Luminescence in Li2O under ion beam irradiation

Nuclear Instruments and Methods in Physics Research B 141 (1998) 392±397

Luminescence in Li2 O under ion beam irradiation Victor Grishmanov, Satoru Tanaka *, Koji Ogikubo, Quanli Hu Department of Quantum Engineering and Systems Science, The Faculty of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

Abstract The luminescence emitted under light ion (H‡ , He‡ ) irradiation in Li2 O single crystal has been measured at 313± 773 K by an optical multi-channel detector. The temperature and absorbed dose dependences of the luminescence spectrum were investigated. The luminescence emission in Li2 O induced by both H‡ and He‡ ion beam irradiations was observed at about 5.0±1.8 eV. The luminescence intensity decreased with absorbed dose. This e€ect is associated with the diminution of the concentration of oxygen vacancies and concentration quenching due to the high damage density and stresses in the ion track region. The luminescence intensity of the 2.35 eV band (F-center aggregates) gradually increases with irradiation dose at temperatures below 423 K. Alternatively, the intensities of the luminescence emissions originated from F‡ (3.3 eV) and F2 (2.9 eV) centers increase with irradiation dose in the temperature range of 423±573 K. These phenomena are thought to be connected with the ecient temperature for the generation of di€erent radiation defects. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.82.Ms; 78.55.Hx Keywords: Lithium oxide; Ion irradiation; Luminescence; Radiation defects

1. Introduction The Li2 O ceramics is one of the candidates as a tritium breeding material for a fusion reactor. It will be exposed to a heavy irradiation dose at high temperatures leading to the production of various radiations defects [1]. This will result in the swelling and degradation of mechanical properties of Li2 O ceramics, and can a€ect the tritium release process ± the most important characteristic of the

* Corresponding author. Tel./fax: +81 3 5800 6860; e-mail: [email protected].

tritium breeding material. Thus, the research on radiation defects in Li2 O is required for both engineering design and fundamental interests. The luminescence emission in Li2 O has been studied under ion beam [2], reactor [3,4] c-ray [5] and synchrotron [6] irradiations. The luminescence emission bands emitted in Li2 O have been assigned in the following way: the 4.7 eV band to a selftrapped exciton [5±7], the 4.1 eV band to impurity or intrinsic defect [4,5], the 3.65 eV band to F0 center [2,3,5,7], the 3.3 eV band to F‡ center [2,3,5,7], the 2.9 eV band to F2 center [5,7] and the 2.35 eV band to F-center aggregates (Fn center) [8]. The ®rst results of in situ luminescence measurements for poly- and single crystals of Li2 O

0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 9 1 - 8

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bombarded with 2 MeV He‡ were reported by Asaoka et al. [2]. The luminescence spectra have been observed in the temperature range from 300 to 850 K. However, the attention has not been given to the high dose rate and heavy absorbed dose which are characteristic for ion irradiation. Thus, the present paper deals with the study of the temperature and absorbed dose dependences of the luminescence spectrum of Li2 O. 2. Experimental The Li2 O single crystal was used for the investigation. The sample was obtained from the Japan Atomic Energy Research Institute. The ¯oating zone growth method was used for the preparation of Li2 O single crystal [9]. The crystal rod of about 8 mm diameter was sawed along the (1 1 1) plane into discs of 1.5 mm thickness. The luminescence was emitted by H‡ or He‡ ion beam irradiation (energy of ion: 1 MeV; beam current: 6±320 nA/cm2 ; temperature of the sample: 303±773 K). The ion beam was swept in order to irradiate uniformly over the all area of the sample. The luminescence from the sample was guided through a quartz window, a quartz lens and a quartz optical ®ber, and then measured by an optical multi-channel detector (5.6±1.55 eV) [8]. The absorbed dose in Li2 O single crystal was calculated according to the maximum penetration depth of an incident ion which was computed by TRIM code with the assumption that an ion energy was fully absorbed by the target. Before the luminescence measurement was carried out at the certain irradiation conditions (ion beam current, temperature), the sample was heat treated at 773 K for 15±20 min. It was found that these conditions are sucient for an annealing of the radiation defects in Li2 O matrix produced by previous irradiation.

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ferent bands were superimposed. To accurately separate the spectrum into di€erent bands, it was assumed that each emission peak is Gaussian. The luminescence spectrum of Li2 O at low absorbed dose (Fig. 1(a)) was similar to those observed under c-ray [8] and reactor [5] irradiations. With the increase in absorbed dose, the disappearance of the 4.7 eV band, the strong drop in the intensities of 4.1, 3.7, 3.3 and 2.9 eV bands, and the occurrence of a new luminescence band at 2.35 eV were observed (Fig. 1(b)). It is pertinent to note that the luminescence spectra emitted by 1 MeV He‡ and H‡ ion irradiations at the same conditions (temperature, dose rate and absorbed dose) were similar in shape. This observation indicates that the luminescence emission in Li2 O is caused by electronic excitations. Thus, the experimental results will be discussed in the subsequent text from the viewpoint of irradiation temperature, dose rate and absorbed dose. The effect of ion type (H‡ or He‡ ) on luminescence emis-

3. Results and discussion Fig. 1 shows the luminescence spectra emitted by 1 MeV He‡ ion irradiation at 423 K. In every case the observed spectra showed that several dif-

Fig. 1. Luminescence spectra of Li2 O emitted under He‡ ion irradiation at 423 K.

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sion, which is usually signi®cant in the case of low energy ions, will be omitted in the present discussion. The Fig. 2 gives a comprehensive idea of the change of the luminescence emission spectrum of Li2 O with irradiation dose. In regard to the decrease of the luminescence intensity at an initial stage of irradiation (Fig. 2(a)), the main reason can be as follows: Firstly, the decrease of the luminescence intensity is connected with diminution of a concentration of oxygen vacancies, which mainly determine a yield of the luminescence emission of F-centers. Secondly, the accumulation of point radiation defects eventually results in their aggregation. The formation of defect clusters also contributes the quenching of the luminescence, since it causes a change in the lattice properties. Thirdly, the energetic ions penetrating into the Li2 O cause a local radiation heating along their tracks leading to the thermal excitation of the radiative decay into a non-radiative one. The above

Fig. 2. Luminescence spectra of Li2 O emitted under He‡ ion irradiation at 423 K.

reasons of the decrease of luminescence intensity are valid for the emission bands of the F0 center (3.7 eV), F‡ center (3.3 eV) and F2 center (2.9 eV). The luminescence emission of the self-trapped exciton (4.7 eV) is suppressed by a formation of defect clusters and by an e€ect of radiation heating. The observed decrease of the luminescence emission at 4.1 eV, which was attributed to an impurity or intrinsic defect [5], has been led to the suggestion that the origin of this luminescent defect is associated with oxygen vacancy. The luminescence emission approximately at 2.35 eV (Fig. 1(b)) has been attributed to F-center aggregates [8], because its intensity increases with absorbed dose (see Fig. 2(b)). It should be emphasized, that the luminescence band of Fn center is observed only under ion irradiation marked by the high dose rate of irradiation. It is pertinent to note that the measured luminescence intensity under ion beam bombardment regards only the presence of defects, since the studies on ion induced luminescence of silica (SiO2 ) and sapphire (Al2 O3 ) showed that there is not direct correlation between the concentration of defects and the emission intensity [10,11]. However, the case of the emission band at 2.35 eV suggests that the luminescence intensity is dependent on a concentration of radiation defects. Although the emission intensity at 2.35 eV is certain to be dependent on concentration of radiation defects, this is not meant that the value of luminescence intensity is direct proportional to the concentration of radiation defects. The enhancement of the emission intensity of Fn centers with absorbed dose just testi®es to the fact that their concentration increases. Further inspection of the luminescence spectra displayed in Fig. 2(b) discloses that the luminescence intensity at around 3.1 eV increases with absorbed dose beginning at about 30 MGy. According to the Gaussian ®tting of the luminescence spectrum presented in Fig. 1(b), the increase of the emission intensity should be valid for the F2 (2.9 eV) and F‡ (3.3 eV) centers. Fig. 3 shows the absorbed dose dependence of the luminescence intensity of emission bands emitted in Li2 O crystal under He‡ ion irradiation at 423 K. The luminescence intensities of the 4.1 and 3.7 eV bands decrease monotonically with adsorbed dose. On the

V. Grishmanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 392±397

Fig. 3. The absorbed dose dependence of the intensities of luminescence bands emitted in Li2 O crystal under He‡ ion irradiation at 423 K.

other hand, the luminescence intensity at 2.35 eV increases monotonically with adsorbed dose. As for 2.9 eV and 3.3 eV bands, the luminescence intensities decrease with irradiation dose but pass through a minimum as the absorbed dose reached a value of 20±30 MGy (Fig. 3). The enhancement of the luminescence intensity of F2 center (2.9 eV) with absorbed dose can be explained by the high concentration of simple F-centers and hence the probability of the generation of F2 center increases. Unexpected feature is that the emission intensity of F‡ center increases with absorbed dose. The similar observational data have been obtained in the studies of ion-luminescence from Al2 O3 [11]. It has been suggested that the luminescence intensity of the F0 band is reduced more eciently than one of F‡ band due to the stress generated by the ion implantation. Minor changes in lattice symmetry may o€er a non-radiative decay route to excited centers. The 6 nm shift of the emission peak, relative to uniformly excited material, was noted as an evidence for a reduction in conventional defect symmetry [11]. Since the shifts of emission peaks were observed for several bands in Li2 O Fig. 1, it seems likely that the stress generated by the buried damage strongly suppresses the luminescence emission of F0 center (Fig. 3) but it causes the inverse e€ect on the luminescence emission of F‡ center. Turning back to the enhancement of the luminescence intensity at 2.35 eV with irradiation dose,

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it might be well to point out that the formation of Fn centers is strongly temperature dependent [12]. The growth of Fn centers proceeds by the accumulation of F-centers. The F-centers execute a thermally-activated random walk through the crystal until they are trapped at Fn centers or at Li metal colloids. However, the growth of Fn centers at high temperatures is limited by thermal evaporation of simple F-centers back to the lattice and by the recombination of F-centers with hole defects (Oÿ , Oÿ 2 ). The absorbed dose whereby the growth of Fn centers occurs with a maximum rate is shifted to higher irradiation doses by going to higher temperatures. The certain temperature region exists wherein the maximum growth of the F-center aggregates proceeds with sharply de®ned temperature, and above which they are not generated [13]. The thorough analysis of the experimental data presented in Figs. 2, 4 and 5 allows to make a conclusion that the ecient growth of the Fn centers at dose rate of about 100 MGy/s occurs at around 373 K. The eciency of the generation of Fn centers decreases with temperature from 373 to 473 K. On the other hand, the eciency of the production of F2 (2.9 eV) and F‡ (3.3 eV) centers increases with temperature. The above experimental data indicate that the accumulation of F2 centers is highly ecient at around 473 K. In regard to F‡ center, the results are not yet sucient to allow de®nite conclusion.

Fig. 4. Luminescence spectra of Li2 O emitted under He‡ ion irradiation at 373 K.

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Fig. 5. Luminescence spectra of Li2 O emitted under H‡ ion irradiation at 473 K.

Fig. 6 shows the luminescence emission spectra at high dose rate (500 kGy/s). The main contribution to the luminescence emission is given by the F2 and F‡ centers. Dispute the high dose rate the luminescence emission related to Fn centers is insigni®cant, since the irradiation temperature is high enough (473 and 523 K). Two phenomena should be considered to cause this e€ect. Firstly, the generation of Fn centers is restricted by a high irradiation temperature. Secondly, the possibility of thermal excitation of the radiative decay of Fn center into a non-radiative one increases with temperature. In addition, the phase transformation of F-center aggregates into the metal Li colloids can e€ect the luminescence emission of Fn center hence the formation of the colloidal Li was found to be possible at temperatures up to 573 K [8]. 4. Conclusions By in situ luminescence measurement under ion irradiation was determined the ecient conditions for the emission of luminescent defects in Li2 O. It is shown that the absorbed dose of irradiation plays a vital role in the processes of the luminescence emission from Li2 O. The suppression of the luminescence intensity at high irradiation dose is associated with the diminution of the concentration of oxygen vacancies and concentration quenching due to the high damage density and stresses in the ion track region.

Fig. 6. Luminescence spectra of Li2 O emitted under He‡ ion irradiation.

The ecient formation of the F-center aggregates in Li2 O under ion beam irradiation is roughly estimated at temperatures below 423 K. On the other hand, the production of F2 and F‡ centers eciently occurs at temperature region from 423 to 523 K. However, the results of in-situ luminescence measurement re¯ects the generation of radiation defects and does not provide an information on their concentration. Thus, the simultaneous measurement of the luminescence emission and optical absorption is desirable for the fundamental understanding of the formation of F-centers in Li2 O in dependence of irradiation temperature, dose rate and absorbed dose. References [1] K. Noda, J. Nucl. Mater. 179±181 (1991) 37. [2] Y. Asaoka, H. Moriyama, K. Iwasaki, K. Moritani, Y. Ito, J. Nucl. Mater. 183 (1991) 174.

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