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Photon cascade luminescence of Gd3+ in GdBaB9O16
Journal of Alloys and Compounds 308 (2000) 94–97
L
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Photon cascade luminescence of Gd 31 in GdBaB 9 O 16 a a, a b b Z. Yang , J.H. Lin *, M.Z. Su , Y. Tao , W. Wang a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, Department of Materials Chemistry, Peking University, Beijing 100871, PR China b Beijing Radiation Laboratory, Institute for High Energy Physics, Beijing 100030, PR China Received 29 December 1999; accepted 15 April 2000
Abstract The photon cascade emission of Gd 31 ions under 8 S7 / 2 → 6 GJ excitation is observed in GdBaB 9 O 16 . Under the 202 nm excitation, a red and a near infrared radiative transition from 6 GJ states, 6 GJ → 6 IJ and 6 GJ → 6 PJ occur followed by an ultraviolet emission of 6 PJ → 8 S7 / 2 . The absence of emission from 6 DJ and 6 IJ states is attributed to the effective multiphonon 6 DJ → 6 IJ and IJ → 6 PJ relaxation in this lattice. 2000 Elsevier Science S.A. All rights reserved. Keywords: Cascade luminescence; Rare earth borate; Nonradiative decay; Optical properties; Luminescence
The development of phosphors for excitation in vacuum ultraviolet (VUV) region becomes an important challenge in the field of luminescent materials. The initiation of the study is the proposed mercury-free lamps, as well as the plasma display panels. Because the energy of the VUV photon (172 nm for xenon dimer) is much higher than that of the Hg discharge radiation (254 nm), a large part of the energy is lost through nonradiative relaxation processes, even if the quantum efficiency of the phosphor is close to 100%. To make the mercury-free lamp competitive, a quantum efficiency of more than 100% is required for the VUV phosphors. The ability of Pr 31 to generate two photons for every absorbed photon was known for quite a long time. A quantum yield of about 140% upon 185 nm excitation for the Pr 31 activated YF 3 was reported [1]. Recent studies on Pr 31 activated borates and other oxides [2–4] show that the photon cascade luminescence of Pr 31 may occur if the lowest energy component of the 4f5d configuration is located above the 1 S0 state. This phenomenon has also been studied for Tm 31 , but substantial amount of energy is lost in the IR region [5]. Very recently, Wegh et al. observed cascade luminescence of Gd 31 ions in LiYF 4 [6] and demonstrated that an alternative concept for obtaining quantum efficiencies of more than 100% by *Corresponding author. Tel.: 186-10-62751-715; fax: 186-10-62751708. E-mail address: [email protected] (J.H. Lin).
using two lanthanide ions [7]. Through two-step energy transfer from Gd 31 to Eu 31 , two red emission photons of Eu 31 were realized with quantum efficiency close to 200%. As shown in Fig. 1, the concept of this process is based on the cascade transitions of Gd 31 of 6 GJ → 6 PJ and 6 PJ → 8 S7 / 2 , as well as the subsequent energy transfer to
Fig. 1. Energy-level scheme in the range 0–50 000 cm 21 for Gd 31 [6].
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00908-7
Z. Yang et al. / Journal of Alloys and Compounds 308 (2000) 94 – 97
the Eu 31 ions. In terms of application, however, the instability and low absorption ability do not permit the utilization of the fluoride materials in the plasma display panel (PDP) and the mercury-free lamps. Lanthanide borates, such as GdBO 3 and GdB 3 O 6 , are known to show very strong VUV absorption around 170 nm and promising chemical stability. In fact, the Eu 31 doped GdBO 3 has been commercially used in the PDP devices [8]. However, the study of the two-step energy transfer process in lanthanide borates is complicated, which may concern the absorption and energy migration in the host-lattice and the energy transfer and luminescence processes of the lanthanide ions in the materials. In this report we focus on cascade luminescence of the Gd 31 ions in GdBaB 9 O 16 . The LnBaB 9 O 16 family was first reported by Saubat et al. [9], and was studied systematically by Fu et al. [10,11]. The crystal structure of GdBaB 9 O 16 has been studied by using single crystal technique [12]. This compound crystallized in rhombohedral space group R3 with the lattice ˚ The crystal constant of a57.809(2), c546.47(1) A. structure model was established using direct method and subsequent refinement leads to a residual value of R5 0.035. However, some of the boron atoms can not be refined appropriately and further study with neutron diffraction is under the way. The preliminary structure model shows that there are three crystallographic independent Gd ˚ The sites and the shortest Gd–Gd distance is about 4.9 A. long Gd–Gd distance may reduce the probability of the 31 energy migration among the Gd ions, which in turn enhances the cascade emission of the Gd 31 ions. Single crystals of GdBaB 9 O 16 and LaBaB 9 O 16 were synthesized in the flux of B 2 O 3 . The starting materials, Ln 2 O 3 , BaCO 3 and B 2 O 3 , in a ratio of LnBaB 9 O 16 : B 2 O 3 51:1, were mixed and heated to 10708C. After maintained at this temperature for about 2 h, the samples were cooling down to 4008C in a rate of 2–38C / h, then to room temperature in about 12 h. Both GdBaB 9 O 16 and LaBaB 9 O 16 crystals are colorless, transparent and plate like. The Eu 31 doped single crystals were synthesized in a similar way. X-ray powder diffraction was recorded on a Rigaku D/ max–2400 powder diffractometer with Cu-Ka radiation from a rotating anode. The VUV absorption and excitation spectra were measured on the Spectroscopy Station of Beijing Synchrotron Laboratory. An Acton VM502 VUV monochromator with a 1200 lines / mm holographic concave grating was used and the signal was detected by EMI 9635QB PMT with sodium salicylate as fluorescence converter. Luminescence spectra were recorded with a Hitachi M-850 fluorescence spectrophotometer and both emission and excitation spectra were corrected for the excitation source and the photomultiplier. Fig. 2 shows the absorption spectra of GdBaB 9 O 16 and LaBaB 9 O 16 . The absorption spectrum of GdBaB 9 O 16 is composed of sharp lines originated from the f–f transitions of the Gd 31 ion and a broad band originated from the host-lattice. According to the energy diagram of Fig. 1, the
95
Fig. 2. Absorption spectra of LaBaB 9 O 16 and GdBaB 9 O 16 .
sharp lines are assigned to the transitions from ground state S7 / 2 to the excited states of 6 PJ , 6 IJ , 6 DJ and 6 GJ respectively. The broad absorption band at short wavelength (,170 nm) should be related to the inter-band transition. Similar host-lattice absorption was observed for LaBaB 9 O 16 as shown in the figure. From the preliminary structural study [13], we know that GdBaB 9 O 16 and LaBaB 9 O 16 are crystallized in a same crystal structure. For the LnBaB 9 O 16 compound, the valence band is mainly composed of the bonding orbitals of borate groups, while the conduction band may consist of the 5d orbitals of the lanthanide and the antibonding orbitals of the borate groups. The band gap obtained from the absorption spectra is about 7.0 eV. Although the absorption of host lattice is very intense, the energy transfer to the doped activators is prevented largely due to the long Gd–Gd distance in these materials. In Fig. 3, we show the excitation spectrum of Eu 31 doped GdBaB 9 O 16 , monitored by Eu 31 emission. The strong excitation at about 240 nm is the charge transfer transition of O 22 →Eu 31 , and the line at about 274 nm is originated from the 8 S7 / 2 → 6 IJ transition of the Gd 31 ions. In comparison with the O 22 →Eu 31 charge transfer transition, the excitation efficiency of the host-lattice is relatively low. Fig. 4 shows proposed excitation and energy transfer processes upon excitation of the inter-band absorption. Because the long Gd–Gd distance in GdBaB 9 O 16 , the energy transfer between the Gd 31 ions and that of Gd 31 →Eu 31 are limited, resulting in low Eu 31 emission intensity upon the host-lattice excitation. Under 274 nm ( 8 S7 / 2 → 6 IJ ) and 254 nm ( 8 S7 / 2 → 6 DJ ) excitations, the only detectable emission lines of GdBaB 9 O 16 at about 310 nm are attributed to the 6 PJ → 8 S7 / 2 transitions. The absence of the radiative transition from 6 DJ and 6 IJ states implies that the nonradiative 6 6 6 decay from DJ and IJ to PJ is predominant. For weak coupling interaction, the nonradiative decay is most likely through the multiphonon relaxation. The rate of multi8
96
Z. Yang et al. / Journal of Alloys and Compounds 308 (2000) 94 – 97
Fig. 3. VUV excitation spectrum of Gd 0.95 Eu 0.05 BaB 9 O 16 , monitored by Eu 31 emission.
phonon relaxation can be calculated using modified energy-gap law of Dijk and Schuurmans [14,15] Wnr 5 bel exp[2a (DE 2 2hvmax )]
(1)
where the bel and a are constants for a given host lattice, DE is the energy difference of the energy level considered, and hvmax is the highest fundamental phonon frequency. For the borates, bel 516.8310 7 s 21 and a 54.43310 23 s 21 [14]. The hvmax obtained from IR spectrum of GdBaB 9 O 16 is about 1450 cm 21 . The energy difference DE between the 6 D9 / 2 and 6 I11 / 2 is about 2900 cm 21 , and
that between 6 I7 / 2 and 6 PJ is about 3900 cm 21 . Inserting these values in Eq. 1 yields the nonradiative decay of Wnr 51.68310 8 s 21 for the 6 D9 / 2 → 6 I11 / 2 and of Wnr 5 6 21 6 6 2.0310 s for the I7 / 2 → P7 / 2 . The radiative decay 31 time for the Gd ions in solids is about 1 ms, which gives a decay rate Wr 51310 3 s 21 [16]. Since the multiphonon relaxation rate exceeds remarkably the radiative decay rate, the quenching of the 6 D9 / 2 and 6 I7 / 2 states is mainly attributed to the multiphonon relaxation in GdBaB 9 O 16 . As far as the nonradiative decay is concerned, the multiphonon relaxation rate between the excited states of Gd 31 in fluorides should be relatively smaller because the vibration frequency hvmax is low. In the LiYF 4 , for 6 8 31 example, the IJ → S7 / 2 radiative transitions of Gd have been observed [6], however, the radiative transition rate is still 5 times smaller than the nonradiative transition. Upon 202 nm excitation ( 8 S7 / 2 → 6 GJ ), three emission 31 6 6 6 6 transitions of Gd ions, GJ → IJ , GJ → PJ and 6 8 PJ → S7 / 2 , have been observed as shown in Fig. 5. The emission of 6 PJ → 8 S7 / 2 is the strongest transition, and its integrated intensity is approximately 5 times higher than that of the 6 GJ → 6 PJ and 6 GJ → 6 IJ transitions. The emission spectrum of 6 P3 / 2 → 8 S7 / 2 shown in Fig. 5a consists of three peaks, originated from the three crystallographically different Gd positions in the structure [12]. The emission spectrum of 6 GJ → 6 PJ shown in Fig. 5b is more complicated because of the splitting of the final 6 PJ states and the multiple Gd sites. Fig. 5c shows the emission spectrum of 6 GJ → 6 IJ up to 800 nm and, the longer wavelength part was not measured due to the limitation of the present spectrophotometer. In order to exclude the possible contamination from europium, the excitation spectrum of each emission lines in the 6 GJ → 6 PJ transitions were measured and all of them show a single and characteristic
Fig. 4. Energy transfer scheme in GdBaB 9 O 16 : Eu 31 .
Z. Yang et al. / Journal of Alloys and Compounds 308 (2000) 94 – 97
Fig. 5. Emission spectra of Gd 31 in GdBaB 9 O 16 upon 202 nm excitation, 6 PJ → 8 S7 / 2 (a), 6 GJ → 6 PJ (b) and 6 GJ → 6 IJ (c).
S7 / 2 → 6 GJ excitation as shown in Fig. 6. The absence of the typical O 22 →Eu 31 charge transfer transition clearly shows that there is no detectable contamination in the samples. The absence of the other excitation transitions, such as 8 S7 / 2 → 6 DJ or 8 S7 / 2 → 6 IJ in Fig. 6 is expectable since the emission lines are originated from the 6 GJ states. The luminescence processes of Gd 31 in GdBaB 9 O 16 can be understood in the following. Upon 8 S7 / 2 → 6 GJ excitation, cascade luminescence within the f-configuration of the Gd 31 ions occurs. Based on the energy level diagram in Fig. 1, there are several possibilities for the Gd 31 ions to decay from the 6 GJ states, i.e. 6 GJ → 6 DJ , 6 GJ → 6 IJ and 6 GJ → 6 PJ . The emission of 6 GJ → 6 DJ should locate at about 1000 nm, which is beyond the instrument limitation. The 6 GJ → 6 IJ and 6 GJ → 6 PJ transitions were observed,
97
which are located at about 730–800 nm and 560–650 nm respectively. Following the first emission photon, the excitation energy on the 6 DJ and 6 IJ state decays nonradiatively through the multiphonon relaxation to the 6 PJ states and than the ultraviolet radiative emission of 6 PJ → 8 S7 / 2 occurs. Although the quantitative analysis of the quantum efficiency was not performed, the quantum efficiency of this material may possibly exceed over 100%. However, above results indicated that considerable branch of emissions occur in the near IR region for this material, for example the 6 GJ → 6 DJ and 6 GJ → 6 IJ transitions, so that the materials based on the emission of Gd 31 alone may not be favorable as the visible quantum cutter. Nevertheless, it has been clearly demonstrated that the photon cascade luminescence can occur in borate materials, and this may be very helpful in understanding the luminescent process of the other borates or oxide under VUV excitation.
8
Fig. 6. Excitation spectrum of Gd 31 in GdBaB 9 O 16 , monitored by the 6 GJ → 6 PJ emission (608 nm).
Acknowledgements We are thankful to the financial support from the Youth Scientist Excellency Foundation (29625101), NSFC (29731010), Rhodia Rare Earth Co. and the State Key Basic Research Program.
References [1] W.W. Piper, J.A. DeLuca, F.S. Ham, J. Lumin. 8 (1974) 344. [2] A.M. Srivastava, D.A. Doughty, W.W. Beers, J. Electrochem. Soc. 143 (1996) 4113. [3] A.M. Srivastava, D.A. Doughty, W.W. Beers, J. Electrochem. Soc. 144 (1997) L190. [4] A.M. Srivastava, W.W. Beers, J. Lumin. 71 (1997) 185. [5] R. Pappalardo, J. Lumin. 14 (1976) 159. ¨ ¨ ¨ [6] R.T. Wegh, H. Donker, A. Meijerink, R.J. Lamminmaki, J. Holsa, Phys. Rev. B 56 (1997) 13841. [7] R.T. Wegh, H. Donker, D. Oskam, A. Meijerink, Science 283 (1999) 663. [8] R.P. Rao, D.J. Devine, Abstracts of Int. Conf. Lumin. Opt. Spectr. Condens. Matter, P152, August 23–27, 1999, Osaka, Japan. [9] S. Saubat, M. Vlasse, C. Fouassier, J. Solid State Chem. 34 (1980) 271. [10] W.T. Fu, C. Fouassier, P. Hagenmuller, Mat. Res. Bull. 22 (1987) 389. [11] W.T. Fu, C. Fouassier, P. Hagenmuller, Mat. Res. Bull. 22 (1987) 899. [12] Z. Yang, J.H. Lin, C. Zheng, unpublished results. [13] Z. Yang, J.H. Lin, M.Z. Su, L.P. You, Mater. Res. Bull., submitted 1999. [14] J.M.F. van Dijk, M.F.H. Schuurmans, J. Chem. Phys. 78 (1983) 5317. [15] M.F.H. Schuurmans, J.M.F. van Dijk, Physica 123B (1984) 131. [16] J. Sytsma, G.F. Imbush, G. Blasse, J. Phys.: Condens. Matter 2 (1990) 5171.
L
www.elsevier.com / locate / jallcom
Photon cascade luminescence of Gd 31 in GdBaB 9 O 16 a a, a b b Z. Yang , J.H. Lin *, M.Z. Su , Y. Tao , W. Wang a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, Department of Materials Chemistry, Peking University, Beijing 100871, PR China b Beijing Radiation Laboratory, Institute for High Energy Physics, Beijing 100030, PR China Received 29 December 1999; accepted 15 April 2000
Abstract The photon cascade emission of Gd 31 ions under 8 S7 / 2 → 6 GJ excitation is observed in GdBaB 9 O 16 . Under the 202 nm excitation, a red and a near infrared radiative transition from 6 GJ states, 6 GJ → 6 IJ and 6 GJ → 6 PJ occur followed by an ultraviolet emission of 6 PJ → 8 S7 / 2 . The absence of emission from 6 DJ and 6 IJ states is attributed to the effective multiphonon 6 DJ → 6 IJ and IJ → 6 PJ relaxation in this lattice. 2000 Elsevier Science S.A. All rights reserved. Keywords: Cascade luminescence; Rare earth borate; Nonradiative decay; Optical properties; Luminescence
The development of phosphors for excitation in vacuum ultraviolet (VUV) region becomes an important challenge in the field of luminescent materials. The initiation of the study is the proposed mercury-free lamps, as well as the plasma display panels. Because the energy of the VUV photon (172 nm for xenon dimer) is much higher than that of the Hg discharge radiation (254 nm), a large part of the energy is lost through nonradiative relaxation processes, even if the quantum efficiency of the phosphor is close to 100%. To make the mercury-free lamp competitive, a quantum efficiency of more than 100% is required for the VUV phosphors. The ability of Pr 31 to generate two photons for every absorbed photon was known for quite a long time. A quantum yield of about 140% upon 185 nm excitation for the Pr 31 activated YF 3 was reported [1]. Recent studies on Pr 31 activated borates and other oxides [2–4] show that the photon cascade luminescence of Pr 31 may occur if the lowest energy component of the 4f5d configuration is located above the 1 S0 state. This phenomenon has also been studied for Tm 31 , but substantial amount of energy is lost in the IR region [5]. Very recently, Wegh et al. observed cascade luminescence of Gd 31 ions in LiYF 4 [6] and demonstrated that an alternative concept for obtaining quantum efficiencies of more than 100% by *Corresponding author. Tel.: 186-10-62751-715; fax: 186-10-62751708. E-mail address: [email protected] (J.H. Lin).
using two lanthanide ions [7]. Through two-step energy transfer from Gd 31 to Eu 31 , two red emission photons of Eu 31 were realized with quantum efficiency close to 200%. As shown in Fig. 1, the concept of this process is based on the cascade transitions of Gd 31 of 6 GJ → 6 PJ and 6 PJ → 8 S7 / 2 , as well as the subsequent energy transfer to
Fig. 1. Energy-level scheme in the range 0–50 000 cm 21 for Gd 31 [6].
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00908-7
Z. Yang et al. / Journal of Alloys and Compounds 308 (2000) 94 – 97
the Eu 31 ions. In terms of application, however, the instability and low absorption ability do not permit the utilization of the fluoride materials in the plasma display panel (PDP) and the mercury-free lamps. Lanthanide borates, such as GdBO 3 and GdB 3 O 6 , are known to show very strong VUV absorption around 170 nm and promising chemical stability. In fact, the Eu 31 doped GdBO 3 has been commercially used in the PDP devices [8]. However, the study of the two-step energy transfer process in lanthanide borates is complicated, which may concern the absorption and energy migration in the host-lattice and the energy transfer and luminescence processes of the lanthanide ions in the materials. In this report we focus on cascade luminescence of the Gd 31 ions in GdBaB 9 O 16 . The LnBaB 9 O 16 family was first reported by Saubat et al. [9], and was studied systematically by Fu et al. [10,11]. The crystal structure of GdBaB 9 O 16 has been studied by using single crystal technique [12]. This compound crystallized in rhombohedral space group R3 with the lattice ˚ The crystal constant of a57.809(2), c546.47(1) A. structure model was established using direct method and subsequent refinement leads to a residual value of R5 0.035. However, some of the boron atoms can not be refined appropriately and further study with neutron diffraction is under the way. The preliminary structure model shows that there are three crystallographic independent Gd ˚ The sites and the shortest Gd–Gd distance is about 4.9 A. long Gd–Gd distance may reduce the probability of the 31 energy migration among the Gd ions, which in turn enhances the cascade emission of the Gd 31 ions. Single crystals of GdBaB 9 O 16 and LaBaB 9 O 16 were synthesized in the flux of B 2 O 3 . The starting materials, Ln 2 O 3 , BaCO 3 and B 2 O 3 , in a ratio of LnBaB 9 O 16 : B 2 O 3 51:1, were mixed and heated to 10708C. After maintained at this temperature for about 2 h, the samples were cooling down to 4008C in a rate of 2–38C / h, then to room temperature in about 12 h. Both GdBaB 9 O 16 and LaBaB 9 O 16 crystals are colorless, transparent and plate like. The Eu 31 doped single crystals were synthesized in a similar way. X-ray powder diffraction was recorded on a Rigaku D/ max–2400 powder diffractometer with Cu-Ka radiation from a rotating anode. The VUV absorption and excitation spectra were measured on the Spectroscopy Station of Beijing Synchrotron Laboratory. An Acton VM502 VUV monochromator with a 1200 lines / mm holographic concave grating was used and the signal was detected by EMI 9635QB PMT with sodium salicylate as fluorescence converter. Luminescence spectra were recorded with a Hitachi M-850 fluorescence spectrophotometer and both emission and excitation spectra were corrected for the excitation source and the photomultiplier. Fig. 2 shows the absorption spectra of GdBaB 9 O 16 and LaBaB 9 O 16 . The absorption spectrum of GdBaB 9 O 16 is composed of sharp lines originated from the f–f transitions of the Gd 31 ion and a broad band originated from the host-lattice. According to the energy diagram of Fig. 1, the
95
Fig. 2. Absorption spectra of LaBaB 9 O 16 and GdBaB 9 O 16 .
sharp lines are assigned to the transitions from ground state S7 / 2 to the excited states of 6 PJ , 6 IJ , 6 DJ and 6 GJ respectively. The broad absorption band at short wavelength (,170 nm) should be related to the inter-band transition. Similar host-lattice absorption was observed for LaBaB 9 O 16 as shown in the figure. From the preliminary structural study [13], we know that GdBaB 9 O 16 and LaBaB 9 O 16 are crystallized in a same crystal structure. For the LnBaB 9 O 16 compound, the valence band is mainly composed of the bonding orbitals of borate groups, while the conduction band may consist of the 5d orbitals of the lanthanide and the antibonding orbitals of the borate groups. The band gap obtained from the absorption spectra is about 7.0 eV. Although the absorption of host lattice is very intense, the energy transfer to the doped activators is prevented largely due to the long Gd–Gd distance in these materials. In Fig. 3, we show the excitation spectrum of Eu 31 doped GdBaB 9 O 16 , monitored by Eu 31 emission. The strong excitation at about 240 nm is the charge transfer transition of O 22 →Eu 31 , and the line at about 274 nm is originated from the 8 S7 / 2 → 6 IJ transition of the Gd 31 ions. In comparison with the O 22 →Eu 31 charge transfer transition, the excitation efficiency of the host-lattice is relatively low. Fig. 4 shows proposed excitation and energy transfer processes upon excitation of the inter-band absorption. Because the long Gd–Gd distance in GdBaB 9 O 16 , the energy transfer between the Gd 31 ions and that of Gd 31 →Eu 31 are limited, resulting in low Eu 31 emission intensity upon the host-lattice excitation. Under 274 nm ( 8 S7 / 2 → 6 IJ ) and 254 nm ( 8 S7 / 2 → 6 DJ ) excitations, the only detectable emission lines of GdBaB 9 O 16 at about 310 nm are attributed to the 6 PJ → 8 S7 / 2 transitions. The absence of the radiative transition from 6 DJ and 6 IJ states implies that the nonradiative 6 6 6 decay from DJ and IJ to PJ is predominant. For weak coupling interaction, the nonradiative decay is most likely through the multiphonon relaxation. The rate of multi8
96
Z. Yang et al. / Journal of Alloys and Compounds 308 (2000) 94 – 97
Fig. 3. VUV excitation spectrum of Gd 0.95 Eu 0.05 BaB 9 O 16 , monitored by Eu 31 emission.
phonon relaxation can be calculated using modified energy-gap law of Dijk and Schuurmans [14,15] Wnr 5 bel exp[2a (DE 2 2hvmax )]
(1)
where the bel and a are constants for a given host lattice, DE is the energy difference of the energy level considered, and hvmax is the highest fundamental phonon frequency. For the borates, bel 516.8310 7 s 21 and a 54.43310 23 s 21 [14]. The hvmax obtained from IR spectrum of GdBaB 9 O 16 is about 1450 cm 21 . The energy difference DE between the 6 D9 / 2 and 6 I11 / 2 is about 2900 cm 21 , and
that between 6 I7 / 2 and 6 PJ is about 3900 cm 21 . Inserting these values in Eq. 1 yields the nonradiative decay of Wnr 51.68310 8 s 21 for the 6 D9 / 2 → 6 I11 / 2 and of Wnr 5 6 21 6 6 2.0310 s for the I7 / 2 → P7 / 2 . The radiative decay 31 time for the Gd ions in solids is about 1 ms, which gives a decay rate Wr 51310 3 s 21 [16]. Since the multiphonon relaxation rate exceeds remarkably the radiative decay rate, the quenching of the 6 D9 / 2 and 6 I7 / 2 states is mainly attributed to the multiphonon relaxation in GdBaB 9 O 16 . As far as the nonradiative decay is concerned, the multiphonon relaxation rate between the excited states of Gd 31 in fluorides should be relatively smaller because the vibration frequency hvmax is low. In the LiYF 4 , for 6 8 31 example, the IJ → S7 / 2 radiative transitions of Gd have been observed [6], however, the radiative transition rate is still 5 times smaller than the nonradiative transition. Upon 202 nm excitation ( 8 S7 / 2 → 6 GJ ), three emission 31 6 6 6 6 transitions of Gd ions, GJ → IJ , GJ → PJ and 6 8 PJ → S7 / 2 , have been observed as shown in Fig. 5. The emission of 6 PJ → 8 S7 / 2 is the strongest transition, and its integrated intensity is approximately 5 times higher than that of the 6 GJ → 6 PJ and 6 GJ → 6 IJ transitions. The emission spectrum of 6 P3 / 2 → 8 S7 / 2 shown in Fig. 5a consists of three peaks, originated from the three crystallographically different Gd positions in the structure [12]. The emission spectrum of 6 GJ → 6 PJ shown in Fig. 5b is more complicated because of the splitting of the final 6 PJ states and the multiple Gd sites. Fig. 5c shows the emission spectrum of 6 GJ → 6 IJ up to 800 nm and, the longer wavelength part was not measured due to the limitation of the present spectrophotometer. In order to exclude the possible contamination from europium, the excitation spectrum of each emission lines in the 6 GJ → 6 PJ transitions were measured and all of them show a single and characteristic
Fig. 4. Energy transfer scheme in GdBaB 9 O 16 : Eu 31 .
Z. Yang et al. / Journal of Alloys and Compounds 308 (2000) 94 – 97
Fig. 5. Emission spectra of Gd 31 in GdBaB 9 O 16 upon 202 nm excitation, 6 PJ → 8 S7 / 2 (a), 6 GJ → 6 PJ (b) and 6 GJ → 6 IJ (c).
S7 / 2 → 6 GJ excitation as shown in Fig. 6. The absence of the typical O 22 →Eu 31 charge transfer transition clearly shows that there is no detectable contamination in the samples. The absence of the other excitation transitions, such as 8 S7 / 2 → 6 DJ or 8 S7 / 2 → 6 IJ in Fig. 6 is expectable since the emission lines are originated from the 6 GJ states. The luminescence processes of Gd 31 in GdBaB 9 O 16 can be understood in the following. Upon 8 S7 / 2 → 6 GJ excitation, cascade luminescence within the f-configuration of the Gd 31 ions occurs. Based on the energy level diagram in Fig. 1, there are several possibilities for the Gd 31 ions to decay from the 6 GJ states, i.e. 6 GJ → 6 DJ , 6 GJ → 6 IJ and 6 GJ → 6 PJ . The emission of 6 GJ → 6 DJ should locate at about 1000 nm, which is beyond the instrument limitation. The 6 GJ → 6 IJ and 6 GJ → 6 PJ transitions were observed,
97
which are located at about 730–800 nm and 560–650 nm respectively. Following the first emission photon, the excitation energy on the 6 DJ and 6 IJ state decays nonradiatively through the multiphonon relaxation to the 6 PJ states and than the ultraviolet radiative emission of 6 PJ → 8 S7 / 2 occurs. Although the quantitative analysis of the quantum efficiency was not performed, the quantum efficiency of this material may possibly exceed over 100%. However, above results indicated that considerable branch of emissions occur in the near IR region for this material, for example the 6 GJ → 6 DJ and 6 GJ → 6 IJ transitions, so that the materials based on the emission of Gd 31 alone may not be favorable as the visible quantum cutter. Nevertheless, it has been clearly demonstrated that the photon cascade luminescence can occur in borate materials, and this may be very helpful in understanding the luminescent process of the other borates or oxide under VUV excitation.
8
Fig. 6. Excitation spectrum of Gd 31 in GdBaB 9 O 16 , monitored by the 6 GJ → 6 PJ emission (608 nm).
Acknowledgements We are thankful to the financial support from the Youth Scientist Excellency Foundation (29625101), NSFC (29731010), Rhodia Rare Earth Co. and the State Key Basic Research Program.
References [1] W.W. Piper, J.A. DeLuca, F.S. Ham, J. Lumin. 8 (1974) 344. [2] A.M. Srivastava, D.A. Doughty, W.W. Beers, J. Electrochem. Soc. 143 (1996) 4113. [3] A.M. Srivastava, D.A. Doughty, W.W. Beers, J. Electrochem. Soc. 144 (1997) L190. [4] A.M. Srivastava, W.W. Beers, J. Lumin. 71 (1997) 185. [5] R. Pappalardo, J. Lumin. 14 (1976) 159. ¨ ¨ ¨ [6] R.T. Wegh, H. Donker, A. Meijerink, R.J. Lamminmaki, J. Holsa, Phys. Rev. B 56 (1997) 13841. [7] R.T. Wegh, H. Donker, D. Oskam, A. Meijerink, Science 283 (1999) 663. [8] R.P. Rao, D.J. Devine, Abstracts of Int. Conf. Lumin. Opt. Spectr. Condens. Matter, P152, August 23–27, 1999, Osaka, Japan. [9] S. Saubat, M. Vlasse, C. Fouassier, J. Solid State Chem. 34 (1980) 271. [10] W.T. Fu, C. Fouassier, P. Hagenmuller, Mat. Res. Bull. 22 (1987) 389. [11] W.T. Fu, C. Fouassier, P. Hagenmuller, Mat. Res. Bull. 22 (1987) 899. [12] Z. Yang, J.H. Lin, C. Zheng, unpublished results. [13] Z. Yang, J.H. Lin, M.Z. Su, L.P. You, Mater. Res. Bull., submitted 1999. [14] J.M.F. van Dijk, M.F.H. Schuurmans, J. Chem. Phys. 78 (1983) 5317. [15] M.F.H. Schuurmans, J.M.F. van Dijk, Physica 123B (1984) 131. [16] J. Sytsma, G.F. Imbush, G. Blasse, J. Phys.: Condens. Matter 2 (1990) 5171.