METAL-CLUSTER COMPOUNDS AND UNIVERSAL FEATURES OF THE HOPPING CONDUCTIVITY OF SOLIDS

In this paper we present an overview of data on the DC conductivity and on the frequency dependent (up to 1 MHz) dielectric properties of polynuclear metal cluster compounds. These material, can be viewed as model systems for aggregates of identical metal clusters embedded in a dielectric matrix. Comparison is made with results on related narrow band materials, such as ceramic metals (cermets), various other metal-nonmetal composites, and doped or amorphous semiconductors. Remarkable similarities are observed in the behavior of these at first sight quite different systems. In order to understand this apparent universality an overview is made of the available predictions from theoretical models for the hopping conductivity in random composites and other disordered media. Although these models are often based on quite different assumptions regarding the microscopic transport mechanisms, the nature of the carriers, etc.., one finds the same similarities in the predicted behavior as found in the experiments. Thus, for a system of carriers hopping between sites separated by random barriers in a diffusive motion, the nature of carriers, sites and barriers does not appear to be very important, except that external variables such as temperature, strength and frequency of the electric field have to be properly scaled. From the observed dependences of the dielectric properties on these parameters, it is concluded that the behavior is well explained in terms of stochastic models for the hopping conductivity. The possible relevance of an extended Hubbard model for the description of these materials is discussed.