Polarization Effects in the Ionic Conductivity of Alkali Halide Crystals. II. Current-Time Dependence

The dc polarization of KCl at high temperatures has been investigated for the first time. Current-time curves may be resolved as the sum of a steady-state current and the time-dependent polarization current: the latter may be represented as the sum of two exponential terms. The relaxation times for these two processes are of the order of 40 and 250 sec at around 850 degrees K. The Jaffe (constant-field) theory has been extended to include the case of low fields and the resulting equations have been solved numerically. The calculated relaxation times are an order of magnitude larger than those found experimentally. The reason for this discrepancy is probably the severity of the constant-field approximation. The polarization is attributed to the slowness of the electrode processes compared to the velocity of the carriers in the bulk crystal and numerical values of the discharge parameter are derived. These are consistent with an activation energy for the electrode process of about 2 eV.