Imperfections in Solids

• Constituent particles in crystalline solids are arranged in a short range as well as long range
  order yet crystals are not perfect.
• They possess small crystals have defects that are irregularities in the arrangement of constituent
  particles in them when crystallization process occurs at fast or moderate rate.
• These defects are of two types- point defects and line defects.
• The irregularities or deviations from ideal arrangement around a point or an atom in a
  crystalline substance.
• The irregularities or deviations from ideal arrangement in entire rows of lattice points are known as line defects.
• These irregularities are called crystal defects.
• Point defects can be classified into three types: (i) stoichiometric defects
  (ii) impurity defects and (iii) non-stoichiometric defects

Defects in Solid

Electrical Properties of soilds

Solids exhibit electrical conductivities that extend from 27 orders of magnitude ranging from 10^–20 to 10⁷ ohm^–1 m^–1.

Conductors:

• Solids with conductivities ranging between 10⁴ to 10⁷ ohm^–1m^–1 are conductors.
• Metals have conductivities in the order of 10⁷ ohm^–1m^–1 are good conductors.
• For example, Iron, Copper, Aluminum.

A man touching the electric pole with a metal rod will get an electric shock because metal rod is a conductor whereas a man touching the same with a wooden plank will be safe because wood is an insulator.

• Semiconductors:
• Solids with conductivities in the intermediate range from 10–6 to 104 ohm^–1 m^–1.
• For example, Gallium, Germanium, Silicon

• Metals conduct electricity both in solid state as well as molten state.
• The conductivity of metals depends upon the number of valence electrons available per atom.
• The atomic orbitals of metal atoms form molecular orbitals that are close in energy to each other as to form a band.
• Partial filling or overlapping with a higher energy unoccupied conduction band enables the electrons to flow easily under an applied electric field.
• This results in conductivity of metals.

If the gap between valence band and the conduction band is large, electrons cannot jump to it and such a substance has very low conductivity that makes it behave like an insulator

Number of cation vacancies in lattice of NaCl = Number of divalent Sr²⁺ ions added.

Concentration of cation vacancy on doping with 10^-3 mol% of SrCl₂. = 10^-3 mol% = 10^-3 / 100 = 10^-5 mol.

Number of Sr²⁺ ion in 10^-5 mol = 10^-5 x 6.023 x 10²³ = 6.023 x 10¹⁸

Hence number of cationic vacancies = 6.023 x 10¹⁸

Electron – rich impurities

In a periodic table Silicon and germanium belongs to group 14 with four valence electrons each.  In their crystals each atom forms four covalent bonds with surrounding atom.

When doped with a group 15 element like P or As, which contains five valence electrons,

Applications of n-type and p-type semiconductors

• n-type and p-type semiconductors finds a great use in manufacturing electronic components.
• Diode is a combination of n-type and p-type semiconductors extensively used as a rectifier.
• Transistors are manufactured by keeping a layer of one type of semiconductor between two layers of another type of semiconductor.
• npn and pnp type of transistors are used to detect or amplify radio or audio signals.
• The solar cell is an efficient photo-diode used for conversion of light energy into electrical energy.
• Gallium arsenide (GaAs) semiconductors have very fast response and have transformed the design of semiconductor devices.
• Transition metal oxides show marked differences in electrical properties.
• TiO, CrO₂ and ReO₃ behave like metals.
• Rhenium oxide, ReO₃ resembles metallic copper in terms of its conductivity and appearance.
• Certain other oxides like VO, VO₂, VO₃ and TiO₃ exhibit metallic or insulating properties depending on temperature.

• its spin around its own axis.
• Electron being a charged particle undergoes these motions and can be considered as a small loop of current possessing a magnetic moment.
• Therefore, each electron has a permanent spin and an orbital magnetic moment associated with it.
• Magnitude of this magnetic moment is very small and is measured in the unit called Bohr magneton, μ B and is equal to 9.27 × 10–24A m².

On the basis of their magnetic properties, substances can be classified into five categories:

• Paramagnetic
• Diamagnetic
• Ferromagnetic
• Antiferromagnetic
• Ferrimagnetic.

Ferromagnetism

• Ferromagnetic substances get strongly attracted towards magnetic field.
• They can be permanently magnetized.
• In solid state, the metal ions of ferromagnetic substances are grouped together into small regions and are known as domains that act as a tiny magnet.
• In an unmagnetised ferromagnetic substance the domains are randomly oriented that cancels out their magnetic moments.
• When placed in a magnetic field all the domains of the substance get oriented in the direction of the magnetic field producing a strong magnetic effect which persists even on removal of the magnetic field and the ferromagnetic substance becomes a permanent magnet.
• For example, iron, cobalt, nickel, gadolinium and CrO2 are ferromagnetic substances.

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