Valence NiAs structure

The structure of MnP is a distorted variant of the NiAs type the metal atoms also have close contacts with each other in zigzag lines parallel to the a-b plane, which amounts to a total of four close metal atoms (Fig. 17.5). Simultaneously, the P atoms have moved up to a zigzag line this can be interpreted as a (P-) chain in the same manner as in Zintl phases. In NiP the distortion is different, allowing for the presence of P2 pairs (P ). These distortions are to be taken as Peierls distortions. Calculations of the electronic band structures can be summarized in short 9-10 valence electrons per metal atom favor the NiAs structure, 11-14 the MnP structure, and more than 14 the NiP structure (phosphorus contributes 5 valence electrons per metal atom) this is valid for phosphides. Arsenides and especially antimonides prefer the NiAs structure also for the larger electron counts. 

As mentioned earlier, the NiAs structure can readily accommodate additional cations in the bipyramidal holes. The upper limit of the homogeneity range of these phases is obviously determined by the total number of valence electrons since more cations are accepted if the B metal carries fewer valence electrons (147) ... 

Still further distortions can take place. In NiP, the chains of Ni and P atoms discernible in the MnP structure break up into Ni2 and P2 pairs. For phosphides, it is experimentally clear that the number of available electrons tunes the transition from one structural type to another. Nine or 10 valence electrons favor the NiAs structure (for phosphides), 11-14 the MnP, and a greater number of electrons the NiP alternative. For the arsenides this trend is less clear. 

A number of important structure types are found in transition-metal sulphides which have no counterparts among oxide structures, notably the various layer structures and the pyrites, marcasite, and NiAs structures. Further, many sulphides, particularly of the transition metals, behave like alloys, the resemblance being shown by their formulae (in which the elements do not exhibit their normal chemical valences, as in 0983, Pd4S, TiSa), their variable composition, and their physical properties-metallic lustre, reflectivity, and conductivity. The crystal structures of many transition-metal sulphides show that in addition to M-S bonds there are metal-metal bonds as, for example, in monosulphides with the NiAs structure (see later), in chromium sulphides, and in many sub-sulphides such as Hf2S,... 

Arsenides.—Some indications of the complexity of binary arsenide structures are apparent from recent X-ray investigations. In the new intermetallic compound Ca2As3, As4 and Asg chains are present in a 1 1 ratio, with As—As distances ranging between 2.47 and 2.58 As units occur in the simple arsenide CaAs, which has an anti-NiAs structure, and here the As—As separation is 2.563 The ternary arsenides CasGa2As6 and Ca4Ga3Ass have also been synthesized, the structure of the former containing layers of Ca—As octahedra and tetrahedra. As expected from valence considerations, As2 units are present... 

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