Study of oxygen vacancies in silica using ultra soft pseudopotentials

Defect free as well as defective silica have been studied by an ab initio m ethod using ultra-soft pseudopotentials implanted in the Vienna Ab-Initio S imulation Package (VASP). In this way, a good agreement with experiment has been found For the cohesive energy (better than I eV) and the lattice para meter of alpha-quartz (better than 0.2%). The use of supercells has allowed us to evaluate the energy of formation and the partial molar volume of neu tral oxygen vacancies. The use of effective potentials to study defects is discussed. (C) 1999 Published by Elsevier Science B.V. All rights reserved.