Structure of NiAs and related compounds

In the crystal structure of nickel asenide, NiAs, the As atoms are in hep with all octahedral interstices occupied by the Ni atoms, as shown in Fig. 10.2.3(a). An important feature of this structure is that the Ni and As atoms are in different coordination environments. Each As atom is surrounded by six equidistant Ni atoms situated at the corners of a regular trigonal prism. Each Ni atom, on the other hand, has eight close neighbors, six of which are As atoms arranged octahedrally about it, while the other two are Ni atoms immediately above and belowitatz = c/2. The Ni-Ni distance is c/2 = 503.4/2 = 251.7 pm, which corresponds to the interatomic distance in metallic nickel. Compound NiAs is semi-metallic, and its metallic property results from the bonding between Ni atoms. In the NiAs structure, the axial ratio da = 503.4/361.9 = 1.39 is much 

Structure of NiAs (a) origin at As atom (white circle) (b) origin at Ni atom (black circle). 

The space group of NiAs is DA6h — P63/mmc. The atomic coordinates are 

When a translation is applied so that the origin of the unit cell now resides on the Ni atom, as shown in Fig. 10.2.3(b), the atomic coordinates become 

Some compounds with the NiAs structure and their unit-cell parameters are listed in Table 10.2.3. 

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