# Disruption of the $sp^2$ bonding by the compression of the $\pi$-electronic orbitals of graphene at various stacking orders

## TL;DR

This study explores how compression affects the electronic orbitals and stacking order in graphene, revealing that $ extit{sp}^2$ bonding disruption varies with stacking and impacts phonon frequencies, emphasizing the importance of 3D effects.

## Contribution

It provides new insights into the behavior of $ extit{sp}^2$ electrons under compression in different stacking orders of graphene, highlighting non-monotonic bonding changes and 3D effects.

## Key findings

- Electrons resist squeezing through the $ extit{sp}^2$ network regardless of stacking.
- Interlayer interactions increase similarly in A-A and Bernal stacking under compression.
- Out-of-plane compression significantly shifts phonon frequencies differently for stacking types.

## Abstract

We investigate the behaviour of the $\pi$-electrons under compression and the effect of the stacking order of graphene layers. First we find that electrons can hardly be squeezed through the $sp^2$ network, regardless of the stacking order. The largely deformed electronic orbitals (mainly those of $\pi$-electrons) under compression along the $\textit{c}$-axis increase interlayer interaction between graphene layers as expected, but surprisingly in a similar way for the A-A and Bernal stacking. On the other hand, the large out-of-plane compression shifts the in-plane phonon frequencies of A-A stacked graphene layers significantly and very differently from Bernal stacked layers. We attribute these results to the $sp^2$-electrons filling the low-density central area in a carbon hexagon under compression for the A-A stacking, hence resulting in a non-monotonic change of the $sp^2$-bonding. The results strongly suggest not to ignore 3D features of a 2D material.