THEORETICAL STUDIES ON THE EXTRINSIC DEFECTS OF MONTMORILLONITE IN SOFT ROCK

Using the first-principles methods, we study the formation energetics and charge doping properties of the extrinsic substitutional defects in montmorillonite. 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 montmorillonite. Among them, MgAl has the shallowest transition energy level at 0.10 eV above the valence band maximum. The transition-elemental defects FeAl, CrAl, and MnAl are found to have relatively low formation energies, suggesting their easy formation in montmorillonite under natural surrounding conditions. Our calculations show that the defects CuAl and ZnAl have high formation energies, which exclude the possibility of their formation in montmorillonite.