Possible centers of broadband near-IR luminescence in bismuth-doped   solids: $Bi^{+}$, Bi$_5^{3+}$, and Bi$_4^0$
  Subvalent bismuth centers (interstitial $Bi^{+}$ ion, Bi$_5^{3+}$ cluster ion, and Bi$_4^0$ cluster) are examined as possible centers of broadband near-IR luminescence in bismuth-doped solids on the grounds of quantum-chemical modeling and experimental data. 
V.O.Sokolov, V.G.Plotnichenko, E.M.Dianov
cond-mat.mtrl-sci physics.optics