Dehydration-related proton conductivity in kaolinite

Abstract

Abstract Well-crystallized kaolinite was found to exhibit marked proton conductivity at pre-dehydration temperatures. Direct current conductivity and thermopotential measurements were performed. By comparing the results obtained with Au electrodes (proton blocking) to those obtained with Pd electrodes (proton injecting), non-protonic contributions could be separated from proton conductivity. The majority charge carriers appeared to be defect proton (negative sign), corresponding chemically to an O 2− replacing OH − in the lattice, but excess protons were also noted (positive sign). Below 275°C the activation energy was of the order of 7–8 kcal mole −1 ; between 275–450°C it was about 18–20 kcal mole −1 . This indicated a two-step defect proton-excess proton conductivity mechanism involving, in succession, the two kinds of OH − in the kaolinite structure: (i) OH − located within the six-membered ring formed by the SiO 4 tetrahedral sheet, (ii) the close OH − array formed by the octahedral sheet. The conductivity results were supported by IR spectroscopic measurements.