NiAs structure

Other hydrides with interstitial or metallic properties are formed by V, Nb and Ta they are, however, very much less stable than the compounds we have been considering and have extensive ranges of composition. Chromium also forms a hydride, CrH, though this must be prepared electrolytically rather than by direct reaction of the metal with hydrogen. It has the anti-NiAs structure (p.. 555). Most other elements... 

Nickel sulfides are very similar to those of cobalt, consisting of NiS2 (pyrites structure, p. 680), Ni3S4 (spinel structure, p. 247), and the black, nickel-deficient Nii-j S (NiAs structure, p. 555), which is precipitated from aqueous... 

As expected from the similarity of ionic radii between Ag+ (1.15 A) and Na+ (1.01 A), one form has the NaCl structure (it is trimorphic) with other forms having the CsCl and inverse NiAs structures. Unlike the other silver(I) halides, it is very soluble in water (up to 14 M) and forms di- and tetra-hydrates it is decomposed by UV rather than visible light and melts unchanged at 435°C. 

Fig. 12-6. The Nonmagnetic Unit Cell of the NiAs Structure. 0, magnetic...

Fig. 1.2 Crystal structures of the major sulfides (metal atoms are shown as smaller or black spheres) (A) galena (PbS) structure (rock salt) (B) sphalerite (ZnS) structure (zinc blende) (C) wurtzite (ZnS) strucmre (D) pyrite structure and the linkage of metal-sulfur octahedra along the c-axis direction in (/) pyrite (FeSa) and (//) marcasite (FeSa) (E) niccolite (NiAs) structure (F) coveUite (CuS) structure (layered). (Adapted from Vaughan DJ (2005) Sulphides. In Selley RC, Robin L, Cocks M, Plimer IR (eds.) Encyclopedia of Geology, MINERALS, Elsevier p 574 (doi 10.1016/B0-12-369396-9/00276-8))...

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 NiAs structure and distorted variants. The images for MnP and NiP show the same section as the image for NiAs in the upper left. 

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). 

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). 

MnAs exhibits this behavior. It has the NiAs structure at temperatures exceeding 125 °C. When cooled, a second-order phase transition takes place at 125 °C, resulting in the MnP type (cf. Fig. 18.4, p. 218). This is a normal behavior, as shown by many other substances. Unusual, however, is the reappearance of the higher symmetrical NiAs structure at lower temperatures after a second phase transition has taken place at 45 °C. This second transformation is of first order, with a discontinuous volume change AV and with enthalpy of transformation AH. In addition, a reorientation of the electronic spins occurs from a low-spin to a high-spin state. The high-spin structure (< 45°C) is ferromagnetic,... 

The NiAs structure is an important reference type because of its several (filled-up and defect) derivative structures. If atoms are left out of the metal layers in an ordered way, defects superstructures are obtained the Cdl2 type is obtained when all the metal atoms are omitted in alternate layers. On the contrary other derivative structures (filled-up structures) may be obtained by adding atoms in the same layers as... 

The structure of MnP along [100] can be compared with the NiAs structure along [001], In comparison with Ni in NiAs, Mn in MnP is surrounded by a distorted octahedron of P atoms (6 P) phosphorus is surrounded by a distorted trigonal prism of Mn atoms (6Mn). 

Figure 5.11 Ordered superstructure arrangements of vacant sites in cation layers of chalcogenides of NiAs structure. Fraction of occupied sites (a) 1 (b) (c)...

The Jensen symbols are very important in helping to unravel the relationship between the different structure types in neighbouring domains. For example, it is not fortuitous that the NaCl and NiAs domains adjoin each other. Their Jensen symbols 6/6 and 8IV/6 tell us immediately that in NaCl the Na and Cl sites are octahedrally coordinated, whereas in NiAs the Ni site is octahedrally coordinated (but with two extra capping atoms), and the As site is trigonally coordinated. It is also not surprising that at the boundary between cF8 (NaCl) 6/6 and hP4(NiAs) IV/6 we find the two much smaller domains of hP8(TiAs) 7/6, 6 and tI8(NbAs) 6 /6. Nor is it unexpected to find the two islands of oP8(MnP) 10 78 " stability in the hP4(NiAs) 8rv/6 domain. A distorted NiAs structure type, MnP leads to the bicapping of the trigonal prismatic coordination about the As site, that is 6 - 8W (cf Fig. 1.9). Further, we see that the cP8(FeSi) 13713 domain adjoins a cP2(CsCl) 14/14 domain they are related structure types as mentioned earlier. 

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