na order and co charge disproportionation in sodium...
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Na order and Co charge disproportionation in sodium cobaltates NaxCoO2
Irek Mukhamedshin
A.V. Dooglav, I.F. Gil’mutdinov, S.A. Krivenko
Institute of Physics, Kazan Federal University, Russia
H. Alloul, P. Mendels, F. Bert G. Collin, N. Blanchard
Laboratoire de Physique des Solides, Universite Paris-Sud, France
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Cobaltates – why they are interesting?
~1985: Ionic Mobility of Li, Na →batteries
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1997: High conductivity + Large thermoelectric power:
Cobaltates – why they are interesting?
Terasaki et al., PRB 56, R12685 (1997).
Seebeck effect (∆T →∆V) : thermogenerators Peltier effect (∆V→∆T) : cooling systems
ρκ =
2 S Z Figure of merit
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Na0.35CoO2 +1.3H20 : superconductivity
x=0.35
x=0.35+1.3 H2O
Takada, et al., Nature 422, 53 (2003).
Cu and Co are 3d elements!
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Sodium Cobaltates NaxCoO2: x=0..1
Na
Co Na1
Na2
CoO2
CoO2
CoO2 B
B
A
A
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Initial phase diagram
CoO2 : Co4+ (S=1/2) Mott insulator
T (K)
80
60
40
20
0
superconductivity
normal metal (Pauli-like)
0 0.25 0.50 0.75 1 x (sodium content)
magnetic orderings
magnet. correlated metal (Curie-Weiss-like)
Na1CoO2 : Co3+ (S=0) band insulator
~2eV
t 2g
e g
3d 5
t 2g
e g
3d 6 ~2eV Co3+ x S=0
Co4+ 1-x S=1/2
Na1CoO2 band
insulator + (1-x) holes
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7 7
Na1CoO2 – band insulator
Lang et al., PRB 2005
Only Na2 positions are occupied All cobalts are the Co3+
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Na0.5CoO2
Bobroff et al., PRL 06
Rows of Na1 and Na2
at T>TN : Co3.5±ε (ε<0.2) No clear charge disproportionation
Two kinks at Tc1=88K and Tc2=53K in χ Resistivity shows insulating behavior
below T=53K
0 50 100 150 200 250 3000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
T (K)
NaxCoO2
0.68
0.31
0.50
0.75
88K53K
χ (10
-3em
u/m
ole.O
e)
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x=0.71
Some limited compositions give stable phases Composition gap=mixture of phases
Powder X-ray diffraction patterns
20,0 30,0 40,0 50,0 60,0 70,0
2θ
Inte
nsity
(a.u
.)
incommensurate satellites
Ref.31: R. Berthelot et al., Nature Mater. (2011) Ref.32: F. C. Chou et al., Phys. Rev. Lett. (2008)
0.4 0.5 0.6 0.7 0.8 0.9 1.0
10.4
10.6
10.8
11.0
11.2
0.68 0.72 0.76 0.80
10.8
10.9
11.0
c (A
)
Sodium content x
o
NaxCoO2 Our data Ref. 31 Ref. 32
H75
H72O71
H67
PRB 2012
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NMR spectra: magnetic and quadrupole effects
23Na: I=3/2 Q=0.42
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05
10152025
0 50 100 150 200 2500.0
0.1
0.2
0.3
0.4
x=2/3 x=0.77
106 *M
/H (E
mu/
g*G
)
Bulk susceptibility χ
Spin susceptibility 23K~Ahfχ
23K s (
%)
T (K)
2/3 and 0.77 phases comparison: susceptibility
11
NaxCoO2:
x=2/3=0.67 no magnetic
transitions down to 50 mK
x=10/13=0.77
TN=22K
EPL 2008
TN=22K
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2/3 and 10/13 phases comparison: (Т1Т)-1
12
3/ 2
1
1 KT T
∝
for ferromagnetic fluctuations in
2D metal
Analogous to Sr2Ru2O7 Kitagawa et al, PRL 2005
Hatatani and Moriya 1995
Na Co
A-type AF order
EPL 2008
0.1
0.1
1
x=2/3 x=10/13
0.04
(T1T
)-1 ((
Sec
K)-1)
23K (%)
K3/2
0.4high T low T
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x=2/3: 23Na NQR and NMR
x=2/3 → 8 Na / 12 Co
3 unequivalent Na positions
1.6 1.7 1.8 1.9 2.0 2.1 2.2
Spin
-ech
o in
tens
ity (a
rb. u
nits
)
Na1Na2a
Frequency (MHz)
Na2b
EPL 2009
5.70 5.75 5.80 5.85 5.90 5.95 6.00
SumNa1
Na2aNa2b
Sum
T = 5 K ν = 66.3 MHz Na0.66CoO2
∗Href
Na2bNa2a
Na1
H⊥c
Spin
-ech
o in
tens
ity (a
.u.)
H0 (T)
H||c
∗
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3.6 4.0 4.4 6.4 6.8 7.2 7.6 8.0
Co2b
Co2a
Co1bCo1a
Frequency (MHz)
Exp: 25(3) 17(3) 49(4) 9(2)
Co2a Co1b Co1a Co2b
3 2 6 1
25 16.7 50 8.3
x=2/3: 59Co NQR
EPL 2009
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15 T.A.Platova et al., PRB 2010
x=2/3: Structure confirmation by x-ray
20 40 60 80 100 120
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
25 30 35 40 45 50 55300
400
500
600
700
2θ (degree)
2θ (degree)
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x=2/3: electronic properties of the Co plane
Co1a and Co1b = non magnetic Co 3+ 3 sites/cell Co2a and Co2b = identical magnetism
9 sites/cell ; 8Na+ = 4 holes / cell ; formal charge Co 3.44+
PRB 2011
0 25 50 75 100 125
2
4
6
8
10
12 K2a, x
K2a, y
K1, ab
K2b, y
Na2/3CoO2
59K
(%)
T (K)
K2b,x
delocalised holes Co 3.44+ non magnetic Co 3+
Kagomé doped band with 0. 44 hole per site
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x=0.77: 23Na NMR and 2D order
x=0.77 → 10 Na / 13 Co
4 unequivalent Na positions
PRB 2012
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x=0.77: 59Co NMR
18
130 132 134 136 138 140 142
Co3b
Co3aCo2cCo2bCo2a
Spin
-ech
o in
tens
ity (a
rb. u
nits
)
ν (MHz)
Co1
SC1H||cT=80 KB0=13.2 Tτ=5µS
137 138 139 140 141 142 143
136 137 138 139 140 141 142
SC4H||cT=80 Kν0=136 MHz
SC1H||cT=80 KB0=13.2 T
5/2-3/2Co2c
Spin
-ech
o in
tens
ity (a
rb. u
nits
)
ν (MHz)
Co3b7/2-5/2
ν (MHz)
25µS40µS
15µSτ=5µS
What is the 3D order? No 3D order!
6 unequivalent Co positions
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0.0 0.4 0.8 1.2 1.6 2.0
0.1
1
0.0 0.1 0.2 0.3 0.4 0.5
0.1
1
Co2b7/2-5/2
Co1a7/2-5/2
x=2/3T=80KH||c
1-
M(t)
/M0
Time (ms)
x=0.77T=80KH||c
Co1 central line
Co3b7/2-5/2
Co2a5/2-3/2
1-M
(t)/M
0
Time (mS)
2/3 and 0.77 phases comparison: 59Co NSLR
19
Model of Co3+/Co4+ charge segregation doesn’t work! Cobalt charge disproportionation and itinerant magnetism!
x=2/3 x=0.77
2 types of cobalt: slow and fast relaxing
Slow is only 25% of total signal!
3 types of cobalt: slow, intermediate and fast relaxing
Slow is only 20% of total signal!
x=2/3: PRB 2011
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Correlations
20
0.0 0.5 1.0 1.5 2.0 2.50
1
2
2.0
2.2
2.4
2.6
νQ (MHz)
T 1-1/2 (m
s-1/2)
x=0.77
K ZZ (%
)
Co1Co2abc Co3b
Co3a
0.0 0.5 1.0 1.5 2.0 2.50
1
2
2.0
2.2
2.4
2.6
νQ (MHz)
T 1-1/2 (m
s-1/2)
x=2/3
x=0.77 Co2b
K ZZ (%
)
Co1Co2abc Co3b
Co3a
Co2a
Co1b
Co1a
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Correlations
21 A. Abragam and F. Bleaney, Electron Paramagnetic Resonance of Transition Ions (1970).
T. Kiyama and M. Itoh, Phys. Rev. Lett. 91, 167202 (2003).
0.0 0.5 1.0 1.5 2.0 2.52.0
2.2
2.4
2.6 Co2bK ZZ
(%)
νQ (MHz)
Co1Co2abc Co3b
Co3a
Co2a
Co1b
Co1a
x=0.77
x=2/3
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Nuclear spin-lattice relaxation correlation
22
0.0 0.5 1.0 1.5 2.0 2.50
1
2
2.0
2.2
2.4
2.6
νQ (MHz)
T 1-1/2 (m
s-1/2)
x=2/3
x=0.77 Co2b
K ZZ (%
)
Co1Co2abc Co3b
Co3a
Co2a
Co1b
Co1a
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Charge disproportionation: hole concentrations
23
t 2g
e g
3d 6
S=0
t 2g
e g
3d 6
0.0 0.5 1.0 1.5 2.0 2.50
1
2
νQ (MHz)
T 1 sp
in-1
/2 (m
s-1/2)
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Charge disproportionation in Na0.5CoO2
24
Bobroff et al., PRL 06 Na0.5CoO2: Co3.5±ε (ε<0.2)
Our model:
as νQ=2.8 and 4.0MHz then ε=0.18
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Conclusion
Many experimental aspects are solved – it is time for good theory!
0
20
40
60
80
100
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00
20
40
60
80correlated metal (C-W-like)
AFM correlations
Na order & Co chargedisproportionation
T N
Sodium content x
Co chargehomogenious
FM correlations
SC
normal metal (Pauli-like)
Co3+ (%
)
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THE END
Thank you
for your attention!