crystal structure and magnetism of layered...

��Crystal structure and magnetism of ��

layered thermoelectric materials ��

1

F. C. Chou •  Center for Condensed Ma2er Sciences, Na8onal Taiwan University •  Na8onal Synchrotron Radia8on Research Center •  Taiwan Consor8um of Emergent Crystalline Materials

outline

2

•  Introduction to crystal growth •  Introduction to thermoelectricity •  A few selected layered TE materials :

•  Bi2Se3: structure variation and defects •  PbBi2Te4: (PbTe)m(Bi2Te3)n homologous series •  Bi2Sr2Co2Oy: hole-doped misfit •  NaxCoO2: spin entropy •  BiTeI: inversion asymmetry and impurities

•  Summary

Taiwan Consortium of Emergent Crystalline Materials

Focus, melt and grow into single crystals

Optical Floating Zone Crystal Growth

optical floating zone furnace

(top view)

Floa8ng zone crystal growth

3

•  congruent vs. incongruent melt

Bridgman crystal growth

4

-‐140 -‐120 -‐100 -‐80 -‐60 -‐40 -‐20 0 20 40 60 80

400 500 600 700 800 900

Posi%o

n(mm)

Temperature(oC) m.p.

•  nucleation •  congruent melt •  constituents m.p. and v.p.

Brief review of thermoelectricity

ZT =S2�

(e + ph)T

e

�= LT

S =�V

�T

•  Heat ó Electricity •  high ZT, high Carnot efficiency •  raise power factor S2σ •  reduce κph

semiconductor

Seebeck coefficient

Wiedemann-Franz law

Seebeck effect

6

S =�V

�T

Uchida et al., Nat. Mater. 2010

•  thermocouple •  Spin voltage •  Inverse spin Hall effect

Mott’s formula:

candidates of layered TE material

7

more TE material: •  Bi2Te3 and its variation: Bi2Se3 , PbTe-Bi2Te3 •  misfit hole-doped layered oxide : Bi2Sr2Co2Oy •  Spin entropy contribution: NaxCoO2 •  Inversion-asymmetry and impurities: BiTeI

NaxCoO2 crystal

Terasaki2005 IEEE

Bi2Sr2Co2Oy

Ca3Co4O9

NaxCoO2 ceramics

ZT =S2�

(e + ph)T

misfit layered transi8on metal oxides

8

Yamauchi et al., 2011 J. Sol. St. Chem.

•  misfit parameter q = bH/bRS •  Mixed valence, local strain, and incommensurability •  S2σ: increased conducting layer power factor •  κph: more incommensurate interfaces to reduce phonon contribution of

thermal conductivity

New variable: quantum confinement

9

Koga1998

•  quantum confinement: 2D QM well •  significant increase of S •  more interfaces: reduced κph •  quantum dot superlattice QDSL

Hicks and Dreselhause, PRB 1993

Hoogland, Photonics Spectra 2008

10

•  Introduction to crystal growth •  Introduction to thermoelectricity •  A few selected layered materials :

•  Bi2Se3: structure variation and defects •  PbBi2Te4: (PbTe)m(Bi2Te3)n homologous series •  Bi2Sr2Co2Oy: hole-doped misfit •  NaxCoO2: spin entropy •  BiTeI: inversion asymmetry and impurities

•  Summary

Bi-‐Se binary phase diagram

11

•  also topological insulator •  low T Bi2-layer staging •  low T metastable phases

Okamoto, J.Phase Equilibria 1994

Intercala8on Defect

5

2

5

2

2

5

5

525252

(Bi2)1(Bi2Se3)1

Bi4Se3

5

2

5

525

(Bi2)1(Bi2Se3)2

Bi2Se2

5

5

5

5

ab

c

Bi+3Se-1Se-2

555 Bi2Se3

Quituple

Bi2 layer intercalated (Bi2)m(Bi2Se3)n 777

Bi3Se4 (BiSe)1(Bi2Se3)2

7

7 ab

cBi+0

Se- 1

Se- 2

7

7

Septuple

Bi metal

[Xe]4f145d106s26p3

10 20 30 40 50 60 70 80

0

20000

40000

60000

80000

100000

120000

(0017)(0016)

(0015)

(0014)

(0012)

(0011)

(0010)

(009)

(007)

(005)

(003)

(0021)

(0018)

(0015)

(0012)

(009)

(006)

Inte

nsity

(arb

uni

t)

2θ(Degree)

Bi2Se2 Bi2Se3

(003)

Crystal a(Ǻ) b(Ǻ) c(Ǻ) Bi2Se3 4.138 4.138 28.664 BiSe 4.180 4.180 22.800

Bi2Se2

staged Bi2-‐Bi2Se3

5

2

5

525

(Bi2)1(Bi2Se3)2

Bi2Se2

Bi excess

Se-rich Bi-rich

T300

T600

28.54

28.56

28.58

28.60

28.62

28.64

28.66

4.13

4.14

4.15

4.16

4.17

4.18

C

a-la

ttice

(Å)

c-la

ttice

(Å)

B

4.13

4.14

4.15

4.16

4.17

4.18

28.54

28.56

28.58

28.60

28.62

28.64

28.66

424.0 424.5 425.0 425.5

c-la

ttice

(Å)

a-la

ttice

(Å)

B

C

Volume (Å3)

c a

28.54

28.56

28.58

28.60

28.62

28.64

28.66

4.13

4.14

4.15

4.16

4.17

4.18

C

a-la

ttice

(Å)

c-la

ttice

(Å)

B

4.13

4.14

4.15

4.16

4.17

4.18

28.54

28.56

28.58

28.60

28.62

28.64

28.66

424.0 424.5 425.0 425.5

c-la

ttice

(Å)

a-la

ttice

(Å)

B

C

Volume (Å3)

c a

crystal structure analysis of Bi2Se3 growth

F.-T. Huang et al., PRB-Rapid Comm. 2012

•  Bi:Se=2:3+δ and Bi:Se=2+δ:3 growth •  local Bi intercalation in the vdW gaps •  300C annealing removes excess Bi

1960 2009

STEM imaging of Bi2Se3

15

Bi excess removed by 300C annealing

20nm

Bi excess in grain boundary + Bi2 patches within vdW gap

initial flux Bi:Se=2+δ:3

low density of Bi2 patches within vdW gap

initial flux Bi:Se=2:3 Bi:Se=2:3+δ

an8site defect BiSe in Bi2Se3

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•  STEM-HAADF imaging •  anomalous Se1 column contrast •  STEM-EDX chemical mapping •  unavoidable Se1 vacancies •  more BiSe1 antisite defects near

the surface

BiSe1

Mo8va8on an8site and carrier doping in Bi2Te3 /Bi2Se3

n-‐type p-‐type

at % Te 50 60 70

Bi2Te3

Bi Te Te Bi

Bi: [Xe]4f145d106s26p3

Se: [Ar]3d104s24p4

Te: [Kr]4d105s25p4

•  BiSe1 antisite p-type? No ! •  m.p. of Bi2Se3=705C but v.p. of Se=685C •  n-type carrier = Se1 vacancy + Bi0

18


• Bi2Se3: defects • PbBi2Te4: (PbTe)m(Bi2Te3)n homologous series • Bi2Sr2Co2Oy: hole-doped misfit • NaxCoO2: spin entropy • BiTeI: inversion asymmetry and impurities

•  Summary

PbSe-‐Bi2Se3 misfit layers ?

19 Shelimova et al., Inorg. Mater. 2008

•  (PbSe)m(Bi2Se3)n •  monoclinic [5(PbSe)]m[3(Bi2Se3)]n

Intercala8on Defect

Septuple

777 PbBi2Se4

7

7

7

7

(PbSe)1(Bi2Se3)1

ab

c

Bi+3

Pb+2

Se-2

Bi Pb Se

PbBi4Se7 (PbSe)1(Bi2Se3)2

75

7

5

7

Bi2Se3

PbBi2Se4

(PbSe)m(Bi2Se3)n homologous series

777 Bi3Se4

7

7 ab

cBi+0

Se- 1

Se- 2

7

7

(BiSe)1(Bi2Se3)1

21



•  Summary

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•  Bi2Sr2CaCu2O8 •  Incommensurate •  misfit [Bi0.87SrO2]–[CoO2] •  highly strained CoO2 plane

ZT of misfit layered Bi2-‐xPbxSr2Co2Oy

Hsu et al., JAP 2012

heavily hole doped Bi2-‐xPbxSr2Co2Oy

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•  Pb substitution vs. Sr vacancy •  anomalously enhanced χ0 •  not χPauli contribution, heavy fermion, or

FM impurities

EPMA+titration

•  quasi-2D itinerant FM ordering below 4 K and itinerant FM domain clusters at high T?

magne8sm of Bi2-‐xPbxSr2Co2Oy

24

•  Co4+/Co3+ population inversion •  spin glass: geometrical frustration of

AF coupling Hsu et al., submitted to PRB

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•  Summary

semiconductor

Why is NaxCoO2 so special?

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NaxCoO2 crystal

Terasaki2005 IEEE

Bi2Sr2Co2Oy

Ca3Co4O9

NaxCoO2 ceramics

•  spins: Curie-Weiss law •  electrons/holes: Fermi surface with hole pockets, metallic transport •  Curie-Weiss metal: electrons localized and itinerant, in space and time window? •  high S not of electronic origin?

spin entropy contribu8on to Seebeck

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•  field suppressed Seebeck coefficient •  spin entropy •  Heikes formula: S ~ kBln(gsgc) •  enhanced Peltier conduction α ∼ S σ •  S peaks at x~0.85 but not at x=0.5 ?

Extra spin moment beyond S=1/2?

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Composi8on X=0.67 X=0.71 X=0.75 X=0.82

Ficng range 60-‐300K 65-‐300K 60-‐300K 125-‐300K

χ0 0.00045 0.00034 0.00034 0.00014

Curie const. 0.06751 0.10635 0.13141 0.14215

Weiss temp. -‐101.965 -‐65.077 -‐106.846 -‐101.105

µeff per Co 0.7350 0.9225 1.0255 1.0666

µeff per Co+4 1.280 1.713 2.051 2.514

•  S=1/2 with g=2 è µeff = 1.732 µB •  If all Co4+ spins localized, part of the Co3+ with thermally induced PM spins? •  C=Nµ2/3kB and µ2 = (1-x) (1.732µΒ)2 + x [(1-α) 02 + α µ2

2] , µ2 = ?

29

Co4+ ó strain ó Na-‐vacancy

di-vacancy

tri-vacancy quadri-vacancy

1 2

1

2 3 4 1

2 3

Na1 site (on top Co) unfavorable, but

Na2 site favorable

Roger et al., Nature 2007 Chou et al., PRL 2008

Na vacancy superlacce and strain reduc8on

30 Chou et al., PRL 2008, Huang et al., PRB 2009

•  √12 a superlattice •  Tri- and Quadri-vacancy

Na0.71CoO2 Na0.77CoO2

•  √19 a superlattice •  (T+D) and (Q+D)-vacancy

Na0.820-0.859CoO2

•  √13 a superlattice •  Di-vacancy

Thermopower from spin entropy contribu8on

Shu et al., manuscript in preparation.

Lee et al., Nat. Mater. 2006

Wang et al., Nature, 2003

•  Na hops with Co4+/3+ together? •  field locks hopping? •  S should peak at x=0.5 instead?

0.6 0.7 0.8 0.9 1.0

0

2

4

6

8

10

Exc

ited

Co3+

(%)

Na Content (x)

Pha

se S

epar

atio

n

√13a √19a √12a

PS

& S

tagi

ng

rand

om o

r PS

•  C=Nµ2/3kB and µ2 = (1-x) (1.732µΒ)2 + x [(1-α) 02 + α µ2

2] , µ2 è S=1 of Co3+

•  higher Seebeck due to higher fraction of thermally excited Co3+

Summary and conclusions

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•  Crystal growth of layered single crystals of layered TE materials •  Material design of layered system: S enhancement and κph reduction

•  Intrinsic carrier doping of antisite defect BiSe in Bi2Se3 •  Material design of Bi2Se3 structural variants : staging and misfit

•  Hole doping of Pb2Sr2Co2Oy with incommensurate layered structure •  Spin entropy contribution to the thermopower in NaxCoO2 •  strain-assisted narrow gap thermal excitation of Co3+

Acknowledgment and Collaborators

•  G. J. Shu, F. T. Huang, R. Sankar, H. C. Hsu •  National Taiwan University: M. W. Chu, G. Y. Guo •  Academia Sinica: W. L. Lee •  National Chiao-Tung University: J. Y. Lin •  National Dong-Hwa University: Y. K. Kuo •  NSRRC: H. S. Sheu

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•  Taiwan Consortium of Emergent Crystalline Materials •  National Science Council •  National Taiwan University •  Academia Sinica thermoelectric theme project

Acknowledgement:

crystal structure and magnetism of layered...

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