Transformations CsCl-NaCl

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 ... 

Although the structure of CsCl is quite different from that of NaCl, even CsCl can be transformed into the sodium chloride structure when heated to temperatures above 445 °C. Some of the other alkali halides that do not have the sodium chloride structure under ambient conditions are converted to that structure when subjected to high pressure. Many solid materials exhibit this type of polymorphism, which depends on the external conditions. Conversion of a material from one structure to another is known as a phase transition. 

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.)...

Figure 2. Dialatation transformation from CsCl structure to NaCl structure of an AB-type compound. Symmetry about the the unique axis of dilatation (3m) is preserved.

Pniim-Fniim transformations of solid solutions of CsCl with KC1 and CsBr exhibit different behaviours. With increasing percentages of KC1, the NaCl structure gets stabilized in the CsCl+KCl system. In the CsCl+CsBr system, the transformation temperature increases with % CsBr and AH essentially remains constant. Both these behaviours can be satisfactorily explained in terms of the Born treatment of ionic solids. The Pmlm-Fniim transformation retains its first-order characteristics in the CsCl+KCl system, but higher-order components seem to be present in the CsCl+CsBr system. Incorporation of vacancies do not affect the transformation of CsCl markedly. 

We have studied the transformations of the CsCl + KCl and CsCl + CsBr solid solutions in order to find the limitations and applicability of the Born treatment in explaining the two entirely different behaviours of the solid solutions of these two systems. Such a study is of value since theoretical approaches to explain the relative stabilities of structures of ionic solids have not been quite successful, and it is important to explain the relative stabilities of at least the two simplest structure types in ionic solids, viz., the NaCl and CsCl structures. We also wished to find out whether the first order characteristics of Pm3m-Fm3m transitions are retained in the solid solutions. We have therefore examined the crystallography of the Pm3m and Fm3m phases of the solid solutions as functions of temperature from these data, coefficients of expansion of the two structures have been calculated. 

Ammonium chloride, NH4CI, crystallizes in a CsCl lattice up to a temperature of 174-3 0 and above that temperature, in the NaCl lattice. Similar behaviour is observed with ammonium bromide (temperature of transformation 137-8 C) and ammonium iodide (temperature of transformation — /I7-6 C). The interionic distances in the CsGl lattice are actually found to be approximately 3 per cent greater than in the NaGl... 

FIGURE 15.15. The transformation of (a) a CsCl-type structure to (b) an NaCl-type structure. This occurs at 465° C when the arrangement of ions found in CsCl crystals distorts to give the arrangement of ions found in NaCl crystals. A.H — 2.90 kJ mol and AF = 17% (Ref. 100). The filled circles represent Cs ions. 

At room temperature and pressure Rbl crystallizes with the NaCl-type structure, (a) Use ionic radh to predict the length of the cubic unit cell edge, (b) Use this ue to estimate the density, (c) At high pressure the structure transforms to one with a CsCl-type structure, (c) Use ionic radii to predict the length of the cubic unit cell edge for the high-pressure form of Rbl. (d) Use this value to estimate the density. How does this density compare with the density you calculated in part (b) ... 

A Dilatational phase transition is a special displacive transformation in which there is a coordination change. The symmetry is not necessarily increased at the higher-temperature side of the transition point. The transition of the CsCl structure to the NaCl structure is one example. These transitions are rapid and may have high latent heats. 

Table 10.1 lists the pressures of phase transitions in MX compounds from the ZnS structure type (Ac = 4) to the NaCl or NiAs (Ac = 6), and into the CsCl (Ac = 8) types. This table shows that the pressures of phase transitions (Ttr) fall with the increase of ionic radii of both the cations (which agrees with the ionic model) and anions (which contradicts it). This contradiction disappears, if we take into account the polar character of M-X bonding the increase of the bond covalency in the successions MF MI or MO -> MTe rises r(M+) and reduces r(X ), thus increasing the kr = r+lr- ratio and diminishes Ftr- In alkali hydrides [18, 19] the pressure of the structural transformation from the NaCl into the CsCl type decreases with the increase of the cation size, viz. 29.3 GPa in NaH, 4.0 in KH, 2.7 in RbH and 1.2 in CsH. In CsH one more phase transition was discovered at 17.5 GPa [70] whereupon the CsCl structure converts into the CrB type this phase is stable up to 253 GPa and its compressibility at the this pressure is the highest for the MX compounds. Wo = 0.26. 

Polymorphic transformations into the structure with Ac > 8 were observed in cesium halides [71-74] atfollowing pressures CsCl 65, CsBr 53, Csl 39 GPa besides, Csl at P > 200 GPa undergoes a smooth second-order transition into a hep structure with Ac = 12. Very peculiar transitions were observed in samarium chalcogenides SmX under high pressures the electron transition Sm(II) Sm(III) takes place with a sharp isomorphic decrease in the lattice parameters. Further compression leads to the transformation of the NaCl CsCl type. Values of pressures at the isomorphic (Pitr) and polymorphic (Pptr) transitions according to [75, 76] are following ... 

Now let us consider specific features of these transformations. The pressure of phase transition, Pn, depends on the direction in which the shock wave front propagates through a crystal. Thus, the transitions from a NaCl to a CsCl type structure occur at lower pressures if the crystallographic axis < 111 > is normal to the shock wave front, i.e. the unit cell is compressed along its spacial diagonal, because this is... 

Zahn D, Leoni S (2004) Mechanism of the pressure induced reconstructive transformation of KCl from the NaCl type to the CsCl type structure. Z Krist 219 345-347... 

Sm VSm Transformation in Chloride Melts Stability of samarium ions (Sm % Sm " ) in the alkaline chloride melts changes as functions of the solvent salt cations and temperature [4]. Sm " exhibits a higher stability for a larger solvent salt cation and lower temperature. Electrochemical reduction of Sm " into Sm in KQ-NaCl-CsCl melt at an inert cathode has been found to occur in two steps as shown in Eqs. 12 and 13. And the reduction of Sm " to Sm° takes place at near the decomposition potential of the supporting electrolyte. In addition, Sm " losing one electron to form Sm " takes place at the anode in terms of reaction Eq. 14, making Sm " Sm /transformation at the electrodes therefore, this process can circulate in the cathode and anode, and therefore nearly no Sm metal can be obtained at the cathode, resulting an extremely low current efficiency. This is the rea-sOTi why samarium caimot be produced from the chloride melts by molten salt electrolysis. It is reported that when the concentration of Sm " ions reach 0.1 wt% in the chloride melts, the current efficiency will be substantially decreased. Eu " behaves in nearly the same manner as Sm " in the chloride melts. 

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