layers of close-packed spheres: Figure 5.5a is a ‘space-filling’
representation, while in Figure 5.5b, the sizes of the spheres
have been reduced so that connectivity lines can be shown
(a ‘ball-and-stick’ diagram). This illustrates that the spheres
lie at the corners of either tetrahedra or octahedra; conversely,
the spheres pack such that there are octahedral and tetrahedral
holes between them. There is one octahedral hole per
sphere, and there are twice as many tetrahedral as octahedral
holes in a close-packed array; the octahedral holes are larger
than the tetrahedral sites. Whereas a tetrahedral hole can
accommodate a sphere of radius 0.23 times that of the
close-packed spheres, a sphere of radius 0.41 times that of
the close-packed spheres fits into an octahedral hole.