Structure evolution of h.c.p./c.c.p. metal oxide interfaces in solid-state reactions.

The structure of crystalline interfaces plays an important role in solid-state reactions. The AlO/MgAlO/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAlO layers have been grown between AlO and MgO, and the atomic structure of AlO/MgAlO interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (h.c.p.) stacking in AlO to cubic close-packed (c.c.p.) stacking in MgAlO. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the AlO/MgAlO interface into AlO is accomplished by the glide of partial dislocations accompanied by the exchange of Al and Mg cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAlO grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent Σ3 grain boundaries form. The newly grown MgAlO grains compete with each other, leading to a growth selection and successive coarsening of the MgAlO grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar h.c.p./c.c.p. transition.