The Effect of Shear Deformation on C-N Structure under Pressure up to 80 GPa

We studythe effect of shear deformation on graphitic g-C₃N₄ under pressures of up to 80 GPa at room temperature. g-C₃N₄ samples are transformed from initial amorphous flakes into onion-like structures, in which the nitrogen content in the quenched samples decreases with increasing pressure (from 42% in the initial conditions to 1% at 80 GPa). The concentration of the sp² bonds also decreases from 1 (the initial sample) to 0.62 with increasing pressure to 80 GPa. This transformation of the sample is due to the fact that in the pressure range of 55-115 GPa, the equilibrium phase is not a diamond, but instead, carbon onions cross-linked by sp³ bonds, which are denser than diamonds. The results of our study show that the presence of nitrogen in sp³-bonded structures at pressures of higher than 55 GPa reduces the density and, accordingly, carbon structures without nitrogen become thermodynamically favorable.