Electrical conductivity in ionic solids

At temperatures above OK, the thermodynamic balance favours the presence of defects, and the minimum value of AG is attained at a given equilibrium concentration of defects, the concentration being temperature-dependent. 

The presence of defects in a crystal lattice facilitates ion migration and enhances electrical conductivity (i.e. lowers the resistance). 

Anion migration could also occur by the first mechanism, but for the second, it is usually the cation that is small enough to occupy an interstitial site, for example, the tetrahedral holes in an NaCl-type structure. 

The conductivity of a fast-ion conductor typically lies in the range 1(T to 10 m  

For an ionic solid to be a fast-ion conductor, it must meet some or all of the following criteria  

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A variety of techniques has been employed to investigate aliovalent impurity-cation vacancy pairs and other point defects in ionic solids. Dielectric relaxation, optical absorption and emission spectroscopy, and ionic thermocurrent measurements have been most valuable ESR studies of Mn " in NaCl have shown the presence of impurity-vacancy pairs of at least five different symmetries. The techniques that have provided a wealth of information on the energies of migration, formation and other defect energies in ionic solids are diffusion and electrical conductivity measurements. Electrical conductivity in ionic solids occurs by the motion of ions through vacancies or of interstitial ions. In the case of motion through vacancies, the conductivity, a, is given by... 

Many ionic crystals have an appreciable electrical conductivity in the solid state, due to the motion of anions or cations or both. We have already given an extreme example of this effect in the case of the high-temperature form of silver iodide ( 8.05), where the silver atoms are... 

Figure 12.4 Electrical conductivity, (a) Ionic solids do not conduct electrical current but (b) are good conductors when melted or dissolved in water. 

In ionic solids, electrons are held in place around the ions so they don t conduct electricity. However, in aqueous solution and molten state, they do conduct electricity. Electrical conductance of ionic compounds is not due to movement of electrons but to the movement of ions. 

Electrical conductance in solids (other than ionic conductors) depends on the availability of delocalised orbitals close enough together in energy to form bands. 

I. Ionic compounds tend to have very low electrical conductivities as solids but conduct electricity quite well when molten. This conductivity is attributed to the presence of ions, atoms charged either positively or negatively, which are free to move under the influence of an electric field. In the solid, the ions are... 

Following the introduction of basic kinetic concepts, some common kinetic situations will be discussed. These will be referred to repeatedly in later chapters and include 1) diffusion, particularly chemical diffusion in different solids (metals, semiconductors, mixed conductors, ionic crystals), 2) electrical conduction in solids (giving special attention to inhomogeneous systems), 3) matter transport across phase boundaries, in particular in electrochemical systems (solid electrode/solicl electrolyte), and 4) relaxation of structure elements. 

We begin our discussion by characterizing the electrical conduction in solid electrolytes. These are solids with predominantly ionic transference, at least over a certain range of their component activities. This means that the electronic transference number, defined as... 

Another troublesome borderline area is that between ionic solids and three-dimensional polymers. The distinction cannot be made from the structure alone. Electrical conductivity in the molten state does not, as already mentioned, necessarily demonstrate the presence of ions in the solid state and such a test is inapplicable where, as often happens, the substance sublimes or decomposes before melting. There can rarely be any objective means of assigning a compound to one category or the other. We are often persuaded towards one description on aesthetic grounds. For example, the structure of sodium chloride cannot easily be rendered in terms of localised, electron-pair bonds (but this is true also of many unequivocally covalent compounds). Its structure is eminently plausible for an array of cations and anions, however. 

Haering, C., A. Roosen, H. Schichl, M. Schnoller (2005), Degradation of the Electrical Conductivity in Stabilized Zirconia System Part II Scandia-stabilised Zirconia , Solid Sate Ionics, Vol. 176, No. 3-4,... 

The transformation of /3-AgI to a-Agl is accompanied by a dramatic increase in the ionic electrical conductivity of the solid, which leaps by a factor of nearly 4000from3.4 x 10 4to 1.3 ohm- em-1. This arises because in /1-AgItheAg... 

Electrical conductivity of metals is very high and is of the order of 106 108 ohm-1 cm-1 while that of insulators is of the order of 10-12 ohm-1 cm-1. Semi-conductors have intermediate conductivity which lies in the range 102 10-9 ohm-1 cm1. Electrical conductivity of solids may arise through the motion of electrons and positive holes (electronic conductivity) or through the motion of ions (ionic conductivity). The conduction through electrons is called n-type conduction and through positive holes is called p-type conduction. Pure ionic solids where conduction can take place only through motion of ions are insulators. However, the presence of defects in the crystal structure increases their conductivity. 

The phenomenological equation for electrical conduction in an ionic solid is the same as that for electronic conduction, as shown in Eq. 6.21. However, the universal expression for electrical conductivity (Eq. 6.23) must be modified to include the transport contribution by all the species, including cations, anions, and electrons ... 

Subsequent findings that even conventional ionic solids, such as sodium chloride, have measurable conductivities that are not electronic stimulated the development of theories for ionic motion in solids. Early in this century, Ioffe introduced the concept of interstitial ions and vacancies (see Defects in Solids), which was the starting point of the theory of defects. Frenkel and Schottky used these theories to develop their classic mechanisms to explain how electricity can be conducted through ionic solids by the flow of ions (see Frenkel Defects, Schottky Defects) They proposed that ionic solids are not perfect, with every lattice site occupied by its appropriate ions, but contain defects in which either ions... 

Why do ionic crystals conduct electric current in the liquid phase or when dissolved in water but do not conduct electric current in the solid phase ... 

Charged particles must be free to move for a material to conduct an electric current. In the solid state, ionic compounds are nonconductors of electricity because of the fixed positions of the ions. However, in a liquid state or when dissolved in water, ionic compounds are electrical conductors because the ions are free to move. An ionic compound whose aqueous solution conducts an electric current is called an electrolyte. You will learn more about solutions of electrolytes in Chapter 15. 

Equations 22.7 and 22.8 are valid for all electrical conduction processes motion of positive and negative ions in solution and in ionic solids, and motion of electrons in solids. These equations are used in subsequent sections to discuss conduction in a variety of materials. These equations should remind you that the conductivity in any material depends on two separate microscopic parameters the number of charge carriers present and their mobilities. 

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