Chemical and Structural Defects in Crystalline Solids

Introduction to Chemical and Structural Defects in Crystalline Solids 

Morris E. Fine Department of Materials Science Northwestern University Evanston, Illinois 

The understanding that masses of most solid substances are assemblages of a large number of crystals or grains is quite old. It was clearly stated by Savart in 1829. This led to the first crystal defect to be recognized namely the grain boundary or interface between two differently oriented grains of the same phase in a piece of solid material. We will return to this subject later. Each type of defect will be mentioned in this introduction and then be discussed in detail later in this chapter. 

Soon after Von Laue s discovery of X-ray diffraction Darwin in 1914 realized that crystals themselves were not perfect, because the measured intensities of the diffracted beams from rocksalt crystals were substantially greater than the theoretical values. Darwin  

The idea of point defects in crystals goes back to Frenkel, who in 1926 proposed the existence of point defects to explain the observed values of ionic conductivity in crystalline solids. In a crystal of composition MX such as a monovalent metal halide or a divalent metal oxide or sulfide, volume ionic conductivity occurs by motion of positive or negative ions in the lattice under the influence of an electric field. If the crystal were perfect, imperfections, such as vacant lattice sites or interstitial atoms, would need to be created for ionic conductivity to occur. A great deal of energy is required to dislodge an ion from its normal lattice position and thus the current in perfect crystals would be very, very small under normal voltages. To get around this difficulty, Frenkel proposed that point defects existed in the lattice prior to the application of the electric field. This, of course, has been substantiated by subsequent work and the concept of point defects in all classes of solids, metals, ionic crystals, covalent crystals, semiconductors, etc., is an important part of the physics and chemistry of crystalline solids, not only with respect to ionic conductivity but also with respect to diffusion, radiation damage, creep, and many other properties. 

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