Nonlinear Ionic Conductivity in Alkali-Halide Crystals

Nonlinearity in the electrical conductivity of NaCl and other alkali-halide crystals has been measured by a sensitive ac method. There is a component of current proportional to the cube of the voltage. The ratio of this component to the total current is about +10^-5, at an electric field of 2000 V/cm and a frequency of 10³ cps. This ratio is only slightly dependent on temperature, over a range which includes both extrinsic and intrinsic ionic conduction. The nonlinearity is about 10³ times what would be expected on the basis of simple ionic-conductivity theory. The type of dependence upon temperature, voltage, and specimen thickness, and the nondependence upon electrode material or preparation, favor an interpretation that the non-linearity is a bulk property. But there is a frequency dependence, at least at the lower temperatures, which favors an interpretation in terms of space charge or boundary layers. A number of conceivable mechanisms have been examined, but none seem to offer an explanation.