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Photoluminescence Properties of Yb2+ Ions Doped in the Perovskites Cscax3 and Cssrx3 (X = Cl, Br, and I) – a Comparative Study

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Photoluminescence Properties of Yb2+ Ions Doped in the Perovskites Cscax3 and Cssrx3 (X = Cl, Br, and I) – a Comparative Study Showcasing Research from the laboratory of Prof. Dr Claudia As featured in: Wickleder, Inorganic Chemistry, University of Siegen, Germany, and the laboratory of Prof. Dr Claude Daul and Prof. Dr Werner Urland, Theoretical Chemistry, University of Fribourg, Switzerland. Title: Photoluminescence properties of Yb2+ ions doped in the perovskites CsCaX3 and CsSrX3 (X = Cl, Br, and I) – a comparative study Yb2+ ions generally exhibit two emission bands stemming from the excited high spin and low spin 4f135d1 states. The high symmetry 2+ of Yb -doped halidoperovskites CsCaX3 and CsSrX3 (X = Cl, Br, I) leads to extremely detailed spectra, which allows a systematic analysis of the electronic states and especially their positions in See Claudia Wickleder et al., relation to the crystal structure of the host lattices reported in Phys. Chem. Chem. Phys., this paper. The respective results are also of interest for future 2016, 18, 13196. luminescent materials, e.g. for Yb2+-based scintillators. www.rsc.org/pccp Registered charity number: 207890 PCCP View Article Online PAPER View Journal | View Issue Photoluminescence properties of Yb2+ ions doped in the perovskites CsCaX3 and CsSrX3 (X = Cl, Br, Cite this: Phys. Chem. Chem. Phys., 2016, 18,13196 and I) – a comparative study† Markus Suta,a Werner Urland,b Claude Daulb and Claudia Wickleder*a 2+ The Yb -doped perovskite derivatives CsMX3 (M = Ca and Sr; X = Cl, Br, and I) are ideal systems for obtaining a detailed insight into the structure–luminescence relationship of divalent lanthanides. The investigation of the respective photoluminescence properties yielded two emission bands in the violet and blue spectral range for all compounds, which are assigned to the spin-allowed and spin-forbidden 5d–4f transitions, respectively. The impact on their energetic positions is dependent on both the covalency of the Yb2+-halide bond and the corresponding bond length in agreement with expectations. The excitation spectra provide a detailed fine structure at low temperatures and can be partly Received 5th January 2016, interpreted separating the 4f13 core from the 5d electron in the excited state. The local crystal field in Creative Commons Attribution 3.0 Unported Licence. Accepted 2nd February 2016 2+ CsSrI3:Yb provides a special case due to the trigonal distortion induced by the crystal structure that is 2+ DOI: 10.1039/c6cp00085a clearly evident in the luminescence features of Yb . The structure–property relationship of several spectroscopic key quantities of Yb2+ in this series of halides is analyzed in detail and parallels the proper- www.rsc.org/pccp ties of Eu2+ ions doped in the given perovskites. 5,6 2+ 1 Introduction CsCaX3 (X = Cl, Br, and I). Tm is also known to show upconversion in halides.7,8 In particular, the relatively low This article is licensed under a The photoluminescence properties of divalent lanthanides have number of literature studies on the photoluminescence of attracted a lot of attention both from theoretical and applica- Yb2+ and other divalent lanthanide ions compared to Eu2+ is tional perspectives due to the parity-allowed character of the mainly due to the high preparative challenge in stabilizing Open Access Article. Published on 03 February 2016. Downloaded 6/4/2019 6:56:44 PM. 5d–4f transitions they exhibit in the UV and visible range. Among these ions in inorganic compounds. them, Eu2+ is by far the most studied ion due to its rather high Yb2+ exhibits a closed-shell 4f14 configuration in its ground stability and the fact that it shows luminescence in essentially state and hence it is also stabilized in many solids. Contrary to any visible color solely dependent on the appropriate choice of thecaseofEu2+, however, its luminescence is often quenched or the respective host compound. A nice overview of the photo- related to excitonic features due to the proximity of the excited luminescence of many Eu2+-activated compounds was given by states to the conduction band of the respective host compound.9,10 Dorenbos,1 while we recently presented a review of the lumi- This extraordinary behavior is known as anomalous luminescence nescence properties of Eu2+-doped nanoparticles.2 In contrast, and has been recently investigated in much detail in fluorides the luminescence properties of other divalent lanthanides are where exciton trapping is often encountered.11–13 reported to a much lesser extent and basically limited to Sm2+, Due to its closed-shell ground state configuration, Yb2+ only Tm2+ and Yb2+. Rubio reviewed the electronic spectra of Sm2+, shows broad-band 5d–4f luminescence from an excited 4f135d1 Eu2+ and Yb2+ in mainly alkali halides up to 1991,3 whereas configuration, which is electric-dipole allowed for the free ion, and other representative examples of the luminescence of Sm2+ and no 4f–4f emission. Although the respective emission bands are at 2+ 4 2+ 2+ Tm in halides were reported for MZnCl4 (M = Sr and Ba) and very similar energies compared to those of Eu in a given host, Yb mayshowtwoemissionbandsduetothefactthatitexhibitsamore a Inorganic Chemistry, Faculty of Science and Technology, than half-filled ground state configuration. This is related to the fact University of Siegen, Adolf-Reichwein-Str. 2, 57068 Siegen, Germany. that the lowest energetic excitation occurs under reversal of spin, E-mail: [email protected]; Fax: +49-271-7402555; whichistermedasaspin-forbiddenor high-spin (HS) transition. The Tel: +49-271-7404217 corresponding spin-allowed or low-spin (LS) transition lies at b Department of Chemistry, University of Fribourg, Chemin du Muse´e9, 1700 Fribourg, Switzerland higher energies according to Hund’s rules. This special beha- 2 † Electronic supplementary information (ESI) available: X-ray powder diffraction vior has been shown very illustratively for the 4f 5d transi- 14–16 patterns of the presented Yb2+-doped compounds. See DOI: 10.1039/c6cp00085a tions of the heavy trivalent lanthanides in the VUV region. 13196 | Phys. Chem. Chem. Phys., 2016, 18, 13196--13208 This journal is © the Owner Societies 2016 View Article Online Paper PCCP The photoluminescence of Yb2+ had already been investigated (Acros Organics, 57 wt%) to remove traces of residual carbo- 17,18 in detail in selected alkaline-earth halides such as MgF2, nates that form upon standing of the hydrate in air. The Yb(II) 19 20–22 23 CaF2, SrCl2 or fluorohalides. Another largely investigated halides YbX2 (X = Cl, Br, and I) were obtained in good yields class of compounds are Yb2+-activated alkali halides.24 In the cases (B80–85%) by dissolution of the pre-dried desired ammonium 2+ 22 2+ 25 of SrCl2:Yb and NaCl:Yb , both spin-forbidden and spin- halides NH4Cl (Th. Geyer, 99.99%), NH4Br (Alfa Aesar, 99.99%) allowed emissions are observed as expected. It was found that they and NH4I (Riedel de-Ha¨en, 99.9%), respectively, in liquified NH3 show a sensitive temperature-dependent intensity variation of the (Messer, 99.999%) at À60 1C and addition of Yb metal (smart- emission transitions indicating a phonon-assisted population elements, 99.999%) using the Schlenk technique.53 In all cases, mechanism of the excited states. In fact, this phenomenon has an orange-colored solid precipitated that changed its color to 2+ been so far only rarely observed for Yb . Besides halides, other greenish-yellow upon evaporation of NH3. The thus obtained 26 materials have also been investigated. Among them are borates, intermediate YbX2ÁxNH3 (X = Cl, Br, and I) was finally decom- haloborates,27 sulfates,28 phosphates29 or nitridosilicate-based posed under vacuum at 250 1C for 3 h in each case and afforded 30,31 host materials to mention a few. Dorenbos also summarized the desired Yb(II) halide ranging from faint green (YbCl2) over 2+ 32 the reported Yb -doped compounds up to 2003, where it is slight yellow (YbBr2) to an intense yellow color (YbI2). All also revealed that halides form the largest class of investigated starting materials have been checked for their purity by X-ray compounds so far. powder diffraction (XRD) (Stoe Stadi P, Cu Ka radiation). Due to the small number of only 140 microstates in the The Yb2+-doped host compounds were prepared by usage of excited 4f135d1 configuration, many calculations have been the Bridgman technique fusing the respective halides CsX and 2+ attempted on high-symmetry sites occupied by Yb in halides. MX2 (M=CaandSr;X=Cl,Br,andI)instoichiometricamounts Most of them include semi-empirical crystal-field approaches together with 0.1 mol% YbX2 (X = Cl, Br, and I) in sealed with parameters fitted to the experimental spectra, as in the case evacuated silica ampoules. The ampoules had been graphitized 24 22 of alkali halides or SrCl2. More recently, Sa´nchez-Sanz et al. inside before by pyrolysis of acetone in order to avoid the contact Creative Commons Attribution 3.0 Unported Licence. have reported about ab initio wavefunction-based relativistic of the Yb(II) halide with the glass at higher temperatures that leads calculations also including the low-lying excited 4f136s states to oxidation to the respective trivalent halide. All mixtures were 2+ 33,34 35–37 À1 for Yb embedded in SrCl2 and CsCaBr3. heated at a rate of 20 1Ch to temperatures higher than their Unlike the binary halides, the luminescence of Yb2+ in perovskite- melting points and kept in the melt for 36 h.54–56 After this, they analogue halides has been so far considered only for selected were slowly cooled (2 1ChÀ1) to 100 1C below their melting points 38 39 2+ fluorides, RbCaCl3 and the purely Yb -based iodides AYbI3 in order to provide good crystallinity of the desired products. (A = K, Rb, and Cs).40 As recent publications show, especially Finally, the mixtures were cooled down to room temperature Yb2+-doped iodides have become interesting as potential new with a cooling rate of 10 1ChÀ1.Thephasepurityofallobtained scintillating materials.41–43 Based on the previously mentioned Yb2+-doped compounds was verified by XRD (see ESI†).
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