Structure types nickel arsenide

The nickel arsenide structure is shown in Figure 1.18. Arsenide ions in identical close-packed layers are stacked directly over each other, with nickel ions filling all the octahedral holes. The larger arsenide anions are in a trigonal prism of nickel cations. Both types of ion have a coordination number of six. 

The octahedrons surrounding the Ni " " ions in NiAs, on the other hand, are edge-shared in one direction while face-shared in another, leading to a layered structure with an ABABAB-type repeating motif. The following molecules all crystallize in the nickel arsenide structure TiS, TiSe, TiTe, CrS, CrSe, CrTe, CrSb, NiS, NiSe, NiTe, NiSb, and NiSn. 

The symmetry reduction to the mentioned hettotypes of diamond is necessary to allow the substitution of the C atoms by atoms of different elements. No splitting of Wyckoff positions, but a reduction of site symmetries in necessary to account for distortions of a structure. Let us consider once more MnP as a distorted variant of the nickel arsenide type (Fig. 17.5, p. 197). Fig. 18.4 shows the relations together with images of the structures. 

Fig. 1.10 The sodium chloride (a) and nickel arsenide (b) structure types. From Wells (1986).

The intermetallic compounds MnAs, MnSb, and MnBi crystallize with a nickel arsenide-type structure, and they have many features in common regarding their structure and nature of physical properties. At room temperature, all these compounds are ferromagnetic. The Curie point of manganese arsenide is 45°C of MnSb and MnBi it is 314 and 360°C, respectively. Their ferromagnetism is due to the uncompensated spins of the 3d electron shells of the manganese atoms, located at the crystal lattice points with the coordinates 0, 0, 0 and 0, 0, i. 

The more important structures of heterodesmic t5 e are cuprite type, galene type, nickel arsenide and the structure types of the molecular compoimds. The molecular compoimds were previously discussed in subsection 4.4.1.4. the other compoimds will be shortly presented in the following. 

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