Intervalence Charge Transfer Luminescence: Interplay between anomalous   and 5d-4f emissions in Yb-doped fluorite-type crystals
  We report the existence of intervalence charge transfer (IVCT) luminescence in Yb-doped fluorites associated with Yb2+-Yb3+ mixed valence pairs. We show that the very broad band, anomalous emission of Yb2+-doped CaF2 and SrF2, usually associated with impurity-trapped excitons, is, rather, an IVCT luminescence associated with Yb2+-Yb3+ mixed valence pairs. It is very efficiently excited by a two-photon upconversion mechanism. The IVCT vertical transition leaves the pair moieties very far from their equilibrium structures; this explains the unexpectedly large band width of the emission band and its low peak energy, because the large reorganization energies are substracted from the normal emission. The ab initio IVCT energy diagrams explain the different luminescent properties of Yb-doped CaF2, SrF2, BaF2, and SrCl2: the presence of IVCT luminescence in Yb-doped CaF2 and SrF2; its coexistence with regular 5d-4f emission in SrF2; its absence in BaF2 and SrCl2; the quenching of all emissions in BaF2; and the presence of additional 5d-4f emissions in SrCl2 which are absent in SrF2. They also allow to interpret and reproduce recent experiments on transient photoluminescence enhancement in Yb2+-doped CaF2 and SrF2, the appearence of Yb2+ 4f -5d absorption bands in the excitation spectra of the IR Yb3+ emission in partly reduced CaF2:Yb3+ samples, and to identify the broad band observed in the excitation spectrum of the so far called anomalous emission of SrF2:Yb2+ as an IVCT absorption, which corresponds to an Yb2+ 4f5/2 -> Yb3+ 4f7/2 electron transfer. 
Zoila Barandiaran and Luis Seijo
physics.chem-ph cond-mat.mtrl-sci