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Atomic scale understandings on hydrogen behavior in Li 2 O - toward a multi-scale modeling -

Atomic scale understandings on hydrogen behavior in Li 2 O - toward a multi-scale modeling -. Satoru Tanaka, Takuji Oda and Yasuhisa Oya The University of Tokyo. Background. 6 Li + n → 4 He (2.1 MeV) + T (2.7 MeV).

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Atomic scale understandings on hydrogen behavior in Li 2 O - toward a multi-scale modeling -

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  1. Atomic scale understandings on hydrogen behavior in Li2O- toward a multi-scale modeling - Satoru Tanaka, Takuji Oda and Yasuhisa Oya The University of Tokyo

  2. Background 6Li + n →4He (2.1 MeV) + T (2.7 MeV) To establish a secure and efficient fuel cycle in a fusion reactor, produced tritium must be recovered rapidly from the breeding blanket. In the case of a solid breeding material (Li2O, Li2TiO3 etc), radiation defects created in the severe radiation conditions affect the tritium behavior strongly. Hence, behaviors of tritium and defects in Li2O have been extensively studied. However, …. • The evaluated tritium diffusivities are scattered. • The concrete influence of each defect is not understood sufficiently. Our aim is to model the hydrogen isotope behavior precisely, based on the atomic-scale understandings (= multi-scale modeling).

  3. Multi-scale modeling (i) Quantitative analysis by the ab initio calculation <high accuracy in the statics; ~1 nm> (ii) Quantitative analysis by the molecular dynamics <inclusion of the dynamics; ~10 nm, ~ms> (iii) Integration in mesoscale by the kinetic Monte Carlo simulation <inclusion of the statistics; ~1 mm, ~s> (iv) Extrapolation into the real scale by theoretical modeling<engineering goal; e.g. ~1 m, ~year> (v) Model validation by referring to experimental results (a) In the bulk <diffusion, de-/trap> (a) (b) (c) (b) On the surface <diffusion, ad/ab/de-sorption> (c) At the grain boundary <diffusion, retention at pores>

  4. Subjects H2O H2 (1) Radiation behavior (MD simulation) (2) Interaction with Li vacancy (FT-IR exp. & DFT calculation) (3) Interaction with O vacancy (DFT calculation) (4) Surface processes (XPS/UPS exp. & MC) *not a topic today (4) surface OT- T- OT- bulk T+ n O (1) T+ Li F (3) T+ (LiOT)n (2) VLi Tritium in defective Li2O

  5. Method-1 (experiment) : FT-IR under ion irradiation Sample：Li2O s.c. φ10mm, 1mm IR absorption experimental system OD stretching vibrations showsmultiple peaks by interaction with a specific defect. The behaviors of hydrogen isotopes in various chemical states can be analyzed individually.

  6. O : Li : Method-2 (ab initio calculation): plane-wave pseudopotential DFT Software: CASTEP code Functional: GGA-PBE K-point grid: 3x3x3 Energy cutoff: 380 eV Calculation cost was reduced by use of plane-wave basis and pseudopotential technique (O 1s). 2x2x2 Conventional cell (Li8O4) 2x2x2 supercell (Li64O32)

  7. Method-3 (molecular dynamics): MD for cascade simulation In the classical MD, electrons are not described explicitly. As a result, the calculation cost is enough reduced to perform the dynamics simulation. < Buckingham pair potential model> q1q2/r+A×exp(-r/ρ) - C/r6 Inter-ionic potential (Li-O) (i) Coulombic interaction (ii)Short range interaction(10 Å cutoff) Software: DL-POLY System: 5x5x5 or 10x10x10 supercell (Li1000O500 or Li8000O4000) Ensemble: NpT or NEV Time step: 1 fs or variable step Simulation time: ~5 ns or ~4 ps In the case of radiation simulation, the Buckingham potential was connected to the ZBL potential by polynomial at around 0.6-1 Å.

  8. Subject-1; (1) Radiation behavior of Li2O (1) Radiation behavior (MD simulation) (2) Interaction with Li vacancy (FT-IR exp. & DFT calculation) (3) Interaction with O vacancy (DFT calculation) H2O H2 surface OT- T- OT- bulk T+ n O (1) T+ Li F (3) T+ (LiOT)n (2) VLi Tritium in defective Li2O

  9. (1) Radiation behavior of Li2O; 102.9 eV Li PKA along <110> (MD) Movie 1. Li PKA along [110] (PKA energy: 102.9 eV, NEV with 0K initial temp.)

  10. (1) Radiation behavior of Li2O; threshold displacement energy (MD) Angle dependence of the threshold displacement energy was obtained: angular resolution of 6x6=36 for each under NEV ensemble (0 K initial temp.) ( 0 eV 80 eV ) Vacant O [505] [555] Li [550] [500] O displacement Li displacement (left: vac., right: O) Threshold displacement energies Li2O crystal • O requires much more high energy for displacement than Li. • The threshold energy can be ordered as [111] ＞ [110] ＞ [100].

  11. (1) Radiation behavior of Li2O; key points for the modeling (MD) Number of Li vac. survived after 4 ps Variation of the maximum energy • The PKA energy is immediately spread into the system. * [relaxation time] ∝ [PKA energy]2 • Number of stable defects are sensitively dependent on the PKA energy. (due to the self-annealing effect, etc) The threshold energy is not enough to describe the radiation event. • The self-annealing is occurred within the relaxation time.

  12. Subject-2; (2) Interaction with Li vacancy (1) Radiation behavior (2) Interaction with Li vacancy (FT-IR exp. & DFT calculation) (3) Interaction with O vac. (DFT calculation) H2O H2 • The threshold displacement energy: O > Li, [111] ＞ [110] ＞ [100]. • The PKA energy is rapidly spread into the system. surface OT- T- OT- bulk T+ n O (1) T+ Li F (3) T+ (LiOT)n (2) VLi Tritium in defective Li2O

  13. (2) Interaction with Li vac.; FT-IR during 3keV D2+ irradiation O-D peaks during 3keV D2+ irradiation Intensity variation of each peak O-D is stabilized in the bulk by interaction with a defect (2605 cm-1) or by mutual aggregation (LiOD phase: 2710 cm-1) • 2710 cm-1 is LiOD phase. • 2660 cm-1 is mainly the surface O-D. • 2605 cm-1 is not attributed.. • [Low fluence] Only the surface O-D. • [High] The LiOD phase becomes dominant. What is the “defect” ??

  14. (2) Interaction with Li vacancy ; FT-IR during heating after the D2+ irradiation decrease increase Variation in O-D peaks during heating By the heating, the 2605 cm-1 peak decreased, while the 2710 cm-1 peak increased. O-D aggregated each other: (LiO- -D + )n [2605 cm-1] → LiOD phase [2710 cm-1] By the aggregation, (LiO- -D + ) can be really stabilized ??

  15. Li : O : H : (2) Interaction with Li vacancy ; stabilization by aggregation (DFT) A: 1 isolated (LiO- - H+) C: (LiO- - H+)2 Electronic density ⊿E　＝ EC － EB ＝ - 0.38 eV B: 2 isolated (LiO- - H+) Stabilization by aggregation is confirmed ! How many (LiO- - H+) for the 2605 cm-1 peak ? >> Frequency analysis

  16. (2) O-D stretching frequency in LiOD(as a validation of frequency analysis) A potential energy curve near the equilibrium position was obtained by ab initio calculation, and the Schrodinger equation of anharmonic oscillator is solved to analyze the O-D stretching frequency. Table Calculated O-D frequency Peak sift of O-D vibration in LiOD (FT-IR experiment) The plane-wave pseudopotential DFT with PBE/PW91 can provide good predictions.

  17. (2) O-D stretching frequency for O-D of sub./int. D+ in Li63O32D O-D of a substitutional D+ for Li+ in Li63O32D supercell O-D in Li2O at high temperatures Substitutional D+; 2440 cm-1 , Interstitial D+; 2493 cm-1 (LiO- -D + )n [2605 cm-1] → LiOD phase [2710 cm-1] → int. or sub. D+ [2510 cm-1]

  18. Subject-3; (3) Interaction of H+ with F centers (1) Radiation behavior (2) Interaction with Li vacancy (3) Interaction with O vacancy (DFT calculation) H2O H2 • The threshold displacement energy: O > Li, [111] ＞ [110] ＞ [100]. • The PKA energy is rapidly spread into the system. surface OT- T- OT- bulk T+ • Li vacancy heightens the stability of D+ (formation of subs. D+). n O (1) • (LiO- - H+) becomes more stable by aggregation. T+ Li F (3) T+ (LiOT)n (2) VLi Tritium in defective Li2O

  19. (3) Interaction with F centers ; locally stable positions near F centers (DFT) <O-H site> Li: , O: , H: , F centers: <O defect site> Sub. H+ neighboring F center in Li2O *By controlling the system charge, O vac., F+, and F0 are modeled.

  20. (3) Interaction with F centers ; stability around F centers (DFT) <O-H site> <O defect site> Stability of sub. H+ near F center F centers trap H strongly, and reduce it to H-.

  21. Summary (1) Radiation behavior (2) Interaction with Li vacancy (3) Interaction with O vacancy H2O H2 • The threshold displacement energy: O > Li, [111] ＞ [110] ＞ [100]. surface OT- T- • The PKA energy is rapidly spread into the system. OT- bulk T+ • Li vac. heightens the stability of D+ (formation of subs. D+). n O (1) T+ • (LiO- - H+) becomes more stable by aggregation. Li F (3) T+ (LiOT)n (2) • F centers trap T+ and reduce it to T-. VLi • Capturing force depends on the charge state of F centers: F0 > F+ > O vacancy Tritium in defective Li2O

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