Stability of ionic solids

Ion size plays an important role in determining the structure and stability of ionic solids, the properties of ions in aqueous solution, and the biological ef-... 

Ion size plays an important role in determining the structure and stability of ionic solids, the properties of ions in aqueous solution, and the biologic effects of ions. As with atoms, it is impossible to define precisely the sizes of ions. Most often, ionic radii are determined from the measured distances between ion centers in ionic compounds. This method, of course, involves an assumption about how the distance should be divided up between the two ions. Thus you will note considerable disagreement among ionic sizes given in various sources. Here we are mainly interested in trends and will be less concerned with absolute ion sizes. 

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

Structure at high temperatures. A distorted perovskite would be expected to transform to the cubic structure at high temperatures. The Born model of ionic solids with the appropriate repulsive and van der Waals parameters can explain the relative stabilities of crystal structures in partly covalent solids, an ionicity parameter would have to be used to predict the preferred crystal structure (see Chapter 1, Section 1.3). 

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. 

The value of (8bl + 6b2) thus calculated is 76867 x 10-12 ergs mol-1. The A /ss of solid solutions of CsCl and CsBr were evaluated (eqn. (1)) for different values of b2 (in the range 0-900 x 10-12 erg mol-1) using the r0 values from table 2. The results are shown diagramatically in fig. 3, the values of b, and b2 are 9257 x 10-12 and 468 x 10-12 erg mol-1 respectively when (6b2/%b1 + 6b2)csci (6b2l%bi + 6b2)csB,- The AUs s remains nearly constant with the % CsBr when b2 is between 400 and 500 x 10-12 erg mol-1. Thus, curves 3 and 4 of fig. 3 closely represent the experimentally observed situation. Thus, the Born model of ionic solids describes the relative stabilities of structures of alkali halides in widely different situations as manifested here by the CsCl+KCl and CsCl + CsBr systems. 

Knotek and Feibelman [94] examined the modification to a surface when exposed to ionising radiation and assesed the damage that can be produced. They addressed the stability of ionically bonded surfaces, where the KF mechanism applies, and concluded that Auger induced decomposition only occurs when the cation in the solid is ionised to relatively deep core levels. In the case of non-maximal oxides as with NiO, Freund s group [95] showed that whilst desorption of neutral NO and CO from NiO(lOO) and (111) surfaces has thresholds at the C Is, N Is and O Is core levels, it proceeds mainly on the basis of the MGR model, involving an excited state of the adsorbate. An overview of electronic desorption presented by Feibelman in 1983 [96] examined particularly the stability of the multiple-hole final state configuration leading to desorption. The presence of multiple holes, and associated hole-hole correlation... 

The creation and destruction of charged interfaces between ionic, electronic, and dielectric materials is a central problem where electrochemical principles should be brought to bear. Phenomena embraced in this area include deposition and dissolution, growth of dendrites, bubble evolution, wetting, sintering of ionic solids and ceramic powders, and phase stability. 

The stability of the solid polyhalide depends on numerous factors, among them the size of the cation, the size and the nature of the polyhalide ion, and the chemical resistance of the compound to atmospheric moisture. The most stable salts are formed when the ionic sizes of the cation and the anion are similar. In the alkali metal series the stability of the polyhalides decreases in the order Cs > Rb > NH4 > K > Na, which corresponds to the order of decrease in cationic size. Not enough work has, as yet, been done on the substituted onium poly halides to allow any convincing generalizations. 

Sveijensky DA, Sahai N (1996) Theoretical prediction of single-site surface protonation equilibrium constants for oxides and silicates in water. Geochim Cosmochim Acta 60 3773-3797 Tasker PW (1979) Stability of ionic crystal surfaces. J Physics C Solid State Physics 12 4977-4984 Toukan K, Rahman A (1985) Molecular-dynamics study of atomic motions in water. Phys Rev B- Con Mat 31 2643-2648... 

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