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Details for article 1 of 1 found articles
| Title: | |
| Author: | |
| Appeared in: |
Advances in physics |
| Paging: | Volume 11 (1962) nr. 42 pages 83-201 |
| Year: |
1962-04 |
| Contents: |
The evidence for vacancy-pair formation in alkali halide crystals is reviewed. Existing information on the dielectric properties and structure of thin films is discussed with a view to using the high defect concentrations in vacuum-deposited films to confirm vacancy-pair orientation effects. Experimental measurements of the dielectric constant and dissipation factor of evaporated films have been made at room temperature over a range of frequencies from 100 c.p.s. to 100 kc/s. Even under vacuum these films show pronounced losses at low frequencies which are attributed to excess vacancy concentrations. Such losses are contrary to the theory of ideal lattices and are not shown by single crystals. In spite of the excess defect concentrations no evidence of vacancy-pair orientation has been found. Ageing effects have been observed in all cases, the losses decreasing with time as the defect concentration decreases. In the alkali halides the magnitude of the losses depends on the cation mobility but they decrease at a rate determined by the anion mobility. This leads to a proposed dielectric relaxation mechanism in which individual crystallites form the polarizable units, becoming polarized by migration of the cation vacancies towards one end. The losses decrease as the defects are gradually eliminated by simultaneous condensation of positive and negative vacancies at grain boundaries. This is essential to maintain electrical neutrality and the rate is determined by the diffusion of the slower anion vacancies. The measured rates are in agreement with anion activation energies obtained by tracer methods. These results cannot be explained by vacancy-pair formation even if it is assumed that vacancy pairs can form but are incapable of orientation and hence we must conclude that there is little or no vacancy-pair formation. Measurements at atmospheric pressure show that moisture has a pronounced effect in all cases, producing dielectric losses which completely obscure the vacancy effects. The changes in dielectric properties during and after adsorption cannot be explained as conductivity effects and are in complete opposition to any modification of the Maxwell-Wagner theory. The ageing effects show that after the initial adsorption, the water molecules migrate over crystallite surfaces to positions where they are more strongly bound and contribute to the dielectric polarization by a form of hindered rotation closely analogous to mechanisms proposed for ice. At these equilibrium sites, H-bonds are frequently formed between the adsorbed molecules and the halide ions of the crystal lattice. |
| Publisher: |
Taylor & Francis |
| Source file: |
Elektronische Wetenschappelijke Tijdschriften |
Details for article 1 of 1 found articles