Adjacent to the Td hole, three
atoms in the B layer touch three atoms in the A layer such that a trigonal
antiprismatic polyhedron (a regular octahedron) is formed. At the
center of the octahedron is a cavity called the Octahedral Hole
(a common symbol for "octahedral" is Oh). A guest sphere will
just fill this cavity and touch the six host spheres if its radius is approximately
0.41r. It can be shown that there are twice as many Td
holes as there are Oh holes in any closest packed bilayer.
