Speaker
Prof.
Jorma Hölsä
(University of Turku)
Description
1. Introduction
Impurities and dopants’ inappropriate valences may deteriorate the performance of luminescent materials, cause waste of the high purity (rare earth) material and then incur significant financial losses [1]. The methods used to detect elements’ valence (XPS, Mössbauer and XANES) are not sensitive enough for low concentrations and EPR is not suitable for powders. Obtaining quantitative data leaves a lot to hope for, too. To make the things worse, the two most common rare earth dopants in phosphors, Eu3+ and Tb3+, may exist in different oxidation states, Eu2+ and TbIV [1], as well. For the Eu3+ or Tb3+ doped R2O2S:Eu3+ and R 2O2SO4, the Eu2+ or TbIV may be formed since the manufacture of these phosphors involve reducing and/or oxidizing conditions. The qualitative observation of Eu2+ can usually be made based on its intense luminescence due to the parity-allowed electric dipole 4f6↔4f55d1 transitions. In contrast, the Eu3+ line emission is weaker despite the high quantum yield. TbIV does not luminesce, but this species may absorb the emission of Tb3+ and, in addition, may facilitate non-radiative processes reducing the efficiency of Tb3+ doped phosphors even further [1].
2. Results
In this work, the comparison between the experimental and calculated temperature-dependent paramagnetic susceptibilities was used to obtain quantitatively the concentrations of the valence impurity in Eu2O2S (Eu2+) and Tb2O2SO4 (TbIV), both containing nominally only R3+. The wave functions for the calculations were obtained from the analysis (Fig. 1) of the Eu3+ luminescence spectra [2,3]. Minute (ppm level) Eu2+ impurities could be defined due to the huge difference in the paramagnetic susceptibility of Eu2+ and Eu3+. However, temperatures below 50 K are then needed (Fig. 2). In contrast, the TbIV impurity in a Tb3+ host can be determined already at higher temperatures with similar susceptibility measurements. The latter method is based on comparing the slopes of the Tb3+/TbIV paramagnetic susceptibility vs. temperature curves. The results for the Tb3+/TbIV couple are less sensitive than for the Eu2+/Eu3+ one, however. Finally, the host independent temperature evolution of the paramagnetic susceptibility was calculated for Gd3+ (or Eu2+ or TbIV) to yield an analytical expression to be used universally.
3. References
[1] W. M. Yen, S. Shionoya and H. Yamamoto (Eds.). Phosphor Handbook, 2nd ed., (CRC Press, 2007).
[2] J. Sovers and T. Yoshioka. J. Chem. Phys. 51 (1969) 5330.
[3] P. Porcher, D. R. Svoronos, M. Leskelä and J. Hölsä. J. Solid State Chem. 46 (1983) 101.
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Primary authors
Cássio C.S. Pedroso
(University of São Paulo, Institute of Chemistry, São Paulo-SP, Brazil)
Hermi F. Brito
(University of São Paulo, Institute of Chemistry, São Paulo-SP, Brazil)
Jivaldo R. Matos
(University of São Paulo, Institute of Chemistry, São Paulo-SP, Brazil)
Prof.
Jorma Hölsä
(University of Turku)
Lucas C.V. Rodrigues
(University of Turku, Turku, Finland)
Mika Lastusaari
(University of Turku, Turku, Finland)
Miroslav Maryško
(The Academy of Sciences of the Czech Republic, Institute of Physics, Cukrovarnická 10, CZ-162 53 Praha 6, Czech Republic)
Petriina Paturi
(University of Turku, Department of Physics and Astronomy, Wihuri Physical Laboratory, FI-20014 Turku, Finland)
Rodrigo V. Rodrigues
(University of São Paulo, Institute of Chemistry, São Paulo-SP, Brazil)
