The d-f emission from Ce 3+ and Pr 3+ in garnets is attracting considerable attention, especially in relation to application in white light LEDs and scintillators. An important aspect is the luminescence quenching temperature T Q . It is not trivial to determine T Q and to unravel the quenching mechanism. In this paper the T Q of d-f emission for Ce 3+ and Pr 3+ are determined by temperature dependent lifetime measurements. The results show a T Q for Pr 3+ of 340 K for Y 3 Al 5 O 12 :Pr 3+ (YAG:Pr) and 680 K for Lu 3 Al 5 O 12 :Pr 3+ (LuAG:Pr). For Ce 3+ the T Q is too high to measure. An onset of quenching above 600 K (YAG:Ce) or 700 K (LuAG:Ce) is observed. The differences in T Q between YAG and LuAG are explained by a smaller Stokes shift for the d-f emission in LuAG (∼2300 cm −1 ) compared to YAG (∼2750 cm −1 ) derived from low temperature luminescence spectra. The large difference in T Q between Ce 3+ and Pr 3+ is related to the smaller energy difference between the lowest energetic fd state of Pr 3+ and the next lower 4f 2 state ( 3 P 2 ) compared to the 5d -4f 1 ( 2 F 7/2 ) energy difference for Ce 3+ . Both observations are consistent with luminescence temperature quenching by non-radiative relaxation from the 5d state to the 4f state described by a configurational coordinate diagram and not by thermally induced photoionization.
a b s t r a c tThe silicates Ca 3 Sc 2 Si 3 O 12 , Ca 3 Y 2 Si 3 O 12 and Ca 3 Lu 2 Si 3 O 12 , both undoped and doped with Pr 3 + ions, have been synthesized by solid-state reaction at high temperature. The luminescence spectroscopy and the excited state dynamics of the materials have been studied upon VUV and X-ray excitation using synchrotron radiation. All doped samples have shown efficient 5d-4f emission upon direct VUV excitation of 5d levels, but only Ca 3 Sc 2 Si 3 O 12 :Pr 3 + shows luminescence upon interband VUV or X-ray excitation. The VUV excited emission spectra of Ca 3 Y 2 Si 3 O 12 :Pr 3 + and Ca 3 Lu 2 Si 3 O 12 :Pr 3 + show features attributed to emission from two distinct sites accommodating the Pr 3 + dopant. The decay kinetics of the Pr 3 + 5d-4f emission in Ca 3 Sc 2 Si 3 O 12 :Pr 3 + upon VUV excitation across the band gap are characterized by decay times in the range 25-28 ns with no significant rise after the excitation pulse. They appear to be faster upon X-ray irradiation than for VUV excitation. Weak afterglow components are attributed to defect luminescence.
Lu(3)Al(5)O(12) (LuAG) doped with Ce(3+) is a promising scintillator material with a high density and a fast response time. The light output under X-ray or γ-ray excitation is, however, well below the theoretical limit. In this paper the influence of codoping with Tb(3+) is investigated with the aim to increase the light output. High resolution spectra of singly doped LuAG (with Ce(3+) or Tb(3+)) are reported and provide insight into the energy level structure of the two ions in LuAG. For Ce(3+) zero-phonon lines and vibronic structure are observed for the two lowest energy 5d bands and the Stokes' shift (2 350 cm(-1)) and Huang-Rhys coupling parameter (S = 9) have been determined. Tb(3+) 4f-5d transitions to the high spin (HS) and low spin (LS) states are observed (including a zero-phonon line and vibrational structure for the high spin state). The HS-LS splitting of 5400 cm(-1) is smaller than usually observed and is explained by a reduction of the 5d-4f exchange coupling parameter J by covalency. Upon replacing the smaller Lu(3+) ion with the larger Tb(3+) ion, the crystal field splitting for the lowest 5d states increases, causing the lowest 5d state to shift below the (5)D(4) state of Tb(3+) and allowing for efficient energy transfer from Tb(3+) to Ce(3+) down to the lowest temperatures. Luminescence decay measurements confirm efficient energy transfer from Tb(3+) to Ce(3+) and provide a qualitative understanding of the energy transfer process. Co-doping with Tb(3+) does not result in the desired increase in light output, and an explanation based on electron trapping in defects is discussed.
Ca(9)Lu(PO(4))(7):Ce (3+) and Ca (9)Lu (PO (4))(7):Pr (3+) polycrystalline materials were synthesized by solid state reaction at high temperature. The materials were characterized by powder x-ray diffraction (XRPD). The luminescence spectroscopy and the excited state dynamics of these compounds were investigated upon excitation with UV/VUV synchrotron radiation. Both materials showed efficient and fast 5d-4f emission upon direct VUV excitation into the 5d levels but only Ca(9)Lu(PO(4))(7):Ce (3+) revealed luminescence upon excitation across the bandgap. The decay kinetics of the 5d-4f emission upon VUV intra-center excitation is characterized by a decay time of 29 ns for Ce (3+) and 17 ns for Pr (3+) with no significant build-up after the excitation pulse. For the both compounds, no significant temperature dependence of the 5d-4f emission lifetime was observed within the range 8-300 K.
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