- Email: [email protected]
Quantum chemical analysis of hydrogen isotopes in lithium oxide
Fusion Engineering and Design 61 /62 (2002) 789 /794 www.elsevier.com/locate/fusengdes
Quantum chemical analysis of hydrogen isotopes in lithium oxide Hisashi Tanigawa , Satoru Tanaka Department of Quantum Engineering and Systems Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract In order to clarify the existing states and the diffusion process of hydrogen isotopes in Li2O, the quantum chemical analysis has been conducted especially emphasising interaction with charged defects such as F-centers. Using two calculation codes based on different theories, treatment of charged defects was discussed. From the comparison with FT-IR or diffusion model studies, the interaction of hydrogen isotopes with defects was explained taking electron transfer between them into consideration. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen isotopes; Quantum chemical analysis; Defects; Li2O
1. Introduction For establishment of an efficient fusion reactor fuel cycle, it is required to understand the tritium diffusion behaviour in the blanket materials. The authors have previously investigated existing states of hydrogen isotopes in bulk Li2O using Fourier transform infrared absorption spectroscopy (FTIR) [1,2]. The FT-IR study gives us direct information of /OH. Recent development of computational environment makes it possible to treat a large system including several kinds of defects using ab-initio quantum chemical calculation codes. In order to explain multiple peaks observed by FT-IR and to inquire into the existing states of
Corresponding author. Tel.: /81-3-5841-6970; fax: /81-33818-3455 E-mail address: [email protected] (H. Tanigawa).
hydrogen isotopes, a quantum chemical analysis has been conducted [3,4]. In the present paper, a series of our quantum chemical analyses is reported. The simple structure of Li2O is advantage in ab-initio calculation. The existing states and the diffusion process of hydrogen isotopes in Li2O are investigated from the comparison with FT-IR or model studies. A potential of quantum chemical analysis in the field is discussed.
2. Calculation scheme 2.1. Computational methods Two calculation codes were used for the investigation. One is CRYSTAL-95 code using all-electron Gaussian-type basis sets [5]. Another is CASTEP code based on density functional theory
0920-3796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 2 ) 0 0 2 6 3 - 6
790
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
(DFT) using pseudopotentials and plane wave basis sets. Using CRYSTAL, the calculations can be performed at the Hartree-Fock (HF), DFT and hybrid levels. In the present paper, HF method and basis sets that were optimized to Li2O were used [6]. Concerning DFT calculation in CASTEP, the generalized gradient approximation was used. It is easier to obtain a relaxed configuration in CASTEP than CRYSTAL, taking advantage of the automatic scheme. In CRYSTAL, atoms or ions are put on several positions in the input data and the most stable configuration should be searched. As regards quantitative analysis, estimation of activation energy for instance, CRYSTAL has an advantage because of optimized basis sets.
2.2. Treatment of charged defects The way to treat defects and charge configuration are quite different between the two codes. For each defect, atom or ion, electronic states can be set in CRYSTAL. While in CASTEP, only total charge states of the system can be inputted. For instance, the total charge should be zero when the Li2O system has one F0 center. If the situation is reasonable, the oxygen vacancy site traps two electrons and F0 center is formed at the end of the calculation. In both codes, the most stable electronic structure is to be found for the inputted geometrical structure. Concerning the basis sets, the proper ones should be considered for the vacancy in CRYSTAL. In a series of our studies, the basis sets for oxygen ion in Li2O have been used for F-centers. It was reported that the formation energy of oxygen vacancy in MgO crystal was estimated accurately using the optimized basis sets for oxygen ion in MgO [7]. However, uncertainties are still left. On the other hand, it is not necessary to input the basis sets for the vacancy in CASTEP. The electronic structure around the vacancy is made up of the expanding wave functions of adjacent atoms or ions. In this stage, the combination of these two codes and comparison of their results would be a good way to make the quantum chemical analysis more reliable.
2.3. Calculated system Fig. 1 shows the conventional cell of Li2O with anti-fluorite structure. Both the codes, CRYSTAL and CASTEP, are periodic programs and can treat an infinite crystal system as a series of consecutive supercells. Supercells consist of several unit cells and suitable sizes to solve problems are decided. In the calculations, a hydrogen ion (proton) was used, instead of deuterium or tritium ions. The quantum effects on tritium were estimated by Shah et al. [8]. It was concluded that the classical picture in their calculation would probably be fairly accurate. Concerning our analysis, the result for a proton could be equivalent to those for deuterium or tritium ions.
3. FT-IR observation of O /D Before going to quantum chemical analysis, results of our infrared absorption spectroscopy will be briefly presented. Fig. 2 shows O /D stretching vibration region observed by FT-IR in Li2O single crystals which were subjected to thermal absorption of deuterium gas and consequent quenching [1,2]. Deuterium was used in order to avoid the high background of O /H in the air. In the previous work, observed peaks were attributed to /OD present as a LiOD phase
Fig. 1. Crystalline structure of Li2O.
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
791
Using CRYSTAL, hybrid calculation (B3LYP) and a limited anharmonic analysis were carried out. For vibrational frequencies of O /D in LiOD, 2740 cm 1 was obtained. The result represents well the O /D frequency in a LiOD phase observed at 2715 cm 1. The future direction of the study will be one that deals with /OH in the Li2O crystal.
4.2. Stable position of /OH in Li2O with defects
Fig. 2. IR spectra of D2 absorbed Li2O single crystal (modified from Ref. [2]).
(2715cm 1 at room temperature) or Li2O /D (2510 cm 1 at 473 K) in bulk Li2O. Here, Li2O / D means O /D existing as a separate D bonded with an oxygen ion in the Li2O crystal. Multiple peaks were observed at room temperature between 2700 and 2550 cm 1, for instance, 2651, 2607 and 2565 cm 1. These peaks were assigned to /OD that was affected by defects, such as F-centers. To elucidate the effects of defects on /OD suggested by FT-IR study is the aim in our quantum chemical analysis.
4. Ab-initio analysis of
/
In order to elucidate how the defects interact with hydrogen isotopes in Li2O, the stable position of a proton and the electronic charge density map were calculated [4]. In the investigation, three varieties of F-centers and lithium vacancy were considered. It was concluded that the relaxation of the proton was explained by taking account of the two effects: the replacement of lithium ion caused by production of the defects and the interaction with the electrons trapped by F-centers. In addition, it was suggested that the proton could be trapped by F0 center. Fig. 3 summarises the most stable configurations of /OH obtained with each defect. It should be noted that the proton is not on (100) plane and orientated toward the vacancy site in the case of lithium vacancy. The position ‘iii’ corresponds to the proton keeping O /H bond and ‘iv’ to trapped one by F0 center. Fig. 4 is an electronic charge density map of the trapped proton by F0 center calculated by CASTEP.
OH in Li2O
4.1. Calculation of vibrational frequency For comparison with observed spectra, an analysis of the stretching vibrational frequency of O /H is effective. However, it had been difficult to estimate the frequency accurately. One reason was that our calculations were limited in HF level, and another was that the relaxation of ions or charge of the system had significant effects on the analysis. For the first step of the study on O /H frequencies, LiOH crystal was considered [3].
Fig. 3. Stable positions of -OH with defects on Li2O (100) plane. /OH with, i: F2 center, ii: F center, iii: F0 center, iv: F0 center (trapping), v: perfect crystal, vi: Li vacancy.
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
792
Fig. 4. Electronic charge density map of trapped proton by F0 center on Li2O (100) plane.
4.3. Charge configuration of /OH with defects In the present paper, Mulliken population analysis has been conducted in order to estimate electron transfer between the proton and the defects. In the population analysis, eight neighbours of each ion were taken into account. Table 1 shows atomic charges of the proton, bond populations and O /H distance calculated by CRYSTAL. From the table, it is shown that atomic charges and bond populations between the proton and F0 center is the largest in the position ‘iv’. It was found that the proton reacted to F0 center, and electrons trapped by the defect transferred to the
proton. In other words, the proton was trapped and reduced by F0 center. In the cases of perfect crystal and lithium vacancy, the protons bond to oxygen ions strongly. Especially /OH with lithium vacancy has the largest O /H bond populations and its O /H length is similar to that of OH ˚ ). The result supports premolecular ion (0.97 A vious energetic analysis, where the stability of / OH with lithium vacancy was significant high [4]. In the light of atomic charge of the proton and bond populations with F center, it is considered that F center could reduce the proton weakly. However, understanding bond populations with oxygen ion is difficult because it bonds to neither oxygen ion nor any other ions. Sometimes d shell orbital in the basis sets for oxygen has an important role in O /H bond, however, the orbital was not considered in the present study. In the next stage, the effects of d shell orbital will be checked. 4.4. Effect of Li relaxation on H trapping by F0 center The reduction process of the proton by F0 center discussed in the above section could be considerably related to diffusion of the proton in the Li2O crystal. In the neutron irradiated Li2O, two reactions are proposed for tritium diffusion in the grain [9]. LiOTF0 0 LiT; LiT1=2O2 0 LiOT After a number of conversions between LiT and LiOT, tritium diffuses to the grain surface. From the viewpoint of the electron transfer, the former equation corresponds to reduction of tritium by F0 center. As regards the process, the potential curve
Table 1 Mulliken population analysis of a proton in Li2O with defects and O /H distance
i. F2 center ii. F center iii. F0 center iv. F0 center v. Perfect crystal vi. Li vacancy
Atomic charges of H (e)
Bond populations O /H (e)
Bond populations F center /H (e)
˚) O /H distance (A
0.836 0.940 0.989 1.411 0.790 0.560
0.018 0.002 0.033 0.000 0.045 0.053
0.000 0.005 0.023 0.113
0.935 0.950 0.956 2.750 0.939 0.971
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
was estimated between the positions ‘iii’ and ‘iv’ (in Fig. 3) in our previous work [4]. The obtained activation energy was about 1.2 eV but hardly probable for the diffusion process, because reported activation energy is ranging from 0.85 to 1.0 eV in the neutron-irradiated Li2O [10]. The relaxation of lithium ions neighbouring the proton was considered, which had been neglected in the previous analysis. The following is the procedure of the calculation. The proton is fixed at the position which is the nearest from the two lithium ions between the oxygen ion and F0 center, ˚ . Fig. 5 shows geomewhere O /H length is 1.3 A trical configuration on (110) plane. In the condition, the stable positions of the two lithium ions were searched. The stable position was confirmed by both CRYSTAL and CASTEP. ˚ After relaxation, the lithium ions moves 0.22 A outward from the proton and the energy of the system calculated by CRYSTAL decreases 0.45 eV. Fig. 6 is calculated potential energy curve. The activation energy for trapping of the proton became 0.75 eV. The results confirm that a proton is easily trapped and reduced by neighbouring F0 center. If not related to diffusion of a proton directly, it could be concluded that the obtained activation energy stands within the reasonable range for the diffusion. To discuss whole of tritium diffusion, several uncertain processes are still left. For instance, it is not clear whether the reduced proton bonds to a lithium ion and forms LiT. The diffusion and recovery of defects also should be considered. The quantum chemical analysis presented in the paper affords new approach to analyse the inter-
793
Fig. 6. Energy of the system with F0 center as a function of O / H distance.
action of hydrogen isotopes with charged defects. From the analysis, the role of defects in the diffusion of hydrgen isotopes in Li2O is going to be clear from the quantum chemical point of view. The combination of the quantum chemical calculation, FT-IR and model studies has a potential to elucidate tritium behaviour in the blanket materials.
5. Conclusions A series of investigations for the interaction of a proton with defects in Li2O was reported. From Mulliken population analysis, it was shown that F0 center around a proton could trap and reduce the proton. Taking the relaxation of lithium ions into consideration, the activation energy for trapping of the proton by F0 center was estimated to be 0.75 eV.
References
Fig. 5. Relaxation of Li during H trapping by F0 center on Li2O (110) plane.
[1] H. Tanigawa, M. Taniguchi, S. Tanaka, et al., Interaction of hydrogen isotopes with defects in Li2O, Fusion Technol. 34 (1998) 872. [2] H. Tanigawa, S. Tanaka, FT-IR study on interaction of hydrogen isotopes with defects in lithium oxide, Fusion Eng. Des. 51 /52 (2000) 193. [3] H. Tanigawa, S. Tanaka, Quantum chemical calculation of O /H vibration in LiOH, J. Nucl. Sci. Technol. 38 (2001) 1004.
794
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
[4] H. Tanigawa, S. Tanaka, Ab-initio study on interaction of hydrogen isotopes with charged defects in lithium oxide, J. Nucl. Mater., in press, 2002. [5] R. Dovesi, V.R. Saunders, C. Roetti, et al., CRYSTAL-95 code, 1996. [6] R. Dovesi, Ab-initio Hartree-Fock extended basis set calculation of the electronic structure of crystalline lithium oxide, Solid State Commun. 54 (1985) 183. [7] A. De Vita, M.J. Gillan, J.S. Lin, et al., Defect energetics in MgO treated by first-principles methods, Phys. Rev. B 46 (1992) 12964.
[8] R. Shah, A. De Vita, M.C. Payne, et al., Ab initio study of tritium defects in lithium oxide, J. Phys. Condens. Matter 7 (1995) 6981. [9] H. Moriyama, T. Kurasawa, Tritium release kinetics of solid lithium ceramics with irradiation defects, J. Nucl. Mater. 212 /215 (1994) 932. [10] T. Tanifuji, K. Noda, T. Takahashi, H. Watanabe, et al., Tritium release from neutron-irradiated Li2O: diffusion in single crystal, J. Nucl. Mater. 149 (1987) 227.
Quantum chemical analysis of hydrogen isotopes in lithium oxide Hisashi Tanigawa , Satoru Tanaka Department of Quantum Engineering and Systems Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract In order to clarify the existing states and the diffusion process of hydrogen isotopes in Li2O, the quantum chemical analysis has been conducted especially emphasising interaction with charged defects such as F-centers. Using two calculation codes based on different theories, treatment of charged defects was discussed. From the comparison with FT-IR or diffusion model studies, the interaction of hydrogen isotopes with defects was explained taking electron transfer between them into consideration. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen isotopes; Quantum chemical analysis; Defects; Li2O
1. Introduction For establishment of an efficient fusion reactor fuel cycle, it is required to understand the tritium diffusion behaviour in the blanket materials. The authors have previously investigated existing states of hydrogen isotopes in bulk Li2O using Fourier transform infrared absorption spectroscopy (FTIR) [1,2]. The FT-IR study gives us direct information of /OH. Recent development of computational environment makes it possible to treat a large system including several kinds of defects using ab-initio quantum chemical calculation codes. In order to explain multiple peaks observed by FT-IR and to inquire into the existing states of
Corresponding author. Tel.: /81-3-5841-6970; fax: /81-33818-3455 E-mail address: [email protected] (H. Tanigawa).
hydrogen isotopes, a quantum chemical analysis has been conducted [3,4]. In the present paper, a series of our quantum chemical analyses is reported. The simple structure of Li2O is advantage in ab-initio calculation. The existing states and the diffusion process of hydrogen isotopes in Li2O are investigated from the comparison with FT-IR or model studies. A potential of quantum chemical analysis in the field is discussed.
2. Calculation scheme 2.1. Computational methods Two calculation codes were used for the investigation. One is CRYSTAL-95 code using all-electron Gaussian-type basis sets [5]. Another is CASTEP code based on density functional theory
0920-3796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 2 ) 0 0 2 6 3 - 6
790
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
(DFT) using pseudopotentials and plane wave basis sets. Using CRYSTAL, the calculations can be performed at the Hartree-Fock (HF), DFT and hybrid levels. In the present paper, HF method and basis sets that were optimized to Li2O were used [6]. Concerning DFT calculation in CASTEP, the generalized gradient approximation was used. It is easier to obtain a relaxed configuration in CASTEP than CRYSTAL, taking advantage of the automatic scheme. In CRYSTAL, atoms or ions are put on several positions in the input data and the most stable configuration should be searched. As regards quantitative analysis, estimation of activation energy for instance, CRYSTAL has an advantage because of optimized basis sets.
2.2. Treatment of charged defects The way to treat defects and charge configuration are quite different between the two codes. For each defect, atom or ion, electronic states can be set in CRYSTAL. While in CASTEP, only total charge states of the system can be inputted. For instance, the total charge should be zero when the Li2O system has one F0 center. If the situation is reasonable, the oxygen vacancy site traps two electrons and F0 center is formed at the end of the calculation. In both codes, the most stable electronic structure is to be found for the inputted geometrical structure. Concerning the basis sets, the proper ones should be considered for the vacancy in CRYSTAL. In a series of our studies, the basis sets for oxygen ion in Li2O have been used for F-centers. It was reported that the formation energy of oxygen vacancy in MgO crystal was estimated accurately using the optimized basis sets for oxygen ion in MgO [7]. However, uncertainties are still left. On the other hand, it is not necessary to input the basis sets for the vacancy in CASTEP. The electronic structure around the vacancy is made up of the expanding wave functions of adjacent atoms or ions. In this stage, the combination of these two codes and comparison of their results would be a good way to make the quantum chemical analysis more reliable.
2.3. Calculated system Fig. 1 shows the conventional cell of Li2O with anti-fluorite structure. Both the codes, CRYSTAL and CASTEP, are periodic programs and can treat an infinite crystal system as a series of consecutive supercells. Supercells consist of several unit cells and suitable sizes to solve problems are decided. In the calculations, a hydrogen ion (proton) was used, instead of deuterium or tritium ions. The quantum effects on tritium were estimated by Shah et al. [8]. It was concluded that the classical picture in their calculation would probably be fairly accurate. Concerning our analysis, the result for a proton could be equivalent to those for deuterium or tritium ions.
3. FT-IR observation of O /D Before going to quantum chemical analysis, results of our infrared absorption spectroscopy will be briefly presented. Fig. 2 shows O /D stretching vibration region observed by FT-IR in Li2O single crystals which were subjected to thermal absorption of deuterium gas and consequent quenching [1,2]. Deuterium was used in order to avoid the high background of O /H in the air. In the previous work, observed peaks were attributed to /OD present as a LiOD phase
Fig. 1. Crystalline structure of Li2O.
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
791
Using CRYSTAL, hybrid calculation (B3LYP) and a limited anharmonic analysis were carried out. For vibrational frequencies of O /D in LiOD, 2740 cm 1 was obtained. The result represents well the O /D frequency in a LiOD phase observed at 2715 cm 1. The future direction of the study will be one that deals with /OH in the Li2O crystal.
4.2. Stable position of /OH in Li2O with defects
Fig. 2. IR spectra of D2 absorbed Li2O single crystal (modified from Ref. [2]).
(2715cm 1 at room temperature) or Li2O /D (2510 cm 1 at 473 K) in bulk Li2O. Here, Li2O / D means O /D existing as a separate D bonded with an oxygen ion in the Li2O crystal. Multiple peaks were observed at room temperature between 2700 and 2550 cm 1, for instance, 2651, 2607 and 2565 cm 1. These peaks were assigned to /OD that was affected by defects, such as F-centers. To elucidate the effects of defects on /OD suggested by FT-IR study is the aim in our quantum chemical analysis.
4. Ab-initio analysis of
/
In order to elucidate how the defects interact with hydrogen isotopes in Li2O, the stable position of a proton and the electronic charge density map were calculated [4]. In the investigation, three varieties of F-centers and lithium vacancy were considered. It was concluded that the relaxation of the proton was explained by taking account of the two effects: the replacement of lithium ion caused by production of the defects and the interaction with the electrons trapped by F-centers. In addition, it was suggested that the proton could be trapped by F0 center. Fig. 3 summarises the most stable configurations of /OH obtained with each defect. It should be noted that the proton is not on (100) plane and orientated toward the vacancy site in the case of lithium vacancy. The position ‘iii’ corresponds to the proton keeping O /H bond and ‘iv’ to trapped one by F0 center. Fig. 4 is an electronic charge density map of the trapped proton by F0 center calculated by CASTEP.
OH in Li2O
4.1. Calculation of vibrational frequency For comparison with observed spectra, an analysis of the stretching vibrational frequency of O /H is effective. However, it had been difficult to estimate the frequency accurately. One reason was that our calculations were limited in HF level, and another was that the relaxation of ions or charge of the system had significant effects on the analysis. For the first step of the study on O /H frequencies, LiOH crystal was considered [3].
Fig. 3. Stable positions of -OH with defects on Li2O (100) plane. /OH with, i: F2 center, ii: F center, iii: F0 center, iv: F0 center (trapping), v: perfect crystal, vi: Li vacancy.
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
792
Fig. 4. Electronic charge density map of trapped proton by F0 center on Li2O (100) plane.
4.3. Charge configuration of /OH with defects In the present paper, Mulliken population analysis has been conducted in order to estimate electron transfer between the proton and the defects. In the population analysis, eight neighbours of each ion were taken into account. Table 1 shows atomic charges of the proton, bond populations and O /H distance calculated by CRYSTAL. From the table, it is shown that atomic charges and bond populations between the proton and F0 center is the largest in the position ‘iv’. It was found that the proton reacted to F0 center, and electrons trapped by the defect transferred to the
proton. In other words, the proton was trapped and reduced by F0 center. In the cases of perfect crystal and lithium vacancy, the protons bond to oxygen ions strongly. Especially /OH with lithium vacancy has the largest O /H bond populations and its O /H length is similar to that of OH ˚ ). The result supports premolecular ion (0.97 A vious energetic analysis, where the stability of / OH with lithium vacancy was significant high [4]. In the light of atomic charge of the proton and bond populations with F center, it is considered that F center could reduce the proton weakly. However, understanding bond populations with oxygen ion is difficult because it bonds to neither oxygen ion nor any other ions. Sometimes d shell orbital in the basis sets for oxygen has an important role in O /H bond, however, the orbital was not considered in the present study. In the next stage, the effects of d shell orbital will be checked. 4.4. Effect of Li relaxation on H trapping by F0 center The reduction process of the proton by F0 center discussed in the above section could be considerably related to diffusion of the proton in the Li2O crystal. In the neutron irradiated Li2O, two reactions are proposed for tritium diffusion in the grain [9]. LiOTF0 0 LiT; LiT1=2O2 0 LiOT After a number of conversions between LiT and LiOT, tritium diffuses to the grain surface. From the viewpoint of the electron transfer, the former equation corresponds to reduction of tritium by F0 center. As regards the process, the potential curve
Table 1 Mulliken population analysis of a proton in Li2O with defects and O /H distance
i. F2 center ii. F center iii. F0 center iv. F0 center v. Perfect crystal vi. Li vacancy
Atomic charges of H (e)
Bond populations O /H (e)
Bond populations F center /H (e)
˚) O /H distance (A
0.836 0.940 0.989 1.411 0.790 0.560
0.018 0.002 0.033 0.000 0.045 0.053
0.000 0.005 0.023 0.113
0.935 0.950 0.956 2.750 0.939 0.971
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
was estimated between the positions ‘iii’ and ‘iv’ (in Fig. 3) in our previous work [4]. The obtained activation energy was about 1.2 eV but hardly probable for the diffusion process, because reported activation energy is ranging from 0.85 to 1.0 eV in the neutron-irradiated Li2O [10]. The relaxation of lithium ions neighbouring the proton was considered, which had been neglected in the previous analysis. The following is the procedure of the calculation. The proton is fixed at the position which is the nearest from the two lithium ions between the oxygen ion and F0 center, ˚ . Fig. 5 shows geomewhere O /H length is 1.3 A trical configuration on (110) plane. In the condition, the stable positions of the two lithium ions were searched. The stable position was confirmed by both CRYSTAL and CASTEP. ˚ After relaxation, the lithium ions moves 0.22 A outward from the proton and the energy of the system calculated by CRYSTAL decreases 0.45 eV. Fig. 6 is calculated potential energy curve. The activation energy for trapping of the proton became 0.75 eV. The results confirm that a proton is easily trapped and reduced by neighbouring F0 center. If not related to diffusion of a proton directly, it could be concluded that the obtained activation energy stands within the reasonable range for the diffusion. To discuss whole of tritium diffusion, several uncertain processes are still left. For instance, it is not clear whether the reduced proton bonds to a lithium ion and forms LiT. The diffusion and recovery of defects also should be considered. The quantum chemical analysis presented in the paper affords new approach to analyse the inter-
793
Fig. 6. Energy of the system with F0 center as a function of O / H distance.
action of hydrogen isotopes with charged defects. From the analysis, the role of defects in the diffusion of hydrgen isotopes in Li2O is going to be clear from the quantum chemical point of view. The combination of the quantum chemical calculation, FT-IR and model studies has a potential to elucidate tritium behaviour in the blanket materials.
5. Conclusions A series of investigations for the interaction of a proton with defects in Li2O was reported. From Mulliken population analysis, it was shown that F0 center around a proton could trap and reduce the proton. Taking the relaxation of lithium ions into consideration, the activation energy for trapping of the proton by F0 center was estimated to be 0.75 eV.
References
Fig. 5. Relaxation of Li during H trapping by F0 center on Li2O (110) plane.
[1] H. Tanigawa, M. Taniguchi, S. Tanaka, et al., Interaction of hydrogen isotopes with defects in Li2O, Fusion Technol. 34 (1998) 872. [2] H. Tanigawa, S. Tanaka, FT-IR study on interaction of hydrogen isotopes with defects in lithium oxide, Fusion Eng. Des. 51 /52 (2000) 193. [3] H. Tanigawa, S. Tanaka, Quantum chemical calculation of O /H vibration in LiOH, J. Nucl. Sci. Technol. 38 (2001) 1004.
794
H. Tanigawa, S. Tanaka / Fusion Engineering and Design 61 /62 (2002) 789 /794
[4] H. Tanigawa, S. Tanaka, Ab-initio study on interaction of hydrogen isotopes with charged defects in lithium oxide, J. Nucl. Mater., in press, 2002. [5] R. Dovesi, V.R. Saunders, C. Roetti, et al., CRYSTAL-95 code, 1996. [6] R. Dovesi, Ab-initio Hartree-Fock extended basis set calculation of the electronic structure of crystalline lithium oxide, Solid State Commun. 54 (1985) 183. [7] A. De Vita, M.J. Gillan, J.S. Lin, et al., Defect energetics in MgO treated by first-principles methods, Phys. Rev. B 46 (1992) 12964.
[8] R. Shah, A. De Vita, M.C. Payne, et al., Ab initio study of tritium defects in lithium oxide, J. Phys. Condens. Matter 7 (1995) 6981. [9] H. Moriyama, T. Kurasawa, Tritium release kinetics of solid lithium ceramics with irradiation defects, J. Nucl. Mater. 212 /215 (1994) 932. [10] T. Tanifuji, K. Noda, T. Takahashi, H. Watanabe, et al., Tritium release from neutron-irradiated Li2O: diffusion in single crystal, J. Nucl. Mater. 149 (1987) 227.