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Some Common Crystal Structures

The conventional simple cubic (SC) unit cell. This structure is relatively rare amongst the metallic elements and only Po appears to crystallize in the SC structure at room temperature and pressure. This is likely because the packing efficiency and coordination number for this structure are low at 0.52 and 6 respectively.
Of much more interest are compounds based on the SC lattice. For example, the CsCl structure is based on the SC lattice with a two atom basis. The Cl atom is at position 0 0 0 and the Cs at 0.5 0.5 0.5. Here, and in the structures below, the atoms are shown at a reduced relative diameter to emphasize the structure. Other materials which crystalize in this structure are TlBr, TlI, NH4Cl, RbCl, CuZn (beta-Brass), AgMg, LiHg, AlNi and BeCu.
The conventional face-centered cubic (FCC) lattice. This structure is very common amongst metallic elements because it maximises nearest neighbor interactions (coordination number of 12). The unit cell has a packing efficiency of 0.74. The FCC structure is also known as the cubic close-packed (CCP) structure.
The conventional FCC lattice cut along the (111) plane (cell diagonal). This view indicates the relationship between the conventional cubic lattice and a lattice formed by close-packed hexagonal layers in an ABCABC stacking arrangement.
The conventional body-centered cubic (BCC) lattice, another common crystal structure adopted by metallic solids such as Fe, Cr, Mo and W. The packing efficiency is slightly worse than the FCC lattice at 0.68 and the coordination number is lower at 8.
The conventional NaCl unit cell. This cell can be described as a simple FCC lattice with a two atom (Na, Cl) basis or two interpenetrating FCC lattices, one of Na and one of Cl, displaced from each other by 0.5 of the body diagonal. This is a common structure for ionic compounds inclusing LiH, KCl, PbS, AgBr, MgO and MnO.
The conventional CaF2 (fluorite) unit cell. The fluoride ions form a FCC lattice while the Ca ions are placed in a simple cubic arrangment in the tetrahedral holes. The anti-fluorite structure has the atomic positions reversed.
The diamond unit cell is adopted by C (diamond), Si, Ge and grey Sn. It is formed by elements which have a strong covalent bonding tendency. The directionality of these covalent bonds favors structures other than simple close-packed. The structure can be visualized as an FCC lattice with two atom basis (both C atoms). It is a curious quirk of geometry that this produces local tetrahedral bonding around each atom. Alternatively, the structure can be considered as an FCC lattice with half of the tetrahedral holes filled by interstitial C atoms. This structure has a packing efficiency of 0.34, much lower than any close-packed lattice.
The cubic ZnS unit cell (zincblende) is closely related to the diamond unit cell. If we consider the diamond lattice as two interpenetrating FCC lattices, then in zincblende one lattice is composed of Zn atoms and the other of S atoms.

This page created January 14, 1998 by Simon J. Garrett.