Close packed structures with bases

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SUMMARY

Close-packed structures can indeed incorporate bases, as demonstrated by the face-centered cubic (FCC) structure of rock salt (NaCl), which features a two-atom basis. However, the ionic radii of sodium ions prevent them from achieving a truly close-packed arrangement. In contrast, binary colloidal crystals exhibit unique structural formations when mixing particles of different sizes, behaving similarly to hard spheres. The study of hexagonal close packing also confirms the viability of two-atom bases in such structures.

PREREQUISITES
  • Understanding of face-centered cubic (FCC) crystal structures
  • Knowledge of ionic radii and their implications in crystal packing
  • Familiarity with binary colloidal crystals and their behavior
  • Basic concepts of close-packed structures in crystallography
NEXT STEPS
  • Research the properties of face-centered cubic (FCC) structures in detail
  • Explore the implications of ionic radii on crystal packing efficiency
  • Investigate the formation and characteristics of binary colloidal crystals
  • Study hexagonal close packing and its applications in materials science
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Materials scientists, crystallographers, and researchers in solid-state physics who are exploring the complexities of crystal structures and their packing efficiencies.

hokhani
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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?
 
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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
 
hexagonal close packing has a two atom base.
 

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