Transient and steady electron currents limited by ionic space charge in electrolytically coloured potassium halides

Abstract

Transient electrical currents have been measured when potassium halides are electrolytically coloured under constant temperature and applied voltage. An analysis of the characteristics of these currents leads to the conclusion that they are electron currents limited by ionic space charge. A very detailed study of the transient current as function of temperature ( T ⪅ 825°K) and applied voltage ( V ⪅ 10 3 V) has been accomplished in KBr, KI and KC1 single crystals. A quadratic law of the type J s αV 2, has been observed to be obeyed in the stationary level of the current, J s . From the dependence of the steady value ( J s ) on temperature we have obtained activation energies ( W) for the process responsible for the propagation of the colour ( F-centre) cloud from the cathode to the anode. The experimental values are W = 1.6, 1.6 and 1.8 eV for KBr, KI and KC1, respectively. According to these results and others obtained by other authors, it is proposed that the F-centre diffusion takes place by movement of the F-electron from the centre to the nearest anion vacancy under the action of the electric field and without passing through the conduction band. The propagation process appears to be controlled by the anion vacancy density and mobility. From the above values of W, we have obtained activation energies for the diffusion of anion vacancies of 0.8, 0.8 and 0.7 eV for KBr, KI and KC1, respectively.