the compound is highly incompressible. Based on macroscopic and microscopic calculations, its hardness may achieve 47.5 or 79.7 GPa, respectively, making it a superhard material. Incompressibility of CN₄ tetrahedra in all experimentally observed C₃N₄ polymorphs is found to be greater than that of the CC₄ and BN₄ tetrahedra forming the structures of diamond and cubic boron nitride. Density functional theory calculations provide further insight into the electronic, vibrational, and mechanical properties of oP28-C₃N₄, as well as their stability relative to other C─N phases.