Atomic-scale mechanism of rhombohedral twinning in sapphire

Deformation twinning is a fundamental plastic deformation mode in crystals. Upon twinning, individual atoms must move in different directions to satisfy the twin symmetry, which is called atomic shuffling. However, actual atomic motions during shuffling are still unknown, especially for ionic compounds. Here, we report the dynamic twinning behavior dominated by the atomic shuffling in sapphire ( α -Al 2 O 3 ). The propagation and annihilation of twins are revealed to be mediated by migration of step structures on the matrix/twin interfaces. The step migration is driven by cooperative motions of a group of five atoms with relatively few recombination of Al–O bonds. Our findings imply that the atomic shuffling associated with twinning is determined by a collective property of a group of several atoms.