Afterglow Luminescence of Lu2O3:Eu Ceramics ... - ACS Publications

Feb 17, 2010 - Three series of Lu2O3:Eu ceramic materials doped with different concentrations of Eu3+ ions (0.05−5 atom %) were prepared by sinterin...
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J. Phys. Chem. C 2010, 114, 4215–4220

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Afterglow Luminescence of Lu2O3:Eu Ceramics Synthesized at Different Atmospheres J. Trojan-Piegza* and E. Zych Faculty of Chemistry, UniVersity of Wrocław, 14 F. Joliot-Curie Street, 50383 Wrocław, Poland ReceiVed: October 23, 2009; ReVised Manuscript ReceiVed: February 1, 2010

Three series of Lu2O3:Eu ceramic materials doped with different concentrations of Eu3+ ions (0.05-5 atom %) were prepared by sintering at the temperature of 1700 °C of nanocrystalline powders. The heat-treatments were performed in oxidizing, slightly reducing, and strongly reducing atmosphere of air, vacuum, and a N2-H2 mixture (9:1 by volume), respectively. The radioluminescent properties of these materials have been systematically studied. After exposure to X-rays, independent of the atmosphere of the preparation, the ceramics exhibited an extensive afterglow, especially long and strong for low Eu concentrations. The afterglow and radioluminescence spectra differed significantly with the former showing much more emission resulting from Eu3+ ions occupying the S6 symmetry site in the host compared to the activator in the C2 position. The effect was especially significant within the range of low and medium Eu concentrations, 0.05-1 atom %. From the decay traces of the persistent luminescence of Lu2O3:Eu ceramics it was concluded that the mechanism of the process is governed by the second order kinetics. It is postulated that only Eu3+ ions located within the layer containing both S6 and C2 metal ion sites are active in the afterglow emission, while those placed within the layer consisting of only the C2 metal ion sites do not contribute to the afterglow. Another option is that electronic levels of Eu3+ in S6 site are more favorably positioned to intercept migrating from their traps excited carriers. Introduction Lu2O3 crystallizes in a cubic C-type structure,1,2 which host offers two different sites for the Eu3+ dopant ion replacing Lu3+, each of them with 6-fold coordination: noncentrosymmetric, C2, and centrosymmetric, C3i (S6),1,3,4 as is shown in Figure 1. The population of the C2 site triples the abundance of the S6 one in the host. It was shown by Mossbauer spectroscopy that Eu3+ entering the host at high temperatures has some tendency to preferentially occupy the C2 site.5 This was, however, found for heavily doped materials (10 atom %) and may not be valid for low concentration systems. As a matter of fact, contrary to the experimental results, recently published theoretical analysis suggested that Eu3+ may tend to preferentially occupy the S6 position in the host, especially at low concentration systems.6 All of that is important for analysis of the afterglow emission in Lu2O3:Eu. Unfortunately, as will be shown shortly, the contradictory findings hamper analysis of the afterglow phenomenon in Lu2O3:Eu ceramics. For the noncentrosymmetric Eu(C2) ions the electric dipole induced f-f transitions are only partially forbidden and they appear quite strong in both absorption and emission spectra.4,7-9 The selection rules for them are not that restricting and the spectra of the Eu3+ ion positioned in the C2 symmetry site of Lu2O3, similarly to isostructural Y2O3 and Gd2O3, are quite rich as was proved in numerous papers.4,9-11 The situation is quite different for the centrosymmetric Eu(S6) ions. The selection rules are much more restricted in this case and allow only for magnetic dipole induced transitions which are characterized by ∆J ) 0, (1 (yet, 0 f 0 is also forbidden).7-9 In practice for Lu2O3:Eu only two emission lines resulting from radiative relaxation of excited Eu(S6) ions, both related to the 5D0 f 7F1 transition, can be recorded and they were found to appear at * To whom correspondence should be addressed. Tel: +48 71 3757265. E-mail: [email protected].

Figure 1. Top: Arrangements of the oxygen ligands and vacancies around the S6 and C2 symmetry sites of Lu3+ ion. Bottom: A general view of the unit cell of Lu2O3 showing the mutual arrangement of Lu with C2 symmetry (blue balls) and with S6 symmetry (brown balls).

582.8 and 593.6 nm.4,9 The transitions within the Eu(S6) are not only much less numerous, but they are also significantly less probable, and consequently emissions related to them are considerably longer and much less intense compared to those characteristic for Eu(C2).4,9

10.1021/jp910126r  2010 American Chemical Society Published on Web 02/17/2010

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Lutetium oxide, Lu2O3, was recognized as a very good host lattice for X-ray phosphors. Exceptionally high absorption coefficient for ionizing radiation, very good photofraction, and high effective atomic number make lutetia-based compositions especially attractive for medical imaging. Particularly Eu-doping converts this host into efficient X-ray phosphor attractive for modern planar digital imaging,12-22 as its efficient, red luminescence matches perfectly the highest quantum efficiency of the CCD camera. However, the Lu2O3:Eu phosphor is not free of drawbacks, the most important of which is quite a significant afterglow appearing after irradiation with X-rays.14,23,24 This problem was studied quite thoroughly but only for materials containing 5% of the Eu dopant.23,24 Recently, in scant studies, it was shown that important information about the afterglow luminescence characteristics could be obtained from experiments performed on lightly doped compositions.25 The main observation was that for low Eu concentrations (0.05-1%) the afterglow luminescence spectra differed strongly from analogous emissions of samples containing 3-5% of Eu. The lightly doped compositions produced afterglow luminescence with Eu(S6) emission intensity comparable or even stronger than that from Eu(C2). These limited studies were restricted to materials sintered in air, which opened the question if the atmosphere of preparation could alter this property. In general terms, an afterglow results from a temporal interception of excited carriers in traps from which they can be continuously released regaining the ability to migrate to the activator, excite it, and thus produce delayed emission. According to the Arrhenius equation the mean time, τ, a carrier spends in its trap at a specific temperature, T, is given by eq 1:

( kTE )

p ) τ-1 ) s exp -

(1)

where p gives the probability per unit of time that a carrier escapes the trap, s is a constant called the frequency factor, and E is the trap depth. Hence, to observe an afterglow the traps cannot be too deep as then the intercepted carriers (electrons and/or holes) are not at all able to escape from them. On the other hand, the traps cannot be too shallow as then the carriers could not be immobilized for a time long enough to produce a delayed emission.26 The traps can be of various characters. Mostly, these are lattice defects of various types always present in materials. Cation or anion vacancies, interstitial ions, and impurities (intentional or unintentional) can produce local potentials able to serve as traps for electrons or holes migrating in the material. The traps (defects) may also be created upon the impact of a high-energy particle, as frequently occurs in halides. Population of some of the defects can be strongly altered changing the material preparation conditions, for example, the fabrication atmosphere. Therefore we decided to significantly broaden the superficial research we performed in the past25 to learn more about the properties of the afterglow in Lu2O3:Eu ceramics. We hoped to get some indications about the possible methods of reducing the phenomenon intensity and/or duration in this phosphor as well as to learn more about the mechanism governing the afterglow phenomenon in the Lu2O3:Eu phosphor. Materials and Experiments Three sets of Lu2O3:Eu ceramics were prepared by sintering of cold-pressed powders at 1700 °C. The series differed only

in the atmosphere at which the sintering was performed: ambient air (oxidizing), vacuum with pressure of about 10-2 hPa (slightly reducing), and N2-H2 mixture (9:1 by volume) (strongly reducing; normal, atmospheric pressure). The starting powders for sintering were prepared by coprecipitation method as was previously reported.25 Each series consisted of samples containing 0.05, 0.1, 0.2, 0.5, 1, 3, and 5 atom % of Eu with respect to Lu. The radioluminescence and luminescence afterglow spectra were recorded with an Ocean Optics HR2000-CG Spectrometer equipped with 25 µm slits assuring the resolution of about 1.2 nm. The samples were irradiated with white X-rays taken from a copper lamp of a DRON-1 powder diffractometer applying a voltage of 160 V and 10 mA current. The sample-lamp distance was 4 cm. All spectra were recorded in the 200-1100 nm range but only the Eu3+-related emissions were observed therefore they are presented truncated to the 550-650 nm region to expose their most important features. Both luminescence afterglow spectra and kinetics of the afterglow decays were measured following 10 min irradiation of the samples with the white X-rays from the tube. For technical reasons, the measurements started 10 s after ceasing the irradiation. For the afterglow kinetics measurements the Eu(C2) luminescence was monitored at 611 nm and the Eu(S6) one at 582.8 nm. Results Figure 2a shows a series of the radioluminescence (RL) spectra of the samples sintered in air. These spectra are indistinguishable from RL recorded for the two other sets of samples (sintered in vacuum and in N2-H2 mixture). Also the intensity of RL is not affected by the atmosphere in which the materials were fabricated. All spectra within a series are very similar with the dominant emission located around 611 nm and resulting from the 5D0 f 7F2 transition within the Eu3+ ion located at the C2 symmetry site. The RL from Eu(S6) appears only as a vestige and with increasing concentration of Eu3+ it almost completely vanishes. After irradiation with X-rays it could be seen by eye that samples exhibited a profound afterglow, most significant for the lightly doped specimens. Panels b, c, and d of Figure 2 show the afterglow luminescence spectra of the three series of materials recorded 3 min after ceasing their stimulation. Two effects are evident. First, the afterglow emission spectra strongly differ from their regular RL counterparts, at least for the Eu content in the range of 0.05-1%. Second, the afterglow spectra are strongly concentration dependent. Taking into account the literature data discussed in the Introduction,8,9,25 the reasons for the differences appear obvious: for materials with lower Eu contents a significant fraction of the afterglow emissions comes from Eu(S6) ions. The positions of the two characteristic lines are indicated with arrows in Figure 2. Yet another observation is that the afterglow luminescence spectra do not practically change with the atmosphere of the materials preparation. Both the intensities as well as spectral distributions of the afterglow emissions are very similar for samples prepared at the three different atmospheres. In each series the relative intensity of the Eu(S6) afterglow emission is the highest for materials containing 0.2% and 0.5% Eu. When the Eu concentration is 3% or 5% the afterglow from the Eu(S6) could not be recorded 3 min after ceasing the irradiation. This effect is not a surprise as it is known that an efficient Eu(S6) f Eu(C2) energy transfer for higher concentrations takes place leading to a strong quenching of the Eu(S6) luminescence.9 Consequently, even if the energy happens to reach Eu(S6) it is being transferred to a

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Figure 2. Eu concentration dependence of radioluminescence (a) and persistent luminescence spectra (b-d) recorded 3 min after X-ray excitation of the Lu2O3:Eu materials produced in air (b), vacuum (c) and N2-H2 mixture (d). Radioluminescence does not change with the atmosphere of preparation.

Figure 3. Concentration dependence of ratios of emission intensities of Eu(S6) and Eu(C2) ions in radioluminescence and afterglow spectra. The latter were calculated 30 s and 10 min after ceasing irradiation of the materials with X-rays. Materials were prepared in air (a), vacuum (b), and N2-H2 mixture (c).

nearby located Eu(C2) and thus this is the latter which sends off the photon of light. Hence, what is observed for higher concentrations is not surprising. However, the difference between RL and afterglow spectra for lower Eu concentrations is spectacular as is the lack of dependence of this property on the synthesis atmosphere within oxidation and reduction conditions that were applied. The Eu(S6)/Eu(C2) luminescence intensity ratios for the regular RL spectra as well as for the afterglow emissions recorded 30 s and 10 min after ceasing the irradiation were calculated and are presented in Figure 3. Within the experimental error, the concentration dependences are practically the same for all series. It can be easily noted that for lightly doped samples the Eu(S6)/Eu(C2) afterglow luminescence intensity ratio is even higher at longer delay time. Hence, the differences between RL and afterglow spectra, as seen in Figure 2 (3 min after irradiation), become even more profound at later stages of the afterglow. Results presented in Figure 3 once more prove that the behavior of samples prepared at different atmospheres is very similar. It is seen in Figure 3 that shortly after the

irradiation the highest ratio of the afterglow intensities from both Eu sites, Eu(S6)/Eu(C2), is observed for 0.5% Eu concentration, while at later stages of the measurement this ratio is uppermost for the 0.2% material. Again, this effect does not much depend on the atmosphere of preparation of the materials. As a consequence of these observations it can be stated that the kinetics of the afterglow emissions generated by Eu(S6) and Eu(C2) ions differ to some extent and are concentration dependent becoming much faster for higher Eu contents. Having concluded that the afterglow spectra are not only concentration but also time dependent it was decided to record decay traces of the emissions from both Eu sites. The results of such measurements are presented in Figure 4 for all three series of samples. Clearly, there are striking similarities between materials of different series. Again it has to be concluded that the preparation atmosphere does not have any significant influence on the properties of the afterglow emissions in the materials. For higher Eu contents, 3% and 5%, the traces for both Eu(S6) and Eu(C2) ions decay relatively quickly. Yet, the afterglow from the Eu(S6) site disappears much faster, practically within 1 min after stopping the irradiation. This observation confirms what was already concluded from the spectra presented in Figure 2 and the data shown in Figure 3. An interesting situation is seen for the materials with very low Eu contents, 0.05-0.2%, hence when the Eu(S6) f Eu(C2) energy transfer is negligible. Right after ceasing irradiation the intensity of the emission from the Eu(C2) is noticeably higher than that from Eu(S6). This is true independent of the atmosphere of preparation of the materials. The effect becomes progressively stronger when the content of the dopant decreases. However, both traces cross after some time and consequently at later stages of the phenomenon this is the Eu(S6) afterglow intensity, which surpasses the Eu(C2) one. The lower the Eu content the later the crossing point comes into view. For the 0.2% materials the intersection of both decay lines is observed about 4 min after ceasing the irradiation. After this time the Eu(S6) afterglow intensity becomes stronger than from Eu(C2). However, for 0.1% materials it takes about 8-10 min for the Eu(S6) afterglow intensity to surpass the one resulting from Eu(C2). When the Eu content is further reduced to 0.05% both traces cross yet later, after about 15-20 min. Again, not much influence of the preparation atmosphere on these effects can be noted.

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Figure 4. Persistent luminescence decays traces of sintered Lu2O3:Eu ceramics obtained in air (a), vacuum (b), and N2-H2 mixture (c). Simultaneously the 611 nm Eu(C2) (9) and 582.8 nm Eu(S6) (red O) emission intensities were recorded after X-ray excitation.

may be taken as a proof for the second order kinetics of the afterglow phenomena of both Eu(S6) and Eu(C2) ions. Discussion

Figure 5. Time dependence of the reciprocal of the square root of the afterglow intensity (I-1/2 vs t) of the Lu2O3:Eu3+ ceramics sintered in N2-H2 mixture (Eu(C2) (black solid symbol 9) and Eu(S6) (red open symbol O)). Data for materials prepared in air and vacuum are practically identical. For the highest concentrations (not shown) the dependence is no longer linear, most probably due to an efficient Eu(S6) f Eu(C2) energy transfer.

A closer analysis of the afterglow decay kinetics delivers new information. In Figure 5 the first 20 min of the decay traces of the lightly doped materials (0.05-1%) sintered in the N2-H2 mixture drawn as the time dependence of the reciprocal of the square root of the afterglow intensity (I -1/2(t)) are presented. For the two other series (sintered in air and in vacuum) the results are identical and therefore are not shown in the figure. It is striking that the I -1/2(t) relationships for the afterglow of both the Eu(S6) and Eu(C2) ions are linear, with maybe some small disparity at the very beginning (1-2 min) of the process. This perturbation may well result from the Eu(S6) f Eu(C2) energy transfer between spatially correlated (closely positioned) ions. Even for the lowest Eu concentration, 0.05%, a certain number of Eu3+ ion pairs may exist in the materials.27 According to the literature,28 such a linear dependence of the I -1/2 vs t

The time spent by the electron and/or hole carrying the excessive energy in its trap circumscribes the afterglow duration while the trap population defines the phenomenon intensity. By an appropriate chemical treatment it is often possible to alter types of traps, their population, and even depths modifying materials behavior.29-33 In this context it is interesting that Tbactivated Lu2O3 shows a profound afterglow when prepared in a reducing atmosphere and no afterglow at all when fabricated in air.34,35 Starting the research it was believed that treating the Lu2O3: Eu ceramics at atmospheres ranging from oxidizing (air) through mildly reducing (vacuum) to strongly reducing (N2-H2 mixture) it would be possible to modify either the type of defects acting as traps for excited carriers (electrons and/or holes) or at least alter their population and thus change (presumably reduce) the intensity and/or duration of the afterglow luminescence. As lutetia is an oxide material, it was supposed that the different atmospheres of preparation would strongly change the anticipated defect (traps);oxygen vacancies and/or oxygen interstitials;and consequently that improvement of the decay kinetics of Lu2O3:Eu ceramics radioluminescence could be achieved, as it was done for Lu2O3:Tb.35 From the results presented above beyond any doubts it can be stated that (1) the Eu(S6) ions are relatively more active in the afterglow process than the Eu(C2) ones, especially when compared to regular RL and (2) the intensity, duration, and kinetics of the afterglow in Lu2O3:Eu ceramics is not affected by the atmosphere at which the materials were prepared. From (1) it appears that, although the population of Eu(C2) roughly triples the population of Eu(S6), the energy from traps is being delivered preferentially to Eu(S6) rather than to Eu(C2) ions. This may further indicate that the carriers possessing excessive energy are preferentially trapped in the vicinity of Eu(S6) ions at the expense of the Eu(C2) ones. On the other hand, the Eu(S6)/

Afterglow Luminescence of Lu2O3:Eu Ceramics Eu(C2) ratio of afterglow intensities was not much different from unity up to the Eu content of 1%, hence when the Eu(S6) f Eu(C2) energy transfer does not play a significant role. This observation may signify that the populations of Eu(S6) and Eu(C2) ions actiVe in the afterglow luminescence are similar. This temporary conclusion can be confronted with the Lu2O3 structure. In the Introduction it was already mentioned that in the Lu2O3 lattice the metal ions are organized into two types of layers. Those containing exclusively ions of C2 symmetry are separated by layers with equal numbers of ions of C2 and S6 symmetry, see Figure 1. Altogether this gives the 3:1 population of both types of sites in Lu2O3. It is postulated that the temporary immobilized carriers after being thermally freed migrate either exclusively or at least with much preference to Eu3+ ions located within the layers consisting of both S6 and C2 sites, basically omitting those with only the C2 metal site. Consequently only these Eu3+ ions (whose populations may be taken as roughly 1:1 in the investigated systems) are active in the afterglow. It is pure speculation but it corresponds to the observed similar intensities of the afterglow from Eu(S6) and Eu(C2) sites for low concentration systems. Unfortunately, at present we do not see a good experiment that could settle the veracity of this hypothesis. Yet another possibility is that the electronic levels of Eu(S6) ions are simply positioned more favorably to be reached by charge carriers escaping their traps compared to levels of the Eu(C2). This may well be as indeed the levels of Eu(S6) were reported to be situated noticeably above their counterparts of the Eu(C2),36 hence closer to the conduction band of the host lattice. The second clear observation that the intensity duration and kinetics of the afterglow of Lu2O3:Eu ceramics are not influenced by the atmosphere at which the materials were prepared indicates that the traps in which the excited carriers are being temporary immobilized are not connected with oxygen vacancies or interstitials, as their populations have to significantly vary when the oxide is treated at high temperatures at oxidizing and strongly reducing atmospheres. Moreover, it was also found, but not presented in this paper, that analogous afterglow properties as reported here for ceramics occur for powders of different sizes of crystallites (50-1000 nm) prepared at various atmospheres and temperatures in the range of 800-1300 °C. Altogether, the obtained picture convinces that the afterglow is an intrinsic property of Lu2O3:Eu materials. It cannot be excluded that defects (traps) standing behind the afterglow in Lu2O3:Eu are created upon the impact of the ionizing radiation or the carriers are temporarily immobilized in the (neighborhood of) empty ligand sites inherited to the structure, as is shown in Figure 1. While all that does not exclude that the afterglow phenomenon still can be altered by some other technological tricks, now there is much less hope for that. It is a pity, as reducing the afterglow would make Lu2O3:Eu materials even more attractive for practical applications. The afterglow in Lu2O3:Eu materials cannot be triggered irradiating the materials into the Eu3+ f O2- charge transfer absorption band located around 250-260 nm. Only high energy radiation leads to afterglow. This allows us to conclude that the carriers reach their traps taking advantage of some mobility when they are raised into conduction (electron) and valence (hole) bands. This differentiates the afterglow mechanism in Lu2O3:Eu from a similar effect in Lu2O3:Tb,34,35 where the tunneling seems to take place. Finally, it is noteworthy that Euactivated Y2O3 does not show similar behavior despite an

J. Phys. Chem. C, Vol. 114, No. 9, 2010 4219 analogous crystal structure and great similarity in other spectroscopic properties. Conclusions Lu2O3:Eu ceramics produce the efficient radioluminescence, which is associated with a significant afterglow lasting for more than 1 h in lightly doped compositions. Up to the Eu concentration of 1 atom % the afterglow spectra are characterized by roughly equally intense emissions from Eu3+ occupying both C2 and S6 sites while in regular radioluminescence luminescence from Eu3+ in the C2 site totally dominates the spectra. The afterglow phenomenon appears to be totally independent of the atmosphere of the materials fabrication: its intensity, duration, and kinetics, as well as spectral distribution, could not be altered by changing the preparation atmosphere from oxidizing to strongly reducing. Analysis of the I -1(t) dependence leads to the conclusion that the phenomenon is governed by the second order kinetics. Acknowledgment. Financial support by Minister of Science and Higher Education under Grant No. N205 024 31/1207 and partially under Grant No. N N205 015 934 is gratefully acknowledged. References and Notes (1) Saiki, A.; Ishizawa, N.; Mizutani, N.; Kato, M. Acta Crystallogr. B 1984, 40, 76. (2) FIZ Karlsruhe & Gmelin Inst. 1990 ICSD Collection Code 40471, release 99/1. (3) Heber, J.; Ko¨in, U. Luminescence of Crystals, Molecules and Solutions; Plenum Press: New York, 1973. (4) Zych, E.; Karbowiak, M.; Domagala, K.; Hubert, S. J. Alloys Compd. 2002, 341, 381. (5) Concas, G.; Spano, G.; Zych, E.; Trojan-Piegza, J. J. Phys.: Condens. Matter 2005, 17, 2597. (6) Stanek, C. R.; McClellan, K. J.; Uberuaga, B. P.; Sickafus, K. E.; Levy, M. R.; Grimes, W. R. Phys. ReV. B 2007, 75 (13), 134101. (7) Forest, H.; Ban, G. J. Electrochem. Soc. 1969, 116, 474. (8) Buijs, M.; Meijerink, A.; Blasse, G. J. Lumin. 1987, 37, 9. (9) Zych, E. J. Phys.: Condens. Matter 2002, 14, 5637. (10) Garcia-Murillo, A.; Le Luyer-Urlacher, C.; Dujardin, C.; Pedrini, C.; Mugnier, J. J. Sol-Gel Sci. Technol. 2003, 26, 957. (11) Bazzi, R.; Flores, M. A.; Louis, C.; Lebbou, K.; Zhang, W.; Dujardin, C.; Roux, S.; Mercier, B.; Ledoux, G.; Bernstein, E.; Perriat, P.; Tillement, O. J. Colloid Interface Sci. 2004, 273, 191. (12) Derenzo, S. E.; Moses, W. W.; Weber, M. J.; West, Z. C. Mater. Res. Soc. Symp. Proc. 1994, 39, 348. (13) Zych, E.; Hreniak, D.; Strek, W. J. Alloys Compd. 2002, 341, 385. (14) Strek, W.; Zych, E.; Hreniak, D. J. Alloys Compd. 2002, 344, 332. (15) Garcia-Murillo, A.; Le Luyer, C.; Dujardin, C.; Martin, T.; Garapon, C.; Pedrini, C.; Mugnier, J. Nucl. Instrum. Methods Phys. Res., Sect. A 2002, 486, 181. (16) Lempicki, A.; Brecher, C.; Szupryczynski, P.; Lingertat, H.; Nagarkar, V. V.; Tipnis, S. V.; Miller, S. R. Nucl. Instrum. Methods Phys. Res., Sect. A 2002, 488, 579. (17) van Eijk, C. W. E. Nucl. Instrum. Methods Phys. Res., Sect. A 2003, 509, 17. (18) Nagarkar, V. V.; Miller, S. R.; Tipnis, S. V.; Lempicki, A.; Brecher, C.; Lingertat, H. Nucl. Instrum. Methods Phys., Sect. B 2004, 213, 250. (19) Zych, E.; Trojan-Piegza, J.; Dorenbos, P. Radiat. Meas. 2004, 38, 471. (20) Trojan-Piegza, J.; Zych, E. J. Alloys Compd. 2004, 380, 118. (21) Trojan-Piegza, J.; Zych, E.; Hreniak, D.; Strek, W. J. Alloys Compd. 2004, 380, 123. (22) Dujardin, C.; Le Luyer, C.; Martinet, C.; Garapon, C.; Mugnier, J.; Murrillo, A. G.; Pedrini, C.; Martin, T. Nucl. Instrum. Methods Phys. Res., Sect. A 2005, 537, 237. (23) Brecher, C.; Bartram, R. H.; Lempicki, A. J. Lumin. 2004, 106 (2), 159. (24) Bartram, R. H.; Lempicki, A.; Kappers, L. A.; Hamilton, D. S. J. Lumin. 2004, 106 (2), 169. (25) Zych, E.; Trojan-Piegza, J. J. Lumin. 2007, 122-123, 576. (26) Zych, E.; Brecher, C.; Glodo, J. J. Phys.: Condens. Matter 2000, 12, 1947.

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(27) Suyver, J. F.; Meester, R.; Kelly, J. J.; Meijerink, A. J. Lumin. 2003, 102, 182. (28) Mott, N. F.; Gurney, R. W. Electronic processes in ionic crystals; Dover Publications: New York, 1964. (29) Ferri, J. L.; Mathers, J. E.; Yale, R. L. U.S. Patent 3 974 389, 1976. (30) Rossner, W.; Grabmaier, B. C. J. Lumin. 1991, 48&49 (1), 29. (31) Lempicki, A.; Brecher, C.; Wisniewski, D.; Zych, E.; Wojtowicz, A. J. IEEE Trans. Nucl. Sci. 1996, 43 (3), 1316. (32) Lynch, M. J.; Duclos, S. J.; Greskovich, C. D.; Srivastava, A. M. U.S. Patent 5 882 547, 1999.

Trojan-Piegza and Zych (33) Creasey, J. P.; Tyrrell, G. C. Proc. SPIE 2000, 3942, 114. (34) Zych, E.; Trojan-Piegza, J.; Hreniak, D.; Strek, W. J. Appl. Phys. 2003, 94 (3), 1318. (35) Trojan-Piegza, J.; Niittykoski, J.; Ho¨lsa¨, J.; Zych, E. Chem. Mater. 2008, 20, 2252. (36) Karbowiak, M.; Zych, E.; Ho¨lsa, J. J. Phys.: Condens. Matter 2003, 15, 2169.

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