NiAs type

All three metals form a wide variety of binary chalcogenides which frequently differ both in stoichiometry and in structure from the oxides. Many have complex structures which are not easily described, and detailed discussion is therefore inappropriate. The various sulfide phases are listed in Table 22.4 phases approximating to the stoichiometry MS have the NiAs-type structure (p. 556) whereas MS2 have layer lattices related to M0S2 (p. 1018), Cdl2, or CdCl2 (p. 1212). Sometimes complex layer-sequences occur in which the 6-coordinate metal atom is alternatively octahedral and trigonal prismatic. Most of the phases exhibit... 

Figure 1. Crystallographic relation (schematic) between the structure types of RhB (anti-NiAs type), TaFeB (ordered Fe2P type) and ZrIrjB4 type. Numbers given indicate heights in projection along [001]. Large circles are metal atoms, small circles are B atoms. Metal sublattice of RhB and different modes of filling the voids (squares) generate the different structure types (see text).

Titanium monosulfide, TiS, assumes two forms, both of which are of the NiAs type. In 384, the packing of sulfur is of the ABAC type, with alternate layers of metal sites being fully occupied but the intermediate sites half filled. A series of intermediate phases Ti2+xS4 (0.2 < x < 1) also occurs. The trisulfide TiS3 is best represented as TiS (S2) . The trichalcogenides of Group IVA elements are typified... 

A number of selenium and tellurium compounds of the presently discussed metals show a quite different behavior from the Fe-S system. Iron and selenium form two compounds FeSe with a broad stoichiometry range and FeSe2 with a much narrower composition field. Below 400 the non-stoichiometric Fei xSe exists by creation of iron vacancies and can have compositions lying between FeySes and Fe3Se4. At low temperatures there exist two phases an a (PbO type) and a f) (NiAs type) phase. The crystal sUiicture of the diselenide, FeSe2, is an orthorhombic, C18 (marcasite) type. In the Fe-Te system, the defect NiAs structure is found at a composition close to FeTei.s, as about one-third of the Fe atoms are missing. At compositions around FeTe the behavior is complex, and the f)-phase has the PbO structure (like FeSe) but with additional metal atoms (i.e., FeuTe). 

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. 

Compounds which have the NiAs structure often exhibit a certain phase width in that metal atom positions can be vacant. The composition then is M X. The vacancies can have a random or an ordered distribution. In the latter case we have to deal with superstructures of the NiAs type they are known, for example, among iron sulfides such as Fe9S10 and Fe10Sn. If metal atoms are removed from every other layer, we have a continuous series from Mj 0X with the NiAs structure down to M0 5X (= MX2) with the Cdl2 structure phases of this kind are known for Co Te (CoTe NiAs type CoTe2 Cdl2 type). 

Distorted variants, similar to the distorted variants of the NiAs type, are known for the Cdl2 type. For example, Zrl2 has a distorted Cdl2 structure in which the Zr atoms form zigzag chains. Therefore, every Zr atom is involved in two Zr-Zr bonds which is in accordance with the d2 configuration of divalent zirconium. 

By the addition of, , 0 to the coordinates listed in Fig. 18.4 for the space group Cmcm, we obtain ideal values for an undistorted structure in Pmcn. However, due to the missing distortion the symmetry would still be Cmcm. The space group Pmcn is only attained by the shift of the atoms from the ideal positions. First of all, the deviations concern the y coordinate of the Mn atom (0.214 instead of ) and the z coordinate of the P atom (0.207 instead of ). These are rather small deviations, so we have good reasons to consider MnP as being a distorted variant of the NiAs type. 

The relation between diamond and zinc blende shown above is a formal view. The substitution of carbon atoms by zinc and sulfur atoms cannot be performed in reality. The distortion of the NiAs structure according to Fig. 18.4, however, can actually be performed. This happens during phase transitions (Section 18.4). For example, MnAs exhibits this kind of phase transition at 125 °C (NiAs type above 125 °C, second-order phase transition another transition takes place at 45 °C, cf. p. 238). 

The aristotype can be considered to be either the packing of spheres itself, or the NiAs type which corresponds to the packing in which all octahedral interstices are occupied by Ni atoms (Wyckoff position 2a). In the aristotype these interstices are symmetry equivalent subgroups result if the interstices are occupied only partially or by different kinds of atoms (or if the Ni atoms of NiAs are partially removed or substituted). By this procedure the sites of the interstices become non-equivalent. 

TiN, NaCl type FeP, MnP type FeSb, NiAs type CoS, NiP type CoSb, NiAs type. 

Our focus is on the most comprehensively studied series, the monophosphides of the first-row transition metals, whose structures successively distort from NaCl-type (ScP) to TiAs-type (TiP), NiAs-type (VP), MnP-type (CrP, MnP, FeP, CoP), and NiP-type, forming stronger metal-metal and phosphorus-phosphorus bonding with greater electron count (Fig. 11) [63-65], The P atoms are six-coordinate, but... 

Fig. 12 Displacement of M atom positions in MnP-type structure relative to NiAs-type structure, with formation of zigzag chains of M-M bonds...

Derived structures may also be formed by the ordered introduction of vacant sites. As an example, consider the hP3-CdI2 type structure (see Chapter 7) which can be related to the hP4-NiAs type structure in which the set of equivalent points 0,0,0 and 0, 0, M is considered as being subdivided into two groups (each of one site) 0,0,0 (occupied by one atomic species) and 0, 0, M (vacant). We can therefore regard the hP3-CdI2 type structure as a defect derivative form of the hP4-NiAs type (see 7.4.2.4.3). Similar considerations maybe extended to include (besides substitution and subtraction) ordered addition of atoms. In this case stuffed or filled-up derivative... 

Similar considerations may be made with reference to the other simple close-packed structure, that is to the hexagonal Mg-type structure. In this case two basic derived structures can be considered the NiAs type with occupied octahedral holes and the wurtzite (ZnS) type with one set of occupied tetrahedral holes (compare with the data given with an origin shift in 7.4.2.3.2). For a few more comments about interstices and interstitial structures see 3.8.4. See Fig. 3.35. 

In several Fe, Co, Ni alloy systems, phases having structures pertaining to the inter-related Laves type and a and // types are formed (often homogeneous in certain ranges of compositions). For compositions around 1 1, a number of solid solution phases with the CsCl-type structure and (with semi-metals) with the NiAs-type are found. 

Sulphides. The partially ionic alkali metal sulphides Me2S have the anti-fluorite-type structure (each Me surrounded by a tetrahedron of S, and each S atom surrounded by a cube of Me). The NaCl-structure type (6/6 coordination) is adopted by several mono-sulphides (alkaline earth, rare earth metals), whereas for instance the cubic ZnS-type structure (coordination 4/4) is observed in BeS, ZnS, CdS, HgS, etc. The hexagonal NiAs-type structure, the characteristics of which are described in 7.4.2.4.2, is observed in several mono-sulphides (and mono-selenides and tellurides) of the first-row transition metals the related Cdl2 (NiAs defect-derivative) type is formed by various di-chalcogenides. Pyrite (cP 12-FeS2 type see in 7.4.3.13 its description, and a comparison with the NaCl type) and marcasite oP6-FeS2 are structural types frequently observed in several sulphides containing the S2 unit. 

In the schemes of Fig. 7.10, typical sections of a few adjacent cells of this structure are shown these are also compared with those of a number of related hexagonal structures, some of which are described in the following paragraphs. Notice that important filled-up derivatives can be considered among the ordered structures derived from Mg. Typical examples are the hP4-NiAs type with occupied octahedral holes and the wurtzite (hP4-ZnS) type with one set of occupied tetrahedral holes. 

In order to have around each atom in this hexagonal structure four exactly equidistant neighbouring atoms, the axial ratio should have the ideal value (8/3 that is 1.633. The experimental values range from 1.59 to 1.66. This practical constancy of the axial ratio, in contrast with what is observed for other families of isostructural compounds such as those of the NiAs type, may be attributed to a sort of rigidity of the tetrahedral (sp3) chemical bonds. As for the atomic positional parameters, the ideal value of one of the parameters (being the other one fixed at zero by conventionally shifting the origin of the cell) is z = 3/8 = 0.3750. The C diamond, sphalerite- and wurtzite-type structures are well-known examples of the normal tetrahedral structures (see 3.9.2.2). 

On passing from Cdl2 to the NiAs type the insertion of a new layer is found at level 14 as well as, from NiAs to Ni2In, the ordered addition of atoms at levels 14 and 34. 

NiAs distortion structures. The characteristics of a number of structures have been described by Pearson (1972) in terms of distortions of the NiAs type. Different variants have been considered, of which the following new prototypes having a 1 1 stoichiometry are typical examples mS8-CrS, oP8-MnP, oP16-NiP, hP24-aFeS. The oP8-MnP type is described here below. 

This structure and its relation with the NiAs type is described in 7.4.2.4.3. 

Sulphides and Selenides. The new compounds VySg and VySeg have been prepared and shown to have NiAs-type structures with ordered vacancies. 

Dependencies of the /I-NiH phase volume fraction on the nature and concentration of the promotor, changing current density and changing duration were presented and the phenomena observed were explained with the assumption that NiAs-type compounds take part in the formation of nickel hydride. 

---