Tag: solid state

The addition of small amount of foreign impurity in the host crystal is called as doping. It results in an increase in the electrical conductivity of the crystal. Doping of group 14 elements (such as Si, Ge etc.) with elements of group 15 (such as As) produces an excess of electrons in the crystals, thus, giving n-type semiconductors. Doping of groups 14 elements with group 13 elements (such as Indium) produces holes (electron deficiency) in the crystals. Thus, p-type semiconductors are produced. The symbol 'p' indicates flow of positive charge. Positive holes are responsible for the semiconducting properties like conduction.

A compound may have excess metal ion if an anion (negative ion) is absent from its appropriate lattice site creating a 'void' which is occupied by an electron. The ionic crystal which is likely to possess Schottky defect may also develop this type of metal excess defect. When alkali metal halides are heated in an atmosphere of vapours of the alkali metal, anion vacancies are created. The anions (halide ions) diffuse to the surface of the crystal from their appropriate lattice sites to combine with the newly generated metal cations. The electron lost by the metal atom diffuse through the crystal is known as F-centres. The main consequence of a metal excess defect in the development of colour in the crystal. For example, when NaCl crystal is heated in an atmosphere of Na vapours, it becomes yellow. Similarly, KCI crystal when heated in an atmosphere of potassium vapours, it appears violet.
The addition of a small amount of foreign impurity in the host crystal is called doping. It increases the electrical conductivity of the crystal. Doping of group 14 elements (such as Si, Ge etc.) with elements of group 15 (such as As) produces an excess of electrons in the crystals, thus, giving n-types semiconductors. Doping of groups 14 elements with group 13 elements (such as Indium) produces holes (electron deficiency) in the crystals. Thus, p-type semiconductors are produced. Then symbol 'p' indicates the flow of positive charge.
Paramagnetic (weakly magnetic): Such materials contain permanent magnetic dipoles due to the presence of atoms, ion or molecules with unpaired electrons.
Due to unpair electron of F-centers, solids containing F-centers (Farbe) are paramagnetic.

We know that resistance of a conductor is directly proportional to the temperature. So, when it is cooled below a certain temperature called critical temperature, the resistance tends to drop to zero and superconductivity can be observed.

Around $1993$, the highest temperature superconducting material was known. It has been a ceramic material consisting of $Hg, Ba, Ca, Cu$ and $O$ i.e, the compound was $HgBa _2Ca _2Cu _3O _2$

Intrinsic (pure) semiconductors are undoped semiconductors. The charge carriers i.e electrons and holes in this kind of semiconductor are equal in number. The electrical conductivity of these type of semiconductors is due to electron excitation.

Eg: $Si, \ Ge, \ etc…$

Silver halides show both Frenkel and Schotkky defects. For Frenkel defect the reason is that there is size difference between the sizes of silver and halide.

Schottky Defect: This defect occurs when oppositely charged ions leave their lattice site creating vacancies in such a way that electrical neutrality of crystal is maintained. It is generally seen in highly ionic compounds where a difference in size of cation and anion is small. 

So, an equal number of cations and anions vacancies are present in the crystals with Schottky Defect.

Frenkel defect is one in which atom is displaced from its lattice point to the interstitial site, creating a vacancy at the lattice point. Here, since dislocation of atom lattice point happens. So, it is also called as dislocation defect.

Given, sample of ferrous oxide is $Fe _{0.93}O _{1.00}$.

Let number of $Fe^{2+}$ ions is $x$ then number of $Fe^{3+}$ ions is $(0.93-x)$

Now, for electrical neutrality, $(+2)x + (+3)(0.93-x)=+2$

$\Rightarrow 2x+2.79 – 3x = 2$

$\Rightarrow x=0.79$

So, number of $Fe^{2+}$ ions $=0.79$

Number of $Fe^{3+}$ ions $=0.14$

So, fraction of $Fe^{2+}$ ions $= \cfrac {0.79}{0.93}=0.849$

$FeO$ has non stochiometric metal deficiency defect in which number of $Fe^{2+}$ ions are lesser than $O^-$ ions as compared to stochiometric formula. The neutrality is maintained to variable oxidation state possessing capability of $Fe$.