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 Al ₂O ₃/MgAl ₂O ₄/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl ₂O ₄ layers have been grown between Al ₂O ₃ and MgO, and the atomic structure of Al ₂O ₃/MgAl ₂O ₄ 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 Al ₂O ₃ to cubic close-packed (c.c.p.) stacking in MgAl ₂O ₄. 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 Al ₂O ₃/MgAl ₂O ₄ interface into Al ₂O ₃ is accomplished by the glide of partial dislocations accompanied by the exchange of Al ³⁺ and Mg ²⁺ cations. The interface migration may be envisaged as a plane-by-plane zipperlike motion, which repeats along the interface facilitating its propagation. MgAl ₂O ₄ 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 3 grain boundaries form. The newly grown MgAl ₂O ₄ grains compete with each other, leading to a growth selection and successive coarsening of the MgAl ₂O ₄ 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.