Nickel arsenides

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 

The nitrides (and phosphides) of many of the transition metals form crystals with the sodium chloride structure. This description, however, must not be interpreted as implying that they are ionic in character, for in fact they display many of the properties of intermetallic systems. For this reason a discussion of these nitrides is deferred to chapter 13. 

Many carbides and silicides of composition AX are formed by transition metals. These carbides and silicides are characterized by very high melting points, extreme hardness, optical opacity and relatively high electrical conductivity. Many of them have the sodium chloride structure but they are not ionic compounds rather do they resemble the corresponding nitrides and phosphides in simulating alloy systems in many of their properties. For this reason they will be discussed later. 

Other carbides, notably those of the more electropositive metals, are quite different in their properties, being colourless, transparent insulators resembling inorganic salts rather than metal systems. These carbides are ionic some typical structures will be described in the sections of this chapter devoted to AX2 and AmXz compounds. 

Silicon carbide (carborundum, SiC) is of especial interest on account of its rich polymorphism, no fewer than six structures being known. As is to be expected, each carbon and silicon atom is tetra-hedrally co-ordinated by four atoms of the other kind, and two of the forms of carborundum have the zincblende and wurtzite structures. The close relationship between these two structures has already been discussed ( 4.13), and is emphasized by the many AX compounds (including ZnS itself) in which both are found. It is illustrated in fig. 8.03, where the cubic zincblende structure has been drawn with one of the cube diagonals vertical and parallel to the principal axis of the wurtzite structure. When viewed in this way it will be seen that both structures can be visualized as formed by the superposition of a series of puckered sheets of atoms, but that in zincblende successive sheets are identical (albeit translated) whereas in wurtzite they differ and are related by a rotation through 180° about the principal axis. In the two structures the sequence of sheets can therefore be symbolized as 

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Structure of nickel arsenide showing (a) 3 unit cells, (b) a single unit cell NiaAsa and its relation to (c) the unit cell of the layer lattice compound Cdia (see text). 

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.

FIGURE 5.44 The structure ol nickel arsenide, NiAs. Atypical structures such as this one are often found when the covalent character of the bonding is important and the ions have to take up specific positions relative to one another to maximize their bonding. 

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 type (NiAs) is the result of linking layers of the kind as in cadmium iodide. Continuous strands of face-sharing octahedra perpendicular to the layers... 

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. 

A. Kj eskhus, W. B. Pearson, Phases with the nickel arsenide and closely-related structures, Prog. Solid State Chem. 1 (1964) 83. 

Arsenopyrite Cobaltite Nickel Arsenide Unknovm Distilled water 277 303 323 Iwasaki et al. (1989)... 

Nakazawa and Iwasaki (1985) and Pozzo, Malicsi and Iwasaki (1988) investigated a pyrite-pyrrhotite contact and a pyrite-pyrrhotite-grinding media contact on flotation, respectively. They found that the floatability of pyrrhotite increased in the presence of pyrite, whereas it decreased in the presence of both pyrite and grinding media (mild steel). Similarly, a galvanic contact between nickel arsenide and pyrrhotite decreased the floatability of pyrrhotite (Nakazawa and Iwasaki, 1986). 

Nakazawa, H. and Iwasaki, I., 1986. Galvanic contact between nickel arsenide and pyrrlotite and its effects on flotation. Inter. J. Miner. Process, 18 203 - 215 Nakazawa, H. and Iwasaki, 1., 1985. Effect of pyrite-pryyhotite contact on their floatabilities. 

The compounds of Group V elements are often volatile, and loss of, for example, arsenic, selenium and tellurium during ashing of the sample can be reduced by the addition of nickel, to form nickel arsenide. Such stabilization procedures are called matrix modification (see Section 3.6.4). 

Hexagonal close-packed 6 6 MO All oct. Nickel arsenide NiAs, FeS, FeSe, CoSe... 

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). 

FIGURE 1.36 (a) The unit cell of nickel arsenide, NiAs. (For undistorted hep c/a=1.633, but this ratio is found to vary considerably.) Ni, blue spheres As, grey spheres, (b) The trigonal prismatic coordination of arsenic in NiAs. 

MX 6 6 All octahedral Sodium chloride NaCl, FeO, MnS, TiC Nickel arsenide NiAs, FeS, NiS... 

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

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