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Molecular simulation on radiation behavior of Li 2 OPowerPoint Presentation

Molecular simulation on radiation behavior of Li 2 O

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### Molecular simulation on radiation behavior of Li2O

Takuji Oda, Yasuhisa Oya, Satoru Tanaka

Department of Quantum Engineering & Systems Science

The University of Tokyo

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 on the radiation effect.

Subjects

(1) Radiation behavior

(MD simulation)

(2) Interaction with Li vac.

(FT-IR exp. &

DFT calculation)

surf.

(3) Interaction with F centers

(DFT calculation)

(4) Influence of the dynamic

Frenkel defects

(MD simulation)

bulk

n

T-

(1)

(3)

O

F

Li

VLi

(4)

T+

T+

Li+

VLi

(2)

(LiOT)n

Fig. 1. Tritium in Li2O

Experimental ; FT-IR with an ion gun

Sample：Li2O s.c.

φ10mm, 1mm

Fig.2. 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.

Li :

Calculation details-1; plane-wave pseudopotential DFTSoftware: CASTEP code

Functional: 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)

Calculation details-2; classical molecular dynamics (MD)

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

Fig. 2. Inter-ionic potential (Li-O)

(i) Coulombic interaction

(ii)Short range interaction(10 Å cutoff)

Software: DL-POLY

System: 5x5x5 or 7x7x7 supercell

(Li1000O500 or Li2744O1372)

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.

Subjects

(1) Radiation behavior

(MD simulation)

(2) Interaction with Li vac.

(FT-IR exp. &

DFT calculation)

surf.

(3) Interaction with F centers

(DFT calculation)

(4) Influence of the dynamic

Frenkel defects

(MD simulation)

bulk

n

T-

(1)

(3)

O

F

Li

VLi

(4)

T+

T+

Li+

VLi

(2)

(LiOT)n

Fig. 1. Tritium in Li2O

(2) Interaction with Li vac.; FT-IR during 3keV D2+ irradiation

Fig.3. O-D peaks during 3keV D2+ irradiation

Fig.4. 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” ??

(2) Interaction with Li vac. ; FT-IR during heating after the D2+ irr.

decrease

increase

Fig.5. 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 ??

(2) Interaction with Li vac. ; stabilization by aggregation (DFT)

A: 1 isolated (LiO- - H+)

C: (LiO- - H+)2

Fig.6. Electronic density

B: 2 isolated (LiO- - H+)

Stabilization by aggregation is confirmed !

(3) Interaction with F centers ; locally stable positions near F centers (DFT)

<O-H site>

Li: , O: , H: , F centers:

<O defect site>

Fig. 7. H+ neighboring F center in Li2O

*By controlling the system charge, O vac., F+, and F0 are modeled.

(3) Interaction with F centers ; stability around F centers (DFT)

<O-H site>

<O defect site>

Fig. 8. Stability of H near F center

F centers trap H strongly, and reduce it to H-.

(4) Influence of the dynamic Frenkel defect; what is “the superionics” in Li2O ? (MD)

2600 K

(liquid)

1600 K

(superionics)

1000 K

(solid)

Vacant

site

O

Li

O

Li

Fig. 10. Li2O crystal

Fig. 9. Projected ionic densities on (100) plane

- Just Li behaves like liquid even below the melting point >> the superionics.
- Most Li migrates along [100] (~90%), assisted by the dynamic Frenkel defects.

(4) Influence of the dynamic Frenkel defect; what is “the dynamic Frenkel defect” ? (MD)

(a) Extrinsic region(by a Li vacancy) >>0.25 eV

(b) Below the critical temp. (by the dynamic defect) >> 1.9 eV

(c) Above the critical temp. >> 0.62 eV

(d) Liquid state >> 0.40 eV

f: correlation factor

>> ~ 0.653 in theory

d: distance in a jump

>> ~0.25 nm along <100>

[Freq.]: vibration frequency

>> ~ 3x1013 s-1 from MD

Ed: diffusion barrier

Ndefect / Natom: defect density

Fig. 11. Variation of Li diffusion coefficients

(4) Influence of the dynamic Frenkel defect; the dynamic defect, a defect cluster, etc (MD)

Fig. 12. Contribution of the dynamic Frenkel defect to Li diffusivities

- Even in the highly Li-burnup conditions, the contribution of the dynamic
Frenkel defect in the Li diffusivity reaches 50 % above 1200 K.

The participation of the dynamic defect is significant above 1200 K.

The dynamic defects may also affect T+ behavior, due to the similarity.

(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.)

(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)

Fig. 14. Threshold displacement energies

Fig. 13. Li2O crystal

- O requires much more high energy for displacement than Li.
- The threshold energy can be ordered as [111] ＞ [110] ＞ [100].

(1) Radiation behavior of Li2O; key points for the modeling (MD)

Fig. 15. Number of Li vac. survived after 4 ps

Fig. 16. Variation of the maximum energy

- The PKA energy is immediately spread
into the system.

- Number of stable defects are sensitively
dependent on the PKA energy.

(due to the self-annealing effect, etc)

This behavior could be related to

the self-annealing effect,

the radiation induced diffusion, etc.

The threshold energy is not enough to describe the radiation event.

Summary

surf.

(1) Radiation behavior

(MD simulation)

(3) Interaction with F centers

(DFT calculation)

bulk

- F centers trap T+ strongly and
reduce it to T-.

- O requires much higher energy
for displacement than Li.

n

T-

- The threshold energy:
[111] ＞ [110] ＞ [100].

(1)

(3)

O

- Capturing force depends on
the charge state of F centers:

F0 > F+ > O vac.

F

Li

- The PKA energy is rapidly spread into the system.

VLi

(4) Influence of the dynamic

Frenkel defects

(MD simulation)

(2) Interaction with Li vac.

(FT-IR exp. &

DFT calculation)

(4)

T+

T+

Li+

VLi

(2)

- The dynamic defect assists
Li diffusion strongly, over 1200 K..

- Li vac. heightens the stability
of T+ (formation of subs. T+).

(LiOT)n

- The dynamic defect may also
affect T+ behavior.

- (LiO- - T+) becomes more
stable by aggregation.

Fig. 1. Tritium in Li2O

Future works

(1) Radiation behavior

- How about electron excitation >> ??
- How about model dependences >> checking by other models

(2) Interaction with Li vac.

- How to aggregate each other >> classical MD
>> modeling “T+ in Li2O”

(3) Interaction with F centers

- How to detrap >> ab-initio MD
>> FT-IR & UV absorption experiment

(4) The dynamic Frenkel defect

- How to interact with T+>> classical MD
>> modeling “T+ in Li2O”

Acknowledgements

We are very grateful to

Dr. R. Devanathan, Dr. F. Gao, Dr. W.J. Weber and Dr. L.R. Corrales

for help and support during the present research.

This research was performed in part using the MSCF in EMSL, a national scientific user facility sponsored by the U.S. DOE, OBER and located at PNNL.

- the 21st Century COE Program, “Mechanical Systems Innovation,”
by the Ministry of Education, Culture, Sports, Science and Technology

- the Tokyo Denryoku Zaidan
- the Atomic Energy Society of Japan

We are also grateful to

for financial support on the present research.

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