electronic, magnetic and thermoelectric properties of

Electronic, Magnetic and Thermoelectric Properties ofPerovskite: BaTbO3

Dhurba Raj Jaishi1,2, and Madhav Prasad Ghimire1,2

1Central Department of Physics, Tribhuvan University, Kathmandu, Nepal2Condensed Matter Physics Research Center (CMPRC), Butwal, Nepal

Dhurba Raj Jaishi 1 / 15

Outline

1 Introduction

2 Computational Details

3 Results and Discussion

4 Conclusions


Introduction:Thermoelectric (TE) Effect

The Seebeck effect ( process thatconverts temperature gradientdirectly in to electricity).

Peltier effect (process that convertselectrical energy in to temperaturegradient).

TE performance of a materialmeasured by the figure of merit(ZT)

ZT =α2σT

κe + κl

α=8π2k2

B3eh2 m∗T( π

3n )23

Fig. 3: Schematic representation of a) Power generation and b) Cooling.


Introduction: Requisites for Better TE Materials

Insulator

Semiconductor

Metal

Fig.1: Classification of the materials on the basis of energy gapFig.2: Schematic dependence of the electrical conductivity, Seebeckcoefficient, power factor, and thermal conductivity on the carrierconcentration (Snyder et al. Nature 7, 105-114, (2008)).


Introduction:Perovskite

TE materials have applications:cooler, power generation,temperature sensor etc.

Silicon-Germanium, chalcogenides,Heusler alloys, Clathrates used forfabricating TE devices.

Perovskite have various interestingproperties such as:

I FerromagneticI Half-metallicI SemiconductorI Topological insulatorI Thermoelectric

Oxides Perovskite:I Structural stabilityI Low costI Naturally abundanceI Non-toxic

Fig.4: Ideal Cubic perovskite crystal structure of ABO3.


Computational DetailsDensity functional theory (DFT) is used to study the electronic,magnetic and transport properties based on the full-potentiallinearized augmented plane wave + local orbital (FP-LAPW+lo)using generalized gradient approximation (GGA) as exchangecorrelation functional.

WIEN2K code used for the electronic/magnetic properties calculation.

BoltzTraP code to study the transport properties based on constantrelaxation time approximation (CRTA) and the rigid bandapproximation (RBA).


Crystal Structure and Optimization

Space group Pm3m (223)

a= 4.36 A, α= 90

Fig.5: Optimized crystal structure of BaTbO3.

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

500 520 540 560 580 600 620 640 660 680

E0=-40169.175310

E-E

0 [

Ry]

Volume [bohr3]

EOS-fitCalculated energy

Fig.6: Structure optimization.

The calculated value of theenergy with respect to thevolume, fitted using theMurnaghan equation of state.


Results and DiscussionGround state − antiferromagnetic, and direction of easy axis is foundalong [010] direction.

Table3: Total magnetic moment, spin and orbital magnetic moment of antiferromagnetic BaTbO3

Magnetic moment (µB)ms ml

GGA GGA+SOC

Total 0.0 0.0

Tb1 6.25 6.18 0.32

Tb2 -6.25 -6.18 -0.32

Fig.8: Crystal structure of antiferromagnetic BaTbO3 with direction of easy axis along [010]. The sphere in black, red, andgreen color represents Tb1, Tb2, and Ba atoms, respectively.


Results and DiscussionDensity of States

Conduction region is mainly contributed by Tb−4f orbital.Valence region is contributed by O-2p orbital near Fermi level.

-20

-10

0

10

20

a)Spin-up

Spin-down

GGA

Total

-20

-10

0

10

20

Spin-up

Spin-down

b)

Den

sity

of

stat

es (

stat

es/e

V)

Tb1Tb2

-6

-4

-2

0

2

4

6

-4 -3 -2 -1 0 1 2 3 4

Spin-up

Spin-down

c)

Energy (eV)

O-2p

d)Spin-up

Spin-down

GGA+SOC

Total

Spin-up

Spin-down

e) Tb1Tb2

-4 -3 -2 -1 0 1 2 3 4

Spin-up

Spin-down

f)

Energy (eV)

O-2p

Fig.7: Density of states.


Results and DiscussionBand Structure

0.81 eV band gap

W L Γ X W K -2

-1

0

1

2

Ener

gy(e

V)

GGA

EF

W L Γ X W K -2

-1

0

1

2

Ener

gy(e

V)

GGA+SOC

EF


Results and DiscussionTransport Properties

The Seebeck coefficientdecreases with the temperature.

σ increased with thetemperature signifiessemiconducting in nature.

200

210

220

230

240

250

260

270a)

S (

µV

K-1

)

GGA+SOC

0

5

10

15

20

25

30

35

b)

σ/τ

x 1

01

6 (

Ω c

m s

)-1

GGA+SOC

20

40

60

80

100

120

140

400 600 800 1000 1200

c)

S2σ

/τ x

10

14 (

µW

cm

-1 K

-2 s

-1)

Temperature (K)

GGA+SOC

0

0.05

0.1

0.15

0.2

0.25

400 600 800 1000 1200

d)

κe/

τ x

10

14(W

cm-1

K-1

s-1)

Temperature (K)

GGA+SOC

Fig.9: Variation of a) Seebeck Coefficient (α) b) Electrical Conductivity (σ/τ) c) Power Factor (α2σ/τ) and d) Electrical

Thermal Conductivity (κe/τ).



0

20

40

60

80

100

120

140

-0.9 -0.6 -0.3 0 0.3 0.6 0.9

1200 K S

2 σ

/τ x

10

14 (

µW

cm

-1K

-2s

-1)

µ-µ0 (eV)

GGAGGA+SOC

Fig.10: Variation of the power factor with the chemical potential.



Positive value of α is due to p-type charge carrier.

High value of the ZT is obtained due to the flat nature of degeneratestates in valence region near EF .

0.76

0.77

0.78

0.79

0.8

0.81

400 600 800 1000 1200

ZT

Temperature (K)

GGAGGA+SOC

Fig.11: Variation of the figure of merit (ZT) with the temperature (K).


Conclusions

We investigate the electronic, magnetic and thermoelectric propertiesusing density functional theory.

The ground-state found as antiferromagnetic with the energy gap of0.81 eV.

Large value of the Seebeck coefficient ∼ 205 µVK−1 observed even at1200 K signifies suitable for a high temperate TE devices.

Based on the CRTA, obtained value of the power factor ∼ 124 µWcm−1 K−2 at 1200 K indicates suitability for potential TE devices.

We only consider the electronic part of the thermal conductivity,lattice part is dominant over the high temperature region, whichaffect the final value of the ZT.


Acknowledgments

Central Department of Physics, Tribhuvan University, Nepal

Condensed Matter Physics Research Center (CMPRC) Butwal, Nepal

THANK YOU !!!


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