Imperfections, in solids

In practice, all crystals have imperfections. If a substance crystallizes rapidly, it is likely to have many more imperfections, because crystal growth starts at many sites almost simultaneously. Each small crystallite grows until it runs into its neighbors the boundaries between these small crystallites are called grain boundaries, which can be 

Vacancies are missing atoms and are the simplest defects. Because higher temperatures increase vibrational motion and expand a crystal, more vacancies are formed at higher temperatures. However, even near the melting point, the number of vacancies is small relative to the total number of atoms, about 1 in 10,000. The effect of a vacancy on the rest of the lattice is small, because it is a localized defect and the rest of the lattice remains unaffected. Self-interstitials are atoms displaced from their normal location and appear in one of the interstices in the lattice. Here, the distortion spreads at least a few layers in the crystal because the atoms are much larger than the available space. In most cases, the number of these defects is much smaller than the number of vacancies. 

Substitution of one atom for another is a common phenomenon. These mixtures are also called solid solutions. For example, nickel and copper atoms have similar sizes and electronegativities and the same fee crystal structures. Mixtures of the two are stable in any proportion, with random arrangement of the atoms in the alloys. Other combinations that can work well have a very small atom in a lattice of larger atoms. In this case, the small atom occupies one of the interstices in the larger lattice, with small effects on the rest of the lattice but potentially large effects on behavior of the mixture. If the impurity atoms are larger than the holes, lattice strains result and a new solid phase may be formed. 

Edge dislocations result when atoms in one layer do not match up precisely with those of the next. As a result, the distances between the dislocated atoms and atoms in adjacent rows are larger than usual and the angles between atoms are distorted for a number of rows on either side of the dislocation. A screw dislocation is one that has part of one layer shifted a fraction of a cell dimension. This kind of dislocation frequently causes a rapidly growing site during crystal growth and forms a helical path, which leads to the name. Because they provide sites that allow atoms from the solution or melt to fit into a corner where attractions from three directions can hold them in place, screw dislocations are frequently growth sites for crystals. 

In general, dislocations are undesirable in crystals. Mechanically, they can lead to weakness that can cause fracture. Electrically, they interfere with conduction of electrons and reduce reliability, reproducibility, and efficiency in semiconductor devices. For example, one of the challenges of photocell manufacture is to raise the efficiency of cells made of polycrystalline silicon to levels that are reached by single crystals. 

In practice, all crystals have imperfections. If a substance crystallizes rapidly, it is likely to have many more imperfections, because crystal growth starts at many sites almost simultaneously. Each small crystallite grows until it runs into its neighbors the boundaries between these small crystallites are called grain boundaries, which can be seen on microscopic examination of a polished surface. Slow crystal growth reduces the number of grain boundaries, because crystal growth starts from a smaller number of sites. However, even if a crystal appears to be perfect, it will likely have imperfections on an atomic level caused by impurities in the material or by dislocations within the lattice. 

Interstices refers to the spaces between adjacent atoms in a crystalline lattice. 

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In this section we will explore the use of the ion microscope for studying adsorption phenomena, with special emphasis on the ability to detect single gas atoms. So far the microscope has been applied principally to the examination of the surface structure of metals and of imperfections in solids. These, as well as other bulk phenomena, will not be touched upon. ... 

Various imperfections in solid-state structures include the Schottky and Frenkel defects. The latter can lead to nonstoichiometric compounds. Edge dislocations can make a metal more malleable. Examples include lead, white tin, and the iron of horseshoes. 

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