The configurational images e -h in Figure show that the intralayer Co migration follows the interlayer migrations, indicating that the Co vacancy causes the surrounding oxygen atoms to vibrate more easily, which could further induce the migration of the Co ions adjacent to the vacancy. This intralayer migration order also exhibits a local characteristic within the x-y plane. Subsequently, once two neighboring Co vacancies (i.e., denoted as a Co vacancy cluster in this work) are present, we observe the oxygen dimer formation in the trajectories of our MD simulations. Two oxygen atoms both from the interlayers and the intralayer can combine to form an oxygen dimer (the configurational images h, j and k in Figure ). Co vacancy cluster is present in images g -h and i -k. Visualizations are done by the VESTA software. Our atomic-scale simulations suggest that the formation of Co vacancy clusters within a TM layer is a prerequisite for the oxygen dimer formation, which is consistent with earlier experimental observations. To provide more quantitative analysis for the correlations of the TM migration and the oxygen sublattice's instability revealed by the above MD simulations, we further investigate the impact of the Co vacancy's number on the kinetic barriers of the oxygen dimer formation by the CI-NEB method, with the presence of a single Co vacancy or a two-vacancy cluster in a TM layer. All possible paths corresponding to an intralayer oxygen dimer formation process are considered (see Figure and). We can see that two adjacent Co vacancies lead to significantly lower barriers of the oxygen dimer combination. The nearest and next nearest neighboring oxygen dimer combination paths in the Co vacancy cluster model, denoted as the path 1 and path 3 in Figure , exhibit lower barriers (0.43 eV and 0.14 eV) compared with that in the single Co vacancy model (1.23 eV and 0.45 eV, Figure ).