On the polaron-mechanism in iron-bearing trioctahedral phyllosilicates: an investigation of the electrical and optical properties

The electrical conductivity in the temperature range from 298 to 873 K and the optical absorption in the spectral range from 4000 to 20000 cm^-1 (at T = 298 K) of four natural samples of trioctahedral phyllosilicates with varying iron contents have been investigated. It is observed that the values of anisotropy systematically increase with increasing iron content. The increase in conductivity for the electrical field (E) polarized parallel to the (001) plane is explained by using a two dimensional percolation model with conducting paths related to the Fe ions in the octahedral layer of the structure. The conductivity perpendicular to the (001) plane is suggested to be due to defects (conducting bridges between the layers).The E (001) polarized absorption spectra exhibit a strong increase in the absorption with increasing wavenumbers with features at 9000, 12000 and 14000 cm^-1. The E (001) polarized spectra show similar line profiles with, however, lower intensities and without the 14000 cm^-1 feature. The analysis of the spectra is based on the model of small polaron absorption in disordered systems. The result shows that the polaron absorption is always the dominant contribution in the near infrared spectral range. A polaron stabilization energy of about 4800 cm^-1 (0.6 eV) is deduced and is related to the energetical separation between Fe²⁺ and Fe³⁺ states. The 9000, 12000 and 14000 cm^-1 features are superimposed to the polaron absorption.