Structural, magnetic and superconducting properties of graphite nanotubes and their encapsulation compounds

1. Phys. Chem. Solids Vol. 54, No. 12. pp. 1861-1870, 1993 0 1994 ~&vier Science Ltd Printed in Great Britain. All rinhts reserved 0022-3697/93 -$6.00 + 0.00

Pergamon

STRUCTURAL, MAGNETIC AND SUPERCONDUCTING PROPERTIES OF GRAPHITE NANOTUBES AND THEIR ENCAPSULATION COMPOUNDS Y. MURAKAMI,~

T. %IIBATA,~ K. OKWAMA,t

T. ARAI,t H. SuEMATsUt and

Y. YOSHIDA~ TDepartment of Physics, Faculty of Science, University of Tokyo, Hongo, Bunkyoku, Tokyo 113, Japan IDepartment of Materials Science, College of Science and Engineering, Iwaki Meisei University, Iwaki, Fukushima 970, Japan (Received

18 August 1993)

Abstract-X-ray

diffraction and magnetic susceptibility measurements have been made for graphite nanotubes and encapsulation compounds of TaC and CeC,. In the nanotubes the graphite layers expand 2.27% (q, = 6.852 A) in the interlayer spacing, and only 0.09% in the in-plane direction. The observed (hk0) diffraction peaks show a pronounced asymmetric lineshape of the Warren type, which results from complete stacking disorder. The carbide inside the nanotube, which is a single crystal in most cases, has a lattice parameter very close to that of the carbide in the bulk, indicating no significant effect due to encapsulation. The superconducting T, of encapsulated TaC is very close to that of the bulk sample. An antiferromagnetic transition has been observed in the CeC, encapsulation compound; the observed TN is consistent with that found in an earlier neutron study of a bulk sample. Keywords: Graphite nanotube, encapsulation compound, X-ray diffraction superconductivity, antiferromagnetic transition.

1.

INTRODUCTION

Graphite nanotubes are a very attractive material for study because of their mesoscopic structure, which may show some quantum effects due to their small diameter of less than 100 A, and also because of the novelty of their network structure and their utility as cluster composites in materials science [l]. Furthermore, some groups have succeeded very recently in encapsulating many kinds of carbides and a few pure transition metals into graphite nanotubes [2-4]. The most striking feature is that the carbide grows as a single crystal in the nanotube. Such a complex material also attracts our interest from the viewpoint of both the mesoscopic physics and the potential new materials applications such as isolated nanometer clusters and composite materials. For instance, some carbides are superconducting in the bulk, while others are ferromagnetic or antiferromagnetic, and these physical features are expected to be strongly affected in a nanotube due to the size effect. In this article we have carried an X-ray diffraction study of the nanotube itself and some carbides encapsulated in the nanotubes, observed a very asymmetric lineshape which is characteristic of the “tube” structure, and determined the crystallographic parameters such as the unit cell dimensions, mean crystallite sizes and the stacking correlation of the graphite layers in

the nanotubes. Furthermore, we have studied the magnetic and superconducting properties of the nanotubes and encapsulation compounds of TaC and CeC,. The latter compounds are known to be a superconductor and an antiferromagnet, respectively, and our major interest is in the mesoscopic effect of these properties at the nanotube scale.

2. EXPERIMENTAL Graphite

nanotubes

PROCEDURES

and encapsulation

compounds

were synthesized by d.c. arc discharge in helium gas at about 100 torr [4]. For graphite nanotubes carbon

rods were used as the positive and negative electrodes. For encapsulation compounds the positive electrode was replaced by a carbon rod filled with a mixture of graphite powder and metal powder(Ta) or metal oxide(Ce0,) in the core. This rod was preheated above 2000°C by resistance heating in vacuum in order to form the carbide. Graphite nanotubes and encapsulation compounds were deposited on the negative electrode. As-grown compounds were treated in sulfuric acid at 180°C for 70 h in order to remove pristine metal carbide and/or metal itself. X-ray fluorescence analysis of the samples showed that only the metal element and carbon were present with no contaminants such as oxygen.

1861

Y. MURAKMU

1862

X-ray diffraction measurements were carried out using synchrotron radiation X-rays at beamline BL-7 at the Photon Factory, KEK, Tsukuba. The radiation was monochromatized at a wavelength of 0.9998 A, and the instrumental resolution was 0.007 A-‘. The magnetic susceptibility and magnetization measurements were performed by means of a SQUID magnetometer (MPMS, Quantum Design). The susceptibility is represented by the value for the whole weight of material, including both nanotubes and encapsulation compounds.

.I 3 8

20000

z

i

0

.

3

a

, 15000 d 2 x .2 2 10000

et al.

3. EXPERIMENTAL

5000

AND DISCUSSION

3.1. Structural study of graphite nanotubes and the TaC encapsulation compound bJ> X-ma), d@iaction 3.1.1. Graphite nanotubes. X-ray diffraction patterns of graphite nanotubes are shown in Fig. l(a)+c). We observe both significantly asymmetric peaks as well as the usual symmetric powder diffraction peaks. The asymmetric diffraction lines are assigned to (hkO), while the symmetric ones are

’

:

:

. ..

. 1: :

3 ,d

RESULTS

.. . ;.

lb)

2ooo F”

Q(A-‘1 Fig. I(a, b)

1863

Properties of graphite nanotubes and their encapsulation compounds (c) 3000

2500

2000

1500

1000

500

0 2.8

3.4

3.2

3

Q(A-‘)

2500

2000

1500

1000

500

0

11

It 5

II’1

II’)

I”1

5.2

5.1

III

1

5.3

Q&-r)

Fig. l(c, d). Fig. 1. (a) X-ray diffraction pattern from nanotubes. (b) The (002) diffraction peak. The solid line is the best fit to a Lorentzian. (c), (d) The (100) and (110) diffraction peaks. The solid line is the Warren-type lineshape assuming an in-plane correlation of 250 A and no stacking correlation between graphite layers.

assigned to (001). We will first discuss the symmetric (002) and (004) diffractions. As shown in Fig. l(b) they can be fitted to a Lorentzian peak-shape function, from which we obtain a lattice parameter c,, = 6.852 A. This value for the nanotubes is 2.27% larger than that for pure

graphite. The crystal size obtained from the linewidth is 30 A, which corresponds to the mean thickness of the tube wall. These crystallographic parameters are consistent with the results of electron microscopy measurements, which will be published elsewhere.

1864

Y. MURAKAMI

The asymmetric lineshape observed for (l&O) diffraction peaks can be ascribed to the powder average ofthe “tube” structure, which is practicaily identical to the Warren lineshape far powdered crystals with a two-dimensional structure [5,6]. In the tubes graphite layers are rolled cylindrically, and the stacking order between graphite layers is restricted to a finite number of layers, namely, the coherence length :,. Thus, the in-plane @kD) Bragg diffraction for tubes extends in the qcdirection to the extent of 5 ; ’ , that is, it makes a Bragg “rod” instead of a Bragg point. Moreover, the tube axes in a powder sample are distributed randomly in all directions. The diniaction intensity is calculated from the following equation.

et

al.

s 2n

f(Q)=

dtJlcosGfa(QMQ 1

(1)

F

where

Lte

1 = {@ ax

I,(Q ) = {(Q sin II/)’ -t (l/&)*}-‘,

jl - qoj2 4” (I/L,)‘}

-‘,

L, is the in-plane coherence length and yO is the wave number of the Bragg rod. In eqn (1) the distribution function of the crystal axis in the original Warren-type calculation is repked by a Lorentzian describing the interlayer correlation in the qcdirection. This argument is valid irrespective of the cross-sectional form of the tubes such as polygons, as long as they have a finite correlation length in the out-of-plane diiection. Figure I(c) and jd) shows the observed lineshape for the (100) and

Fig. 2(a)

Properties of graphite nanotubes and their encapsulation compounds

1865

Fig. Z(b). Fig. 2. Electron micrographs of the TaC encapsulation compound. A single crystal of TaC grows epitaxially in a graphite nanotube. The crystal size is 89 x 135A2 (a), or ‘75x 450 A* (b). The electron diffraction from sample (a) shows that the [IOO]axis of the TaC crystal coincides with the tube axis.

(110) diffraction peaks, respectively. The solid lines are the best-fit curves based on eqn (I). Complete stacking disorder is assumed in the calculation; the precise determination of & is rather difficult, but we can say that it is less than one interlayer spacing in the present case. The lattice parameter a, is determined to be 2.462 A, which is only 0.09% larger than that of pure graphite. 3.1.2. TaC e~cu~~u~ationcompound. Here we have studied the graphite encapsulation compound of TaC. Figure 2(a) and (b) shows transmission electron microscope (TEM) and electron diffraction photo-

graphs. We can see a very beautiful lattice image for TaC, which indicates that one single crystal grows in a graphite tube. A most internsting feature is that in some tubes the crystal axis ([loo] in this case) of the encapsulated TaC aligns along the tube axis, which corresponds epitaxial growth. This feature is frequently observed in compounds which grow in a long graphite tube. We also observe en~psulation compounds surrounded by a polyhedral graphite tube, in which we could not fmd such a definite orientational relation. A typical size of the encapsulation compound is about 100 A in diameter and several

Y. MIRAKAMI er al.

1866

hundred A in length, while the graphite tube outside is usually much longer. We show the X-ray diffraction pattern of this sample in Fig. 3(a). In addition to the diffraction peaks from the graphite tubes, we observe very sharp peaks, which can be assigned to TaC crystals, for which the crystal structure is of NaCl type. As shown in Fig. 3(b) the (311) peak can be fitted to a Gaussian

function with a, = 4.455 & 0.001 A with L = 340 A; the instrumental resolution function is taken into account for the calculation of L. The observed lattice parameter is very close to that of pure TaC [7], so that we conclude that TaC undergoes no change in crystal structure when encapsulated in a graphite tube. However, in Fig. 3(b) we observe an appreciable deviation of the ex~~mental points from the

I ’ ’ I

“0

2

6

4

6

@A-‘)

TaC(31

4.62

4.64

4.66

4.66 Q&-r)

Fig. 3(a, b)

4.7

4.72

1867

Properties of graphite nanotubes and their encapsulation compounds

(cl e

1500

-

1250

I-

1000

-

750

p

I

I

,

t-1

I

TaC(200): R

I

2.5 Q(A-‘)

2.75

2.55

Fig. 3(c). Fig. 3. (a) X-ray diffraction pattern of the TaC en~psulation compound. The background from the glass sample tube has been subtracted. (b) The (31 I) diffraction peak. The solid line is the best-fit to a single Gaussian. (c) The (200) diffraction peak. The solid line is a fit to two Gaussians with different linewidths.

Gaussian curve. The deviation becomes more obvious in the (200) peak shown in Fig. 3(c), in which we

observe a lineshape consisting of two Gaussians with different linewidths, corresponding to L = 890 and 22OA. It seems reasonable to assume that there are two different distributions because their size distribution should follow a single Gaussian in most cases.

XlP 0

,

,

Graphite

Tube

fO-

Fig. 4. The temperature dependence of the magnetic susceg tibility of graphite nanotub-es. The susceptibility of pure graphite, xl1- x,. , is shown for comparison. The solid line

is l/6 of the susceptibility of graphite.

A more plausible interpretation for the two components is that the encapsulated crystals have an elongated form in a certain orientation, for example, the [IOOJdirection as seen in Fig. 2(a). In this case we observe two different crystal sizes depending on whether the tube axis is parallel or perpendicular to the scanning direction of X-ray diffraction. In this sense the observed values give the mean sizes of TaC crystals parallel and perpendicular to the tube axis. 3.2. Magnetic and superconducting properties of graphite nanotubes and encapsulation compounds 3.2.1. Magnetism of graphite nanotubes. We show the temperature dependence of the magnetic susceptibility of pure graphite nanotubes in Fig. 4. We note here that they show a small diamagnetism with a temperature dependence very similar to that of graphite itself except for a reduction in the absolute value. This latter difference, however, can be understood by taking into account the angular average of the susceptibility of graphite layers. For graphite layers of tubes we will assume the parallel susceptibility of pure graphite x,, and neglect the much smaller perpendicular component, where x,, is the susceptibihty for a field applied in the c-axis direction [S]. For a tube whose axis is ~~ndicular to the magnetic field we obtain a factor l/2 after angular averaging around the axis, and obtain another factor l/3 for three-dimensional averaging of the tube axis, giving

1868

Y. MURAKAMIet al.

x 1o-4 I

.

TaC Capsulatian j

.

.

0

r

*

t

r

*

Pure TaC L 1 ’ = .

.

‘I

/

‘3

compound, while pure TaC has a T, of 10.3 K as shown in Fig. 5(b). Whether this small difference is significant or not in relation to the size et%ct is ambiguous because r, of the compound is strongly dependent on the carbon composition x 191.We can estimate the carbon composition from the present lattice canstant as x = 0.99 [7], and from this vaIue we expect a rC of IO.0 K according to f9]*precisely the same as that for the present encapsulation compound. On the other hand, we have observed a remarkable difference in the magnetization process between pure and encapsulated TaC; pure TaC shows a ma~ne~zat~on process with a small hysteresis characteristic of an ideal type fi superconductor,

20

10

TEMPERATURE x10-J I .

Compound

+

[K] *

t

f TEMPERATURE

I

I

200 TEMPERATURE [K]

3

100

[K]

Fig. 5. The temperature dependence of the magnetic suseeptibility of the TaC encapsulation compound {a) and pure TaC @). I~I both cases the upper points correspond to the fieid cooling process, while the lower points correspond to zero-field cooling.

x10-” TN= 28K

73 -5

E a resultant averaging factor of l/6. In Fig 4 we p]ot the calculated value of l/6 xn hy a solid line and obtain

good agreement

with the experimental

value

of the tubes. Thus we can conclude that the susceptibility of the tubes can be approximated by the “tube average” of pure graphite. 3.2.2. ~u~e~~~~~~~t~~t~ of TaC e~~~ps~~~t~~~comporcnb. TaC is a we&known su~reonductor. We are very interested in the effect of microscopic encapsuiation on such physical properties as superconductivity and magnetism. Figure 5(a) shows the temperature dependence of the magnetization for the TaC encapsulation compound around the critical temperature T,. The observed T, is 10.0 K for the enca~s~atjon

s5 tTs m -

fl~‘l”““l”l”‘r”IJ

0

TEMPE!iiAT”RE

[K]

40

Fig. 6. The temperature dependence of the magnetic susceptibility of the CeC, encapsulation compound. Figure [b) the region around TNshown in detail.

Properties of graphite nanotubes and their encapsulation compounds

1869

Figure 6(a) and (b) show the temperature dependence of the magnetic susceptibility of the CeC, encapsulation compound. We observe an anomaly characteristic of an antiferromagnetic transition at TN =28 K. Below 10 K we also observe a steep increase, which may possibly be due to a minor paramagnetic impurity such as cerium oxide. The paramagnetic increase above 100 K seems unusual in an antiferromagnet, and may be ascribed to the temperature dependence for the graphite tube, as shown in the figure for .comparison. The magnetization process {Fig. 7) revealed a spin-flip transition at about 3 Tesla at 5 K, and 3.5 Tesla at 12 K. This fact confirms the antiferromagnetic spin configuration determined by the neutron study.

MAGNETIC FIELD [T]

4. CONCLUSIONS

-5

5 MAGNET,; FIELD [T]

Fig. 7. The magneti~tion process for the CeC, encapsulation compound measured at 5 K. A spin-flip transition can be observed at 3.0 Tesla.

while the encapsulated material shows a large hysteresis characteristic of an inhomogeneous superconductor. The volume fraction of TaC crystals in graphite tubes is estimated as 1.8% from the diamagnetism (shielding effect) measured in the zero-field cooling process. However, when taking account of the penetration depth, which is of the same order as the crystal size, it should be larger than this value. 3.2.3. Antiferromagnetic transition in CeC, encapsulation compound. According to a previous neutron diffraction study [lo], CeC, is an antiferromagnet with a body-centered tetragonal structure. The spin at the corner aligns antiferromagnetically with the body-centered one, and the N&e1 temperature is found to be 30 + 2 K. We have no detailed data of this compound’s magnetic properties.

The X-ray diffraction measurements on graphite nanotubes have revealed that the graphite layers which compose the tube wall expand 2.27% in the interlayer spacing (cO= 6.852 A), and only 0.09% in the in-plane direction. The observed spectra for (MO) diffraction peak show a pronounced asymmetric lineshape of the Warren type, which can be accounted for by assuming complete stacking disorder. In the TaC encapsulation compound the carbide inside the nanotube, which is a single crystal in most cases, has a lattice parameter very close to that of the carbide in the bulk, indicating no significant effect due to encapsulation. However, two different distributions for the crystal size are observed, which can be interpreted in terms of the shape of the carbide crystal extending along the tube axis direction. The diamagnetism of graphite nanotubes shows a very similar temperature dependence to that of the perpendicular susceptibility of graphite but the absolute value is reduced by a factor of l/6. This factor can be understood in terms of the angular average of the tube structure. The superconducting T, of encapsulated TaC is very close to that of the bulk sample, with no evidence of a size effect. In the CeC, encapsulation compound an antiferromagnetic transition is observed at 28 K, and a spin-flip transition is observed at 3.0 Tesla at 5 K. These magnetic properties are consistent with those found in an earlier neutron study of the bulk sample [lo]. Acknowledgemenfs-This work was partially supported by a Grant-in-Aid from the Ministry of Education, Science and Culture.

Y. MURAKAMI el al.

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