Crystal structure of a-Al₂O₃

Figure 31: The rhombohedral crystal structure of a -Al₂O₃ (space group R3,-c). Pairs of face-sharing octahedra extend along the threefold axis. Octahedra of adjacent pairs share corners and edges. Two-dimensionally infinite layers of condensed octahedra may be discerned. Such packages are indicated by braces. At the right-hand side such a layer is projected along the [110] direction.

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