Frenkel defect
Discovered by a soviet physicist Yakov Frenkel.
Frenkel defect (also known as the Frenkel pair/disorder) could be defined as:
When an atom or smaller ion (generally a cation) leaves a place its place in the lattice, creating a vacancy and becomes an interstitial by lodging in a nearby location. Thus, a vacancy is created in the lattice. It is a type of point defect which is also known as dislocation defect.
Or
Could also be defined as a defect in the lattice crystal where an atom or ion occupies a normally vacant site other than its own. As a result, the atom or ion leaves its own lattice site vacant. The cation leaves its own lattice site open and places itself between the area of all the other cations and anions. This defect is only possible if the cations are smaller in size when compared to the anions.
Example of a Frenkel defect
The Frenkel defect within the NaCl structure.
Frenkel defects occur in ionic crystals, which sizes of ions (anions and cations) show large differences. For example, AgI, Zns, NaCl and furthermore.
The number of Frenkel defects can be calculated using this equation:
Nf = Sqrt(NN*) *e^(ΔH/2RT)
Nf = Number of Frenkel defects per unit volume
N = number of lattice sites
N* = number of interstitial sites available.
ΔH = enthalpy formations of one Frenkel defect
R = gas constant
T = temperature
Frenkel defects are intrinsic defects because the existence causes the Gibbs energy of a crystal to decrease, which means it’s favourable to occur.
Frenkel defect does not have any impact on density, mass or volume of the crystals as in this defect migration of ions takes place within the crystal. So density, mass and volume of the ionic crystal remain the same even after the Frenkel defect.
Schottky defect
When oppositely charged ions leave their sites. This leads to point defects.
Schottky defect is one such point defect which is observed in various crystals.
Named after a German physicist called Walter H. Schottky, this defect occurs commonly in ionic crystals where the size of cations and anions is similar. For example, KCl. Potassium (K) has an atomic number of 19 and chlorine (Cl) has an atomic number of 17. Both the ions are of a similar size.
Characteristics of Schottky defects.
Usually occurs when heat is applied to the ionic compound crystals.
- Heat raises the temperature, and hence the thermal within the crystal.
- This creates gaps in the crystal pattern.
- The gaps are created with stoichiometric ratio, i.e., as per like the availability of ions in chemical compounds. For example, in a generic ionic compound with the formula Xn Ym, ‘n’ ions of X and ‘m’ ions of Y will leave to create vacancies.
- A group of vacancies can be referred to as a Schottky cluster.
Reduces the density of ionic compounds because a friction of ions leaves the crystals, hence reduces the overall mass at the same crystal volume.
Concentration of defects.
Schottky defects are formed by applying heat.
At any given temperature, there is a concentration of defects (i.e., Schottky defects per unit volume) given by the following formula:
ns = Ne^ (- ΔHS/2KT)
ns = number of Schottky defects per unit volume at temperature T (in kelvins) in a crystal with N anion and N cations per unit volume and ΔHs is the enthalpy for creating one defect.
Difference between Schottky defect and Frenkel defect.