Luminescence and Energy Transfer Properties of Gd3+ and Tb3+ in

Jun 21, 2007 - f-f transition of Tb3+ ions around the near-UV region, whereas the 4f-5d transition could be neglected. In the Gd3+-Tb3+-activated syst...
91 downloads 0 Views 366KB Size
Download PDF
10682

J. Phys. Chem. C 2007, 111, 10682-10688

Luminescence and Energy Transfer Properties of Gd3+ and Tb3+ in LaAlGe2O7 Yu-Chun Li,† Yen-Hwei Chang,*,† Yee-Shin Chang,‡ Yi-Jing Lin,† and Chih-Hao Laing† Department of Materials Science and Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan, and Institute of Electro-Optical and Materials Science, National Formosa UniVersity Huwei, Yunlin 632, Taiwan ReceiVed: March 9, 2007; In Final Form: May 11, 2007

The novel phosphors of LaAlGe2O7 doped with Gd3+ and Tb3+ were synthesized, and their luminescence properties were investigated. In Tb3+-activated LaAlGe2O7, the unusual excitation spectra showed only intense f-f transition of Tb3+ ions around the near-UV region, whereas the 4f-5d transition could be neglected. In the Gd3+-Tb3+-activated system, the decay time of Tb3+ emission under Gd3+ excitation at the 6IJ energy level was longer than that measured under direct Tb3+ excitation at the 5L10 energy level, which reinforces the existence of effective Gd3+-to-Tb3+ energy transfer. In the 5D4 decay measurements, time-resolved spectra exhibited a “grow-in” behavior as the Tb3+ concentration was diluted (0.05 mol) Tb3+ concentrations. Two linear relationships were found between the energy transfer probability (PGdfTb) and the Tb3+ concentration, which is clear evidence that there were two kinds of efficient Gd3+-to-Tb3+ energy transfer processes in terms of Tb3+ concentration.

Introduction Rare-earth-ion-doped crystallite has attracted considerable research interest owing to its excellent luminescence properties. The rare-earth ions are characterized by a partially filled 4f shell that is well shielded by 5s2 and 5p6 orbitals. Therefore, emission transitions yield sharp lines in the optical spectra.1 The use of rare-earth-element-based phosphor, based on “line-type” f-f transitions, can narrow the emissions to the visible range, resulting in high efficiency and a high lumen equivalence. It is usually considered that the emission of a Gd3+ 6PJ f 8S7/2 transition located at 273 nm overlapps well with the 4f-4f absorption lines of rare earth ions, so the incorporation of Gd3+ could improve the luminescence of rare earth ions. Accordingly, Gd3+ is used as the sensitizer of Sm3+, Eu3+, Eu2+, Tb3+, and Dy3+ in many systems.2-5 The stoichiometric formula of germanates MRGe2O7 (where M ) Al3+, Ga3+, or Fe3+ and R ) rare-earth ion) was reported in the early 1980s to belong to the monoclinic AlNdGe2O7 structure type,6-8 space group P21/c (no. 14). These kinds of compounds are of great interest in laser crystal physics. For instance, the incorporation of R3+ activators into single-centered hosts up to full substitution of all cations gives the possibility of obtaining the so-called self-activated crystals, and the oxygen coordination around rare-earth cations is only 9-fold (CN9). The luminescence study of a series of such compounds provides much valuable information for optical applications. Recently, we have reported that the Tb3+ and Tm3+ ions in LaAlGe2O7 present intense green and blue light emissions, respectively.9,10 Building upon this work, we have investigated the related system (La1-x-yTbxGdy)AlGe2O7. The photoluminescence properties and time-decay measurements clearly show that incorporation * Corresponding author. No. 1, Ta-Hsueh Road, Tainan 70101, Taiwan. Tel.: +886-6-2757575, ext 62941. Fax: +886-6-2382800. E-mail: [email protected]. † National Cheng Kung University. ‡ National Formosa University Huwei.

of Gd3+ and Tb3+ ions into this crystal induces the effective Gd3+-to-Tb3+ energy transfer. Experimental Materials and Synthesis. Samples of Gd3+- and Tb3+-doped LaAlGe2O7 were synthesized by a vibrating, milled, solid-state reaction. The starting materials were La2O3, GeO2, Al2O3, Gd2O3, and Tb4O7 (purity g 99.99%). After they had been mechanically activated by grinding in a high-energy vibromill, the mixtures were calcined at 1250 °C in air for 12 h. For Tb3+doped samples, the heat treatment powders were then fired under a reducing atmosphere (4% H2/96% Ar) at 800 oC for 2 h to convert Tb4+ to Tb3+ in order to obtain higher emission intensity. This reduction process improved the emission intensity by ∼30%. The powders had a white color, which indicates that all terbium ions are in the trivalent state. Characterizations. Conventional X-ray diffraction (XRD) techniques were employed to identify the phases. Both excitation and luminescence spectra of these phosphors were recorded on a Hitachi F-4500 fluorescence spectrophotometer using a 150 W Xe lamp as a source at room temperature. Optical absorption spectra were measured at room temperature using a Hitachi U-3010 UV-visible spectrophotometer. An integrating sphere is suitable for absorbance measurement of a turbid sample and reflection measurement of a solid sample surface. The integrating sphere attachment was used for measuring our samples. Results and Discussion Structural Characterization. The XRD data of the samples reveals a single phase; all of the peaks were identified to be the monoclinic LaAlGe2O7 phase (space group P21/c). The XRD patterns of the extensive substitution of Gd3+ and Tb3+ for La3+ in LaAlGe2O7 are shown in Figure 1. As Gd3+ and Tb3+ content increases, the peaks shift to higher angles. It was recognized that a slight difference in the ionic radius for La3+ (rIX ) 1.216 Å), Gd3+ (rIX ) 1.107 Å), and Tb3+ (rIX ) 1.095 Å) ions

10.1021/jp0719107 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007

Gd3+ and Tb3+ in LaAlGe2O7

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10683

Figure 3. The variation of emission intensity and decay time of the 6 PJ f 8S7/2 transition with Gd3+ concentrations in LaAlGe2O7 under excitation at 273 nm. The signals were detected at 312 nm.

Figure 1. The X-ray diffraction patterns of the LaAlGe2O7:Gd, Tb powders.

Figure 2. The photoluminescence excitation and emission spectra of LaAlGe2O7:Gd measured at room temperature.

resulted in a small difference in the lattice parameters of the solid solution. This is direct experimental evidence of the fact that the crystal can be assigned to the structural nature of the LaAlGe2O7 phase and an indication that the Gd3+ and Tb3+ ions were satisfactorily substituted for the La3+ ions in the lattice. The Luminescence of Gd3+-Doped LaAlGe2O7. Figure 2 presents PL excitation and emission spectra associated with Gd3+ ions in LaAlGe2O7. The sharp peaks between 200 and 400 nm are assigned to the typical 4f f 4f intraconfiguration forbidden transitions of Gd3+. The major emission peak of Gd3+ was at 312 nm, which corresponds to the transition 6PJ f 8S7/2. This luminescence can be excited at the 6IJ (∼273 nm) and 6DJ (∼250 nm) levels, respectively. Samples with various amounts of Gd3+ content had a spectra wavelength pattern similar to those in Figure 2. No wavelength shift or peak for a new site was observed at high Gd3+ concentrations. The emission and decay time behaviors of the 6P f 8S 3+ content under J 7/2 transition dependence of the Gd excitation at 273 nm are illustrated in Figure 3. The intensity of the emission increased proportionally with Gd3+ concentration

until saturation was reached at 0.2 mol. In addition, the lifetime reduced slightly with increasing Gd3+ concentration to 0.2 mol and then begins to decrease rapidly as Gd3+ content increased. This phenomenon (the concentration quenching effect) is due to the rise in the number of nonradiative decay channels, as promoted by the interaction with quenching centers during the cross-relaxation or energy transfer processes between excited and unexcited Gd3+ ions. In many cases, the concentration quenching is due to energy transfer from one activator to another until an energy sink in the lattice is reached, which is related to the interaction between an activator and another ion. For this reason, it is possible to obtain the critical distance (Rc) from the concentration quenching data. Rc is the critical separation between donor (activator) and acceptor (quenching site), for which the nonradiative transfer rate equals the internal decay rate. Blasse11 assumed that for the critical concentration, the average shortest distance between nearest activator ions is equal to the critical distance. LaAlGe2O7 contains a unique crystallographic site available for the activator (substitute for La3+) so that only one type of center is present. Hence, one can obtain the critical distance from the concentration quenching data using the following equation,

( )

Rc ) 2

3V 4πxcN

1/3

(1)

where xc is the critical concentration, N is the number of La3+ ions in the LaAlGe2O7 unit cell (activator ions are assumed to be introduced solely into La3+ sites), and V is the volume of the unit cell (545.45 × 10-30 m3 in this case). The critical concentration was estimated to be ∼20 mol % for Gd3+, for which the emission intensity and decay time measured begin to decrease quickly. Using the above equation, Rc was determined to be about 10.92 Å for Gd3+. The Luminescence of Gd3+- and Tb3+-Doped LaAlGe2O7. The photoluminescence excitation spectrum of LaAlGe2O7:Tb is shown in Figure 4a. The series of sharp excitation peaks between 250 and 500 nm correspond to the Tb3+ intra-4f (4f84f8) transitions. An interesting phenomenon occurs when comparing this result with conventional Tb-doped phosphors.12-14 The particularly noteworthy one is that the remarkably weak 4f-5d transition band of the Tb3+ ion around 235 nm could be detected in the excitation spectra. This phenomenon is uncommon because the typical Tb-activated phosphors always show strong 4f-5d transition band absorption around 200-300 nm.

10684 J. Phys. Chem. C, Vol. 111, No. 28, 2007

Li et al.

Figure 5. Schematic energy levels of the Gd3+-Tb3+ system showing possible energy transfer mechanisms (pathways).

Figure 4. Excitation spectra of LaAlGe2O7:Tb and LaAlGe2O7:Gd, Tb, respectively.

The uneven components mix a small amount of opposite-parity wave functions (such as 5d) into the 4f wave functions. In this way, the intraconfigurational 4fn transitions obtain some intensity. The forbidden 4f-4f transition steals some intensity from the allowed 4f-5d transition.1 This may be the reason that LaAlGe2O7:Tb shows intense f-f absorption. Additionally, it is recognized that only direct excitation of Tb3+ ions could be observed, and no efficient energy transfer occurs between activator and host. Figure 4b shows the excitation spectrum of Gd3+ and Tb3+ co-activated LaAlGe2O7, monitored at the 5D4 f 7F5 (Tb3+) transition. In comparison with Figure 4a, it is clear that there are additional peaks at about 250, 273, and 312 nm, which are assigned to the Gd3+ transitions from the ground level 8S7/2 to excited levels 6DJ, 6IJ, 6PJ, respectively. The presence of the Gd3+ transitions in the excitation spectrum monitored within the Tb3+ transitions indicates that the Gd3+-to-Tb3+ energy transfer channel is active. The excitation spectrum of Tb3+ and the emission spectrum of Gd3+ do not overlap perfectly. It is considered that phonons may be involved in the process. This observation has been previously reported2,5 and suggests that the excitation of Tb3+ occurs mainly through the excitation of Gd3+. Because of the strong interaction between Gd3+ and Tb3+, Tb3+ is easily excited by Gd3+. The possible energy transfer process between Gd3+ and Tb3+ is depicted in Figure 5. The position of the 4f-5d levels is greatly influenced by the crystal field interaction. This effect is known as the crystal field depression of the 5d level, and consequently, the host crystal depresses 4f-5d energy levels for all lanthanides.15 Figure 6 displays the absorption spectra of the LaAlGe2O7 and LaAlGe2O7: Tb at room temperature. The UV-vis optical absorption spectrum of LaAlGe2O7 demonstrates that the absorption bands

Figure 6. Absorption spectra of LaAlGe2O7:20%Tb, LaAlGe2O7: 10%Tb, and blank LaAlGe2O7, respectively. The arrows designate the 4f-5d absorption band of Tb3+.

correspond to the UV region at 215 nm and 300 nm, respectively. The broad absorption band located at ∼300 nm is ascribed to the defects in the germinate host.16 The major absorption edge of pure LaAlGe2O7 is situated at around 250 nm (∼4.96 eV). After Tb3+ was added to LaAlGe2O7, the compounds still exhibited a strong broad host absorption band. Inspecting the absorption spectra on the Tb3+-doped LaAlGe2O7 in detail, however, one observes that there is a slight absorption variation at the host absorption edge. This value is in good agreement with the Tb3+ 4f-5d absorption band; however, there is no strong transition assigned to the host or 4f-5d absorption in the excitation spectrum. Such a mismatch in the wavelength pattern occurs because the absorption spectrum is detected primarily by the numbers of occupied states in the ground level and the numbers of unoccupied states not only in the excited level but also on the transition probability, whether the transition is radiative or not. Nevertheless, only the radiative transition can be measured in the photoluminescence excitation spectrum. A comparison

Gd3+ and Tb3+ in LaAlGe2O7

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10685

Figure 9. Plot of the Gd3+-to-Tb3+ energy transfer probability PGdfTb versus Tb3+ concentration in (La0.8-xTbx)Gd0.2AlGe2O7 using linear curve-fitting. The inset magnifies the PGdfTb-versus-Tb3+ concentration from 0.0005 to 0.03 mol.

TABLE 1: Gd3+-to-Tb3+ Energy Transfer Probabilities and Efficiencies of (La0.8-xTbx)Gd0.2AlGe2O7 Crystals Figure 7. Schematic energy band diagram of Tb3+ energy levels in the LaAlGe2O7 energy band. Photoexcitation to 4f5d (arrow) may be followed by photoionization.

Figure 8. Emission spectra of (La0.8-xTbx)Gd0.2AlGe2O7 under excitation at the Gd3+ 6IJ (273 nm) level.

between doped and undoped LaAlGe2O7 samples indicates that the strong host absorption completely dominates at less than 250 nm. In the experiment (Figure 4), the corresponding 4f5d transition of Tb3+ is situated at 235 nm. It might be acceptable that the Tb3+ 4f-5d absorption band is obscured by

% Tb

lifetime (ms)

PGdfTb (ms-1)

ηGdfTb

0 0.05 0.1 0.5 1 3 5 7 10 20 30 40 50

2.91 2.79 2.68 2.21 1.77 1.09 0.89 0.81 0.73 0.56 0.44 0.38 0.32

0 0.015 0.029 0.109 0.221 0.574 0.779 0.891 1.026 1.442 1.879 2.288 2.781

0 0.04 0.08 0.24 0.39 0.63 0.69 0.72 0.75 0.81 0.85 0.87 0.89

the host lattice absorption band so that the radiationless 4f-5d transition occurred. The absence of 4f-5d luminescence in LaAlGe2O7:Tb3+ has been ascribed to quenching by photoionization,17,18 which implies that the lowest 5d level of Tb3+ lies in the conduction band of the host crystal. The issue of this result is that the bottom of the conduction band of LaAlGe2O7 with a d10 ion (Ge4+) is at lower energy than the lowest 5d level of Tb3+. The principle of photoionization is illustrated in Figure 7. In the excited state, an electron can easily be ionized from the center to the conduction band and may recombine nonradiatively with a hole so that the luminescence is quenched. By analyzing absorption and excitation spectra, it is realized that weakness Tb3+ 4f-5d transition could be observed, and no efficient energy transfer occurs between Tb3+ and the host lattice. The energy absorbed by the host that is not emitted as radiation is dissipated to the crystal lattice (nonradiative transitions). The effects of the concentration of Tb3+ ions emission behavior of (La0.8-xTbx)Gd0.2AlGe2O7 as a function of x under excitation at the Gd3+ 6IJ (273 nm) level are illustrated in Figure 8. The emission spectra exhibit a series of sharp peaks assigned to the 5D4 f 7FJ (J ) 3, 4, 5, 6) transitions of Tb3+, and luminescence from the higher excited-state transitions 5D3 f 7F (J ) 2, 3, 4, 5, 6) are also observed. The spectra also display J a sharp peak (312 nm) attributed to the Gd3+ 6PJ f 8S7/2 transition. It is obvious that the emission spectra show a completely different ratio between the blue 5D3 and the green 5D4 emissions at lower and higher Tb3+ concentrations. As can be seen, the

10686 J. Phys. Chem. C, Vol. 111, No. 28, 2007

Li et al.

Figure 10. Normalized decay curves of 5D3 f 7F5 transition for various Tb3+ concentrations in (La0.8-xTbx)Gd0.2AlGe2O7 under excitation at Gd3+ IJ (273 nm) and Tb3+ 5L10 (370 nm) levels.

6

intensity of the emission from 5D3 decreases with increasing Tb3+ concentration due to cross-relaxation. This is a process whereby excitation energy from an ion decaying from a highly excited state promotes a nearby ion from the ground state to the metastable level. In trivalent terbium, the energy gap between the 5D3 and 5D4 levels (∼5600 cm-1) is close to that between the 7F6 and 7F0 levels (∼5800 cm-1). As a result, if the Tb3+ concentration is sufficiently high, the higher energy level emission can be easily quenched in favor of the lower energy level emission.19 The following cross-relaxation may occur:

Tb3+(5D3) + Tb3+(7F6) f Tb3+(5D4)+ Tb3+(7F0)

(2)

In the case of 5D3 f 7FJ, the cross-relaxation is responsible for concentration quenching; the quenching of the luminescence occurs in ion pairs. By increasing the Tb3+ concentration, the intensity of the 6PJ f 8S7/2 transition decreases, in accord with the increase in the Gd3+ to Tb3+ energy transfer, as the excitation spectra in Figure 4 suggest. The analysis of the intensity of Gd3+ emission relative to the Tb3+ 5D4 f 7F5 transition intensity makes this point very clear. Upon excitation of the Gd3+ absorption, the spectra in Figure 8 exhibit Gd3+ and Tb3+ emissions with relative intensities IGd/ITb ) 2.05, 1.58, 0.96, 0.29, and 0.08, at about 0.05, 0.07, 0.1, 0.2, and 0.4 mol of Tb3+, respectively. The distance between Gd3+ and Tb3+ ions decreases as the Tb3+ concentration increases; subsequently, the Gd3+ to Tb3+ energy transfer becomes more frequent, providing an extra decay

channel, which decreases the Gd3+ emission intensity. This implies that the Gd3+-to-Tb3+ energy transfer is efficient. A simple operational definition of Gd3+-to-Tb3+ energy transfer probability (PGdfTb) in terms of lifetimes is given by20-23

PGdfTb )

() ( ) 1 1 τ τ0

(3)

where τ and τ0 are the Gd3+ donor lifetimes in the presence and absence of a Tb3+ acceptor, respectively. Furthermore, the energy transfer efficiency (ηGdfTb) is defined as

ηGdfTb ) 1 -

() τ τ0

(4)

For comparison, the corresponding Gd3+ 6PJ f 8S7/2 transition lifetimes, energy transfer probabilities, and efficiencies in the series of compounds are listed in Table 1. If only Gd3+ and Tb3+ ions are involved in the energy transfer process responsible for the observed decrease in the lifetime, a plot of PGdfTb values versus the Tb3+ concentration should be linear. As shown in Figure 9, the plot is well fitted by two linear relations between the PGdfTb and the Tb3+ concentration, suggesting that there were two kinds of energy transfer processes at lower (∼0.00050.03 mol) and higher (∼0.05-0.5 mol) Tb3+ concentrations. Decay Analysis. In the decay measurements, luminescence was excited at two different energy levels of Gd3+ 6IJ (273 nm)

Gd3+ and Tb3+ in LaAlGe2O7

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10687

Figure 11. Normalized decay curves of 5D4 f 7F5 transition for various Tb3+ concentrations in (La0.8-xTbx)Gd0.2AlGe2O7 under excitation at Gd3+ IJ (273 nm) and Tb3+ 5L10 (370 nm) levels. The inset shows the “grow-in” in the initial part.

6

and Tb3+ 5L10 (370 nm). The decay curves of the 5D3 f 7F5 transition of Tb3+ monitored as a function of the excitation wavelength for different Tb3+ concentrations are shown in Figure 10. In response to Gd3+ excitation at the 6IJ energy levels, the decay time of Tb3+ emission is much longer than that measured under direct Tb3+ excitation at the 5L10 energy level, which reinforces the existence of effective Gd3+-to-Tb3+ energy transfer. It is noteworthy that the relative lifetimes τex)273 nm/ τex)370 nm decreased as Tb3+ increased. With increasing amounts of Tb3+ dopant, the Gd3+-to-Tb3+ energy transfer probabilities, efficiencies, and rates were found to increase gradually. The lifetimes τex)273 nm close to τex)370 nm as the content of Tb3+ increases. The decay curves had a similar performance under excitation at two different energy levels, even though the lifetimes are different between τex)273nm and τex)370nm. A single-exponential decay was observed for the dilute sample. For higher concentrations (x > 0.01), however, the observed decay curves become nonexponential, and the nonexponential change is getting more prominent as x increases, which indicates that more than one relaxation process exists. When the luminescent centers have different local environments, the associated ions will relax at different rates. If the rates are dramatically different, the difference in the decay curves would also be easily observed. Nevertheless, the low doped samples have a single-exponential decay curve with the longer lifetime, which may rule out this

possibility. Additionally, it is unlikely that only one site with the shorter lifetime is populated for higher concentration. As previously mentioned, it is clear that a study of the crossrelaxation over ion-ion interaction between two neighboring Tb3+ ions would be beneficial to clarify this issue. With an increase of Tb3+ concentration, the distance between Tb3+ ions decreases; subsequently, the energy transfer (5D3-5D4 crossrelaxation) between Tb3+ ions becomes more frequent. Thus, the decay rate will be different, resulting in faster lifetimes and the nonexponential decay curves of the 5D3 emission.24,25 Figure 11 depicts the decay curves of the 5D4 f 7F5 transition of Tb3+ under excitation at Gd3+ 6IJ (273 nm) and Tb3+ 5L10 (370 nm) levels. In addition, the relative lifetimes τex)273 nm/ τex)370 nm decrease as the content of Tb3+ increases, which implies the energy transfer rate from Gd3+ to Tb3+ is equal to the Tb3+ decay rate. In contrast with the 5D3 f 7F5 transition, however, the decay curves are apparently different. As seen in the figure, the intensity of the 5D4 f 7F5 transition clearly rises initially for 1 ms then decays slowly under excitation at the Gd3+ 6IJ (273 nm) level. The inset of Figure 11 shows the rescaled spectra, which shows more precise rising in the initial part of the decay curves. Upon excitation at the Tb3+ 5L10 (370 nm) level, however, the initial rise phenomena was not detected. This rising phenomena of Tb3+ decay curves must be originating from the energy transfer between Gd3+ and Tb3+ ions. It is interesting that the time-resolved spectra exhibit a “grow-in”

10688 J. Phys. Chem. C, Vol. 111, No. 28, 2007 behavior at lower (0.05 mol) Tb3+ concentrations, in perfect agreement with the presence of two kinds of energy transfer processes (we will return to this point later). As already presented in Figure 9, this is clear evidence that there are two kinds of efficient Gd3+-to-Tb3+ energy transfer mechanisms in terms of Tb3+ concentration. The time-resolved spectra display the rise part for 5D4 transitions, but not for 5D3 transitions at lower (∼0.0005-0.03 mol) Tb3+ concentrations under excitation at the Gd3+ 6IJ (273 nm) level. This form of the “grow-in” behavior is ascribed to the difference in the initial population of the 5D4 level through the energy relaxation from 5D to 5D . 3 4 The main factors influencing the radiative emission probability of the excited state are multiphonon relaxation processes and energy transfer processes. The rate of multiphonon relaxation depends on the energy gap between the excited state and the next lower state and the maximum vibration energy of the host.26 The energy gap between the 5D3 and 5D4 states is ∼5600 cm-1, and the approximate frequencies of the highest phonon energy in our host was ∼900 cm-1.8 Using these energies, one obtains a multiphonon relaxation rate of about an order of 101 s-1 at room temperature. The 5D3-5D4 cross-relaxation rate is determined to be of an order of 103 s-1,27 which is much larger than the values for multiphonon relaxation rate. At lower (∼0.0005-0.03 mol) Tb3+ concentrations, the crossrelaxation is negligibly small in comparison with the multiphonon relaxation. When the 5D4 energy level is populated from the upper 5D3 state by a slow multiphonon relaxation rate, an initial increase of the luminescence could be observed (otherwise, the feeding of the 5D4 emission would be slower due to the multiphonon relaxation step, which feeds from the 5D level). For high concentration Tb3+-doped samples, the 3 5D -5D cross-relaxation rate is much larger than that by 3 4 multiphonon relaxation rate, so the fast cross-relaxation process is dominant, whereas the rise part is not seen. This is to be expected, since the effective rate constant for the Gd3+-to-Tb3+ energy transfer includes the relatively slow nonradiative relaxation processes to the 5D4 level on the Tb3+ ion. This is exactly what is observed in the experimental results shown in Figure 9; there were two kinds of efficient Gd3+-to-Tb3+ energy transfer processes in terms of Tb3+ concentration. Conclusions Tb3+- and Gd3+-activated LaAlGe2O7 was synthesized, and its luminescence properties were investigated. In the Tb3+activated photoluminescence excitation spectra, only the intense f-f transitions could be found; no obvious 4f-5d transition band of Tb3+ ions or host absorption band could be detected. The absence of 4f-5d luminescence in this host is ascribed to photoionization. In the Gd3+-Tb3+ system, there were two kinds of efficient Gd3+-to-Tb3+ energy transfer processes in terms of

Li et al. Tb3+ concentration upon excitation at the Gd3+ 6IJ (273 nm) level. In the 5D4 decay measurements, two kinds of Gd3+-toTb3+ energy transfer processes were clearly demonstrated under excitation at the Gd3+ 6IJ (273 nm) level. A “grow-in” behavior was observed as the Tb3+ concentration was diluted (0.05 mol) Tb3+ concentrations. Acknowledgment. The authors thank the National Science Council of the Republic of China for financially supporting this research through contract No. NSC-94-2216-E-006-009. References and Notes (1) Blasse, G.; Grabmaier, B. C. Luminescent Materials; SpringerVerlag: New York, 1994; p 25. (2) Shmulovich, J.; Berkstresser, G. W.; Brandle, C. D.; Valentino, A. J. Electrochem. Soc. 1988, 135, 3141-3150. (3) Wegh, R. T.; Donker, H.; Oskam, K.; Meijerink, A. Science 1999, 283, 663-666. (4) Su, H.; Jia, Z.; Shi, C.; Xin, J.; S. Reid, A. Chem. Mater. 2001, 13, 3969-3974. (5) Sato, Y.; Kumagai, T.; Okamoto, S.; Yamamoto, H.; Kunimoto, T. Jpn. J. Appl. Phys. 2004, 43 (6A), 3456-3460. (6) Jarchow, O.; Klaska, K.-H.; Schenk, H. Naturwissenschaften 1981, 68, 475-476. (7) Jarchow, O.; Klaska, K.-H.; Schenk, H. Z. Kristallogr. 1985, 172, 159-166. (8) Kaminskii, A. A.; Mill, B. V.; Butashin, A. V.; Belokoneva, E. L.; Kurbanov, K. Phys. Status Solidi A 1987, 103, 575-592. (9) Li, Y. C.; Chang, Y. H.; Lin, Y. F.; Chang, Y. S.; Lin, Y. J. Electrochem. Solid-State Lett. 2006, 9 (8), H74-H77. (10) Li, Y. C.; Chang, Y. H.; Lin, Y. F.; Lin, Y. J.; Chang, Y. S. Appl. Phys. Lett. 2006, 89, 081110. (11) Blasse, G. Philips Res. Rep. 1969, 24, 131-144. (12) Mayolet, A.; Zhang, W.; Simoni, E.; Krupa, J. C.; Martin, P. Opt. Mater. 1995, 4, 757-769. (13) Kang, Y. C.; Lenggoro, W.; Okuyamaa, K.; Park, S. B. J. Electrochem. Soc. 1999, 146 (3), 1227-1230. (14) Sohn, K. S.; Lee, J. M.; Jeon, I. W.; Park, H. D. J. Electrochem. Soc. 2003, 150 (8), H182-H186. (15) Dorenbos, P. J. Lumin. 2000, 91, 91-106. (16) Poulios, D. P.; Spoonhower, J. P.; Bigelow, N. P. J. Lumin. 2003, 101, 23-33. (17) Blasse, G.; Schipper, W.; Hamelink, J. J. Inorg. Chim. Acta 1991, 189, 77-80. (18) Blasse, G.; de Mello Donega´, C.; Efryushina, N.; Dotsenko, V.; Berezovskaya, I. Solid State Commun. 1994, 92, 687-688. (19) Shionoya, S.; Yen, W. M. Phosphor Handbook; CRC Press: Boca Raton, 1999; p 185. (20) Ke, H. Y. D.; Birnbaum, E. R. J. Lumin. 1995, 63, 9-17. (21) Ananias, D.; Kostova, M.; Paz, F. A. A.; Ferreira, A.; Carlos, L. D.; Klinowski, J.; Rocha, J. J. Am. Chem. Soc. 2004, 126, 10410-10417. (22) Reisfeld, R.; Greenberg, E.; Velapoldi, R.; Barnett, B. J. Chem. Phys. 1973, 56, 1698-1705. (23) Paulose, P. I.; Jose, G.; Thomas, V.; Unnikrishnan, N. V.; Warrier, M. K. R. J. Phys. Chem. Solids 2003, 64, 841-846. (24) Sohn, K. S.; Choi, Y. G.; Choi, Y. Y.; Park, H. D. J. Electrochem. Soc. 2000, 147, 3552-3558. (25) Park, J. K.; Kim, C. H.; Han, C. H.; Park, H. D.; Choi, S. Y. Electrochem. Solid-State Lett. 2003, 6 (7), H11-H13. (26) Miyakawa, T.; Dexter, D. L. Phys. ReV. B: Condens. Matter Mater. Phys. 1970, 1, 2961-2969. (27) Boutinaud, P.; Mahiou, R.; Cousseins, J. C. J. Lumin. 1997, 7274, 318-320.