Some aspects of polarization in ionic crystals with electrode reactions

Space charge polarization involving an electrode reaction with surface adsorption is examined. As recently demonstrated by Lányi, the presence of such a process may be represented using a complex, frequency-dependent rate constant. Lányi's rate constant is shown to be normally a special case of a more general complex rate constant following from earlier work of Armstrong and others on electrode reactions with an adsorbed intermediate species. An exact expression, based on earlier work of one of the authors, is given for the impedance of a uni-univalent extrinsic-conduction system with mobile charges of only a single sign and for which the electrode processes involve such adsorbed intermediate species. Conditions are then found which reduce the exact results to the approximate expressions for series capacitance and resistance derived by Lányi for his special case of calcium-doped NaCl. These conditions are well satisfied for his specific choices of parameter values. Curves showing the exact frequency response of various series and parallel quantities are presented, together with complex impedance plane results. Finally, it is shown how under conditions that include the range of usual experimental interest the entire space-charge, electron reaction/adsorption system can be well represented by a simple equivalent circuit of frequency-independent elements.