Anomalous phonon softening of G-band in compressed graphitic carbon nitride due to strong electrostatic repulsion

Graphitic carbon nitride (C₂N and C₃N) with various π electron distributions on layers have been studied under pressure through a combined theoretical and experimental approach and a comparison with graphite. It is found that as these materials transform into low compressibility phases in the pressure range from 15 to 45 GPa, strong electrostatic repulsion between π electrons and in-plane sp² electrons may distort and soften the sp² bonds, leading to anomalous pressure evolutions of the intralayer phonon vibrations, such as a plateau-like behavior of E_2g mode (G-band) in C₂N and C₃N. This also causes a slow increase in the resistivity/resistance of C₂N and C₃N as pressure increases, and the gradual interlayer bonding leads to an abrupt increase in resistance of the materials but with different pressure responses due to their different π electron distributions. Moreover, the intensity enhancement of the G band in both CN materials may be related to their electronic structure changes. The results deepen our understanding of the effects of π electron distribution on the structural transition of graphitic materials and may explain some unexplained in previous studies.