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

Graphitic carbon nitride (C2N and C3N) with various πelectron 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 πelectrons 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 E2g 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 πelectron 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 πelectron distribution on the structural transition of graphitic materials and may explain some unexplained in previous studies.