Graphite under uniaxial compression along the c axis: A parameter to relate out-of-plane strain to in-plane phonon frequency

Stacking graphene sheets forms graphite. Two in-plane vibrational modes of graphite, E-1u and E-2g((2)), are derived from the graphene E-2g mode, the shifts of which under compression are considered as results of the in-plane bond shortening. Values of the Gruneisen parameter have been reported to quantify such a relation. However, the reason why the shift rates of these three modes with pressure differ is unclear. In this work, we introduce new parameters gamma'(E2g) = -0.0131 and gamma'(E1u) = 0.0585 to quantify the contribution of out-of-plane strain to the shift of the in-plane vibrational frequencies, suggesting that the compression of the pi- electrons plays a non-negligible part in both graphite and graphene under high pressure.