Reversible electronic & magnetic structures in epitaxial strontium cobaltites

Woo Seok ChoiDepartment of Physics, Sungkyunkwan UniversityMaterials Science & Technology Division,Oak Ridge National Laboratory

Acknowledgements

(Materials Science & Technology Division, Oak Ridge National Laboratory)

Prof. C. U. Jung (HUFS) Film growthDr. D. W. Shin (Oak Ridge Nat’l Lab.) Thermodynamic modelingDr. J. H. Lee (Princeton Univ.) First principles calculationsProf. S. S. A. Seo (Univ. Kentucky) Optical measurementsDr. J. W. Freeland (Argonne Nat’l Lab.) X‐ray absorption spectroscopyDr. M. F. Chisholm (Oak Ridge Nat’l Lab.) Scanning Transmission Electron MicroscopyProf. H. Ohta (Hokkaido Univ.) Transport measurementandmany others…

Dr. H. N. Lee Dr. H. Jeen

K. Vidyasagar et al., J. Chem. Soc. Chem. Comm. 1, 7-8 (1985)

PerovskiteABO3

ABO2.5 ABO2.5 ABO2.5

Topotactic phase transformation

A structural transformation within a crystal lattice, which may include loss or gain of material, so that the final phase has one or more crystallographically equivalent orientational relationships to the lattice of the parent phase.

Advantages• Systematic tailoring of new (meta‐)stable compounds

• Controlled modification of physical properties

• Many technological applicationse.g.) oxide electronics,ionic conductors,cathodes in SOFCs,catalytic converters.

Introduction

Ionic transport

Electrochemical reaction• Catalytic activity• Gas conversion• Cathodes for SOFCs and batteries• Redox of the host material

A. Vojvodic & J. K. Nørskov, Science 334, 1355 (2011)J. Suntivich et al., Science 334, 1383 (2011)

In search for catalytic descriptors

Fundamentalphysical properties

• Structural property• Crystal field

• Stoichiometry• Valence state or Oxidation state

• Electronic structure• Electronic and magnetic property

Topotacticphasereversal

A perfect model system for studying the correlation between fundamental physical properties

and electrochemical activities!i.e., physical signatures for electrochemical processes.

Introduction

Oxygen ion conduction Cathode for solid oxide fuel cells and rechargeable batteries Catalytic gas sensors Oxygen separation membranes

S. B. Adler, Chem. Rev. 104, 4791 (2004).

Oxygen (ion) movement in oxides – Energy materials

J. N. Reimers & J. R. Dahn,J. Electrochem. Soc. 139,

2091 (1992).

Introduction

Multivalencyin transition metal oxides

Multivalent Cobalt in transition metal oxides

8

7

6

5

4

3

2

1

0 Sc Ti V Cr Mn Fe Co Ni Cu Zn

3d Elements

Oxi

datio

n St

ates

Solid dots: the most commonly found states

27

Co3d74s2

27

Co3d74s2

• Multivalent transition metal• Common oxidation states: 2+ & 3+• Less common: 4+

d6: La3+Co3+O3d5: Sr2+Co4+O3d6: Sr2+Co3+O2.5

Multiple sources to control the valence & spin state: A-site ion & oxygen

t2g

eg

Low spin (LS) state Intermediate spin (IS) state High spin (HS) state

S = 0 S = 1 S = 2

t2g

eg

Low spin (LS) state Intermediate spin (IS) state High spin (HS) state

S = 1/2 S = 3/2 S = 5/2

Co3+ (3d6)

Co4+ (3d5)

octahedral crystal field

Introduction

SrCoOx (SCO)

d5: Sr2+Co4+O3 d6: Sr2+Co3+O2.5

Brownmillerite (BM) (110)Perovskite (PV) (110)

Perovskite BrownmilleriteCubic

FM-M

ac = 3.829 Å

Orthorhombic

AFI-I

a = 5.545 Å,

b = 15.738 Å,

c = 5.448 Å

Brownmillerite (BM) (1-10)

1D vacancy channel

27

Co3d74s2

27

Co3d74s2

• Multivalent transition metal• Common oxidation states: 2+ & 3+• Less common: 4+

Introduction

La1-xSrxCoO3-δ

J. Wu & C. Leighton. Phys. Rev. B 67, 174408 (2003)

M. A. Senaris-Rodriguez & J. B. Goodenough,J. Solid State Chem. 118, 323 (1995).

Phase diagram of La1‐xSrxCoO3‐δ

CubicPerovskite

SrCoO3δ = 0

FerromagneticMetallic

Metastable

in La1-xSrxCoO3-δ

RhombohedralPerovskiteFerroelastic

CubicPerovskiteFerromagneticMetallic

1.0

BrownmilleriteAntiferromagneticLocalized Insulator(Co2+/Co4+)

Sr6Co5O15AntiferromagneticSemiconductor

Co3+ Co4+

• Promising cathode for SOFC• Mixed ionic and electronic conductor• Good catalytic activities• Magnetic material

Introduction

Difficult to achieve Co4+

(or, Perovskite SrCoO3)

Attempts to stabilize Co4+

- Quenching : Yakel, Acta. Cryst., 8, 394 (1955) (Co4+ = 3.8 %) : Takeda & Watanabe, J. Phys. Soc. Jpn. 33, 973 (1972) (x = 2.5)

- Electrochemical approach : Bezdicka et al., Z. anorg. Allg. Chem. 619, 7 (1993) (x = 2.5 3.0): Le Toquin et al., J. Am. Chem. Soc. 128, 13161 (2006)

- High pressure/high temperature approach : Takeda & Watanabe, J. Phys. Soc. Jpn. 33, 973 (1972) (x = 2.75 ~ 2.95)

- Hybrid method (high pressure/high temperature + oxidant): Long & Tokura et al., J. Phys.: Condens. Matter 23, 245601 (2011)

First Single Crystal!

However, all of these bulk synthesis methods require post treatmentssuch as high temperature, high pressure, and long time annealing, or chemical treatments.

Epitaxial thin films

0.19 0.20

0.60

0.61

qx || [100](Å-1)

103LSAT

0.19 0.20

0.59

0.60

103P-SCO

q z || [00

1](Å-1

)

qx || [100](Å-1)

106BM-SCO

103STO

22.5 23.4

Inten

sity (

arb.

units

)

(degrees)23.4 24.3

FWHM= 0.04

FWHM = 0.05

(degrees)

SrCoO2.5 epitaxial thin film

• Stable SrCoO2.5 phase readily formed.• Direct synthesis of oxidized perovskite SrCoO3

phase by ozonized growth.

First Epitaxial Thin Film!

Epitaxial thin films

Defect‐free brownmillerite SrCoO2.5 thin film 

STO

[110] BM-SCO [1-10] BM-SCO

STO

Epitaxial thin films

SrCoO2.5 near interface

CoO2(4)

CoO2(2)

0.5 nm

TiO2 (N = 0)SrO (0.5) CoO (1)SrO (1.5)

SrO (2.5)

SrO (3.5) CoO (3)

0.5 nm

ABF imageZ-contrast image

Epitaxial thin films

0.19 0.20

0.60

0.61

qx || [100](Å-1)

103LSAT

0.19 0.20

0.59

0.60

103P-SCO

q z || [00

1](Å-1

)

qx || [100](Å-1)

106BM-SCO

103STO

22.5 23.4

Inten

sity (

arb.

units

)

(degrees)23.4 24.3

FWHM= 0.04

FWHM = 0.05

(degrees)

First SrCoO3 epitaxial thin film, structural properties

• Stable SrCoO2.5 phase readily formed.• Direct synthesis of oxidized perovskite SrCoO3

phase by ozonized growth.

First Epitaxial Perovskite Thin Film!

Epitaxial thin films

Determination of oxidation state: XAS

780 790 800-0.4

-0.2

0.0

Bulk

XMCD

Energy (eV)

0.0

2.0

4.0Co L-edge

O K-edge

L2

Inten

sity (

arb.

units

)

L3

525 530 535 5400.0

1.0

2.0

BM-SCO P-SCO

Inten

sity (

arb.

units

)

Energy (eV)

Karvonen et al., Chem. Mater. 22, 70, (2010)

Bulk data

• Significant change in the pre-peak spectral shape with the increase of the oxidation state (Co 3d - O 2p hybridization).

• Shift of L-edge toward high energy upon introduction of Co4+.

Distinct contrast in physical properties

Electronic and magnetic phase transition in SrCoOx

-10 -5 0

-2

-1

0

-4 -2 0 2 4-2

-1

0

1

2

V (m

V)T (K)

0 100 200 3000.0

0.2

0.4

0.6

0.8

1.0

BM-SCO P-SCO

M (

B/Co)

T (K)

10 10010-410-310-210-1100101102103

(cm

)

T (K)

M

(B/C

o)

H (T)

254 μV/K

9.6 μV/K

H. Jeen & W. S. Choi et al. Nat. Mater. 12, 1057 (2013)

Distinct contrast in physical properties

• Metallic transport behavior for P-SCO• Clear demonstration of successful

stabilization of Co4+.• Four orders of magnitude change in

dc-conductivity already at 300 K.• Insulator-to-metal transition with

incorporation of oxygen in SrCoOx.• A large Seebeck coefficient from BM-SCO.• Positive Seebeck coefficient supports the

p-type conduction (holes), also confirmed by Hall Measurements.

• Ferromagnetic ordering below ~250 K for P-SCO.

Topotactic transformation induced electronic and magneticphase transitions.

Optical properties and electronic structure

PV SCO

O 2p

t2g

Drude

eg

EF

Co 3dCo 3d

O 2p

BM SCO

Coo

Cot

0.5 1.0 1.5 2.0 2.5 3.00

1

2

3

4

5

300 K 5 K

a

(105 cm

-1)

(eV)

• Insulator (BM) to metal (PV) transition is clearly shown.

• BM SCO: A low optical band gap insulator (~0.5 eV)

• Typical BM phase oxides have >2 eV of band gap.

• Advantageous for application: ionic (vacancy channel) + electronic conduction (in the vicinity of IMT)

• PV SCO: Metallic with Drude peak.

-6 -4 -2 0 2 4-6-3036

-6 -4 -2 0 2 4-6-3036

Energy (eV)

DOS

Energy (eV)

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.50

2

4

6

8

10

12

14

Theo

retic

al 2(

) (eV)

• Complicated electronic structure with both CoO6 octahedral and CoO4teterahedral crystal field.

• First principles calculation reproduced the experimental peak features.

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.50

2

4

6

8

BM SCO PV SCO 1

2

Expe

rimen

tal 1(

) & 2(

)

(eV)

W. S. Choi et al. Phys. Rev. Lett. 111, 097401 (2013)

Distinct contrast in physical properties

Topotactic transformation induced optical property change.

PV SrCoO3

+ Oxygen

– Oxygen

BM SrCoO2.5

Fast and reversible phase transformation in SCOTopotactic phase reversal

Real‐time observation of redox processes

Ep

Es

viewport viewport

gas inlet (P(O2) control)

CCD detector

photon source(193 ≤ λ ≤ 1000 nm)

to pump

O2

heater(T control)

vacuum chamber

sample

Spectroscopic ellipsometryfor the electronic structure evolution

during topotatic phase reversal

in collaboration with Prof. Seo of University of Kentucky

Co3+ Co4+

Topotactic phase reversal

@ 10 6 T

2 3 4 5

400

350

300

250

200

150

100

50T

(o C)

(eV)

2.0003.7605.500

2

heating@ 10-6 Torr

Electronic structure evolution

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.50

2

4

6

8

BM-SCO P-SCO 1

2

1() &

2()

(eV)

P. Pasierb et al., J. Phys. Chem. Solids 60, 1835 (1999)

OXIDATION REDUCTION

γ

β

(P)

(BM)

Topotactic phase reversal

2 3 4 5320

300

280

260

240

220

200

180heating@ P(O2) = 500 Torr

T (o C)

(eV)

2.0004.8007.500

2

2 3 4 5350

300

250

200

150

100

50reheating@ 10-6 Torr(after cooldown)

T (o C)

(eV)

Real‐time ellipsometry: BM ↔ PV

2 3 4 5

50

100

150

200

250

300

350 cooling@ 10-6 Torr

@ 10-6 Torr

T (o C)

(eV)

2 3 4 5

400

350

300

250

200

150

100

50

T

(o C)

(eV)

2.0003.7605.500

2

heating@ 10-6 Torr

W. S. Choi et al. Phys. Rev. Lett. 111, 097401 (2013)

γ (P)

β (BM)

β (BM)

β (BM)

β (BM)

β (BM)

γ (P)

γ (P)

Topotactic phase reversal

200 400 600 800

-0.2

-0.4

-0.6

-0.8

-1.0

G SrMn

O 3- -

G SrMn

O 2.5

(eV)

SrMnO3

T (C)

SrMnO2.8

SrMnO2.9

SrCoO2.8

SrCoO2.9

0.2

0.4

0.6

0.8

1.0SrCoO 3

G SrCo

O 3- -

G SrCo

O 2.5 (e

V)

Real‐time XRD: BM ↔ PV

45 46 47 48500

400

300

200

100

T (C

)

2 (degrees)

45 46 47 48400

300

200

100

T

(C)

2 (degrees)

P BM

BM P

Difference in Gibbs free energy between BM and P phase

• Fast and reversible redox reaction at relatively low temperatures for epitaxial SCO thin films.

• Sign and absolute value of the Gibbs energy differenceprovides crucial insight for custom tailoring of topotacticprocesses in multivalent oxides.

• Reduced energy difference at low temperatures was predicted for SCO.

in vacuum

in O2

Topotactic phase reversal

H. Jeen & W. S. Choi et al. Nat. Mater. 12, 1057 (2013)

Catalytic activity

P BM

BM P

• The redox reaction at relatively low temperatures seems to enhance the catalytic activity of epitaxial SCO thin films (BM in this case).

• First evidence of catalytic activity shown form BM-SCO at relatively low temperature.

in vacuum

in O2

Topotactic phase reversal

PV SrCoO3

+ Oxygen

– Oxygen

BM SrCoO2.5

Epitaxial oxygen sponge SrCoOx

• First PLE growth of epitaxial brownmillerite (BM) and perovskite (P) phases of SrCoOx single crystalline thin films!

• Fast, reversible redox reactions at low temperatures!•The topotactic phase transformation accompanies various physical property transitions!

Adv. Mater. Early View doi:10.1002/adma.201302919 (2013).Phys. Rev. Lett. 111, 097401 (2013).Nat. Mater. 12, 1057 (2013).Adv. Mater. 25, 3651 (2013).