The Laves structure

Laves (Cu₂Mg)
Stoichiometry = A₂X
Space Group = Fd3m (227)
Atomic Positions
        A    16d      5/8, 5/8, 5/8
        X    8a         0, 0, 0

Coordination Numbers/Geometry
        A     12       Distorted Icosahedral coordination
        X     12       Icosahedral coordination

Representative Compounds*
        Intermetallics (156) - (Cu₂Mg, Al₂Ca,Ni₂Y,Fe₂Gd, ...) 

*The numbers in parentheses are the number of compounds of each type found in "Structure and Properties of Inorganic Solids" by Francis Galasso, Pergammon Press (1970). 
 

Description 

The Laves structure is simply the spinel structure without the anions. Another way of describing the structure is to start with the X atoms at the FCC positions in the unit cell, as well as 1/2 of the octants of the unit cell. The remaining octants are occupied by a tetrahedron of A atoms. Though this description is useful for visualization one should not consider the tetrahedron of A atoms as an isolated cluster. Rather this structure type is best thought of as an ordered intermetallic structure. In fact it is one of the most common structure types among intermetallics.

	Typical Laves phase called C15 type structure, which belongs to cubic system.  Space group is Fd 3m.  Typical one appears in MgCu2.		Laves phase belonging to hexagonal system, called C14 type structure.  Space group is P 63/mmc.  Typical one appears in MgZn2.
	Hexagonal Laves phase called C36 type structure.  Space group is P63/mmc.  Typical one appears in MgNi2.