Structural, Electronic, and Transport Properties of ...€¦ · Structural, Electronic, and...

Middle East Technical University, Department of Physics Seminar – November 11, 2013

UIC!Materials Modeling Group Physics Department

Structural, Electronic, and Transport Properties of Thermoelectric Ca3Co4O9: Insights from First

Principles Computations and Electron Microscopy

Serdar Öğüt

Department of Physics, University of Illinois at Chicago

Supported by DOE, NSF, Argonne, and NERSC



Outline

q  Z-contract Imaging and EELS q  Increased S and High-Spin Co4+ in 40 nm CCO films q  Supporting Evidence from DFT

Ø  Introduction Ø  Misfit-Layered Cobalt Oxides Ø  DFT Modeling of Ca3Co4O9 (CCO) with Fibonacci approximants

Ø  Characterization of Ca3Co4O9 in the STEM

q  Structural and Electronic Properties q  Size versus Electron Correlations q  Phonons and Thermal Properties

Ø  Summary Ø  Future Prospects

R. F Klie (UIC)

A. Rebola (UIC à Cornell)



Thermopower is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across a material. Seebeck coefficient: Figure of Merit:

S = ΔVΔT

=8π 4

3kB2

3ehm*T 1

3n"

#$

%

&'

23

ρκTSZT2

=

Thermo-electric materials offer direct conversion of waste/excess heat into electrical power.

Achieving low thermal conductivity, high electrical conductivity, and a large effective mass is often difficult in bulk materials.

Introduction: Thermoelectric Materials



Problems with conventional TE materials:

Some important materials: •  Bi2Te3: S= -287 µV/K, ZT~1 •  Bi2Te3/Sb2Te3 superlattice: ZT>2 at 300K •  PbTe/PbSeTe quantum dots: ZT>1.5 •  UO2: S=750 µV/K •  Ca3Co4O9: S= 135 µV/K, ZT~1 at 1000K

Bi2Te3/Bi2Se3

Unstable at high temperature and O-rich environments (most materials are limited to 200 K – 700 K)

Phonon glass/electron crystal or material with different subsystems needed for good TE properties. M. Dresselhaus et al, Adv. Mat. 19 (2007)

Low efficiency at desired temperatures (i.e. T<200K or T>1000K)

Expensive/rare constituent elements

Toxic

Introduction: Thermoelectric Materials



(Bi0.87SrO2)2(CoO2)1.82 (CaOH)1.14CoO2

NaCoO2 Ca3Co4O9 (Ca2CoO3)(CoO2)1.62

Shizuya et al, J. Solid State Chem. 180, 249 (2007)

Layered cobalt oxides are intrinsically nano-structured materials

Incommensurately layered Cobalt Oxides



Misfit-Layered Ca3Co4O9 (CCO)

!

a bCoObRS

b

CoO2

CaO

CoOc

2

Ca1

Ca2

O1O2

O3

O2 O1

a = 4.83 Å

c = 10.84 Å

β = 98.13°

•  Two subsystems, different behaviors:

•  RS ( bRS = 4.558 Å) and CoO2 (bCoO2 = 2.824 Å) are incommensurate along b.

Ø  Ca2CoO3 (RS): distorted, mostly insulating, phonon-glass behavior

Ø  CoO2 (Hex): hole conducting, electron-crystal behavior.



Rational Approximants

τ = (1+ 5) / 2 =1.618...•  The composition ratio for (Ca2CoO3)(CoO2)1.62 , is very

close to the golden mean which is the limit of the sequence of the ratios of consecutive Fibonacci numbers F(n+1)/F(n) = 2/1, 3/2, 5/3, 8/5,13/8,…

•  To model the incommensurate structure of CCO we

construct a series of “rational approximants” with composition (Ca2CoO3)2F(n)(CoO2)2F(n+1) and investigate how various properties evolve as a function of the size.

•  Spin-polarized full structural relaxation (Quantum Espresso & VASP) up to 13/8 approximant (174 atoms in the unit cell). DFT (LDA, PBE) and DFT+U computations



Rational Approximants

3/2

5/3 8/5

13/8

X1

X4

X3 X2

b = 8.58 Å

b = 36 Å

b = 13.71 Å

b = 22.35 Å

(Ca2CoO3)4(CoO2)6 (Ca2CoO3)16(CoO2)26

(Ca2CoO3)6(CoO2)10 (Ca2CoO3)10(CoO2)16

!

a bCoObRS

b

CoO2

CaO

CoOc

2

Ca1

Ca2

O1O2

O2

O1

O2

a = 4.83 Å c = 10.84 Å β = 98.13°



Earlier Studies (Exp. vs Theory)

Takeuchi et al., PRB 69 (2004)

!

a bCoObRS

b

CoO2

CaO

CoOc

2

Ca1

Ca2

O1O2

O2

O1

O2Rock Salt subsystem Experiment (PE)

N(EF)!29.4 eV"1 per unit cell with ten Co atoms; the cor-responding band contribution to the specific heat is70.3 mJ/mol K2, which is smaller than the calculated bandspecific heat of 105 mJ/mol K2 for the 5(NaCo2O4) unit.9The large N(EF), mostly coming from the localized t2g

states in the RS subsystem, causes the Stoner instability forthe nonmagnetic state. The Stoner criterion for the instabilityis given by23,24

n f!U#4J "/10$1. !1"

Using the density of states per Co, n f!N(EF)/10!2.94 eV"1, and the Coulomb and exchange parametersfor the CoO,25 U!4.9 eV and J!0.92 eV, the Stoner crite-rion Eq. !1" is well satisfied as n f(U#4J)/10!2.52.Looking for the most stable state, we tried the following

initial magnetic orderings that are possible under the currentspace group Pm: with an initial up-spin magnetic moment of2#B for all atoms in the unit cell !FM"; with an initial up-spin magnetic moment of 2#B for only Co atoms in theCoO2 subsystem !FM2"; with the same initial spin orderingas FM but with the down spin for Co!3" in the RS subsystem!AM1"; with the same initial spin ordering as FM but withthe down spin for Co!2" !AM2"; and with an initial down-spin magnetic moment of 2#B for Co atoms in the RS sub-system in addition to the FM2 ordering !AM3".After attaining the self-consistency, the magnetic mo-

ments that were initially introduced to any Co atoms in theCoO2 subsystem were reduced to less than 0.1#B so that theFM2 and AM3 orderings resulted in the nonmagnetic andFM states, respectively. The converged spin orderings areshown in Fig. 2. The calculated total energies and magneticmoments within the Co muffin-tin sphere are listed in TableIII. The FM ordering is stabilized by 84.8 mRy from thenonmagnetic state mainly through the exchange splitting ofthe t2g states in the RS subsystem. Among the calculatedorderings, the AM1 ordering yields the lowest total energy,while magnitudes of the Co magnetic moments in the RSsystem are quite similar. We note that the net magnetic mo-ment in the unit cell in the AM1 ordering was calculated tobe 0.17#B per unit cell; this indicates that the ground state isa ferrimagnetic ordering with a small net magnetic momentof about 0.02#B per Co.The detailed projections of the d states in the FM ordering

are shown in Fig. 5. The spin polarization is indeed attributedto the exchange splitting of the d states in the RS subsystem.It should be noted that, contrary to expectations by analogy

with NCO, the Co d states in the CoO2 subsystem do notcontribute to the DOS at the Fermi energy. The Fermi energylies in the crystal-field gap of the d states in the CoO2 sub-system, setting these Co atoms to be in the low-spin !LS"Co3# state (S!0). While three t2g! bands and partial eg!bands in the RS subsystem are filled for the majority- !up-"spin state, the dxy band that is lowered in energy correspond-ing to the short apical Co-O distance is only filled for theminority- !down-" spin state, as schematically shown in Fig.4 !FM". The other t2g" bands, dyz and dzx , as well as egbands for the down-spin state. sit just above the Fermienergy.The local spin-density approximation !LSDA" is known

to fail badly for some transition-metal oxides giving toosmall or zero band gaps. Typically, CoO and FeO appear tobe metal with the Fermi energy that lies at the peak of theminority-spin t2g bands; it was suggested that a populationimbalance due to the high DOS at the Fermi energy couldmake them insulators when the exchange-correlation func-tions are properly treated.24 To deal with such difficulty,methods beyond LSDA like LDA#U !Ref. 26" and self-interaction correction27 have been often employed. Thesetreatments can reduce unphysical self-interactions involvedwithin the LSDA !also the GGA", and deal with unquenchedorbital angular momentum properly.25,26,28,29 In the presentcase, however, the DOS at the Fermi energy in the spin-polarized state is not high, as seen in Fig. 5. This is theconsequence of the strong distortion of the octahedra in theRS subsystem so that the minority-spin dxy orbital is split tobe fully occupied. Therefore, we expect that the populationimbalance in this case is small and the present density-functional theory !DFT" GGA approach may properly de-scribe the Co configurations. In addition, the metallic char-acter coming from the broad eg bands will be unchangedeven with the LDA#U treatment, while the other unoccu-pied t2g will be shifted higher, as seen in calculations forLaCoO3 !Ref. 28" and LaSr2Mn2O7.30The Co atoms in the centered CoO plane are found to be

in the mixed-valence states of Co3# with a high-spin con-figuration (t2g!

3 eg!2 t2g"

1 ) and Co4# with an intermediate-spinconfiguration (t2g!

3 eg!1 t2g"

1 ). With the global charge balance

TABLE III. Magnetic moments !in #B) within the Co muffin-tinspheres in the RS subsystem, Co!1", Co!2", and Co!3", and those inthe CoO2 subsystem, Co!HEX", for several magnetic orderings.Differences in the total energy from the nonmagnetic state ($E inmRy" are also listed.

Co!1" Co!2" Co!3" Co!HEX" $E

FM 1.66 2.38 2.14 0.01 "84.8AM1 1.76 2.38 "2.15 0.00 "89.7AM2 1.69 "2.39 2.12 0.00 "84.4

FIG. 5. The detailed projections of the d states in the ferromag-netic ordering into a Co atom in the CoO2 subsystem, and therocksalt subsystem with its octahedral crystal-field representationsare also shown.

RYOJI ASAHI, JUN SUGIYAMA, AND TOSHIHIKO TANI PHYSICAL REVIEW B 66, 155103 !2002"

155103-4

Asahi et al., PRB 66 (2002) Theory (DFT)

Intensity

(arb. units)

sharp peak at 2.4 eV, a slightly broad one centered at 6.4 eV,and a broad hump extending up to 12 eV. The symmetryabout the cobalt atom in the rocksalt layer gives rise to thet2g and eg bands in the same way as those in the CoO2layers. The sharp peak at !2.4 eV is assigned as the anti-bonding t2g bands and the peak at 6.4 eV mainly as thebonding t2g bands. Widely extended bands over the energyrange 3!E!12 eV is regarded as the bonding eg bands.Obviously the valence band in the rocksalt layer hardlyreaches the Fermi level. This result suggests that the metallicelectrical conduction in CCO arises only from the CoO2layer rather than from the rocksalt layer.The deduced Co 3d partial density of states in the rocksalt

layer shows good consistency with the FLAPW bands calcu-lated by Asahi et al.21 in the point that the eg bands arespread over a wide energy range and the antibonding t2gbands and bonding t2g bands lie at 2 and 6 eV, respectively.However, their calculation shows that the widely spread egbands in the rocksalt layer lies across EF while the Co 3dpartial density of states in our present analysis shows nointensity at EF . Since the widely extended eg bands in theFLAPW calculation provide only a very small density ofstates at EF , our rough assumption of the common electronicstructure in the CoO2 layer among CCO, BSCO, and "-NCOmight cause this inconsistency. Although absence of thewidely extended eg bands at EF cannot be unambiguouslyproved, we decided to employ the electronic structure withno eg bands near EF in our analysis of thermoelectric powerperformed in the Discussion.Co 2p-3d RPES spectra for BSCO, "-NCO, and CCO

suggest that the valence band in an energy range from EF to1 eV is dominated by the electronic structure in the CoO2layer. It is, therefore, very important to compare the energyof the spectrum edge near EF in the RPES spectra for thesethree compounds, because it reflects the carrier concentrationin the CoO2 layer. As shown in Fig. 5#a$, the high-energyedge in the off-resonant spectrum of the CCO is located atthe highest binding energy and that in the "-NCO shows thelowest. This tendency is also confirmed with the energy ofpeak #II$ in the on-resonant spectra; those are observed at3.9, 3.85, and 3.55 eV for the "-NCO, BSCO, and CCO,respectively. These results suggest that the hole concentra-tion in the CoO2 layer increases in the sequence of "-NCO,BSCO, and CCO.The Fermi level EF is located at the high-energy edge of

the peak centered at about 1.0 eV in the off-resonant spectrafor these three cobalt oxides. The metallic electrical conduc-tion commonly observed in these layered cobalt oxides canbe taken as the evidence for the possession of a finite densityof states at EF . We found that the off-resonant spectra ofthese three compounds shown in Fig. 5#b$ possess a commonslope at EF , which would reflect the presence of Fermi edgebroadened by the energy resolution of the measurement.In order to unambiguously confirm the presence of the

Fermi edge in these layered cobalt oxides, we employed UPSmeasurements of an energy resolution better than 20 meV.Figures 6#a$–6#c$ show the UPS spectra of the CCO, BSCO,and "-NCO, respectively, measured with monochromatedHeI% at various temperatures. We observed that the densityof states at the Fermi level shows a finite value at roomtemperature in all compounds, though the Fermi edge was

FIG. 4. Co 2p-3d on-resonant spectra of Ca3Co4O9 #CCO$ and"-Na0.6CoO2 after subtracting the background intensity. The spec-trum of the "-NCO was shifted by 240 meV towards higher bindingenergy, and its intensity was normalized such that the integratedintensity becomes 60% of that of the CCO. By considering NCO’sspectrum as that of the CoO2 layer in the CCO and subtracting itfrom that of CCO, we obtained a spectrum representing the rocksaltlayer in the CCO.

FIG. 5. #a$ Off-resonant photoemission spectra of Ca3Co4O9#CCO$, Bi2Sr2Co2O9 #BSCO$, and "-Na0.6CoO2 measured at 20 K.The leading edge of the 1 eV peak of CCO shows lowest binding-energy, while that of the "-NCO stays at the highest. The spectra inthe vicinity of EF are enlarged in #b$. Spectra of CCO and BSCOare vertically shifted to clearly distinguish each spectrum. Horizon-tal dashed lines indicate background intensity at EF . A finite inten-sity persists at EF in all compounds. All spectra show commonslope at EF that would be caused by the energy resolution of themeasurement #see slopes with thin dashed lines as a guide to eye$.Though its leading edge stays in the lowest binding-energy, theintensity at EF in CCO shows much smaller value than that in theothers. This shows good consistency with the highest electrical re-sistivity of CCO among these three cobalt oxides.

TSUNEHIRO TAKEUCHI et al. PHYSICAL REVIEW B 69, 125410 #2004$

125410-4

Binding Energy (eV) Energy

DO

S (A

rb. U

nits

)



Theory vs. Experiment

q  Two possibilities

Ø  Size effect: In previous study, only the lowest-order approximant (3/2) was considered. How does the electronic structure depend on the order of the Fibonacci approximant?

Ø  Correlation effects: In many cases the mean-field approach of DFT fails to provide a good description of highly localized d orbitals. Previous study did not include any strong correlation effects (even in an approximate way).

How to reconcile experimental findings with theoretical predictions?



Size Effect: Higher order approximants

0.5 1.5 2.5

PDOS

/(Stat

e/eV/

Spin/

Co) d in CoO2

0.5 1.5 2.5

!3 !2 !1 0 1 2 3

d in RS

d in CoO2

!2 !1 0 1 2 3E!E f(eV)

d in RS

Spin Up Spin Down

E!E f(eV) !

a bCoObRS

b

CoO2

CaO

CoOc

2

Ca1

Ca2

O1O2

O3

O2 O1

5/3 Approximant

•  Similar results for higher order approximants.

•  Good agreement with previous DFT calculations, except for the contribution at EF coming from CoO2 spin down.

•  Major contribution to electrical conductivity from the RS substructure, in disagreement with PE experiments.



Adding Correlations: DFT+U

0.5 1.5 2.5

PDOS

/(Stat

e/eV/

Spin/

Co) d in CoO2

0.5 1.5 2.5

!4 !3 !2 !1 0 1 2 3 4E!Ef (eV)

d in RS

d in CoO2

!3 !2 !1 0 1 2 3 4E!Ef (eV)

d in RS

Spin Up Spin Down

•  Finite contribution at EF from the CoO2 subsystem •  Almost negligible contribution from RS subsystem.

•  Better agreement with photoemission experiments.

•  Results insensitive to the value of U

LDA + U, Ueff = 4 eV 5/3 Approximant



Partial DOS (DFT+U) Spin Up Spin Down

dxy

dyz

PDOS

(Arb.

units

)

d3z2!r2

dxz

!4 !3 !2 !1 0 1 2 3 4E!Ef (eV)

dx2!y2

dxy

dyz

d3z2!r2

dxz

!3 !2 !1 0 1 2 3 4E!Ef (eV)

dx2!y2

dxy

dyz

PDOS

(Arb.

units

)

d3z2!r2

dxz

!4 !3 !2 !1 0 1 2 3 4E!Ef (eV)

dx2!y2

dxy

dyz

d3z2!r2

dxz

!3 !2 !1 0 1 2 3 4E!Ef (eV)

dx2!y2

Spin DownSpin Up

CoO2 subsystem

RS subsystem

PD

OS

(Arb

. Uni

ts)

PD

OS

(Arb

. Uni

ts)



Size Evolution (Higher Approximants)

Spin Up Spin Down

3/2

5/3

8/5

!4 !3 !2 !1 0 1 2 3 4

PDOS

/(Arb.

units

)

E!Ef (eV)

13/8

3/2

5/3

8/5

!3 !2 !1 0 1 2 3 4E!Ef (eV)

13/8

Spin Up Spin Down

PDOS

/(Arb.

units

) 3/2

5/3

8/5

!4 !3 !2 !1 0 1 2 3 4E!Ef (eV)

13/8

3/2

5/3

8/5

!3 !2 !1 0 1 2 3 4E!Ef (eV)

13/8

•  Small contribution to the RS PDOS coming from eg orbitals for the 3/2, 8/5, 13/8 approximants, composed of combinations of X1, X2 and/or X4 clusters.

•  5/3 is composed of X3 clusters, fully occupied/unoccupied eg orbitals.

CoO2

RS



Clustering along b 3/2

5/3 8/5

13/8

X1

X4

X3 X2

•  n−unit X ≡ Ca2CoO3 clusters (n = 1 – 4) occurring along the b direction. •  Related to the incommensurate nature of the CoO2 and RS subsystems, and

depends critically on how the system can minimize the total energy globally within the constraints imposed by the ratio of the Fibonacci numbers.

b



Thermal Properties

ZT = σS2

κT

Recalling that:

•  The thermal conductivity is determined by two contributions:

κ =κ ph +κholes

•  From Wiedemann-Franz Law:

•  In CCO: σ ~ 0.1 (mΩ cm)-1 κholes~ 0.1 mW/cmK (at 100K)

•  κ ~ 30 mW/cmK most contributions are due to phonons.

κholes = L0Tσ



Phonons in CCO •  Calculations for the 3/2 (Ca2CoO3)4(CoO2)6 and 5/3 (Ca2CoO3)6(CoO2)10

•  Cell optimizations and force constants:

•  VASP, PBE + U, (Co, Ueff = 4eV). •  3/2 approximant (bopt= 8.81 Å) 2x1x1 supercell, 84 atoms.

•  5/3 approximant (bopt= 14.12 Å) 2x1x1 supercell, 132 atoms.

•  Phonon Calculations:

•  Systems are too big to apply DFPT. •  Finite differences method (Phonopy).



Phonons in CCO

y

z

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 5 10 15 20

Tota

l DO

S (A

rb. U

nits

)

Frequency (THz)

3/2 approximant5/3 approximant

Co, Ca, O O

5/3 approximant

Freq

uency, THz

25

20

10

15

5

0

Γ X Α Γ Ε D

3/2 5/3



Phonons in CCO: Low frequency

Low Frequency Modes: Contributions from all the atoms (Ca, Co, O).



Phonons in CCO: High frequency

High Frequency Modes: Mostly due to O atoms.



Heat Capacity

•  Reasonably good agreement with experimental data for both approximants.

cv = kBω(q, s)kBT

!

"#

$

%&

2

q,s∑ exp(ω(q, s) / kBT )

exp(ω(q, s) / kBT )−1[ ]2

Heat capacity at constant volume:

Experiment: Wu et al. JPCM 24 (2012)

350

400

450

500

550

600

650

700

750

150 200 250 300 350 400Temperature (K)

3/2 approximant5/3 approximant

CV (ȝ

J/m

gK)

Experiment



Thermal Conductivity

kαβ = vαλ (q)vβλ (q)τ λ (q)dnB (ωλq )dTλq

∑

•  Boltzmann Transport Equation within the relaxation time approximation:

•  For each approximant, velocities are obtained from the full dispersion. •  is assumed to be constant, •  Typical values for

•  Also computed “projected thermal conductivity”:

τ λ (q)

τ ≈1ps

τ λ (q) = τ

kαβ ,µ = eµ,i2vαλ (q)vλβ (q)τ λ (q)

dnB (ωλq )dTλq

∑i∑

where refers to the ith Cartesian component of phonon eigenvector corresponding to atom μ.

eµ,i



Thermal Conductivity

a axis

b axis

•  Similar results along a and c for both approximants

•  Differences for κ along the incommensurate direction b (Different clustering).

•  5/3 approximant: Qualitatively good agreement with experiment (different saturation temperatures).

5/3 approximant

b axis

c axis

τ=2ps

a axis

50 100 150 200 250 300 350 400

40

30

20

10

0 Temperature (K)

3/2 approximant

τ=2ps

a axis

b axis

c axis

40

30

20

10

0 100 150 200 250 300 350 400 Temperature (K)

κ (m

W/Kcm

)

50



Characterization of Ca3Co4O9 in the STEM:

A Combined Z-contrast Imaging, EELS, and DFT Study

R.F. Klie, Q. Qiao, T. Paulauskas, A. Gulec, A. Rebola, S. Ogut, M. Prange, J. Idrobo, and S. Pantelides, Phys. Rev. Lett. 108, 196601 (2012)



Incident Probe

Annular Detector

Spectrometer CCD-‐EELS Detector

Z-‐contrast Image

The Key to STEM



Z-contrast image in conventional STEM – Ca3Co4O9 [010]

0.5 nm

Z-contrast image in aberration-corrected TEAM – Ca3Co4O9 [010]

CoO2

CaO

CoO

CaO

CoO2

Z-Contrast Imaging



In materials with strong electron-electron interactions, e.g transition-metal oxides, the Seebeck coefficient can be described using the Heikes form:

⎟⎠

⎞⎜⎝

⎛−

=xx

ekS B

1ln β so gg ⋅=β

Ca3Co4O9

Cobalt valence and spin state control the TE properties of Ca3Co4O9

Koshibae et al, PRB (2000)

Seebeck Coefficient in Layered Cobaltites

: the orbital/spin degeneracy, x fraction of mobile charges.



High quality textured Ca3Co4O9 films were grown, but films appear unstrained. (Q. Qiao, A. Gulec, T. Paulauskas, S. Kolesnik, B. Dabrowski, B.M. Ozdemir, C. Boyraz, D. Mazumdar, A. Gupta, R.F. Klie, J. Phys. Cond. Mat., 23(30), 305005 (2011))

In-plane Seebeck coefficient at 300 K.

Ca3Co4O9 Thin Films

c-lattice parameter for all thin film samples around 1.084 nm Bulk Ca3Co4O9 lattice parameter: 1.084 nm

Thin films grown on SrTiO3, LaAlO3, LSAT and Al2O3 support.



2 n m

Co3Co4O9

SrTiO3

5 n m5 n m

Z-contrast image of 40 nm thick Ca3Co4O9

Unusually high Seebeck coefficient appears to be related to presence of stacking faults and twin boundaries. R.F. Klie, Q. Qiao, T. Paulauskas, A. Gulec, A. Rebola, S. Ogut, M.P. Prange, J.C. Idrobo, and S.T. Pantelides, Phys. Rev. Lett. 108(19), 196601 (2012)

Seebeck coefficient

Ca3Co4O9 Thin Films on SrTiO3



1 n m

Undoped 40 nm Ca3Co4O9 film EELS of O K-edge

O K-edge pre-peak is significantly higher in stacking fault, indicating a change in Co spin state. (Co L-edge remains unchanged)


R.F. Klie et al., Phys. Rev. Lett. 108, 196601 (2012)



1 n m

Undoped 40 nm Ca3Co4O9 film EELS of O K-edge


DFT Calculations

Stacking faults stabilize Co4+ ion in higher spin state.



Summary

Ø  The incommensurate thermoelectric Ca3Co4O9 can be modeled with relatively low-order rational approximants within the framework of DFT+U.

Ø  Good agreement with experiment with respect to structural, electronic, magnetic, and thermal properties.

Ø  Electron correlations are important in accounting for PE experiments. Size of the rational approximant plays a relatively minor role.

Ø  Ultrathin (40 nm) CCO epitaxial film grown on SrTiO3 shows a high concentration of stacking faults which stabilize the Co4+-ion with increased spin state.

Ø  This change in the spin state results in a significant increase in the in-plane Seebeck coefficient of CCO (180 µV/K) at room temperature.

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