In this work, I am showing a faithful atomistic process of estimating the oxygen migration energetics within BSCF, oxygen migration energy exhibit a strong dependence on different local atomic structures of this doped perovskites. In addition, DFT calculations exhibit the reason of cubic phase stability of this doped perovskite in variable oxygen concentration.

1. Atomistic simulations of a promising solid oxide fuel cell
cathode materials Ba0.5Sr0.5Co0.8Fe0.2O3-δ
 Electronic Structure
 Phase Stability
 Oxygen Migration Energetics using Nudged Elastic Band (NEB)
Shruba Gangopadhyay
Email: shruba at gmail.com
University of California, Davis
IBM Research – Almaden
This work performed at University of Central Florida
. 1
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2. Solid Oxide Fuel Cell -SOFC
 Anode - Ceria/Nickel cermet
 Electrolyte - Gadolinia doped Ceria (CGO)
 Cathode - LSCF (a four component oxide based on
La, Sr, Co, and Fe oxides)
2

3. Preferred structure of cathode materials
Cathode materials are Oxygen rich oxides
Perovskites represented by ABO3
A= Lanthanides/Group IIA
B= Transition metals
3
Perovskites (SrCoO3)
For a good SOFC cathode
No phase transition
Ease of oxygen migration

4. BSCF – new perovskite for SOFC
Perovskites represented by ABO3, Ba0.5Sr0.5Co0.8Fe0.2O3-δ
A= Ba or Sr
B= Fe or Co
Constructed a model supercell 2x2x2 cell
4Shao, Z. P.; Haile, S. M., Nature 2004, 431, (7005), 170-173.

5. BSCF - no phase transition
5
Wang, H. H.; Tablet, C.; Feldhoff, A.; Caro, H., Journal of Membrane Science 2005, 262, (1-
2), 20-26.
Dependence of lattice parameter
w.r.t temperature
Closest Analog SrCo0.8Fe0.2O3 shows vacancy ordering

6. Oxygen vacancies in BSCF
6
Oxygen non-stoichiometry ″δ ″
Shao, Z. P.; Haile, S. M., Nature 2004, 431, (7005), 170-173.
BSCF as a function of temperature at the
oxygen partial pressures indicated
We need to remove maximum
Four oxygen from supercell

7. Activation energy of oxygen migration
7
D chem = Rate of diffusion
Ea = Activation energy
D0 = Temperature independent
pre exponential factor depends
on lattice vibrations and jump
distance
k = Boltzmann constant
T = Temperature
Self diffusion coefficient measures ease of oxygen mobility

8. Our goals
 Electronic Structure of BSCF
 Validation with Lattice Parameter
 Ground Spin state of Transition Metal (B)
cations
 Stable Cation Arrangement
 Phase stability of BSCF
 Stable most vacancy position
 Activation Energy of Oxygen Migration
8

9. DFT-simulation of BSCF
Self-Consistent field calculation
 Plane wave basis set
 PBE Functional
 Vanderbilt Ultra-soft Pseudo potential
 Marzari-Vanderbilt cold smearing
9
Structural Optimization
 BFGS algorithm
Population Analysis
 Löwdin population analysis
Activation Energy for Oxygen Migration
 Symmetry Constarined Search and Nuged Elastic band(NEB)
Quantum Espresso (Extensible Simulation Package for
Research on Soft matter) )
http://www.quantum-espresso.org/

10. Spin state of transition metals
10
2Co+4 4Co+4 6Co+4
In BSCF
Co +4 shows intermediate spin states
-249.80
-249.78
-249.76
-249.74
-249.72
-249.70
-249.68
-249.66
-249.64
7.00 7.20 7.40 7.60 7.80
Intermediate
High
Low
Low Intermediate High

11. Co+4 spin state predicted – in
agreement with experiment
Experimental Results
Raman Spectroscopy
Theoretical Validation
Jahn-Teller Distortion
11
O
Co
3.849 Ǻ
3.849 Ǻ
3.652 Ǻ
3.581 Ǻ
3.901 Ǻ
3.581
3.849
3.849
3.9013.652

12. Lattice parameter predicted –in agreement
with experiment
Perovskites Calc
(GPa)
Expt
(GPa)
Calc Expt
(Å)
Bcalc Bexp acalc aexp
BaTiO3 148.34 135 3.98 4.00
SrTiO3 181.57 179 3.93 3.899
SrFeO3 3.88 3.84
SrCoO3 3.85 3.83
Ba 0.5 Sr0.5 Co0.6Fe0.2O3 3.95 4.00
12
No of
Unpaired
Electron
Relative
Energy
Difference
(kJ/mol)
Boltzmann
factor
(%)
Fe+4
(d4)
Co+4
(d5)
2 3 40 2
4 3 0 98
2 5 140 0
4 5 404 0

13. A, B cations in BSCF are distributed
randomly
Fe1 Fe2 Ba1 Ba2 Ba3 Ba4
E
kJ/mol
B
Factor
%
1 5 10 12 14 16 0.00 18
1 6 10 12 14 16 0.10 18
1 8 10 12 14 16 0.76 13
1 5 11 12 13 14 1.42 10
1 6 11 12 13 14 1.41 10
13
There is no preferred cation ordering

14. Plan of action to determine preferred
oxygen vacancy positions
1. First take the lowest energy configuration with no
oxygen vacancy
2. Calculate the energetics of by removing one oxygen
from nonequivalent sites in one oxygen less
supercell
3. Use the lowest energy configuration (obtained after
removing one oxygen) as the starting configuration
to determine second favorable vacancy positions
4. Followed same way……….
14

15. Vacancies prefer to cluster
15
One Vacancy
The most Stable for Co-x-Fe (not Co-x-Co Fe-x-Fe)
Boltzmann Factor for this configuration 67%
Cis cobalt coordination is the most Stable , Than Trans
and cis Fe coordination
Two Vacancies
Co O
O is more stable than
Fe O
O
Boltzmann Factor 54%

16. 16
Three Vacancies
2 cis-octahedral one adjacent to Fe another Co
Three vacancy forms in a plane of octahedral
Vacancies form L shaped trimer
Vacancies prefer to cluster
Boltzmann Factor for this configuration 69%

17. Vacancy forms in square
B cations form tetrahedral geometry
17
Boltzmann Factor for
Square vacancy : 47%
Linear vacancy : 9%

18. Atomistic view of
Oxygen migration inside an perovskite
18
Initial
Intermediate
Metal –O-Metal
450
Final

19. Elementary step for vacancy diffusion
19
0 2 4 6 8
Energy(eV)
NEB image number
83.
72.8
58.
42.8
28.4
14.4
8
Activation
Energy

20. Activation energies for different
cation arrangements
20
Ba Fe Migrating
Vacancy
Permanent
Vacancy
Ea
kJ/mol
10 11 13 16 1 8 22 24 37.9
10 12 14 16 1 8 35 36 52.3
11 12 13 14 3 5 29 24 34.6
11 12 13 14 3 5 38 39 20.2
10 12 14 16 3 5 38 39 41.2
10 12 14 16 3 5 38 39 42.6
10 12 14 16 1 5 38 39 38.8
10 12 14 16 1 5 36 35 23 24 49.2
10 12 14 16 1 5 30 31 23 24 29 29.8
10 12 14 16 1 5 35 40 24 29 36 18.5
Experimental activation energy for
Oxygen Migration 30-50 kJ/mol
Energy from symmetry constrained
Search
Energy from NEB

21. Conclusions: BSCF
 Our DFT calculation shows experimental agreement
 Lattice Parameter of perovskites
 JT distortions
 Cations in stoichiometric BSCF is completely disorder
 Vacancy prefers to form L shaped trimer and square tetramer
 With removal of oxygen transition metals change octahedral to
tetrahedrahal coordinations
 Oxygen migration energy(s) shows agreement with experiment
 Symmetry Constrained pathway and NEB calculations both shows
similar energy value
21
Ref:
S Gangopadhyay et.al. ACS Applied Materials & Interfaces 2009, 1 (7), 1512-1519
S Gangopadhyay et.al. Solid State Ionics 2010, 181 (23–24), 1067-1073.

22. Acknowledgements
22
Ref:
S Gangopadhyay et.al. ACS Applied Materials & Interfaces 2009, 1 (7), 1512-1519
S Gangopadhyay et.al. Solid State Ionics 2010, 181 (23–24), 1067-1073.
 This work have been performed at Professor Artëm E. Masunov’s lab
 Research Collaborators
 Prof. Nina Orlovskaya
 Prof. Ratan Guha
 Prof. Jay Kapat
 Prof. Ahmed Sleiti