CsCl structure

The magnetic FeaNi system was modeled in a tetragonal symmetry with 4 atoms per unit cell (see Fig. la). The CuZn alloy was also considered to have a tetragonal unit cell, in this case c/a = 1 leads to a CsCl structure, which was shown in Fig. lb. 

Thallous halides offer a unique possibility of studying the stereochemistry of the (chemically) inert electron pair, since their structures and their pressure and temperature-dependent phase transitions have been well established. Thallium (1) fluoride under ambient conditions, adopts an orthorhombic structure in the space group Pbcm which can be regarded as a distorted rocksalt structure (Fig. 2.4). In contrast to TIF, the thallium halides with heavier halogens, TlCl, TlBr and Til, adopt the highly symmetric cubic CsCl structure type under ambient conditions [46]. Both TlCl and TlBr, at lower temperatures, undergo phase transitions to the NaCl type of structure [47]. 

Disordered alloys may form when two metals are mixed if both have body-centered cubic structures and if their atomic radii do not differ by much (e.g. K and Rb). The formation of ordered alloys, however, is usually favored at higher temperatures the tendency towards disordered structures increases. Such an arrangement can even be adopted if metals are combined which do not crystallize with body-centered cubic packings themselves, on condition of the appropriate composition. /J-Brass (CuZn) is an example below 300 °C it has a CsCl structure, but between 300 °C and 500 °C a A type transformation takes place resulting in a disordered alloy with a body-centered cubic structure. 

The calculation of M for a three-dimensional array is much more complicated, and depends on the structure of the array. For the particular case of the face-centered-cubic NaCl crystal structure, its value is M = 1.747, whereas, for the body-centered-cubic CsCl structure, it is M = 1.763. 

The hexaboride crystal structure is related to the CsCl structure so by analogy the glide planes are (100) and the glide directions are (100). At the cores of glide disloca-tions the structure becomes quasi-hexagonal. 

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. 

LiHg is one of the MHg amalgams (together with SrCd, BaCd, SrHg, and BaHg [22]) which crystallizes in an undistorted CsCl structure. The term mercuride is... 

Figure 3.8. Crystal structure of CsCl. The positions of the centres of the atoms in the unit cell are shown in (a). In (b) the same cell is described by means of its characteristic sections taken at the height 0, A, and 1 of the third axis. In (c) a projection of the cell on its square basis is presented the values of the third (fractional) coordinate are indicated. In (d) the shortest interatomic distances are shown dCs-ci = a)3/2 = 411.3 X 0.866025. = 356.2. In (e) the subsequent group of interatomic distances (d = a = 411.3) involving six atoms in the adjacent cells is presented. A group of eight cells is represented in (f) to suggest that the actual structure of CsCl corresponds to a three-dimensional infinite repetition of unit cells and to show that the coordination around the white atoms is similar to that around the black ones shown in (d). The unit cell of the CsCl structure is shown as a packed spheres model in (g).

Fig. 1.20 The B2-type CsCl structure of FeTi stoichiometric, ordered, compound (left), and room-temperature hydrogen PCT properties for B2-type FeTi hydrogen storage aUoys amorphous - a, nanocrystalline - b, and crystalhne - c (adopted from [155])...

A large number of solids with stoichiometry AB form the CsCl structure. In this structure, atoms of A define a simple cubic structure and atoms of B reside in the center of each cube of A atoms. Define the cell vectors... 

In the exercises for Chapter 2 we suggested calculations for several materials, including Pt in the cubic and fee crystal structures and ScAl in the CsCl structure. Repeat these calculations, this time developing numerical evidence that your results are well converged in terms of sampling k space and energy cutoff. 

Notice how the coordination numbers of the structures we have observed so far have changed. The coordination number for close-packing, where all the atoms are identical, is twelve. In the CsCl structure, it is eight in NaCl, it is six and in both of the ZnS structures, it is four. Generally, the larger a cation is, the more anions it can pack around itself (see Section 1.6.4). 

The CsCl structure (Fig. 1.5) consisting of interpenetrating primitive cubic arrays of Cs and Cl ions with 8 8 coordination is exhibited by many solids (a) CsCl, CsBr and Csl (b) NH4.CI, NH Br and NH I in their low-temperature modification (c) thallous halides (d) alloys of ) -brass type, CuZn, AuZn (e) CuCN, CuSH and TICN in their high-temperature form and (1) alkali halides (with the exception of lithium salts) at high pressures. 

In displacive transitions only small changes in the arrangement of coordination polyhedra occur. Reconstructive transitions would require the breaking and making of bonds, but the same can be accomplished by a simple dilatational mechanism. Buerger proposed such a mechanism for the transformation from the CsCl structure to the NaCl structure (Fig. 4.10). Such deformational relations are known to exist between... 

Figure 4.10 Dilatational mechanism for the transformation from the CsCl structure to the NaCl structure. (After Buerger 1951.)...

The Group 1 halides have the NaCl structure (6 6 coordination) except for the chloride, bromide and iodide of caesium, which have the CsCl structure (8 8 coordination). The plots shown in Figure 3.3 show a general decrease in the negative value of as the cation radius... 

Cesium, chloride (CsCl) structure (Fig. 4-H)- The CsCl structure can be described as interpenetrating simple cubic arrays of Cs+ and CP. Again, the Cs+ and CP positions are fully interchangeable. The structure is sometimes wrongly called body-centered cubic (bcc). The terminology is appropriate only when the shaded and unshaded atoms of Fig. 4.11 are identical, as in Fig. 4.8. In any case, the coordination number is eight for any atom. The unit cell of CsCl contains one net CsCl unit. 

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