Al2O3

Structure

[[File:Alpha-Al2O3-corundum-crystal-structure.png |thumb |left |upright=1.2 |alt=Oblique three-dimensional view of a repeating network of blue aluminium atoms and red oxygen atoms connected by rods. |Ball-and-stick model of the crystal structure of α-Al₂O₃ (corundum). Aluminium atoms are blue and oxygen atoms are red; sphere sizes are schematic. ]] At ambient conditions, α-Al₂O₃, which has the corundum structure, is the thermodynamically stable form of bulk crystalline aluminium oxide. The oxygen ions form a nearly hexagonal close-packed array, with aluminium ions occupying two-thirds of the octahedral interstices. Each aluminium ion is coordinated by six oxygen ions in a distorted octahedron. Corundum belongs to the trigonal crystal system and has a rhombohedralBravais lattice, with space group R3c (No. 167). Its rhombohedral primitive cell contains two formula units of Al₂O₃, while the conventional cell in the hexagonal setting contains six.