CHECK YOUR PROGRESS : THE KEY

1. Point defects can be divided into Frenkel defects and Schottky defects, and these often occur in ionic crystals. The former are due to misplacement of ions and vacancies. Charges are balanced in the whole crystal despite the presence of interstitial or extra ions and vacancies. On the other hand, when only vacancies of cation and anions are present with no interstial or misplaced ions, the defects are called Schottky defects.

2. Few, if any, crystals are perfect in that all unit cells consist of the ideal arrangement of atoms or molecules and all cells line up in a three dimensional space with no distortion. Some cells may have one or more atoms less whereas others may have one or more atoms than the ideal unit cell. The imperfections of crystals are called crystal defects. The missing and lacking of atoms or ions in an ideal or imaginary crystal structure or lattice and the misalignment of unit cells in real crystals are called crystal defects or solid defects. The two terms are interchangeable. Crystal defects occur as points, along lines, or in the form of a surface, and they are called point, line, or plane defects respectively.

Defects in crystals and Elements of Thin Films

  3. Diffusion is a process where material is transported by atomic motion. One of the

simplest forms of diffusion , "diffusion bonding", occurs when two materials come in contact with each other. There are two basic mechanisms for diffusion: Vacancy diffusion and Interstitial diffusion. The reason for the two types of diffusion stems from the relationship between their relative atomic sizes. Vacancy diffusion occurs primarily when the diffusing atoms are of a similar size, or substitutional atoms. The movement of a substitutional atom requires a vacancy in the lattice for it to move into. Interstitial diffusion occurs when the diffusing atom is small enough to move between the atoms in the lattice. This type of diffusion requires no vacancy defects in order to operate. Diffusion depends on five main variables. These variables are: initial concentration, surface concentration, diffusivity, time, and temperature. The initial concentration of the material that is diffusing into the base ("solid") material is often referred to as the "solute" or impurity which can be but is not always zero. The surface concentration is the amount, weight %, of solute near the surface of the "solid", base, material. Diffusivity is defined as the rate at which the solute or impurity penetrates into the solid, base, material. The time and temperature are fairly self-explanatory, though it should be noted that the temperature increases the rate of diffusion with increasing temperature

  4. A point lattice defect, which produces optical absorption bands in an otherwise                   transparent crystal.

Color centers are imperfections in crystals that cause color (defects that cause color by absorption of light). Due to defects, metal oxides may also act as semiconductors, because there are many different types of electron traps. Electrons in defect region only absorb light at certain range of wavelength. The color seen are due to lights not absorbed. For example, a diamond with C vacancies (missing carbon atoms) absorbs light, and these centers give green color as shown here. Replacement of Al³⁺ for Si⁴⁺ in quartz give rise to the color of smoky quartz.

Defect in Crystal

A high temperature phase of ZnO_x, (x < 1), has electrons in place of the O^2- vacancies. These electrons are color centers, often referred to as F-centers (from the German word farben meaning color). Similarly, heating of ZnS to 773 K causes a loss of sulfur, and these material fluoresces strongly in ultraviolet light.

Some non-stoichiometric solids are engineered to be n-type or p-type semiconductors. Nickel oxide NiO gain oxygen on heating in air, resulting in having Ni³⁺ sites acting as electron trap, a p-type semiconductor. On the other hand, ZnO lose oxygen on heating, and the excess Zn metal atoms in the sample are ready to give electrons. The solid is an n-type semiconductor.

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