Defects in Ionic Crystals

Lattice defects in ionic crystals are interstitial ions and ion vacancies. In crystalline sodium chloride NaCl a cation vacancy Vn - is formed by producing a surface cation NaJ, (Nal - NaJ + Vua ) this is called the Schottky defect. On the other hand, in crystalline silver chloride AgCl a pair of cation vacancy Va,. and interstitial cation Ag is formed, (Ag - Agj + ) this is called the Frenkel... 

Barr L. W. and Liliard A. B. (1971). Defects in ionic crystals. In Physical Chemistry An Advanced Treatise, vol. 10, W. lost (series ed.). 

Let us summarize by modeling the velocity autocorrelation function using Debye-Huckel type interactions between charged point defects in ionic crystals, one can evaluate the frequency-dependent conductivity and give an interpretation of the universal dielectric response. 

I.K. Vitol and U.T. Rogulis, Thermoactivated Spectroscopy of Defects in Ionic Crystals (Latv. Univ. Press, Riga, 1983) p. 83. 

One of the causes of point defects is a temperature increase which results in an increased thermal movement of the atoms which can subsequently lead to the atoms escaping from their place in the lattice. Other causes are the effects of radiation and inbuilt, foreign atoms. In an atomic lattice a vacancy can occur due to the movement of an atom, an absence of an atom or molecule from a point which it would normally occupy in a crystal. In addition to this vacancy an atomic will form elsewhere. This combination of an atomic pair and a vacancy is called the Frenkel defect. In ionic crystals an anion and a cation have to leave the lattice simultaneously due to the charge balance. As a result a vacancy pair remains and this is called the Schottky defect. Both defects can be seen in figure 4.8. 

Point defects in ionic crystals are always paired to preserve electrical neutrality. There are several types of defect pairs ... 

There is a useful notation system for defects in ionic crystals. Either the chemical symbol for the element or V for a vacancy indicates what is on a lattice site. A subscript of either the chemical symbol for the element or i if it is an interstial site indicates what is normally on the site. The charge, relative to the... 

FIGURE 5.21 Frenkel point defect in ionic crystals. 

M. J. Norgett, AERE Harwell Report R7650, Atomic Energy Research Establishment, 1974. A General Formulation of the Problem of Calculating the Energies of Lattice Defects in Ionic Crystals. 

Apart from the point defects, there are impurity defects in ionic crystals due to some impurities in raw materials. The impact of impurity segregation on ionic conductivity of the solid electrolytes will be considered in detail in section 1.4 of this chapter. The vacancies, developed in the solid solutions during the substitution of the main ion (M in the solid solution M(Mi)02 x) by the ion substituent (Mj) of the different valence, have special meaning for solid electrolytes among impurity defects. In this case, the vacancies must appear from one of the solid-state sublattices... 

R. S. Eachus, R. Janes, and M. T. Olm, Papers Summaries, in Fifth Europhysical Topical Conference on Lattice Defects in Ionic Crystals, Madrid, 1986, p. 259. 

Stoichiometric Defects. There are some defects that leave the stoichiometry unaffected. One type is the Schottky defect in ionic crystals. A Schottky defect consists of vacant cation and anion sites in numbers proportional to the stoichiometry thus, there are equal numbers of Na+ and Cl- vacancies in NaCl and 2C1- vacancies per Ca2+ vacancy in CaCl2. A Schottky defect in NaCl is illustrated in Fig. 2-14(a). 

It is possible to imagine a defect in ionic crystals similar to the interstitial defects described above. Such defects are known as Frenkel defects. In this case, an ion from one sublattice moves to a normally empty place in the crystal, leaving a vacancy behind. One Frenkel defect consists of an interstitial ion plus a vacancy (Figure 3.15b). Because the total number of ions present does not change, there is no... 

Leslie, M. (1985) A three-body potential model for the static simulation of defects in ionic crystals, Physica 131B, 145-150. 

There are two typical intrinsic defects in ionic crystals (i) Schottky defect and (ii) Frenkel defect... 

Defects in Ionic Crystals Calculated by the Supercell Method. 

All statements which have been made up to now also apply for ionic crystals. At first sight it would seem that we do not have the prerequisites necessary for a definition of the chemical potentials of the defects in ionic crystals relative to convenient standard states, since firstly the electroneutrality of the crystal must be preserved, and secondly the proper relationships between the number of crystallographic different lattice sites must be preserved. How-... 

Two of the more common types of intrinsic defects in ionic crystals are the Schottky defect and the Frenkel defect. 

There is considerable evidence for the existence of space charge layers near lattice defects in ionic crystals. The experimental work has principally been concerned with the alkali and silver halides. With the alkali halides the experiments have mainly been aimed at determining the charge distribution near dislocations and grain boundaries, while the work on the silver halides arises from their use as photographic materials, although... 

DUPUY Why don t you mention specially dislocations which are important sources of electrostatic defects in ionic crystals (i.e. surrounding space charge...)... 

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