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We report the existence of intervalence charge transfer (IVCT) luminescencein Yb-doped fluorites associated with Yb2+-Yb3+ mixed valence pairs. We showthat the very broad band, anomalous emission of Yb2+-doped CaF2 and SrF2,usually associated with impurity-trapped excitons, is, rather, an IVCTluminescence associated with Yb2+-Yb3+ mixed valence pairs. It is veryefficiently excited by a two-photon upconversion mechanism. The IVCT verticaltransition leaves the pair moieties very far from their equilibrium structures;this explains the unexpectedly large band width of the emission band and itslow peak energy, because the large reorganization energies are substracted fromthe normal emission. The ab initio IVCT energy diagrams explain the differentluminescent properties of Yb-doped CaF2, SrF2, BaF2, and SrCl2: the presence ofIVCT luminescence in Yb-doped CaF2 and SrF2; its coexistence with regular 5d-4femission in SrF2; its absence in BaF2 and SrCl2; the quenching of all emissionsin BaF2; and the presence of additional 5d-4f emissions in SrCl2 which areabsent in SrF2. They also allow to interpret and reproduce recent experimentson transient photoluminescence enhancement in Yb2+-doped CaF2 and SrF2, theappearence of Yb2+ 4f -5d absorption bands in the excitation spectra of the IRYb3+ emission in partly reduced CaF2:Yb3+ samples, and to identify the broadband observed in the excitation spectrum of the so far called anomalousemission of SrF2:Yb2+ as an IVCT absorption, which corresponds to an Yb2+ 4f5/2-> Yb3+ 4f7/2 electron transfer.

Intervalence Charge Transfer Luminescence: Interplay between anomalous and 5d-4f emissions in Yb-doped fluorite-type crystals