Yb 2 + -doped SrCl2: Electronic structure of impurity states and impurity-trapped excitons

Abstract

First principles electronic structure calculations of the excited states of Yb 2+ -doped SrCl 2 crystals up to 65000 cm −1 reveal the existence of unexpected excited states with double-well potential energy surfaces and dual electronic structure lying above and very close in energy to the 4f 13 5d manifold, with which they interact strongly through spin-orbit coupling. The double-well energy curves result from avoided crossings between Yb-trapped exciton states (more stable at short YbCl distances) and 4f 13 6s impurity states (more stable at long Yb-Cl distances); the former are found to be pre-ionization states in which the impurity holds the excited electron in close lying empty interstitials located outside the YbCl 8 moiety. Spin-orbit coupling between the double-well states and lower lying 4f 13 5d impurity states spreads the dual electronic structure character to lower energies and, hence, the instability of the divalent oxidation state is also spread. To some extent, the dual electronic structure (impurity-trapped exciton -impurity state) of some excited states expresses and gives support to hypotheses of interaction between Yb 2+ and Yb 3+ pairs proposed to understand the complex spectroscopy of the material and conciliates these hypotheses with interpretations in terms of the existence of only one type of Yb 2+ defect. The results presented confirm the presence of impurity states of the 4f 13 6s configuration among the 4f 13 5d manifolds, as proposed in the literature, but their energies are very different from those assumed.The Yb-trapped excitons found in this chloride host can be seen as precursors of the luminescent Yb-trapped excitons characterized experimentally in the isomorphous SrF 2 crystals.

Ab initio theoretical study of luminescence properties of Pr3+-doped Lu2O3

“…In the case of the C 2 site, the different Pr-O distances behave differently: two of them (Pr-O(1) and Pr-O(5)) shorten while the third one (Pr-O(3)) remains esentially unchanged. This shortening is in agreement with what has been found in other lanthanides and actinides in high symmetry halide coordination 15,16,18,20,38 , and recently has been experimentally concluded. 39 An extensive study of the factors leading to this result for several ions, including Pr 3+ , can be found in Ref.…”

“…The AIMP embedded cluster method 13,14 has been succesfully applied in a number of structural and spectroscopical studies of lanthanide and actinide-doped ionic hosts. [15][16][17][18][19][20] As a result of the calculations, we obtain the local distortion produced by the Pr 3+ doping and we give support to the assignments of the absorption bands to Pr 3+ occupying different sites in the Lu 2 O 3 lattice 5,6 are supported. The energy minima of the lowest 4f 5d states are calculated and located with respect to the 4f 2 3 P 0 and 1 D 2 levels and possible luminescence after 4f → 5d excitation is predicted.…”

“…16,18 The only exception are those systems and states in which spin-orbit coupling mixes states belonging to different dominant electron configurations. 20 This is not the case here, as the separation between the relevant 4f 2 and 4f 5d manifolds is large (see below).…”

Transient photoluminescence enhancement as a probe of the structure of impurity-trapped excitons in CaF2:Yb2+

“…[4][5][6][7][8] Modern ab initio methods 9,10 are capable of accurately calculating excited-state electronic structure and bond-length variations, giving good agreement with available broad-band 4f N ↔ 4f N−1 5d spectra. The ab initio calculations 10 indicate that transitions between the excited states would give a combination of sharp-line and broad vibronic transitions.…”

Site-selective transient photoluminescence enhancement of impurity-trapped excitons in NaMgF3:Yb2+

“…1 The electron has been assumed to be localized on the neighboring cations 2 but recent ab initio studies of ITE states have shown that the exciton radius may be smaller. 3,4 Many materials have ITE states that play a role in relaxation processes but studying impurity-trapped excitons in optical materials is often difficult since the ITEs that form in these systems decay predominantly nonradiatively and so have to be studied indirectly. This means that their formation and nature are still largely unknown.…”