A pressure-induced superhard SiCN₄ compound uncovered by first-principles calculations

Silicon-carbon-nitride (Si-C-N) compounds are a family of potential superhard materials with many excellent chemical and physical properties; however, only SiCN, Si2CN4 and SiC2N4 were synthesized. Here, we theoretically report a new SiCN4 compound with P4(1)2(1)2, Fdd2 and R (3) over bar structures by first-principles structural predictions based on the particle swarm optimization algorithm. Pressure-induced structural phase transitions from P4(1)2(1)2 to Fdd2, and then to the R (3) over bar phase were determined at 2 GPa and 249 GPa. By comparing enthalpy differences with 1/3Si(3)N(4) + C + 4/3N(2), it was found that these structures tend to decompose at ambient pressure. However, with the increase of pressure, the enthalpy differences of Fdd2 and R (3) over bar structures turn to be negative and they can be stabilized at a pressure of more than 41 GPa. They are also dynamically stable as no imaginary frequencies were found in their stabilized pressure ranges. The calculated band gap is 4.37 eV for P4(1)2(1)2, 3.72 eV for Fdd2 and 3.81 eV for the R (3) over bar phase by using the Heyd-Scuseria-Ernzerhof (HSE06) method and the estimated Vickers hardness values are higher than 40 GPa by adopting the elastic modulus based hardness formula, which confirmed their superhard characteristics. These results provide significant insights into Si-C-N systems and will inevitably promote the future experimental works.