Transcript 投影片 1

Short range magnetic
correlations in spinel
Li(Mn0.976Co0.024)2O4
Short range
magnetic correlations
in spinel Li(Mn0.976Co0.024)2O4
Nanophysics Laboratory, Department of Physics, National Central University
C. C. Yang,a F. C. Tsao,a S. Y. Wu,a
W.-H. Li,a* and K. C. Lee,a
J. W. Lynn,b
R. S. Liu, c
aDepartment
of Physics, National Central Universtiy,
Chung-Li, Taiwan 32054, Republic of China
bNIST
Center for Neutron Research, NIST,
Gaithersburg, Maryland 20899-8562
cDepartment
of Chemistry, National Taiwan University,
Taipei, Taiwan 106, Republic of China
Nanophysics Laboratory, Department of Physics, National Central University
Abstract
The energy material Li(Mn0.976Co0.024)2O4 was prepared by
standard solid-state reaction techniques. The structures are
confirmed by varied temperature neutron scattering_experiments.
From 300 K to 1.8 K, these samples hold cubic Fd3m spinal phase
without any structure change. In ac magnetization experiments,
M(T) may be described using the Curie-Weiss law for antiferromagnetic coupling at high temperatures, which T = 86 K. At low
temperature, two anomaly peaks are observed at 25 K and 13 K,
which are mainly contributed by Mn spins. The neutron magnetic
scattering discovered Li(Mn0.0976Co0.024)2O4.036 with varied temperature which shows the short-range correlation started from 80 K
and saturated around 40 K.
Nanophysics Laboratory, Department of Physics, National Central University
Structural Analysis
As a cathode material for rechargeable lithium-ion batteries, the spinel LiMn2O4 is
known [1,2] to be economically a more suitable material than currently popular LiCoO2.
Improvement in the rechargeable cycle-performance at room temperature has been
reported [3] in Li-rich systems, and a small amount of Co-doping has been found to
stabilize the structure. A polycrystalline sample of Li(Mn0.976Co0.024)2O4 was prepared
by employing the standard solid-state reaction techniques. High purity Li2CO3, MnO2,
and CoO powders were evenly mixed at a stoichiometric molar ratio, and then sintered
in air at 800°C for 24 h, followed by slowly cooling to room temperature. High-resolution
neutron powder diffraction and Rietveld analysis [4] were employed to determine the
detailed structural parameters. The diffraction pattern was collected on the BT-1 powder
diffractometer at the NIST _Center for Neutron Research, employing a Cu(311)
monochromator crystal to extract =1.5402 Å neutrons. The diffraction pattern taken
at 300 K displayed a cubic Fd3m symmetry as Fig. 1, occupy their which is the same
structure as the reported one the for undoped compound [5,6]. Both the Li and Mn/Co
atoms occupy their normal sites, and the Co atoms enter the Mn sites. Analysis of the
occupancy factors gave a chemical formula of Li(Mn0.976Co0.024)2O4.036 for the present
compound as list in the table 1. No traces of any impurity phases were found, as the
temperature was reduced to 7 K, showing that 2.4% Co-doping stabilizing the crystalline
structure against temperature change.
Nanophysics Laboratory, Department of Physics, National Central University
Magnetic Susceptibility
The effects of Co-doping on the magnetic properties of the system were studied by
means of ac magnetic susceptibility and neutron magnetic diffraction measurements.
Neutron magnetic diffraction measurements were also conducted at the NIST Center for
Neutron Research, using the BT-9 triple-axis spectrometers, with a pyrolytic graphite
PG(002) monochromator crystal and PG filters to extract =2.359 Å neutrons.Figure 2
shows the in-phase component of the ac magnetic susceptibility, χ(T), measured at
various applied dc magnetic fields. The main features perceivable in χ(T) are the peaks
at ~15 K. Finite values for χ were obtained at low temperatures, cusps in the χ(T)
curves are clearly seen, and an applied field suppresses the responses in χ at low
temperatures, suggesting the existence of both the ferromagnetic and antiferromagnetic
components for the Mn moments. Although the peaks occur at ~15 K, the correlations
between the Mn spins develop at a much higher temperature, as indicated by the
observations that χ(T) departs from the Curie-Weiss behavior at ~150 K, as can be seen
in the 1/χ curve shown in the inset to Fig. 2.
Nanophysics Laboratory, Department of Physics, National Central University
Magnetic Neutron Diffraction
Figure 3 shows the magnetic diffraction pattern obtained at 1.4 K. Two broad
peaks at around 2θ=31 and 45, with very different widths, are clearly revealed,
signaling the development of short-range magnetic correlations among the Mn spins,
as the temperature was reduced from 160 to 1.4 K. Detail investigations show that the
magnetic intensities include three peaks, as marked by the dashed curves shown in
Fig. 3. The magnetic diffraction pattern observed for the 2.4% Co-doped compound
is similar to that was observed [6] in the undoped compound, but with the widths of the
peaks are much broader. As has been observed [6,7] in the undoped compound, there
are both the ferromagnetic, characterized by the {111} peak, and antiferromagnetic,
characterized by the {01½} and {011} peaks, components for the Mn moments in the
2.4% Co-doped compound. The magnetic correlation lengths that we obtained for the
2.4% Co-doped compound at 1.4 K are 100 Å and 30 Å for the antiferromagnetic and
ferromagnetic components, respectively, which are somewhat smaller than the 120 Å
and 40 Å observed for the undoped compound [6]. The temperature dependence of the
intensity at 2θ=31 is shown in Fig. 3, showing that the magnetic correlations began to
develop below Tm=150 K. The Tm observed for the 2.4% Co-doped compound is
almost a factor of 2 higher than that of the undoped compound, indicating that the Codoping enhancing the couplings between the Mn spins.
Nanophysics Laboratory, Department of Physics, National Central University
Acknowledgements
The work at was supported by the NSC of the ROC under Grant
No. NSC 91-2112-M-008-056.
Reference
1. M. Thackeray et al., Mater. Res. Bull. 18, 461 (1983).
2. D. Guyomard et al., Solid State Ionics 69, 222 (1994).
3. R. J. Gummow et al., Solid State Ionic 69, 59 (1994).
4. H. M. Rietveld, J. Appl. Cryst. 2, 65 (1969).
5. W. I. F. David et al., J. Solid State Chem. 67, 316 (1987).
6. C. C. Yang et al., Mat. Sci. Eng. B 95, 162 (2002).
7. I Tomeno et al., Phys. Rev. B 64, 94422 (2001).
Nanophysics Laboratory, Department of Physics, National Central University
10000
Li(Mn0.976Co0.024)2O4.036
_
T = 300 K, Fd3m
λ = 1.5402 Å, 15'-20'–7'
a = 8.23256(8) Å
Li (¼, ¼, ¼), Mn(½, ½, ½)
O(x, x, x), x=0.26338(4)
Neutron Counts
8000
6000
4000
2000
0
0
20
40
60
80
100
120
140
160
Scattering Angle 2 ( deg. )
The neutron-powder-diffraction pattern of sample
Li0.96(Mn0.976Co0.024)2O4.036
_
at room temperature. Observed (crosses) and Fd3m-fitted (solid lines) patterns with
their differences plotted at the bottom. The inset table shows the fitting parameter
at other different temperatures.
Fig. 1.
Nanophysics Laboratory, Department of Physics, National Central University
(a)
Hac = 10 Oe
1.4
3
f = 10 Hz
150 K
20
16
3
1.2
1 / M' (10 g / emu )
emu / g - Oe )
-4
' ( 10
-6
'' ( 10 emu / g - Oe )
Li(Mn0.976Co0.024)2O4.036
1.6
1.0
0.8
Hdc=0
Hdc=10 KOe
Hdc=50 KOe
Hdc=90 KOe
0.6
0.4
12
Hdc= 0
8
0
50
100
150
200
250
Temperature ( K )
1.2
300
(b)
Temperature dependence of  (a)
and  (b), measured using a probing
field with an rms strength of 10 Oe and
a frequency of 103 Hz, and the insert
shows the dependence of applied field.
The main feature is the cusp at ~13 K,
which signifies the ordering of the Mn
spins with an antiferromagnetic character.
Anomalies observed around 25 K which
is govern by the ratio of Mn3+/ Mn4+ ion.
1.0
0.8
0.6
150 K
0.4
0.2
0.0
0
Fig. 2.
50
100
150
200
250
Temperature ( K )
300
Nanophysics Laboratory, Department of Physics, National Central University
(0 1 1) Li(Mn
Co0.24)2O4.036
0.976
Net counts / min.
200
100
(1 1 1)
(0 1 ½)
0
-100
I1.4 K - I160 K
-200
15
20
25
30
35
40
45
Scattering angle 2 ( deg. )
50
55
Differences between the diffraction patterns token at 1.4 and 140 K. The broaden
peak between 27 ~ 37 show the short-range magnetic ordering of Mn.
Fig. 3.
Nanophysics Laboratory, Department of Physics, National Central University
Peak intensity / 10 min
7800
Li(Mn0.976Co0.024)2O4.036
7600
7400
7200
7000
150 K
6800
 = 2.359 Å
2 = 31°
6600
6400
0
20
40
60
80
100 120 140 160
Temperature ( K )
Temperature dependence of the 31 peak intensity where the solid lines are only
guides to the eye. The 31 intensity disappears at 130 K.
Fig. 4.
Nanophysics Laboratory, Department of Physics, National Central University
8.230
96.30
8.228
96.28
8.226
96.26
8.224
96.24
8.222
96.22
1.955
Lattice parameter ( A )
8.232
Li0.99(Mn0.976Co0.024)2O4.036
o
Arb. Unit
96.32
o
o
1.954
8.230
Mn-O Length ( A )
Lattice parameter ( A )
Li(Mn0.976Co0.024)2O4
Li0.99(Mn0.976Co0.024)2O4.036
Mn-O-Mn angle ( )
96.34
8.232
o
8.228
1.953
8.226
1.952
8.224
8.222
0
1.951
30 60 90 120 150 180 210 240 270 300 330
Temperature ( K )
300 K
250 K
200 K
150 K
100 K
50 K
9K
0
15
30
45
60
75
90
105
120
135
150
165
Scattering angle ( 2 )
Neutron diffraction pattern taken at various temperatures. No structure change observed
between 9 K~300 K. The inset show the temperature dependence of fitted lattice parameters,
Mn-O length, and Mn-O-Mn angle. The lattice parameters and Mn-O length increasing
monotonically as thermo expansion. No obvious changes on the Mn-O-Mn angle were seen.
Fig. 5.
Nanophysics Laboratory, Department of Physics, National Central University
Li0.96(Mn0.953Co0.047)2O3.996
96.30
o
96.28
8.224
8.222
96.26
8.220
96.24
8.218
96.22
Lattice parameter ( A )
8.216
8.228
Arb. Unit
Mn-O-Mn angle ( )
8.226
o
1.954
Li0.96(Mn0.953Co0.047)2O3.996
8.226
o
1.953
o
8.224
8.222
1.952
8.220
1.951
8.218
8.216
Mn-O Length ( A )
Li(Mn0.95Co0.05)2O4
Lattice parameter ( A )
8.228
0
1.950
30 60 90 120 150 180 210 240 270 300 330
Temperature ( K )
300 K
150 K
7K
0
15
30
45
60
75
90
105
120
135
150
165
Scattering angle ( 2 )
Neutron scattering pattern taken at different temperature. No structure change observed
between 7 K~300 K. The inset show the temperature dependence of lattice parameters, Mn-O
length and Mn-O-Mnangle got from structure refinement. The lattice parameters and Mn-O
length increasing monotonically as thermo expansion. No observed change of Mn-O-Mn angle.
Fig. 6.
Nanophysics Laboratory, Department of Physics, National Central University
1.6
Li(Mn0.95Co0.05)2O4
1.2
1.0
-4
 ( 10 emu / g )
1.4
0.8
' = 0' +
0.6
C
T + T
Hac = 10 Oe
Freq. = 1000 Hz
Hdc = 0 KOe
Hdc = 10 KOe
Hdc = 50 KOe
Hdc = 90 KOe
-5
0 = 2.0(3) x 10 emu / g
T = 57(2) K
eff = 3.33(3) B
0.4
1.4
-6
'' ( 10 emu / g )
1.2
1.0
0.8
0.6
Temperature dependence of  (a)
and  (b), measured using a probing
field with an rms strength of 10 Oe and
a frequency of 103 Hz, and the insert
shows the dependence of applied field.
The main feature is the cusp at ~13 K,
which signifies the ordering of the Mn
spins with an antiferromagnetic character.
Anomalies observed around 25 K which
is govern by the ratio of Mn3+/ Mn4+ ion.
The red curve was Curie-Weiss fit of zero
field experiment. We can get the fitting
parameter Tθ~57 K and eff ~3.33 B.
0.4
0.2
0.0
0
25
50
75 100 125 150 175 200 225 250 275 300
Temperature ( K )
Fig. 7.
Nanophysics Laboratory, Department of Physics, National Central University
200
(a)
(b)
Li(Mn0.95Co0.05)2O4
λ = 2.359 A
I1.4 K - I140 K
40’-48’-40’
I35 K - I140 K
(c)
(d)
I50 K - I140 K
I55 K - I140 K
Counts / min
100
0
-100
-200
-300
200
Counts / min
100
0
-100
-200
15
20
25
30
35
40
Scattering angle 2
45
50
55
15
20
25
30
35
40
45
50
55
Scattering angle 2
The neutron magnetic diffraction patterns of Li(Mn0.95Co0.05)2O4 , taken at various temperatures,
owing the development of magnetic correlations when the temperature was reduced.
Fig. 8.
Nanophysics Laboratory, Department of Physics, National Central University
200
(e)
(f)
I 65 K - I 140 K
I80 K - I140 K
(g)
(h)
I110 K - I140 K
I120 K - I140 K
Counts / min
100
0
-100
-200
-300
200
Counts / min
100
0
-100
-200
-300
15
20
25
30
35
40
Scattering angle 2
45
50
55
15
20
25
30
35
40
45
50
55
Scattering angle 2
The neutron magnetic scattering pattern of Li(Mn0.95Co0.05)2O4. The Fig (e), (f), (g), (h)
were collected at 65 K, 80 K, 110 K, 120 K, which show the growth of short range magnetic
ordering.
Fig. 9.
Nanophysics Laboratory, Department of Physics, National Central University
_
Cubic Fd3m Li (¼, ¼, ¼), Mn ( ½, ½, ½), O (x, x, x)
Temp. (K )
a(Å)
O (x)
Mn-O( Å )
Mn-O-Mn
(°)
χ²
7
8.22344(14)
0.26341(6)
1.9518(4)
96.283(28)
1.443
50
8.22347(6)
0.26339(5)
1.9520(4)
96.275(25)
1.365
100
8.22415(6)
0.26340(5)
1.9521(4)
96.279(25)
1.323
150
8.22603(6)
0.26335(5)
1.9528(4)
96.257(25)
1.324
200
8.22872(6)
0.26343(5)
1.9529(4)
96.294(25)
1.328
250
8.23192(5)
0.26346(5)
1.9535(3)
96.308(21)
1.324
300
8.23256(8)
0.26338(4)
1.9542(5)
96.270(30)
1.620
Table. 1.
Nanophysics Laboratory, Department of Physics, National Central University