CsCl type

The Fe—Co alloys exist ia the fee stmeture above 912—986°C to ca 70 wt % Co. Below this temperature range, the stmeture changes to bcc. At ca 50 wt % Co, the material further orders to the CsCl-type B2 stmeture below about 730°C and becomes very brittle. The addition of V ia Permeadur retards the rate of orderiag and imparts substantial ductiHty to the adoy, although quenching is necessary. Vanadium addition also iacreases the resistivity, eg, from 7—26 fifl-cm usiag a 2% addition. 

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

A martensitic transformation from a cubic CsCl-type structure by 110 (lT0> type shears occurs for NlxAli. alloys in the composition range 0.615 < x < 0.64. Precursive... 

Hexafluoroniobates and hexafluorotantalates of rubidium, cesium and ammonium [78, 158, 163, 165] are similar and crystallize in a BaSiF6 type structure. In general, the structure of the compounds can be described as a rhombically distorted CsCl type lattice. 

According to investigations performed by Pakhomov and Kaidalova [204], the crystal structure of NF NbC consists of infinite chains made up of distorted octahedral ions (NbOF4 ) linked by oxygen atoms. Ammonium cations, NH/, occupy the spaces between the chains, as shown in Fig. 30. The packing of the structural units in the NH4NbOF4 crystal can be described as a CsCl type structure in which CF ions are replaced by Nb02F4 complexes and Cs" ions are replaced by ammonium ions. 

Ti3Co5B2 = TiCo3B2(CeCo3B2 type) + 2 TiCo(CsCl type)... 

The classical view of the lone pair is that, after mixing of the s and p orbitals on the heavy metal cation, the lone pair occupies an inert orbital in the ligand sphere [6]. This pair of electrons is considered chemically inert but stereochemi-cally active [7]. However, this implies that the lone pair would always and in any (chemical) environment be stereochemically active, which is not the case. For example, TIF [8] adopts a structure, which can be considered as a NaCl type of structure which is distorted by a stereochemically active lone pair on thallium. In contrast TlCl [9] and TlBr [10] adopt the undistorted CsCl type of structure at ambient temperature, and at lower temperatures the (again undistorted) NaCl type of structure. The structure of PbO [11] is clearly characterized by the stereochemically active lone pair. In all the other 1 1 compounds of lead with... 

CaO experiences a phase transition form the NaCl type to the CsCl type at a pressure of 65 GPa (images in Fig. 7.1, p. 53). What kind of a transformation is this ... 

For compounds of the composition MX (M = cation, X = anion) the CsCl type has the largest Madelung constant. In this structure type a Cs+ ion is in contact with eight Cl-ions in a cubic arrangement (Fig. 7.1). The Cl- ions have no contact with one another. With cations smaller than Cs+ the Cl- ions come closer together and when the radius ratio has the value of rM/rx = 0.732, the Cl- ions are in contact with each other. When rM/rx < 0.732, the Cl- ions remain in contact, but there is no more contact between anions and cations. Now another structure type is favored its Madelung constant is indeed smaller, but it again allows contact of cations with anions. This is achieved by the smaller coordination number 6 of the ions that is fulfilled in the NaCl type (Fig. 7.1). When the radius ratio becomes even smaller, the zinc blende (sphalerite) or the wurtzite type should occur, in which the ions only have the coordination number 4 (Fig. 7.1 zinc blende and wurtzite are two modifications of ZnS). 

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

The CsCl type offers the simplest way to combine atoms of two different elements in the same arrangement as in body-centered cubic packing the atom in the center of the unit cell is surrounded by eight atoms of the other element in the vertices of the unit cell. In this way each atom only has adjacent atoms of the other element. This is a condition that cannot be fulfilled in a closest-packing of spheres (cf. preceding section). 

Although the space filling of the body-centered cubic sphere packing is somewhat inferior to that of a closest-packing, the CsCl type thus turns out to be excellently suited for compounds with a 1 1 composition. Due to the occupation of the positions 0,0,0 and with different kinds of atoms, the structure is not... 

Superstructure of the CsCl type with eightfold unit cell. Left, lower half and right, upper half of the cell in projection onto the plane of the paper, a, b, c, and d designate four different kinds of atomic sites that can be occupied in the following ways ... 

The term Laves phases is used for certain alloys with the composition MM3, the M atoms being bigger than the M atoms. The classical representative is MgCu2 its structure is shown in Fig. 15.4. It can be regarded as a superstructure of the CsCl type as in Fig. 15.3, with the following occupation of the positions a, b, c, and d ... 

Body-centered cubic sphere packing => CsCl type => superstructures of the CsCl type... 

The coordination numbers of the atoms increase from 6 to 8 in the NaCl —> CsCl type conversion therefore, it is a reconstructive phase transition which can only be a first-order transition. 

Figure 11.7 shows schematically the resulting calculated variation of H with p for the NaCl-type and the CsCl-type phases of CaO. The NaCl-type structure, which is stable at low pressures, is the rock salt structure in which the Ca and O atoms are 6-coordinate. In the CsCl structure, stable at high pressures, both cation and anion are 8-coordinate. In the static limit where the entropy is set to zero, the thermodynamically most stable phase at any pressure is that with the lowest value of H at the thermodynamic transition pressure, ptrs, the enthalpies of the two phases are equal. For CaO the particular set of potentials used in Figure 11.7 indicates a transition pressure of 75 GPa between the NaCl-type and CsCl-type structures, which compares with experimental values in the range 60-70 GPa. 

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. 

Unlike CsAu (Cs+Au , CsCl type structure) and others [20] which are aurides with well defined Au ions, CsHg and similar amalgams are not simple mercu-rides but have more complicated structure and bonding properties, as was shown in Section 2.4.2.2. 

As an example, the structure of the CsCl-type has been discussed by Carter using several criteria of evaluation of At. In a geometrical approach a weighted coordination number as a function of atomic radii difference was described. 

Figure 3.17. Interatomic distances in CsCI. The distances are given for the CsCI compound (cubic, cP2-CsCl type, a = 411.3 pm) with Cs and Cl in the representative positions 0, 0, 0, and A, A, A respectively, white and black atoms in Fig. 3.8. In the tables the first two groups of distances (in pm) are given as positions of each atom around the reference atom. Notice that not only atoms in the reference cell but also those in the adjacent cells must be considered (see Figs. 3.8 (d)-(f)). At the right side, the corresponding histograms using the reduced distances d/dmm are shown the first two bars summarize the data contained in the table.

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... 

Figure 3.31. cF16-MnCu2Al type structure (representative of the Heusler type alloys). The unit cell is shown in (a), an eighth of the cell is shown in (b). It degenerates into a cP2-CsCl type cell if the atoms at the cube comers (Mn and Al) are equivalent. If all the atoms were equivalent there would he a further degeneration into the cI2-W type. 

Notice, moreover, that one face-centred cubic cell of atoms X in which all the interstices are occupied (the octahedral by X and the tetrahedral by Z atoms) is equivalent to a block of 8 XZ, CsCl-type cells (see Fig. 3.31). This relationship (and others with other structures such as Li3Bi and MnCu2Al) should be kept in mind when considering, for instance, phase transformations occurring in ordering processes. 

For a review on site preference of substitutional additions to CsCl-type inter-metallic compounds see Kao et al. (1994). In this work dilute additions to NiAl, FeAl and CoAl are especially discussed. The addition of a third element to ordered Ni3Al (cP4-AuCu3 type) occurring in different ways (Ochiai etal. 1984) is another... 

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