Study of the dielectric properties of cobalt-doped sodium chloride crystals

Results of the dielectric loss studies of sodium chloride crystals doped with Co2+ are reported in the temperature range 90-200 °C and frequency range 30 c/s to 100 kc/s. Two peaks are observed in the tan δ-frequency isothermals. The plots of ln fm (fm being the peak frequency) against 1/T are found to be straight lines for both the peaks and yield 0.67 and 0.72 ev as the values for activation energies for the low (A peak) and the high (B peak) frequency peaks respectively The A and B peaks are attributed to the pairs of the substitutional impurity with the second-neighbour vacancy and with the first-neighbour (nearest cation neighbour) vacancy, respectively, as suggested by Dreyfus for the divalent impurity cations smaller in size than the cation of the host crystal. The height of the B peak changes from 0.24 × 10-2 when quenched from 100 °C to 0.70 × 10-2 when quenched from 400 °C or higher temperatures. The effect of the heat treatment on the height of the A peak is smaller. It is shown that, although the results strongly support the model of Dreyfus, the possibility of cobalt being in the interstitial positions cannot be ruled out on the basis of the present results. Experiments performed on crystals of different thicknesses, on the low-frequency variation of the capacitance of these crystals and on the effect of different heat treatments show that the observed loss peaks are not due to Maxwell-Wagner losses or interfacial polarization. The activation energies calculated from the dielectric loss data are found to be of the same order of magnitude as those obtained from the conductivity data, and are much smaller than those obtained from the diffusion data