Substitutional Diffusion Mechanisms
A lot of atoms have to move out of the way for this to occur, and in a real close-packed crystal system there would be even less room, so this mechanism is not observed.
Although ring mechanism takes up less room than direct exchange it is still not observed as all 4 atoms in the ring all have to move together with the same motion, and this is very unlikely to occur. In a close packed structure even more of the atoms have to coordinate in the ring.
When there is a vacant site there is lot less of an energy barrier for the atom to move into it, so this diffusion mechanism is observed. The density of vacancies determines the rate of diffusion. In a real structure, all the atoms are vibrating, and at a high enough temperature, some will gain enough energy to overcome the barrier to move into a vacant site.