Graphite under uniaxial compression along the c axis: A parameter to relate out-of-plane strain to in-plane phonon frequency
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Stacking graphene sheets forms graphite. Two in-plane vibrational modes of graphite, E1u and E2g(2), are derived from the graphene E2g mode, the shifts of which under compression are considered as results of the in-plane bond shortening. Values of the Grüneisen 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 γE2g′=-0.0131 and γE1u′=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 π- electrons plays a non-negligible part in both graphite and graphene under high pressure.