Theoretical Studies on Defects of Kaolinite in Clays

Using the first-principles methods, we study the formation energetics and charge doping properties of the extrinsic substitutional defects in kaolinite. Especially, we choose Be, Mg, Ca, Fe, Cr, Mn, Cu, Zn as extrinsic defects to substitute for Al atoms. By systematically calculating the impurity formation energies and transition energy levels, we find that all group-II defects introduce the relative shallow transition energy levels in kaolinite. Among them, Mg_Al has the shallowest transition energy level at 0.08eV above the valence band maximum. The transition-elemental defects Fe_Al, Cr_Al, and Mn_Al are found to have relative low formation energies, suggesting their easy formation in kaolinite under natural surrounding conditions. Our calculations show that the defects Cu_Al and Zn_Al have the high formation energies and deep transition energy levels, which exclude the possibility of their formation in natural kaolinite.