Enhanced Thermoelectric properties via oxygen non- stoichiometry and strain of La 2 NiO 4+ δ

Enhanced Thermoelectric properties via oxygen non-stoichiometry and strain of La2NiO4+δ

Víctor Pardo March Meeting, Baltimore 2013

Project MAT 2009-08165

Ramón y Cajal Program

Financial support from:

Víctor Pardo, Antía S. Botana, Daniel Baldomir, PRB 86, 165114 (2012).Víctor Pardo, Antía S. Botana, Daniel Baldomir, PRB 87, 125148 (2013).



Daniel Baldomir Antía S. Botana

Francisco Rivadulla’s group:

Paul L. BachJ.M. Vila-FungueiriñoV. Leborán

zT= σS2T/κ- Good electrical conductor- Poor heat conductor- Large thermopower

Picture taken from: Sensors and Actuators A: Physical 145-146, 423 (2008).

zT > 1 for applicationsσT/κ ~ constant for metals → S= 160 μV/K to reach zT= 1. S > 100 μV/K for a high-performance thermoelectric material

Thermoelectric figure of merit (dimensionless quantity): zT= σS2T/κ



- Ab initio electronic structure calculations (DFT-based)WIEN2k package.

- Full-potential, all-electron calculations.

- Comparison of three exchange-correlation potentials: GGA, LDA+U, Tran-Blaha modified version of the Becke-Jonsson potential.

- Transport properties calculated using BoltzTrap.

- We used the virtual crystal approximation to simulate doping effects.

- Structural details taken from experiment.

Methods



Misfit layered cobaltates: NaxCoO2

High thermoelectric performanceExistence of two electronic systems



La2NiO4 layered compound Ni2+: d8 S=1



dx2

-y2

dz2

dxy

dxz,yz

J.B. Goodenough et al., Mat. Res. Bull. 17, 383 (1982).

dx2

-y2

dz2

dxydxz,yz



zT= σS2T/κ

σ/κ constant for metals

S2 enough to analyze trends

La2NiO4+δ

Seebeck vs. hole-doping concentration δ

T= 400 K

Peaks at about δ= 0.05 Negative thermopower at higher doping

Enhanced Thermoelectric properties via oxygen non-stoichiometry of La2NiO4+δ and SrTiO3


Sr6Co5O15: A.S. Botana et al., PRB 83, 184420 (2011)

CrN: A.S. Botana et al., PRB 85, 235118 (2012)



At small hole-doping concentration, there is a peak in thermoelectric figure of merit

Figure of merit:zT= σS2T/κ



Promising high-temperature behavior with S > 100 μV/K for δ ~ 0.05.

VCA calculations



Calculated electronic-only figure of merit shows promising values at high T for δ ~ 0.05.Calculations do not include thermal conductivity due to phonons (largest contribution !!)

zT= σS2T/κ



Comparison with available experiments: difficult due to lack of systematics vs. δ

- Large thermpower at low-doping- Crossing towards negative values at higher-doping levels- S increases at high T and intermediate dopings



zT= σS2T/κ

Using σ/κ constant from experimental valuesand S calculated.

We estimate a figure of merit zT ~ 0.1 at high T and δ ~ 0.05, can this be improved?



Can tensile strain improve zT ?Try to lower the energy of the x2-y2 band and utilize the localized z2 band (large S)

Valence bands get closer together when a is increased





Values of the lattice parameter a= 3.95 – 4.00 A (tensile strain: LNO’s a= 3.86 A) leads to enlarged thermopower

zT= σS2T/κ



Values of the lattice parameter a= 3.95 – 4.00 A (tensile strain) lead to large electronic-only figure of merit, for δ ~ 0.05 – 0.10



Experiments confirm that tensile strain in thin films of different thicknesses (different strains) drives the system towards a larger thermopower without compromising the conductivity (in fact, decoupling them) [Paul L. Bach’s talk on Wednesday N12.12]

Summary:

• La2NiO4+δ can be a possible candidate for high-performance thermoelectric at high T if doping and strain are optimized.

• Seebeck coefficients up to 200 μV/K can be obtained above room temperature for the appropriate tensile strain.

• Conductivity and thermopower can be decoupled via strain engineering !

• Control of the stoichiometry is a new path to improve thermoelectric efficiency in oxides in general.



Enhanced Thermoelectric properties via oxygen non-stoichiometry of La2NiO4+δ and SrTiO3-δ


0 100 200 300-1000

-800

-600

-400

-200

0

0 100 200 3000

3

6

9

12

n= - 4.0 x 1016cm-3

n= - 2.7 x 1017cm-3

n= - 7.1 x 1016cm-3

n= - 2.1 x 1016cm-3

S (

V/K)

Temperature (K)

unnanealed STO

Temperature (K)

(W

/mK)

The agreement is reasonably good for the dopant concentrations analyzed (experimentally obtained via Hall effect measurements)

Very large thermopower is obtained in that very light electron doped region (as an effect in the bulk)



Very large thermopower in STO-2DEG, really a 2D effect?



0 100 200 300-1000

-800

-600

-400

-200

0

0 100 200 3000

3

6

9

12

n= - 4.0 x 1016cm-3

n= - 2.7 x 1017cm-3

n= - 7.1 x 1016cm-3

n= - 2.1 x 1016cm-3

S (

V/K)

Temperature (K)

unnanealed STO

Temperature (K)

(W

/mK)

STO substrates treated in order to tune the oxygen content

Very large Seebeck is observed in pure SrTiO3-δ

The effect is in a bulk (0.5 mm substrate)

See: P.L. Bach et al., arxiv/1211.1615



We study a very light doping case, only 1 oxygen vacancy per 106 sites

We explore the very light electron-doped bulk STO via ab initio calculations



Thermal conductivity is strongly suppressed by the introduction of scattering centers through oxygen vacancies

Similar reduction due to oxygen excess could be active for La2NiO4+δ



La2NiO4+δ

SrTiO3-δ

i) Thin film geometry:

- large conductivity increase due to the larger in-plane conductivity- expected reduction of the thermal conductivity- will the Seebeck coefficient of thin films be different? Further calculations needed in 2D.



Thermoelectric properties of La2NiO4+δ from ab initio techniques

Víctor Pardo Davis, June 2012

100 200 300 400 500 600 700 8000.00E+000

5.00E+010

1.00E+011

1.50E+011

2.00E+011

2.50E+011

3.00E+011

3.50E+011

4.00E+011

a= 11.0 a.u.

a= 10.7 a.u.

S2

T (K)

a= 10.5 a.u.

La2NiO

4+= 0.05

100 200 300 400 500 600 700 8000.00E+000

5.00E+010

1.00E+011

1.50E+011

2.00E+011

2.50E+011

3.00E+011

3.50E+011

La2NiO4+= 0.10

a= 11.0 a.u.

a= 10.7 a.u.

a= 10.5 a.u.

S2

T (K)

100 200 300 400 500 600 700 8000.00E+000

5.00E+010

1.00E+011

1.50E+011

2.00E+011

2.50E+011

3.00E+011

a= 10.5 a.u., = 0.12

a= 10.7 a.u., = 0.08

a= 11.0 a.u., = 0.05

S2

T (K)

S increases as σ decresases, so we need to analyze them combined, e.g. in the form of the power factor S2σ.Highest values at the lower lattice parameter (we need to analyze compressive strain).

100 200 300 400 500 600 700 8000

50

100

150

200

250

300

= 0.08

= 0.04

= 0.01

S_EF S_100 S_160 S_max

S (

V/K

)

T (K)

= 0.0

La2NiO

4 GGA AF tensile a= 10.7 a.u.

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

La2NiO

4 GGA AF tensile a= 10.7 a.u.

S (

V/K

)

p (no. holes per unit cell)

Bands become more closer together and the peak in S vs. p starts to be smeared out

Seebeck coefficients get larger (as conductivity decreases)



0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

S (

V/K

)


La2NiO

4 GGA AF tensile strain a= 11.0 a.u.

100 200 300 400 500 600 700 800

0

50

100

150

200

250

La2NiO

4 GGA AF tensile strain a= 11.0 a.u.

= 0.01

= 0.05S (

V/K

)

T (K)

= 0.0

For a sufficiently large, bands get so close together that the behavior is that of a standard semiconductor, similar to what would be obtained for CrN

Lack of intermediate-doping high S values



0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

S (

V/K

)


La2NiO

4 compressive strain a= 10.3 a.u.

100 200 300 400 500 600 700 8000

50

100

150

200

250

300

La2NiO

4 compressive strain a= 10.3 a.u.

= 0.13

= 0.15= 0.10

= 0.05

= 0.01

S (

V/K

)

T (K)

= 0.0



0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

S (

V/K

)


La2NiO

4 AF GGA a= 10.0 a.u.

100 200 300 400 500 600 700 8000

50

100

150

200

250

300

= 0.05

= 0.10

= 0.15= 0.01

La2NiO

4 AF GGA a= 10.0 a.u.

S (

V/K

)

T (K)

= 0.0



100 200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

a= 11.0

a= 10.3

a= 11.0

a= 10.5

zT

T (K)

a= 10.7

La2NiO

4 AF GGA = 0.05

100 200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

a= 10.0

a= 11.0a= 10.3

a= 10.5

a= 10.7

La2NiO

4 AF GGA = 0.10

zT

T (K)



100 200 300 400 500 600 700 8000.00E+000

5.00E+010

1.00E+011

1.50E+011

2.00E+011

2.50E+011

3.00E+011

3.50E+011

4.00E+011

a= 11.0 a.u.

a= 10.3 a.u.

a= 10.5 a.u.

a= 10.7 a.u.

La2NiO

4+= 0.05

S2

T (K)

100 200 300 400 500 600 700 8000.00E+000

5.00E+010

1.00E+011

1.50E+011

2.00E+011

2.50E+011

3.00E+011

3.50E+011

4.00E+011

a= 10.7, = 0.08

a= 10.5, = 0.12 a= 10.3, = 0.13

S2

T (K)

a= 11.0, = 0.05

S increases as σ decresases, so we need to analyze them combined, e.g. in the form of the power factor S2σ.Highest values at the lower lattice parameter (we need to analyze compressive strain).

100 200 300 400 500 600 700 8000.00E+000

1.00E+011

2.00E+011

3.00E+011

4.00E+011

a= 11.0 a.u.

a= 10.3 a.u.

a= 10.5 a.u.

a= 10.7 a.u.

La2NiO

4+= 0.10

S2

T (K)



0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

S (

V/K

)


La2NiO

4 AF GGA tensile strain a= 10.5 a.u.

100 200 300 400 500 600 700 8000

50

100

150

200

250

300

= 0.07= 0.12

= 0.09

= 0.01= 0.001

S (

V/K

)

T (K)

= 0.00

La2NiO

4 AF GGA tensile strain a= 10.5 a.u.

a= 3.90 Å= 10.44 a.u.a= 3.85 Å= 10.29 a.u.a= 4.00 Å= 10.69 a.u.a= 3.8 0Å= 10.16 a.u.a= 4.11 A= 11.0 a.u.

aexptal = 3.89 Å



z2 ↓

x2 -y2 ↓

x2 -y2 ↑xy ↑

z2 ↑ xy ↓xz,yz



Na cobaltates: NaxCoO2, misfits, etc …

G. Khaliullin and J. Chaloupka

Two types of electrons with different bandwidths: good conductivities and large thermpower

Enhanced Thermoelectric properties via oxygen non- stoichiometry and strain of La 2 NiO 4+ δ

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