Atomic scale understandings on hydrogen behavior in Li2O toward a multiscale modeling

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 lthigh accuracy in the statics 1
nmgt
(ii) Quantitative analysis by the molecular
dynamics ltinclusion of the dynamics 10 nm, msgt
(iii) Integration in mesoscale by the kinetic
Monte Carlo simulation ltinclusion of the
statistics 1 mm, sgt
(iv) Extrapolation into the real scale by
theoretical modeling ltengineering goal e.g. 1
m, yeargt
(v) Model validation by referring to experimental
results
(a) In the bulk ltdiffusion, de-/trapgt
(a)
(b)
(c)
(b) On the surface ltdiffusion,
ad/ab/de-sorptiongt
(c) At the grain boundary ltdiffusion,
retention at poresgt
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
(3)
F
T
(LiOT)n
(2)
VLi
Tritium in defective Li2O
5
Method-1 (experiment) FT-IR under ion
irradiation
SampleLi2O s.c. f10mm, 1mm
IR absorption experimental system
OD stretching vibrations shows multiple peaks by
interaction with a specific defect.
The behaviors of hydrogen isotopes in various
chemical states can be analyzed individually.
6
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.
lt Buckingham pair potential modelgt
q1q2/r Aexp(-r/?) - 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
(3)
F
T
(LiOT)n
(2)
VLi
Tritium in defective Li2O
9
(1) Radiation behavior of Li2O 102.9
eV Li PKA along lt110gt (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 6x636
for each under NEV ensemble (0 K initial temp.)
( 0 eV
80 eV )
O
Vacant
555
505
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
  gt 110 gt 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 energy2

• 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 gt Li, 111 gt 110 gt 100.

• The PKA energy is rapidly spread into the
  system.

surface
OT-
T-
OT-
bulk
T
n
O
(1)
T
Li
(3)
F
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
(2) Interaction with Li vacancy
stabilization by aggregation (DFT)
A 1 isolated (LiO- - H)
C (LiO- - H)2
Electronic density
B 2 isolated (LiO- - H)
?E EC - EB - 0.38 eV
Stabilization by aggregation is confirmed !
How many (LiO- - H) for the 2605 cm-1 peak ?
gtgt 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 gt Li, 111 gt 110 gt 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
(3)
F
T
(LiOT)n
(2)
VLi
Tritium in defective Li2O
19
(3) Interaction with F centers
locally stable positions near F centers (DFT)
ltO-H sitegt
Li , O , H , F centers
ltO defect sitegt
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)
ltO-H sitegt
ltO defect sitegt
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 gt Li, 111 gt 110 gt 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
(3)
F
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 gt F gt O vacancy

Tritium in defective Li2O