= Solution
The conventional lattice-defect description is \b[vacancy-mediated <diffusion>]: an <ion> moves into a neighbouring vacancy and leaves a vacancy behind. <sodium chloride> is another familiar example. In a real alpha-alumina specimen the dominant species and transport path depend on <oxygen> chemical potential, dopants and grain boundaries, so this label should not be read as a universal microscopic assignment for every aluminium and <oxygen> <diffusion> experiment. A vacancy mechanism is different from an interstitial <ion> moving between otherwise empty interstitial sites.
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