# Close packed structures with bases

1. ### hokhani

336
Could we have close-packed structures with bases? Or, is it possible in close-packed structures that atoms of bases have arbitrary sizes? For example could we have a fcc close-packed structure with a two-atomic base (of any types of atoms)? If yes, what would happen if sizes of the bases be larger than Octahedral or tetrahedral sites' volumes?

2. ### M Quack

861
FCC is a close-packed structure. Rock salt NaCl crystallizes in a FCC crystal structure with a 2-atom basis.

I guess, though, that the ionic radii are such that the Na ions do not "touch" each other, so the structure is not really close-packed.

A better example may be binary colloidal crystals. The colloidal particels can behave very much like hard spheres. When you mix two different sizes, interesting structures can form, depending on the ratio of the diameters.

http://www2.ph.ed.ac.uk/~abs/AB2AB13PhysRev.pdf

3. ### DrDu

4,640
hexagonal close packing has a two atom base.