Molecular dynamics study on surface structure of a-SiO 2 by charge equilibration method

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Surface structures on amorphous silica with different thermal histories, i.e., the surface samples were generated at greater than (higher temperature, HT) and less than (lower temperature, LT) the glass transition temperature, were studied by molecular dynamics simulation. The charge equilibration method for taking account of charge transfer among atoms and the Ewald method were applied for calculating electrostatic interaction exactly. Radial distribution functions, bond-angle distribution of O–Si–O, coordination number of silicon and oxygen near the surface, and the surface charge distribution were calculated. The absolute value of charges of Si and O at the surface were smaller than those in the bulk. The data indicate that the near-surface region in the HT sample is more disordered than that of the LT sample: (i) Oxygen atoms are dominant in the near-surface region. (ii) Population of the defect structures, i.e. undercoordinated atoms, in the HT sample is higher than that in the LT sample and defect structures in the HT sample distributed over a wider range from the surface than those in the LT sample. (iii) The surface bond-angle distribution in the HT sample is wider than that in the LT sample and a larger amount of planar three-coordinated silicon exists on the surface than in the LT sample. (iv) The vibration amplitude in the surface region is slightly larger than that in the LT sample.