an octahedral site

An octahedral site is the larger of the two interstitial holes in a close-packed crystal. Take three spheres in one close-packed layer and three in the layer above, staggered so the two triangles point opposite ways; the roomy cavity between them is surrounded by six spheres whose centres form an octahedron. An atom sitting there has coordination number 6.

Being ringed by six neighbours, it is more spacious than the tetrahedral hole. A sphere that just fills it has a radius ratio near 0.414, and it comfortably holds guests up to a radius ratio of about 0.732, beyond which eightfold (cubic) coordination is preferred. In the FCC unit cell there are exactly 4 octahedral sites: one at the body centre (1/2, 1/2, 1/2) and one at the middle of each of the twelve edges, each edge shared by four cells.

Octahedral occupancy defines the rock-salt structure, where sodium fills every octahedral hole of an FCC chloride array, giving the familiar coordination of 6. It is also where carbon sits in gamma-iron, and where the larger B cations sit in spinel. Whenever an oxide or halide shows six-coordinate cations, octahedral interstitials are almost always the explanation.