Luminescence and energy transfer in GdP5O14:Eu3+,Sm3+ single crystals

Journal of Alloys and Compounds, 202 (1993) 47-50 JALCOM 837

47

Luminescence and energy transfer in crystals*

GdPsO14:Eu3+,Sm3+single

Yaqin Yu, Yayan Liu and Mingshu Song Changchun Institute of Applied Chemistry, Academia Sinica, Changchun 130022 (China) (Received April 3, 1993)

Abstract In this paper we report on the luminescence and energy transfer in GdPsOx4:Eua+,Sm 3+ (GdPP:Eu,Sm) in single crystals grown by the hydrothermal method. The room temperature excitation, emission, absorption and IR spectra of the crystals have been measured and analysed. The energy transfer from Gd 3+ and Sm 3+ to Eu 3+ ions in GPP:Eu,Sm crystals is also discussed.

1. Introduction The rare earth ultraphosphates LnPsO14 form a class of luminescent materials with interesting optical and structural properties. They are excellent fluorescence hosts in which the emission of many rare earth ions is much less sensitive to concentration quenching than is usually the case [1-4]. In this paper, the room temperature excitation, emission, absorption and IR spectra of GdPsO14:Eu 3+,Sm 3+ (GPP:Eu,Sm) single crystals have been measured and analysed. Most of the observed emission and absorption can be uniquely assigned in terms of specific transitions between levels in the 4f shell of the Eu 3+ and Sm 3+ ions, and conversely most predicted transitions are observed. The symmetry-determination electric and magnetic dipole selection rules for the C2v site symmetry of the Eu 3+ and Sm 3+ ion locations are obeyed quite well in terms of both the activities of the various transitions and their behaviour. The energy level diagrams of the Eu 3+ and Sm 3+ ions in GdPP: Eu, Sm crystals are given. Energy transfer from Gd 3÷ and Sm 3+ to Eu 3+ ions takes place by nonradiative relaxation and a possible mechanism for this transfer is discussed.

2. Experimental details GdPP:Eu,Sm single crystals were grown by the conventional method in a flux solvent of highly condensed phosphoric acid [2]. *Presented at the International Conference on Luminescence (Luminophor-92), Stavropol, Russia, September 29-October 1, 1992.

0925-8388/93/$6.00

The absorption spectrum was measured using a Japan spectrometer (UV-360). The excitation and emission spectra were obtained with a Japan MPF-4 fluorescent spectrometer. The IR spectrum was recorded with an SP-1050 IR spectrometer. The structure and cell parameters of the crystals were determined from X-ray powder diffraction measurements.

3. Result and discussion

3.1. Absorption spectrum The absorption spectra of various GdPP:Eu,Sm crystals are shown in Fig. 1. The strongest band at 394 nm and the weak sharp band at 361 nm can be immediately assigned to transitions from 7Fo to 5L 6 and 5D 4 respectively. The 7Fo-SD 3 transition is located at 413 nm. The weak bands at 462 and 522 nm can be identified from their close correlation with transitions observed for the Eu 3÷ ion in GdPP:Eu,Sm and GdPP:Eu crystals and are assigned to 7F0-SD 1 and SD2 respectively. As has been shown experimentally, the band at 522 nm (TFo-SD1) arises entirely via a magnetic dipole mechanism. Similarly, for the Sm 3+ ion in GdPP:Eu,Sm crystals the 4L19/2, 4Hll/2, (4I~4L)17/2, 4L13/2, 4F7/2, 6p5/2, 4Klm and 4Ij levels are assigned as shown in Fig. 1. 3.2. Excitation and emission spectra The excitation spectra of the Eu 3+ and Sm 3+ ions are shown in Fig. 2(a) for emission at 594 nm. The strongest band observed for Eu 3+ and Sm 3÷ peaks at 394 nm, corresponding to the 5L6-TF o and 4L13/2, aFT/2, 6Ps,~-6Hs/2 transitions of E u 3+ and S m 3+ res p e c t i v e l y . The excitation spectrum of the E u 3 + fluorescence from 5D o to 7Fi levels shows maxima in

© 1993-Elsevier Sequoia. All rights reserved

Y. Yu / Luminescence in GdPsOu:EuS+,Sm ~+ single

48

crystals

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Fig. 1. Absorption spectra of GdPP:Eu,Sm at room temperature. "

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Fig. 4. Lower energy levels of Eu 3+ and Sm3+ in GdPP:Eu,Sm.

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Fig. 2. (a) Excitation and (b) fluorescence spectra of GdPP:Eu,Sm.

GdPP:Eu,Sm and GdPP:Eu crystals due to electron transfer bands. In the weak lines between 250 and 380 nm we observed additional peaks at about 273 and 318 nm which can be assigned to 6Ij and 6pj to asTt2 excitation transitions of Gd 3+ respectively.

The fluorescence spectra of the Eu 3 ÷ and Sm a ÷ ions are shown in Fig. 2(b). The excitation wavelength is 394 nm. In Fig. 2(b) emissions are observed at 560-570, 587-594, 610.7-620, 642--651 and 687-697.4 nm. These bands are due to the 5Do-TFj (j = 1, 2, 3, 4) transitions o f E u 3+ and the 4Gs/z-6Hj (/=5/2, 7/2, 9/2, 11/2) tran-

sitions of Sm 3+. The strongest band is due to the 5Do-7F 1 and aGs/2-6H7/2 transitions of Eu 3÷ and Sm 3 + respectively observed at 594 nm. Selection rules forbid certain transitions for which very weak emission bands are actually observed. For example 5Do-7F o is disallowed by the electric dipole mechanism for the j = 0 , 0 × 0 transition. The concentration dependence of the fluorescence intensities of Eu 3+ in GdxEul_xPP crystals is given in Fig. 3. The fluorescence intensities increase only slightly

49

Y. Yu I Luminescence in GdPsO14:EuS+,Sm3+ single crystals

with Eu 3+ concentration. Thus the fluorescence quenching is small. The positions of many Stark components for the Eu 3+ and Sm 3+ ions in GdPP:Eu,Sm crystals have been determined from the absorption and emission spectra. The lower energy levels of Eu 3+ and Sm 3+ in GdPP:Eu,Sm crystals are given in Fig. 4. It should be noted that the L S J manifolds of the free rare earth ions are (2j + 1)-fold degenerate. In this matrix the rare earth ions experience C2~ symmetry. This degeneracy should be completely removed for integral j. In our experimental results the (2j + 1)-fold degeneracy of the Eu 3+ and Sm 3+ levels does not show complete removal because of the poor sensitivity of the instrumentation.

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Fig. 8. Fluorescence intensities f r o m 5D0--7Fj transitions of E u 3+ vs. concentrations of S m 3+ in Gd~Eu0.4Smo.6_=PP. 40

3.3. I R spectrum

We have measured the IR spectrum of GdPP:Eu,Sm from 600 to 1400 cm-1 (Fig. 5). The fundamental P - O stretching bands are observed from 975 to 1380 cm-1. GdPP:Eu,Sm crystals are found to be isostructural with NdP50~2, corresponding to monoclinic-1, with space group Pzl/c.

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Fig. 5. IR spectrum of GdPP:Eu,Sm.

Fig. 6. Fluorescence intensity of G d 3+ in Gd~Eul_~PP.

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concentration of E u 3+ Sm 3*

Eu3+ Gd3*

Fig. 9. Scheme of energy transitions of in GdPP:Eu,Srn.

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3.4. Energy transfer 3.4.1. G d 3 + - E u 3÷

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0.2

0.4

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0.8

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Fig. 7. Fluorescence intensities of E u 3+ Vs. concentration of Gd3+ in GdxEul-xPP.

The fluorescence intensity of Gd 3÷ near 310.5 nm vs. the Eu 3÷ concentration in Gd~Eul_xPP is shown

in Fig. 6. The variation in Gd 3÷ and Eu 3+ emission with x in GdxEul_xPP crystals under excitation at 273 nm is presented in Figs. 6 and 7 respectively. The Gd 3+ emission intensities decrease strongly and become very weak when x is about 0.2, whereas the Eu 3+ emission intensities increase. When E u 3 + is excited at 273 nm the fluorescence intensity is a function of the Gd 3+ concentration in GdxEu]_~PP (Fig. 7) and there is a maximum when x is about 0.6. We may note that energy transfer occurs between Gd 3÷ and Eu 3+. Force multipolar transition is possible for the two ions.

Y. Yu / Luminescence in GdPsO14:Eu3÷,Sm 3+ single crystals

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but there is no doubt that the maximum observed is located beyond the point where it really takes place. The energy gap between the sensitizer i.e. the 4G5/2 level (17 953 cm -1) of Sm 3÷, and the activator, i.e. the 5Do (17 361 cm -1) of EU 3+, varies in a small range of energy up to 592 cm -1. The probability of phononassisted transfer may be considered in GdPP:Eu,Sm crystals (see Fig. 9). In this matrix, transfer from donor (sensitizer S, i.e. Sm 3+) to donor cannot be neglected. Because of the resonance condition, the S-S transfer may be even more rapid than the S-A (acceptor A, i.e. activator EU 3+) transfer when the concentrations of the two ions are comparable, especially in the rare earth ions where the Stokes shift is very small. Excitation energy may be able to migrate among the sensitizer ions before passing to the activators. Thus when the concentration of Sm 3÷ increases, the emission intensities from t h e 5 D o - 7 F / t r a n s i t i o n s of Eu 3÷ decrease strongly in GdxEuo.4Smo.6_~PP crystals (see Fig. 10).

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4. Conclusions

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700 500

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Fig. 10. Fluorescence spectra from 4Gs/2-6Hj transitions of Sm3+ in (a) SmPP, (b) Gdo.6Smo.4PP and (c) Gdo.4Smo.4Euo.zPP.

3.4.2. S m S + - E u 3÷

The energy transfer from Sm 3+ to E u 3+ was investigated by recording the emission lines from the 5Do-7Fy transitions of Eu 3÷ and by exciting it at a wavelength of 400 nm in the presence of various concentrations o f Sm 3+ while the Eu 3+ concentration was kept constant (Fig. 8). From Fig. 8 it may be seen that Gd 3+ is not excited at 400 nm, which is only an exciting wavelength for E u 3+ and S m 3+, and the emission band of Sm 3÷ overlaps that of Eu 3+. When the amount of Sm 3+ is increased, the fluorescence of europium exhibits a maximum for an Sm 3÷ concentration of about 0.2. Beyond this value, concentration quenching occurs, indicating that the emission intensity is a function of the S m 3+ concentration. The point where this concentration quenching actually occurs cannot be located exactly,

GdPsO14:Eu,Sm (GdPP:Eu,Sm) single crystals have been grown by the hydrothermal method. The room temperature excitation, absorption, emission and IR spectra of the crystals were measured and analysed. The energy level diagrams of the Eu 3÷ and Sm 3÷ ions in GdPP:Eu,Sm crystals have been constructed from the absorption and fluorescence spectra. The energy transfer from Gd 3+ and S m 3+ to E u 3 + ions has been investigated and a possible mechanism for this transfer is suggested and discussed.

References 1 G. Deshazer and G.H. Dieke, J. Chem. Phys., 38 (1963) 2190. 2 H.G. Danielmeyer, IEEE J. Quantum Electron., QE-8 (1972) 805. 3 B.C. Tofield, H.P. Weber and T.C. Damen, Mater. Res. Bull., 4 (1974) 435. 4 Y.Q. Yu, Q.Y. Wang and S.Z. Liu, Chin. J. Luminesc. Display Devices, 6 (1985) 230.