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 c m(-1)(at T = 298 K) of four natural samples of trioctahedral phyllosil icates with varying iron contents have been investigated. It is observ ed that the values of anisotropy systematically increase with increasi ng 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 perpe ndicular to the (001) plane is suggested to be due to defects (conduct ing bridges between the layers). The E parallel to (001) polarized abs orption spectra exhibit a strong increase in the absorption with incre asing wavenumbers with features at 9000, 12000 and 14000 cm(-1). The E (perpendicular to) (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 ab sorption in disordered systems. The result shows that the polaron abso rption is always the dominant contribution in the near infrared spectr al range. A polaron stabilization energy of about 4800 cm(-1) (0.6 eV) is deduced and is related to the energetical separation between Fe2and Fe3+ states. The 9000, 12000 and 14000 cm(-1) features are superim posed to the polaron absorption.