STATIC DIELECTRIC CONSTANTS OF SOLIDS

As discussed in the previous articles,the dielectric polarization can be considered to arise from three major sources:

Electronic ,ionic and orientational (dipolar) polarizations ,that is

P_total =P_e+P_i+P₀

Thus, the materials may be classified into three types according to the dielectric behaviour of solids is concerned :

I. Solids exhibits only electronic polarizability Element Solid Dielectrics.

II. Solids which possess electronic and ionic polarizabilities Ionic non-polar solid dielectrics (Ionic dielectrics without permanent dipoles).

III. Solids which possess orientational ,electronic and ionic polarizabilities –Polar solids (Solids containing permanent dipole moments)

Let us discuss them one by one:

(i) Elemental solid Dielectics

These are the materials consisting of single type of atoms such as diamond, germanium. These materials contain neither ions nor permanent dipoles and therefore exhibit only electronic polarization.

Under the influence of the internal field E_ithe dielectric solid displays an electric dipole moment P

That is                             P=α_eE_i (1)

Where α_e is electronic polarizability.

If n is the number of molecules per unit volume of the dielectric, then polarization (p) is given by

P=np

By substituiting equation (1) in above equation we get

P=nα_eE_i (2)

As                 E_i=E+P/3ε₀ (3)

By putting equation (3) in equation (2) we get

P=nα_e[E+P/3 ε₀]                                             (4)

If £is the permittivity of the dielectric, the displacement D is given as

D= εE= ε₀E+P

Or  P={ ε- ε₀)E

Or E=P/ε- ε₀ (5)

By putting equation (5) in equation (4), we get

P=nα_e[P/( ε- ε₀₎+ P/3 ε₀]

= nα_eP[2 ε₀+ ε/3 ε₀ (/ε- ε₀)

Or 3 ε₀/nα_e= (ε+2 ε₀)/( ε- ε₀)

Or     (ε- ε₀)/(ε+2 ε₀)=nα_e/3 ε₀ (6)

As   ε₀= ε_r ε₀ (7)

By putting equation(7) in equation (6) we get

(ε_r ε₀– ε₀₎/ (ε_r ε₀+2 ε₀₎=nα_e/3 ε₀

Or ε₀ (ε_r-1)/ ε₀ (ε_r+2)= nα_e/3 ε₀

Or       (ε_r-1)/ (ε_r+2) = nα_e/3 ε₀

This equation is called Clausius –Mossotti equation.

This equation shows that the dielectric constant is determined by n, α_e and γ (Here γ=1/3)

α_e is not the same as the polarizability of the free atoms because the binding between the atoms effect the various electrons. The distance between the atoms in the solid is affected only slightly by temperature and thus n, α_e, γ and relative dielectric constant £_r are independent of temperature for the dielectric material under discussion.