The fluorite structure is comprised of a face-centeredcubic cation lattice, with anions occupying all of the tetrahedral holes (Fig. ). The octahedral holes are vacant, and are usually considered as "interstitial" sites. An alternative structural description is obtained by considering the positions of cations within an anionic substructure (Fig. )): from this perspective, the anions define a simple-cubic lattice, and the cations occupy half the cubic holes, giving 8-fold MX 8 cation coordination.