Intervalence charge transfer luminescence: Interplay between anomalous and 5d − 4f emissions in Yb-doped fluorite-type crystals

In this paper, we report the existence of intervalence charge transfer (IVCT) luminescence in Yb-doped fluorite-type crystals associated with Yb²⁺–Yb³⁺ mixed valence pairs. By means of embedded cluster, wave function theory ab initio calculations, we show that the widely studied, 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. The IVCT luminescence is very efficiently excited by a two-photon upconversion mechanism where each photon provokes the same strong 4f¹⁴–1A_1g→ 4f¹³(²F_7/2)5de_g–1T_1u absorption in the Yb²⁺ part of the pair: the first one, from the pair ground state; the second one, from an excited state of the pair whose Yb³⁺ moiety is in the higher 4f¹³(²F_5/2) multiplet. The Yb²⁺–Yb³⁺ → Yb³⁺–Yb²⁺ IVCT emission consists of an Yb²⁺ 5de_g → Yb³⁺ 4f_7/2 charge transfer accompanied by a 4f_7/2 → 4f_5/2 deexcitation within the Yb²⁺ 4f¹³ subshell: [²F_5/25de_g,²F_7/2] → [²F_7/2,4f¹⁴]. The IVCT vertical transition leaves the oxidized and reduced moieties of the pair after electron transfer 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 subtracted from the normal emission. The IVCT energy diagrams resulting from the quantum mechanical calculations 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 appearance 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 broadband observed in the excitation spectrum of the so far called anomalous emission of SrF₂:Yb²⁺ as an IVCT absorption, which corresponds to an Yb²⁺ 4f_5/2 → Yb³⁺ 4f_7/2 electron transfer.