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

The crystal structure of carbon nitride under high pressure and temperature was investigated up to megabar pressures using graphitic C sub(3)N sub(4) 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) Aa at 1 atm) to those of reported hydrogen-bearing carbon nitride phases such as C sub(2)N sub(2) (NH) and C sub(2)N sub(2) (CH sub(2)). 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 C sub(2)N sub(2)X where NH, CH sub(2), and even potentially vacancies can be flexibly accommodated in the X site.