Graphite under uniaxial compression along c-axis: out-of-plane strain shifts in-plane phonon frequency
Stacking graphene sheets forms graphite. Two in-plane vibrational modes of graphite, E1u and E2g(2), are derived from graphene E2g mode, the shifts of which under compression are all considered as results of in-plane bond shortening. Values of Gruneisen parameter have been reported to quantify such relation. However, the reason why the shift rates of these three modes with pressure differ is unclear. In this work, we quantify the contribution of out-of-plane strain to the in-plane vibrational frequencies, suggesting that the compression of pi-electrons plays a non negligible role in studies of graphite or graphene under strain.