Structure and stability of carbon nitride under high pressure and high temperature up to 125Â GPa and 3000Â K

The crystal structure of carbon nitride under high pressure and temperature was investigated up to megabar pressures using graphitic C3N4 as a starting material. It transformed to an orthorhombic phase above 30 GPa and 1600 K, which has a similar unit cell parameters (a = 7.6251(19), b = 4.4904(8), and c = 4.0424(8) Å at 1 atm) to those of reported hydrogen-bearing carbon nitride phases such as C2N2(NH) and C2N2(CH2). Although the C:N ratio of this orthorhombic phase was carefully determined to be 3:4, FT-IR analysis showed a strong possibility of hydrogen contamination both in the starting and recovered samples. These results suggest that in the studied wide pressure and temperature range, hydrogen-bearing carbon nitride favors the orthorhombic structure with a fundamental composition of C2N2X where NH, CH2, and even potentially vacancies can be flexibly accommodated in the X site.