The Frenkel defect (also known as the Frenkel pair or disorder) is a lattice crystal defect in which an atom or ion occupies a typically unoccupied space that is not its own. As a result, the atom or ion abandons its own lattice location. The defect has a tetravalent cation with four positively charged particles that do not belong to any particular chemical element.
It was first discovered by Lev Frenkel in 1902 when he examined the structure of ice. He found that every water molecule is surrounded by four other water molecules, indicating that there were missing oxygen atoms. He also found that each of these vacant sites was occupied by a small ionic particle called a "disorder point". Thus, a crystal containing such defects is said to be disordered.
These defects can have significant effects on the properties of crystals. For example, they can act as shallow donors or acceptors of electrons, so that doped crystals are conductive. They may also cause stress-induced phase transitions, such as those from rhombohedral to monoclinic symmetry. Finally, Frenkel defects can be responsible for some physical phenomena such as the blue color of cristobalite.
There are two main types of Frenkel defects: vacancy and interstitial. A vacancy is an empty site in the crystal lattice caused by the absence of an atom or group of atoms within the crystal.
A Frenkel defect occurs when one of the ions moves out of its lattice location and into an interstitial void, leaving a vacancy at the lattice site. Because the ion is dislodged from its lattice position, the Frenkel defect is also known as the dislocation defect. Ion vacancies are common in many materials, so this type of defect can occur in a variety of substances.
Frenkel defects can be divided into two categories based on their formation mechanism: intrinsic and extrinsic. Intrinsic Frenkel defects arise from the random motion of atoms during the solidification of a material, while extrinsic Frenkel defects are introduced to a material by other elements or compounds present in the environment. For example, oxygen vacancies are commonly found in metals such as iron, and these vacancies can be responsible for increasing the corrosion resistance of the metal. Similarly, fluorine additions are used in some plastics to improve their chemical resistance. Frenkel defects have been observed in many materials, including semiconductors, ceramics, and polymers. Their presence indicates that there is significant plastic deformation during mechanical testing, which means that the material has potential applications in devices where high stress levels are required.
Frenkel defects can be identified with electron microscopy because of their irregular shape. The edge of a Frenkel defect is called a kink, and it appears as a region of reduced atomic spacing.
A substantial disparity in the size of cations and anions favors the Frenkel defect. A Frenkel defect occurs when an ion is absent from its regular place and occupies an interstitial site between the lattice points. As a result, it is a dislocation defect. Ion vacancies are often associated with such defects.
Ion vacancies are common in many substances used in industry, for example, silica (silicon dioxide), alumina (aluminum oxide), and magnesia (magnesium oxide). These substances are important components of glass, ceramics, and refractory materials. They can also exist as isolated vacancies in some metals such as aluminum and magnesium. Ions can also be incorporated into the crystal structure through substitution by other elements with larger ions. For example, oxygen atoms can be replaced by fluorine or nitrogen atoms to form oxides that do not decompose even at high temperatures. Such compounds include silica, alumina, and magnesia again but this time they are found in dust particles from power plants, factories, and vehicles. Incorporation of foreign atoms into the crystal structure changes the properties of the material. For example, silicon dioxide has different properties than pure oxygen gas.
Ion vacancies can be either intrinsic or extrinsic. Intrinsic ion vacancies are present in the original material and do not affect its properties.
A Frenkel defect develops in ionic crystals with substantial discrepancies in the sizes of ions (anions and cations). Compounds such as KCl, KBr, CsCl, and others Compounds such as NaCl, ZnS, AgI, and others exhibit the Frenkel defect. The size discrepancy can be due to differences in charge or to variations in the masses of anion and cation atoms. Ionic compounds containing large anions or small cations tend to have greater density fluctuations than those with equal-sized anions and cations. These density fluctuations result in some Anions being more isolated from their neighbors than others, which leads to increased anion–anion and anion–cation distances.
Frenkel defects can also appear in non-ionic crystals such as CaF2 and SrF2. Here, the defect arises because calcium and strontium have different valences, which results in different radii. In these cases, the defect does not affect crystal stability but it does change the physical properties of the material. For example, calcium fluoride is less transparent than strontium fluoride and has fewer applications as a lens material.
Defects can also appear in polytypes of crystals. For example, there is a mixed layer of silicon and oxygen atoms near the surface of single-crystal silicon that causes the surface to be less than perfectly smooth.
Ionic crystals with an anion bigger than a cation are seen in the Frenkel defect. An anion and a cation both exit the solid crystal. The solid's density drops. The density remains constant. The defect is not visible under light microscopy.
We must now examine the distinctions between the two flaws.
A Schottky defect develops in ionic crystals with a minor variation in size between the cation and anion. The Frenkel defect is most common in ionic crystals where the anion size is much larger than the cation size. In this case, there is an empty space inside the anion where mobile positive charges can move around, creating a weak point in the crystal structure.
Mobile positive charges are called vacancies and they can come from two sources: interstitials or impurities. Interstitials are atoms that have been removed from their normal location in the crystal lattice and reside at some other position for the length of time it takes for them to find another site where they can fit properly. Common interstitials include oxygen, nitrogen, and silicon. Impurities are atoms that are present in smaller amounts than those found in the pure element and can still contribute vacancies. For example, iron atoms have 30 electrons and so they can leave vacant sites in the crystal structure when they reach their equilibrium position. These vacancies can be filled by other elements or ions if they are available nearby. For example, if calcium atoms arrive before the iron atoms then the vacancies will be filled by the calcium atoms.
Schottky defects can occur in many materials including semiconductors, insulators, and metals. They can change the physical properties of the material by providing points of weakness that could lead to fracture if enough stress is applied.