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.