Analysis of cermet films with large metal packing fractions.

Abstract

Recently reported dielectric properties of deposited Rh films show anomalously small values of the real part of the dielectric function with respect to the imaginary part and also with respect to bulk values. This behavior is substantially different from that expected on the basis of standard isotropic effective-medium theories. Using a strong-coupling model that has an approximate analytic solution, I show that this behavior is consistent with a two-phase, microscopically inhomogeneous but macroscopically homogeneous physical mixture of metal and dielectric, where the dielectric (presumably a grain-boundary oxide or simply open spaces between grains) is restricted essentially to the boundaries of the metal particles and provides optical isolation between them (metal-rich cermet configuration.) The solution is of the Maxwell-Garnett type with the dielectric acting as host phase. Since this solution becomes exact in the limit of vanishing dielectric volume fraction, this model provides a striking demonstration that connectedness, not relative volume fraction, is the physical property determining the identity of the host species. The Rh-film data are best described by an analytic, two-parameter, two-phase effective-medium model that incorporates randomness as well as the coated-particle microstructure and was developed by Sen, Scala, and Cohen to describe the low-frequency conductance of sedimentary rocks.