Influence of electronic and ionic processes on electrical properties of TlBr crystals

The mechanism of electrical conductivity in TlBr at room temperature is investigated. The frequency dependence of the real and imaginary part of conductivity is presented at different temperatures. Persistent conductivity has been observed in the crystals and indicates the existence of inhomogeneities in such crystals. The long-time relaxation of photoconductivity at room temperature is conditioned by ionic-electronic processes, which are sensitive to the polarity of the electric field. The frequency dependence of conductivity at different temperatures and excitations was used to estimate the role of ionic conductivity. We demonstrate experimentally that Tl+ ions are responsible for the ionic conduction mechanism at temperatures higher than 250 K. The main change of the current versus time dependence (500 h) corresponds to the same amount of electric charge that the evaporated Tl layer could carry as Tl+ ions. A new, unexpected result was found in direct Tl+ ion transport: diffusion-limited aggregation is responsible for dendrite structures, which could grow by the electrodiffusion of Tl+ ions. The analysis of fractal formation by the electrodiffusion of Tl+ ions seems very promising to understanding the polarisation phenomena in TlBr. (c) 2005 Elsevier B.V. All rights reserved.