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

2

Cobaltates – why they are interesting?

~1985: Ionic Mobility of Li, Na →batteries

3

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

4

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!

5

Sodium Cobaltates NaxCoO2: x=0..1

Na

Co Na1

Na2

CoO2

CoO2

CoO2 B

B

A

A

6

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

7 7

Na1CoO2 – band insulator

Lang et al., PRB 2005

Only Na2 positions are occupied All cobalts are the Co3+

8 8

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)

9

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

10

NMR spectra: magnetic and quadrupole effects

23Na: I=3/2 Q=0.42

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

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

13

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

∗

14

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

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)

16 16

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

17

x=0.77: 23Na NMR and 2D order

x=0.77 → 10 Na / 13 Co

4 unequivalent Na positions

PRB 2012

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

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

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

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

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

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)

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

25

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+ (%

)

26

THE END

Thank you

for your attention!