Structural type: hP4-NiAs

Hexagonal, space group P6Jmmc, N.194,

Atomic positions:

2 Ni in a) O,O,O; O,O,$;

2 As in c) $,$,$; $8,;;

Coordination formula: 1,3 [Ni,,]As6,,

For the prototype itself, a = 361.9 pm, c = 504 pm, c/a = 1.393.

According to HYDE and ANDERSON [1989], the data reported have to be considered as

correspondiag to an average slightly idealized structure, corresponding for several

compounds to the form which is stable at high temperature. At room temperature, in the

real structure, there are very small displacements of both Ni and As from their ideal

average positions. The structure should, therefore, be better described by:

2 Ni in a) O,O,z; O,O,h + z; (z = 0)

2 As in b) $,%,z;+ ,$,$ + z; (z = $)

in the space group P6,mc, No. 186.

The small (probably correlated) displacements of the atoms produce several sorts of

modulated structures (see sec. 4.4.).

Reference:  http://bookzz.org/book/825631/0a3da3