Schottky Defect

Introduction
Like the human body, lattice structures (most commonly known as crystals) are far from perfection. Our body works hard to keep things proportional but occasionally our right foot is bigger than our left; similarly, crystals may try to arrange it's ions under a strict layout, but occasionally an ion slips to another spot or simply goes missing. Realistically speaking, it should be expected that crystals will depart itself from order (not surprising considering defects occurs at temperature greater than 0 K). There are
many ways a crystal can depart itself from order (thus experiences defects); these defects can be grouped in different categories such as Point Defects, Line Defects, Planar Defects, or Volume or Bulk Defects [2]. We will focus on Point Defects, specifically the defect that occurs in ionic crystal structures (i.e. NaCl) called the Schottky Defect.
Point Defects

Lattice structures (or crystals) undergoing point defects experience one of two types:
1.) atoms or ions leaving their spot (thus creating vacancies) [3].
2.) atoms or ions slipping into the little gaps in between other atoms or ions; those little gaps are known as interstitials--since atoms or ions in the crystals are occupying interstitials, they inherently become (create) interstitials [3].
By the simplest definition, the Schottky defect is defined by type one. For anyone interested, type two is known as the Frenkel defect.Schottky Defect - ChemWiki:

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Point Defects in Ionic Crystals

Even for the most simple ionic crystals of the type A+B– like LiCl or NaCl, we can, in principle, produce arbitrary concentrations of two kinds of vacancies and two kinds of interstitials as shown on the left.

However, as we already learned in dealing with Schottky defects, global charge neutrality must be maintained. Arbitrary concentrations are thus not really allowed, we must demand that the the sum of the positively charged defects equals the sum of the negatively charged defects. In other words: we have to obey the charge conservation law.
If we also keep the number of atoms constant, we must add an A or B atom to the surface of the crystal for every pure vacancy we produce. In other (fancy) words, we have to obey the mass conservation law.

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solid defects

What are crystal defects and how are they classfied?

How do impurities affect the structure and properties of a solid?

What are color centers and how do they affect electric conductivity of solids?

Solid defects

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 imperfection of crystals are called crystal defects.

Crystal defects are results of thermaldynamic equilibrium contributed also by the increase in entropy T S term of the Gibb's free energy:

DG = DH - TDS.

Only at the unatainable absolute zero K will a crystal be perfect, in other words, no crystals are absolutely perfect. However, the degree of imperfection vary from compound to compound.

On the other hand, some solid-like structure called flickering clusters also exist in a liquid. For example, the density of water is the highest at 277 K. The flickering clusters increase as temperature drops below 277 K, and the water density decreases as a result.

Solids are important for many technologies, and this link gives a well organized presentation.

What are crystal defects and how are they classfied?

As mentioned earlier, 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.

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