  First-principle study of possible bismuth-related centers in SiO2 and GeO2
hosts is performed and the results are compared with the experimental data. The
following centers are modeled: trivalent and divalent Bi substitutional
centers; BiO interstitial molecule; interstitial ion, Bi^+, and atom, Bi^0;
Bi...Si-Si and Bi...Ge-Ge complexes formed by interstitial Bi atoms and glass
intrinsic defects, Si-Si or Ge-Ge oxygen vacancies; interstitial dimers, Bi0
and Bi_2^-. Experimental data available on bismuth-related IR luminescence in
SiO2:Bi and GeO2:Bi glasses, visible (red) luminescence in SiO2:Bi glass and
the luminescence excitation are analyzed. Comparison of the calculated spectral
properties of the bismuth-related centers with the experimental data shows that
IR luminescence in SiO2:Bi and GeO2:Bi is most likely caused by Bi...Si-Si and
Bi...Ge-Ge complexes and divalent Bi substitutional center is probably
responsible for the red luminescence in SiO2:Bi.
