���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
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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
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• Introduction to crystal growth • Introduction to thermoelectricity • A few selected layered materials :
• 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
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• Introduction to crystal growth • Introduction to thermoelectricity • A few selected layered materials :
• Bi2Se3: defects • PbBi2Te4: (PbTe)m(Bi2Te3)n homologous series • Bi2Sr2Co2Oy: hole-doped misfit • NaxCoO2: spin entropy • BiTeI: inversion asymmetry and impurities
• 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
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• Co4+/Co3+ population inversion • spin glass: geometrical frustration of
AF coupling Hsu et al., submitted to PRB
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• Introduction to crystal growth • Introduction to thermoelectricity • A few selected layered materials :
• Bi2Se3: defects • PbBi2Te4: (PbTe)m(Bi2Te3)n homologous series • Bi2Sr2Co2Oy: hole-doped misfit • NaxCoO2: spin entropy • BiTeI: inversion asymmetry and impurities
• 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 = ?
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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: