Nickel arsenide structures

Figure 15.18 Comparison of the nickel arsenide structure (a) adopted by many monosulfides MS with the cadmium iodide structure (b) adopted by some disulfides MS2. The structures are related simply by removing alternate layers of M from MS to give MS2.

Newton s second law, L0 nickel, 49, 665 nickel arsenide structure, 201 nickel surface, 189 nickel tetracarbonyl, 665 nickel-metal hydride cell, 520 NiMH cell, 520 nitrate ion, 69, 99 nitration, 745 nitric acid, 629 nitric oxide, 73, 629 oxidation, 549 nitride, 627 nitriding, 208 nitrite ion, 102 nitrogen, 120, 624 bonding in, 108 configuration, 35 photoelectron spectrum, 120... 

The radii in the lowest row of the table were obtained by a number of approximate considerations. For instance, if we assume the bismuth radius to bear the same ratio to the interatomic distance in elementary bismuth as in the case of arsenic and antimony, we obtain (Bi) = 1.16— 1.47 A. A similar conclusion is reached from a study of NiSb and NiBi (with the nickel arsenide structure). Although the structures of the aurous halides have not been determined, it may be pointed out that if they are assumed to be tetrahedral (B3 or Bi) the interatomic distances in the chloride, bromide, and iodide calculated from the observed densities1) are 2.52, 2.66, and 2.75 A, to be compared with 2.19, 2.66, and 2.78 A, respectively, from pur table. 

AuSn has the nickel arsenide structure, B8, with abnormally small axial ratio (c/a = 1.278, instead of the normal value 1.633). Each tin atom is surrounded by six gold atoms, at the corners of a trigonal prism, with Au-Sn = 2.847 A. and each gold atom is surrounded by six tin atoms, at the corners of a flattened octahedron, and two gold atoms, at 2.756 A., in the opposed directions through the centers of the two large faces of the octahedron. 

The nickel arsenide structure is the equivalent of the sodium chloride structure in hexagonal close-packing. It can be described as an hep array of arsenic atoms with nickel atoms occupying the octahedral holes. The geometry about the nickel atoms is thus octahedral. This is not the case for arsenic each arsenic atom sits in the centre of a trigonal prism of six nickel atoms (Figure 1.36). 

Nickel-arsenide structure (shaded) Rock-salt structure (unshaded) vr j Zr Nb Mnf [ Fe - Go nT ... 

Several transition metal sulfides (e.g., FeS, CoS, and NiS) adopt a structure called the nickel arsenide structure, illustrated in Fig. 12-4. In this structure each metal atom is surrounded octahedrally by six sulfur atoms, but also is approached... 

An even more extreme example of nonstoichiometry is provided by the Co—Te (and the analogous Ni—Te) system. Here, a phase with the nickel arsenide structure is stable over the entire composition range CoTe to CoTe2. It is possible to pass continuously from the former to the latter by progressive loss of Co atoms from alternate planes (see Fig. 12-4) until, at CoTe2, every other plane of Co atoms present in CoTe has completely vanished. 

Figure 3 The nickel arsenide structure (reproduced by permission of Oxford University Press from Structural Inorganic Chemistry, 1945, p. 387).

The nickel arsenide structure. The structure most frequently encountered is the NiAs structure (Fig. 17.1), which is also that of many phases MX in which M is a transition metal and X comes from one of the later B subgroups (Sn, As, Sb, Bi, S,... 

The A2 metals and the elements of the earlier B subgroups (Bj metals) form the electron compounds already discussed. With the metals of the later B subgroups the A2 metals, like the Aj, tend to form intermetallic phases more akin to simple homopolar compounds, with structures quite different from those of the pure metals. The nickel arsenide structure has, like typical alloys, the property of taking up in solid solution a considerable excess of the transition metal. From Table 29.12... 

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 nickel arsenide structure occurs not only in the sulphides but also in many other compounds containing a transition metal and one of the elements Sn, Pb, As, Sb, Bi, Se and Te. Many of these systems are essentially intermetallic in their properties and will be discussed further in the chapter devoted to alloys. Here, however, it is interesting to note that as the system becomes more metallic so the bonding in the vertical direction becomes stronger. Thus in FeS the Fe-S and Fe-Fe distances... 

Many T2-B2 systems, at the appropriate composition, display either the nickel arsenide structure or one geometrically closely related to it. This structure has already been described ( 8.12 and fig. 8.02) and it will be remembered that it is characterized by the asymmetry of the co-ordination each of the more metallic T atoms is octahedrally coordinated by six atoms B of the less metallic component, whereas each B atom is surrounded by six neighbours T at the corners of a trigonal prism. For our present purposes, however, it is convenient to describe the structure in somewhat different terms, for it can alternatively be regarded as an expanded hexagonal close-packed array of B atoms with the T atoms disposed in its octahedral interstices. This interstitial picture explains the wide range of composition over which the structure... 

The widespread occurrence of the nickel arsenide and related structures in T2-B2 systems is illustrated by the data in table 13.05. Where the composition is given in the form MN it is to be understood that the simple nickel arsenide structure normally obtains, but that in general a range of solid solution on each side of the equiatomic composition is found. In a few cases, however, the range of homogeneity does not include the simple composition, and phases occur only with an excess of one component. The copper-tin system is an example of this, for the phase shown as CuSn does not exist at this composition but only with an excess of interstitial copper. A similar state of affairs obtains in the other systems marked with an asterisk in the table. 

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