CsCl-type derivative

MnCujAl (Heusler phase)-type = CsCl-type derivative... 

Structures Derived of Body-centered Cubic Packing (CsCl Type)... 

Intuitively, one would expect a volume contraction on forming a strongly bonded compound from the elements. Indeed, Richards 190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume - 2 atomic vol-ume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures 151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

An example where, due to ordering, we observe perhaps in a more immediate way, the increase of the unit cell size (formation of a multiple cell) is the MnCu2Al-type structure (representative of the so-called Heusler alloys) which can be considered a derivative structure (superstructure) of the cP2-CsCl type, which in turn is a superstructure of the W-type structure, corresponding to a non-primitive cubic cI2... 

To compare the experimental lattice parameter more conveniently with the quantum-mechanical and also the classically derived ones, these are all found together in Table 3.1. Indeed, all of them agree, with respect to the [NaCl]-type lattice parameter, to lie around 4.81 A. The best value is the one derived from the empirical ionic radii but it may be argued that the latter data were, in fact, fitted to match the experimental crystal-structure results. The GGA result further allows us to estimate the classical predictions for the h)q5othetical lattice parameters of the [ZnS]/[CsCl] types, and here the fit is acceptable, too, for [ZnS] (fl 5.2 A) and [CsCl] a 3.0 A), with the only exception for the volume-increment method, in particular the [ZnS] entry. Obviously, we have already gone beyond the limits of this simplistic method and will therefore not consider it any further for the present system. 

Thallium(I) halides, TlX, are stable compounds which in some ways resemble Ag(I) halides. Thallium(I) fluoride is very soluble in water, but TlCl, TlBr and Til are sparingly soluble the trend in solubilities can be traced to increased covalent contributions in the ionic lattices for the larger halides, a situation that parallels the trend for the Ag(I) halides (see Section 6.15). In the solid state, TIF has a distorted NaCl-type structure, while TlCl and TlBr adopt CsCl structures. Thallium(I) iodide is dimorphic below 443 K, the yellow form adopts a lattice derived from an NaCl structure in which neighbouring layers are slipped with respect to each other and, above 443 K, the red form crystallizes with a CsCl-type structure. Under high pressures, TlCl, TlBr and TII become metallic in character. 

Fig. 22. Structural relationship between RhsOe, and KjPrCl,. The RujSi- and CsCl-type parts of the structure of RhjOej as derived ftom a primitive sphere packing.

In and T1 monohalides are isoelectronic with the Sn and Pb monochalcogenides. The character of their structures, however, is different, although geometrically, the two structures are closely related. Common to both groups is, furthermore, the occurrence of NaCl-type phases and NaCl-type derivatives (see Table 51). CsCl-type modifications, however, are known only for the In and T1 halides, similar to the alkali halides and rare-earth monochalcogenides, but no such modifications are reported for the isoelectronic Sn and Pb chalcogenides. 

Introduction. A number of common structures, ideally corresponding to a 1 1 stoichiometry, are presented in this chapter. Some of them are not specifically characteristic of intermetallic compounds only. The CsCl and NaCl types, for instance, are observed for several kinds of chemical compounds (from typical ionic to metallic phases). Notice that for a number of prototypes a few derivative structures have also been considered and described, underlining crystal analogies and relationships even if with a change in the reference stoichiometry. 

See also Fig. 3.8. where the nearest-neighbour number (NNN) of the two atomic species is evidently eight. A list and the histogram of the atomic distances with the corresponding number of the equidistant neighbours are shown in Fig. 3.17. The resulting CNE is 14. In Fig. 3.31 the derivative CsCl superstructure MnCu2Al type... 

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