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Study of the europium centers in alkaline earth thiosilicate luminescent powders

Katleen Korthout (UGent) , Anthony Parmentier (UGent) , Philippe Smet (UGent) and Dirk Poelman (UGent)
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Abstract
Due to their efficient yellow-to-red emission, europium doped calcium thiosilicates are very interesting materials for use in phosphor converted light-emitting diodes (pc-LEDs)[1,2]. Their emission spectrum consists of two separate broad emission bands, which are caused by 4f65d 4f7 transitions of the Eu2+ ions, in combination with the presence of two inequivalent lattice sites for Eu2+. Hereby, the emission spectrum and the crystal structure are highly dependent on the europium concentration. Unfortunately no direct investigation of the dopants incorporation was performed yet. Using x-ray absorption spectroscopy combined with photoluminescent measurements, we investigated the local neighborhood of europium ions in Ca2SiS4 phosphors for LED applications. XANES data were collected at the Eu LIII edge at the Dubble beamline, EXAFS spectra were collected at the SNBL and BM23 beamlines, all at the ESRF synchrotron facility in Grenoble (France). This work reports on the concentration dependency of the emission spectra of europium doped alkaline earth thiosilicates and discusses the impact of the structure of the Eu centers on these spectra. At low dopant concentrations (<10%) the orthorhombic structure of Ca2SiS4 is found, whereby the dopant ions substitute for the Ca ions on both inequivalent crystallographic sites, albeit that some preferential substitution is observed. At higher concentrations (>40%) the monoclinic Eu2SiS4 structure is observed, whereby the Ca ions occupy both europium sites. Furthermore an unexpected large fraction of trivalent europium ions is found in the XANES spectra of low doped thiosilicates, although this is not reflected in the photoluminescence spectra. The influence of the europium concentration on the emission spectrum is related to a different crystallographic structure and a difference in occupancy of inequivalent sites by the dopant ions.

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Chicago
Korthout, Katleen, Anthony Parmentier, Philippe Smet, and Dirk Poelman. 2012. “Study of the Europium Centers in Alkaline Earth Thiosilicate Luminescent Powders.” In X-ray Absorption Fine Structure, 15th International Conference, Abstracts.
APA
Korthout, K., Parmentier, A., Smet, P., & Poelman, D. (2012). Study of the europium centers in alkaline earth thiosilicate luminescent powders. X-ray Absorption Fine Structure, 15th International conference, Abstracts. Presented at the 15th International conference on X-ray Absorption Fine Structure (XAFS-15).
Vancouver
1.
Korthout K, Parmentier A, Smet P, Poelman D. Study of the europium centers in alkaline earth thiosilicate luminescent powders. X-ray Absorption Fine Structure, 15th International conference, Abstracts. 2012.
MLA
Korthout, Katleen, Anthony Parmentier, Philippe Smet, et al. “Study of the Europium Centers in Alkaline Earth Thiosilicate Luminescent Powders.” X-ray Absorption Fine Structure, 15th International Conference, Abstracts. 2012. Print.
@inproceedings{3049624,
  abstract     = {Due to their efficient yellow-to-red emission, europium doped calcium thiosilicates are very interesting materials for use in phosphor converted light-emitting diodes (pc-LEDs)[1,2]. 
Their emission spectrum consists of two separate broad emission bands, which are caused by 4f65d 4f7 transitions of the Eu2+ ions, in combination with the presence of two inequivalent lattice sites for Eu2+. Hereby, the emission spectrum and the crystal structure are highly dependent on the europium concentration. Unfortunately no direct investigation of the dopants incorporation was performed yet. 
Using x-ray absorption spectroscopy combined with photoluminescent measurements, we investigated the local neighborhood of europium ions in Ca2SiS4 phosphors for LED applications. XANES data were collected at the Eu LIII edge at the Dubble beamline, EXAFS spectra were collected at the SNBL and BM23 beamlines, all at the ESRF synchrotron facility in Grenoble (France). 
This work reports on the concentration dependency of the emission spectra of europium doped alkaline earth thiosilicates and discusses the impact of the structure of the Eu centers on these spectra.
At low dopant concentrations (<10%) the orthorhombic structure of Ca2SiS4 is found, whereby the dopant ions substitute for the Ca ions on both inequivalent crystallographic sites, albeit that some preferential substitution is observed. At higher concentrations (>40%) the monoclinic Eu2SiS4 structure is observed, whereby the Ca ions occupy both europium sites. Furthermore an unexpected large fraction of trivalent europium ions is found in the XANES spectra of low doped thiosilicates, although this is not reflected in the photoluminescence spectra.
The influence of the europium concentration on the emission spectrum is related to a different crystallographic structure and a difference in occupancy of inequivalent sites by the dopant ions.},
  author       = {Korthout, Katleen and Parmentier, Anthony and Smet, Philippe and Poelman, Dirk},
  booktitle    = {X-ray Absorption Fine Structure, 15th International conference, Abstracts},
  language     = {eng},
  location     = {Beijing, PR China},
  title        = {Study of the europium centers in alkaline earth thiosilicate luminescent powders},
  year         = {2012},
}