Grain size dependence of optical properties and positron annihilation parameters in Bi₂O₃powder

Nanocrystalline Bi₂O₃has been prepared by a ball milling process. The particle size of the ball-milled Bi₂O₃powder has been determined by the x-ray powder diffraction method and transmission electron microscopy. The absorption spectra, in the spectral range 300-1300 nm, indicate an increase of the optical bandgap for both the direct and indirect transitions due to the reduction of grain size. The defects introduced in Bi₂O₃during grinding have been investigated by the positron annihilation technique. Positron annihilation results indicate an increase of defects due to ball milling.