The corundum crystal structure, found for Al2O3, consists of an

The corundum crystal structure, found for Al2O3, consists of an HCP arrangement of O2- ions; the Al3+ ions occupy octahedral positions.
(a) What fraction of the available octahedral positions are filled with Al3+ ions?
(b) Ketch two close-packed O2–planes stacked in an AB sequence, and note octahedral positions that will be filled with the Al3+ ions.