NiAs
Stoichiometry = AX
Space Group = P63/mmc (194)
a = ~ [2.968 - 4.340 angstroms]
c = ~ [4.058 - 6.748 angstroms]
Atomic Positions
        A   2a     0, 0, 0
        X   2c     1/3, 2/3, 1/4

Coordination Numbers/Geometry
        A     CN=6    Octahedral coordination
        X     CN=6    Trigonal prismatic coordination

Representative Compounds*
        Intermetallics (26) - (i.e. MnAs, AuSn, BiPt)
        Borides (1) - (PtB(anti-NiAs))
        Nitrides (1) - (i.e. d'-NbN(not common form))
        Sulfides, Selenides and Tellurides (22) - (i.e. CoS, CrSe, NiTe)

*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 NiAs structure can be generated by starting with a hexagonal close packing of arsenide, and then filling all of the octahedral holes with nickel ions.  The hexagonal ABABAB... stacking of the anions is apparent looking down the c-axis (see the view immediately above). Note the close comparison with the
rock salt structure which is a cubic close packing of ions with all of the octahedral holes filled.

However, the two structures differ in terms of their local coordination and polyhedral connectivity.  Unlike the rock salt structure where both ions are octahedrally coordinated, in the NiAs structure only the nickel ions are octahedrally coordinated. The arsenide ions are surrounded by a trigonal prism of nickel ions.  Another important distinction is the fact that the cation centered octahedra share faces in the NiAs structure as opposed to edges in the rock salt structure. This leads to a rather short cation-cation distance. Consequently the NaCl structure type is much more stable than the NiAs structure type for highly ionic compounds, and the NiAs structure is only observed for MX compounds with highly covalent M-X bonding interactions.

The fact that anions and cations have different coordination environments distinguishes the NiAs structure from most of the AX structures.

Removing 1/2 of the nickel ions would lead to the CdI2 structure.