Synthesis of New Cubic C$_{3}$N$_{4}$ Phase under High Pressure and High Temperature

Synchrotron-based X-ray diffraction studies were carried out on a graphite-like C$_{3}$N$_{4}$ ($g$- C$_{3}$N$_{4})$ phase subjected to high pressures up to 38 GPa and high temperatures of up to 3000 K using the laser-heated diamond-anvil cell. Laser-heating the sample to 1800 K at pressure between 20 and 38 GPa, a new set of diffraction pattern appeared, showing positively that a high-pressure phase was formed. Upon decompression of the post-lasered sample to 1 atmospheric pressure, the X-ray diffraction peaks of high-pressure phase were replaced completely by a new pattern, thus demonstrating a new metastable phase was formed retrogressively. X-ray diffraction data on the recovered phase show that it is a new cubic phase that does not match to any high-pressure phases in C$_{3}$N$_{4}$ predicted theoretically.