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The polarization of a solid dielectric medium, P, is defined as the dipole moment per unit volume averaged over the volume of a crystal cell. A component of P can be expanded as a function of the electric field E :
For relatively small electric fields in isotropic substances P = χeE , where χe is the electric susceptibility. If the medium is made up of N atoms (or ions) per unit volume, the polarization is P = Npm where pm is the average dipole moment per atom. The polarizability α can be defined as pm = αE0 , where E0 is the local field at the position of the atom. Using the Lorentz method to calculate the local field one finds:
P = Nα(E + 4πP) = χeE
Together with the definition of the dielectric constant (relative permittivity), ε = 1 + 4πχe, this leads to the Clausius-Mossotti equation:
α = (3/4πN)(ε - 1)/(ε + 2)
The total polarization associated with atoms, ions, or molecules is due to three different sources:
Because of the different nature of these three polarization processes the response of a dielectric solid to an applied electric field will strongly depend on the frequency of the field. The resonance of the electronic excitation in insulators (dielectrics) takes place in the ultraviolet part of the spectrum; the characteristic frequency of the lattice vibrations is located in the infrared, while the orientation of dipoles requires fields of much lower frequencies (below 1010 Hz). This response to electric fields of different frequencies is shown in Figure 1.
Values of the electronic polarizabilities αe for selected atomic elements and ions are given in the table. Additional data for other atomic elemnts and molecules are found in “Polarizabilties of Atoms and Molecules” in Section 10 of this CRC Handbook.
Figure 1. Schematic graph of the frequency dependence of the different contributions to polarizability.
αe(I)/10-24 cm 3 | αe(II)/10-24 cm 3 | αe(III)/10-24 cm 3 | αe(IV)/10-24 cm 3 | αe(VI)/10-24 cm 3 | αe(VII)/10-24 cm 3 | αe(VIII)/10-24 cm 3 |
He 0.205 | ||||||
Li+ 0.029 | Be2+ 0.008 | B3+ 0.003 | C4+ 0.0013 | O2– 3.88 | F– 1.04 | Ne 0.394 |
Na+ 0.179 | Mg2+ 0.094 | Al3+ 0.052 | Si4+ 0.0165 | S2– 10.2 | Cl– 3.66 | Ar 1.64 |
K+ 0.83 | Ca2+ 0.47 | Sc3+ 0.286 | Ti4+ 0.185 | Se2– 10.5 | Br– 4.77 | Kr 2.48 |
Rb+ 1.40 | Sr2+ 0.86 | Y3+ 0.55 | Zr4+ 0.37 | Te2– 14.0 | I– 7.1 | Xe 4.04 |
Cs+ 2.42 | Ba2+ 1.55 | La3+ 1.04 | Ce4+ 0.73 |
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