Solid State Chemistry and its Applications. Anthony R. West. Читать онлайн. Newlib. NEWLIB.NET

Автор: Anthony R. West
Издательство: John Wiley & Sons Limited
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Жанр произведения: Химия
Год издания: 0
isbn: 9781118695579
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parallel to c as ACBCACBC… where Ni atoms in C positions (red) separate the A and B layers of As (black).

       Each As is surrounded by (Table 1.11):6 Ni in a trigonal prism at distance 0.707a12 As, hcp arrangement, at distance a.Table 1.13 Some compounds with the NiAs structureCompounda/Åc/Åc/aCompounda/Åc/Åc/aNiS3.43925.34841.555CoS3.3675.1601.533NiAs3.6025.0091.391CoSe3.62945.30061.460NiSb3.945.141.305CoTe3.8865.3601.379NiSe3.66135.35621.463CoSb3.8665.1881.342NiSn4.0485.1231.266CrSe3.6846.0191.634NiTe3.9575.3541.353CrTe3.9816.2111.560FeS3.4385.8801.710CrSb4.1085.4401.324FeSe3.6375.9581.638MnTe4.14296.70311.618FeTe3.8005.6511.487MnAs3.7105.6911.534FeSb4.065.131.264MnSb4.1205.7841.404δ′‐NbNa2.9685.5491.870MnBi4.306.121.423PtBa3.3584.0581.208PtSb4.1305.4721.325PtSn4.1035.4281.323PtBi4.3155.4901.272a Anti‐NiAs structure.R. W. G. Wyckoff, Crystal Structures, Vols 1 to 6, Wiley (1971).Figure 1.36 The primitive cubic unit cell of CsCl.

        Each Ni is surrounded by:6 As, octahedrally, at distance 0.707a2 Ni, linearly, parallel to c, at distance 0.816a (i.e. c/2)6 Ni, hexagonally, in ab plane at distance a.

      The main effect of changing the value of the c/a ratio is to alter the Ni–Ni distance parallel to c. Thus, in FeTe, c/a = 1.49, and the Fe–Fe distance is reduced to 0.745a [i.e.c slash 2 equals one half left-parenthesis 1.49 a right-parenthesis], thereby bringing these Fe atoms into close contact and increasing the metallic bonding in the c direction. Simple quantitative calculations of the effect of changing the c/a ratio are difficult to make since it is not readily possible to distinguish between, for example, an increase in a and a decrease in c, either of which could cause the same effect on the c/a ratio.

      1.17.4 Caesium chloride (CsCl)

      Although CsCl is not a cp structure, there is a link between it and the fluorite structure, which can be described as a primitive cubic array of anions with cations in alternate cube body centres, Fig. 1.34; in CsCl, all body centres are occupied.

       Table 1.14 Some compounds with the CsCl structure

Compound a/Å Compound a/Å Compound a/ Compound a/
CsCl 4.123 NH4Br 4.0594 CuPd 2.988 AlNi 2.881
CsBr 4.286 TlCl 3.8340 AuMg 3.259 LiHg 3.287
CsI 4.5667 TlBr 3.97 AuZn 3.19 MgSr 3.900
CsCN 4.25 TlI 4.198 AgZn 3.156
NH4Cl 3.8756 CuZn 2.945 LiAg 3.168

      1.17.5 Other AX structures

      There are five main AX structure types, rock salt, CsCl, NiAs, sphalerite and wurtzite, each of which is found in a large number of compounds. There are also several less common AX structures. Some are distorted variants of one of the main structure types, e.g.:

      1 FeO at low temperatures, <90 K, has a rock salt structure with a slight rhombohedral distortion (the α angle is increased from 90 to 90.07° by a slight compression along one threefold axis). This rhombohedral distortion is associated with magnetic ordering in FeO at low temperatures (see Chapter 9).

      2 TlF has a rock salt‐related structure in which the fcc cell is distorted into a face centred orthorhombic cell by changing the lengths of all three cell axes by different amounts. The distortion arises because, in this structure, Tl+ (Xe core) 4f145d106s2 is a non-spherical cation exhibiting a stereochemically-active inert (or lone) pair effect.

      3 NH4CN has a distorted CsCl structure (as in NH4Cl) in which the CN– ions do not assume spherical symmetry but are oriented parallel to face diagonals. This distorts the symmetry to tetragonal and effectively increases a relative to c.

      Other AX compounds have completely different structures, e.g.:

      1 Compounds of the d 8 ions, Pd and Pt (in PdO, PtS, etc.), often