Diffusion pathway 3, which involves surface-like diffusion along pyramidal 5-fold coordination sites predominantly bound to the same layer (Fig. ), also shifts to a mechanism similar to the second pathway. In the case of the pristine structure with an interlayer spacing of 0.87 nm, the diffusion pathway involves a transition state with short bond distances (d=1.75 Å) to oxygen atoms from the opposing layer (Fig. ) and high activation barriers of 0.51 eV.