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Solid State

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Ion Transport

Ion transport is measured in terms of the ionic diffusion coefficients. In general, the diffusion in solids is much slower than in liquids and gases, but there are important exceptions, when certain compounds can have very high ionic diffusion coefficients. This has a large effect on the conductivity of those compounds.


Zeolites are a class of aluminosilicates which have rigid anionic frameworks containing well defined channels and cavities. These cavities contain metal cations, such as Na+ and K+, which are exchangeable, and also neutral guest molecules, such as water, which can be removed and replaced.

Solid Defects

All solids contain defects, where the ideal lattice as described in terms of an infinitely repeating unit cell is broken. Defects can have a large effect on a range of properties of the material, such as the mechanical strength, electrical conductivity, corrosion and chemical re


When metals and semiconductors conduct, the motion of the electrons has to overcome the vibrational motion of the atoms in the crystal lattice. This lattice vibration, or phonon, leads to scattering of the electrons, or electrical resistance. A superconductor is a substance which conducts electricity without resistance.

Band Gaps

The size of the band gap varies with the type of compound involved. The Band Gap in Ionic Solids The dominant term in determining the band gap is the Madelung energy.


Semiconductors are those compounds with small band gaps between fully filled and fully unoccupied bands. Let us consider the elements of Group 14. These all adopt the structure of diamond, in which the atoms are all tetrahedrally coordinated by other atoms.

Band Structure in Solid State

The electrons in molecules are described in terms of molecular orbital theory. In this the electrons occupy a set of molecular orbitals which belong to a corresponding set of molecular energy levels. These molecular orbitals are made up of linear combinations of atomic orbitals, adapted for the symmetry of the molecule. Often, a given molecular orbital will have the character, ie. be made up from, only a small number of atomic orbitals.