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Crystal structure of graphite bromine lamellar compounds
Carbon
1971, \‘ol. !I. pp. Nl7-416.
Pergamon
Press.
Pnnted
an (Great Briuln
CRYSTAL STRUCTURE BROMINE LAMELLAR
OF GRAPHITE COMPOUNDS
T. SASA, Y. TARAHASHI and T. MURAIBO Department of Nuclear Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo,
Japan (Received 15 August 1970) Abstract-The isotherm of bromine on pyrolytic graphite was studied at 20°C by stepwise bromination, and several phases of graphite bromide were identified, which have compositions of C,Br, C,*Br, C16Br and C,,,Br. X-ray studies show that these possess the socalled 2nd, 3rd, 4th and 5th stage structures respectively. At bromine concentrations between these definite compositions, X-ray diffraction patterns show mixtures of the higher stage and the lower stage structures. The density of the intercalated bromine molecules in a layer is believed to stay unchanged with the change of the composition. During the course of the reaction with the saturated vapor of bromine, an intermediate metastable lamellar compound structure was found in which the bromine layers are intercalated into everv second laver soacing but for which the bromine concentration is believed to be much lo&er than that of’C,BrP 1. INTRODUCTION
Bromine is one of the substances which react with graphite to form lamellar compounds. The composition of the saturated graphite bromide is C8Br, and its structure was clarified by Riidorff [l] to be the second stage structure, where bromine molecules are intercalated into every second layer spacing of graphite, these carbon layer spacings being expanded from 3.35 A to 7.05 A. Eeles and Turnbull [2] more recently investigated the crystal structure of the saturated compound, and reported a crystal system of rhombohedral type with unit cell sides of a,, = 59.5 A, b,, = 34.5 A and co = 10.3 A, but the configuration of atoms within a bromine layer was not fully elucidated. The isotherms of graphite-bromine system have been studied by several investigators [3-51. Maire and Mering[5] concluded from their vapor pressure data and X-ray studies that above the composition of CIGBr, the second stage structure is established, and the bromine density of C&Br within a layer is half that of C,Br. Below the composition of C,,Br, bromine layers of lower density are
intercalated randomly among graphite layer spacings, and from C,,Br to C8Br, bromine layers gradually become more dense. Eeles and Turnbull[2] found a new phase CzsBr after decomposition of C8Br, and investigated its crystal structure in detail. Bromine molecules were found to be inserted into every fourth carbon layer spacing of graphite, and forming highly ordered chain structures in their own layers. They also reported formation of a Ci,Br phase during the course of bromination of graphite, in which bromine layers are intercalated into every second layer spacing of graphite. It was concluded that the structure of bromine layers takes two forms, one which occurs at concentrations in excess of 47 per cent of the saturation composition (C,,Br), the other at lower concentrations. Bach and Herold et al. [ 161 investigated the change in the composition and in the structure of graphite bromide with temperature increase, and reported mediate phases C,,Br the ordered structure fourth The
stages. mechanism
the existence of interand &Br, which take of the third and of the
of bromine
absorption
of
408
T. SASA,
Y. TAKAHASHI
graphite has been investigated by several workers [7-101. Saunders et al. [8] measured the dimensional changes of pyrolytic graphite with bromine absorption, and found a metastable structure which showed practically full expansion along the c-direction at only about one-third of the limiting uptake. Hooley et aZ.[9] also investigated the rate of absorption and the dimensional changes, and proposed a mechanism in which all layers are open to intercalation at a relatively initial stage of reaction. The present authors [lo] measured the changes in the electrical conductivity and in the X-ray diffraction pattern of graphite during the course of bromination, and reached similar conclusions. Graphite bromide goes through an almost continuous range of compositions up to the saturation compound CsBr. The information about the structure of these compounds is however confusing as has been shown above. In the present study, crystal structure of graphite-bromine lamellar compounds with different bromine concentrations is investigated, and the existence of some definite phases is confirmed. 2. EXPERIMENTAL 2.J Materials
The following three types of graphite were used in this work: (1) Natural graphite powder. (2) Pyrolytic graphite, 2 mm in thickness, deposited at 1800°C and heat-treated to 3000°C co = 6.712 & 0.002 A. Pyrolytic graphite was used as chipped flakes in most cases. (3) Kish Graphite flakes, a few mm across. GR grade bromine was distilled and passed through concentrated sulfuric acid before use. 2.2 Apparatus and procedures The apparatus used for the determination of the isotherms for graphite-bromine system has been described in another paper[lO]. A pyrolytic graphite block was chipped into flakes to quicken the attainment of the equilibration of reaction, the flakes were put into a
and T. MUKAIBO
quartz container and suspended by a quartz spring balance (sensitivity 20 mm/g). The weight of absorbed bromine was measured by means of a running microscope. The reaction tube was immersed in a water bath, the temperature of which was kept at 2O.O”Cby an electronic controller. The liquid bromine container was kept at a desired temperature by means of an alcohol bath (from -30°C to +2O”C) to control the vapor pressure of bromine. The vapor pressure was checked using a glass-spoon gauge. For the measurements of the macroscopic thickthe unchipped pyrolytic ness increase, graphite was used, making samples of definite suitable size. For the X-ray diffraction analysis using a Debye-Scherrer camera (11 a46 cm in diameter), natural graphite powder was loaded into a Lindemann glass tube of 0.5 mm in diameter, and allowed to react with bromine vapor of desired pressures. After the equilibrium was attained, the tube was sealed off. For the X-ray diffractometry by means of a G.M. counter with an automatic recorder, a specially designed sample holder system was utilised, which was described in another paper [ll]. It is shown in Fig. 1. The glass sample holder has a drilled hole of 1 mm diameter (Fig. 1) behind the graphite sample, and the bromine container is connected to the back surface of the glass plate. 0.2 mm thick Teflon sheet is placed over the front surface of the holder with “Daifloil’‘-wax (polymonochlorotrifluoroethylene), and the whole apparatus was made gas-tight. Pyrolytic graphite flakes or Kish graphite flakes were loaded into the holder, the whole system was evacuated, the bromine container being held at a desired temperature. The sample holder can be placed in the goniometer as it stands. 3. RESULTS 3.1 Isotherm of bromine on graphite
The isotherm of bromine on pyrolytic graphite at 2O.O”Cis shown in Fig. 2. For each datum point the attainment of sufficient
GRAPHITE
LAMELLAR
-to9
COMPOUNDS
[5] but actually several observed on the curve. spond approximately to C*Br, C,,Br, C&Br and of reducing the bromine steps are not sufficiently part of the curve. 3.2 Dimensional absorfition
Fig. 1. Arrangements of the sample holder 0 glass sample holder; 0 a hole of I mm diameter; 0 graphite sample; 0 “Teflon” sheet (0.2 mm thickness): 0 ground-glass contact; 6 Araldite sealing; 0 bromine; 8 water bath for bromine pressure control; 0 to bejointed to the vacuum system.
indistinct steps are These steps correthe compositions of C,,Br. In the course vapor pressure, such distinct on the lower
change of graphite with bromine
The relative increase in thickness along the c-axis of pyrolytic graphite due to bromine absorption is shown in Fig. 3. The dimensional change is nearly proportional to the bromine concentration, and is consistent with the result of Saunders et (~1.[$I. This observation suggests that the graphite layer spacings are either quite empty or are filled with bromine molecules, and the number of graphite layer spacings occupied fully by bromine increases with the increase in bromine concentration.
12 b x 10
The observed diffraction pattern of CsBr was given in Table 1, following the rhombohedral crystal system reported by Eeles and Turnbull[2]. The sides of the rhombohedral unit cell are taken as a,, = 59.5 A, b,, = 34.5 ,A and c0 = 20.6 A. The repeat distance along the c-axis must be taken at least twice as large
v
&a ;6 a
’
e ,-
4
s!
2
E
m
I
0
i
0
01 -A
’
I
I
I
I
0.2
0.4
0.6
0.8
1.0
Partial
pressure
of bromine
-l
a
I
P/P,
Fig, 2. Isotherm of bromine on pyrolytic graphite (‘WC) 0 on bromination; 0 on debromination. equilibrium conditions was necessary and took about 24 hr. Open circles represent the equilibrium states of graphite-bromine system with increasing bromine vapor pressure, and solid circles the state5 with decreasing bromine vapor pressure (Fig. 2). The isotherm curve is not composed of two stages as was reported by Maire and Mering
0
2 Bromine
4
6 uptake
8 (BrlC
Fig. 3. Thickness increase of pyrolytic sample by bromine absorption.
10
12
X IO21 graphite
410
T. SASA, Y. TAKAHASHI
and T. MUKAIBO
Table 1. X-ray diffraction pattern of graphite bromide CsBr. Rhombohedral system of a, = 59.5 A, b = 34.5 A and c,, = 20.6 A.
2@C,l, Intensity (deg)
(obs.)
20.8 21.7 26.0 31.2 33.0
20.6 21.5 25.9 31.1 33.4
m
m v. St m
42.6
42.6
W
44.6 53.5 59.4 77.8
44.4 53.3 59.2 77.6 84.2 83.3 99.9 148.0
st
(W 0 8 0 14 2 0 0 0 6 012 0 14 10 0 28 0 0 14 14 0 14 14 3 0 012 14 14 9 028 0 0 018 028 6 14 14 18 14 42 3
*Diffraction of CsBr.
83.6 101.1 148.7
line of natural
as that of Riidorff or Eeles et al. This may be understood when the displacements of carbon layers parallel to the layer planes are considered. Riidorff [l] reported that (001) lines were observed clearly, but (Ml) or (MO) lines were completely missing in the diffraction pattern of C8Br. However, the diffraction pattern of C,Br observed by us (Table 1) is quite different from that of Riidorff, in particular lines (Ml) and (MO) are actually observed. But the extra (001) lines characteristic of the second stage structure, observed by Riidorff, are not observed in our X-ray photograph. These lines are indispensable to the investigation of the distribution of bromine layers in the direction of the c-axis. 3.4 X-ray studies of bromide of well-oriented graphite with an automatic recording X-ray dajkxtometry When well-oriented graphite samples are used for the X-ray analysis, the diffraction pattern shows only (001) lines, which are very
Related line*
W
St
m-w m m W v. v. w
graphite
101 004 103 110 006 112 106 211
related with that
strong. For the purpose of detecting the (001) super-structure lines of graphite bromide such as were observed by Riidorff, Kish graphite was taken. Pyrolytic graphite flakes, which were used in the isotherm measurements, did not give sufficient intense patterns of diffraction after the reaction with bromine. The structural analyses were carried out with graphite bromides formed under various vapor pressures of bromine. In the isotherm study, the compositions of C,,Br with n = 2-5 only were found to be of importance. Certain ordered structures were observed at these compositions in the X-ray studies. The results of the structural analyses are shown in Tables 2-5. The observed structure factors are estimated from the intensities of the diffraction lines, with the corrections of the Lorentzpolarization factor, the absorption factor and the absorption by the Teflon sheet. Observed and calculated structure factors are compared on a relative scale. In addition to the confirmation of the second stage structure of C,Br, the structures of
GRAPHITE Table 2.2nd
LAMELLAR
stage structure
411
COMPOUNDS
of graphite
bromide
CsBr.
I
2&b, (degf
2&l,, (de@
Ifii~,loix
lWclca,c
00 1 00 2 00 3 00 4 00 5 00 6 00 7 00 8 00 9 0 0 10 00 11 0 0 12
9.0 17.0 25.7 34.5 43.6 52.8 62.5 72.8 83.7 95.7 109.3 125.7
8.5 17.1 25.7 34.5 436 52.8 62.5 728 83.7 95.7 109.3 125.7
23 23 100 19 3 1 77 14 23 54 6 13 35
17 2 1 100 15 27 68 14 25 5 1 1 1 22 42
00
I, = 10.39 A, Cu - Ka Table 3.3rd 00
1
001 00 2 00 3 00 4 00 5 00 6 00 7 00 8 00 9 0 0 10 00 11 0 0 12 0013 0 0 14 0 0 15 0 0 16 0 0 17
stage structure
of graphite
~~~
2&,,, idegf
12.9 19.2 25.8 32-7 39.4 46.1 53.3 60.8 68.3 76.2 84.5 103.7 114.6 127.5 145.5
6.4 12.9 19.4 25.9 32.7 39.4 46.2 53.3 60.7 68.3 76.4 84.6 93.7 103.5 114.6 127.5 145.1
bromide
ll;lFol0bs 13 23 100 10 18 ::: 8 1 :3 19 47 8 1 1 30 3
&Br.
lFiFotca,e 1 1 14 20 100 8 I 7 23 65 8 17 22 48 5 I 5 20 39 2
f, = 13.74 A, Cu - Ka C&Br, C,,Br and CzoBr were explained as the 3rd stage, the 4th stage and the 5th stage structures, respectively, where the bromine layers are intercalated into every 3rd, 4th and 5th layer spacing of graphite. This agrees with the results of Bach et al. [6]. As it is difficult to know exactly the stacking sequence of the carbon layers, the repeat distance along the c-axis direction, I, is taken for convenience
as that from the position of one bromine layer to the nearest neighbor bromine layer. Thus the indexings of (001) are not consistent with those in Table 1. For the structure of the nth stage, one bromine layer and then it carbon layers are taken repeatedly as shown in Fig. 4. Then I,/ (r$+ 1) has a similar value as the layer spacing of graphite. Thus the (OOn+ 1) lines and the
T. SASA, Y. TAKAHASHI
412
Table4.4th
00
stage structure
1
001 00 2 00 3 00 4 00 5 00 6 00 7 00 8 00 9 0 0 10 0 0 11 0 0 12 0 0 13 0 0 14 0 0 15 0016 0 0 17 0 0 18 0 0 19 0 0 20 0 0 21 I,=
of graphite
2;
2&i,, (deg)
10.6 15.6 20.6 26.1 31.6 37.0 42.3 47.8 53% 59.6 65.8 71.9 78.2 85.1 100.5 108.6 117.9 128.9 143.0
5.2 10.3 15.6 20.8 26.1 31.4 36.8 42.3 48.0 53.6 59.6 65.6 71.9 78.4 85.2 92.5 100.3 108.7 118.1 128.9 142.6
of them come out to be strong, and n super-structure lines are observed between them. The spacing of carbon layers that contain a bromine layer between them is shown in Table 6 for each stage. The observed layer spacings are a little smaller than the value 7.05 A, reported by Riidorff.
t
-----
t
-
-----
4
t
?P
0’
jf-
4_____ (b)
C,zBr
&
+
carbon
---
bromine
layer
-
$ ,-
::I,
_____Y-
-I(cl
-
-----
.d.
c-
--
lf
C,Br
t
-
g-
0, _____
(a)
-----
*a
-
x-
bromide
kWulot,s 15 16 28 100 13 17 20 20 73 4 8 20 29 64 10 1 1 18 45 7
CIGBr.
IF/F&a,c 10 10 13 18 100 4 12 16 2 1 64 4 12 16 20 46 2 10 14 18 37 2
17.08A,Cu-Ka
multiples
04
and T. MUKAIBO
_---C,,Br
(d)
C20Br
layer
Fig. 4. Structures of graphite bromine lamellar compounds (a) 2nd stage structure of CsBr; (b) 3rd stage structure of &Br; (c) 4th stage structure of CIGBr; (d) 5th stage structure of C,,Br.
3.5 The changes in the structure of graphite bromide with the change of bromine concentration Figure 5 shows the change of the profile of (008) diffraction line of graphite with increasing bromine content. When the bromine vapor pressure exceeds the threshold value for the intercalation, the reaction takes place. The concentration of absorbed bromine increases fast with the increase in bromine vapor pressure up to C,,)Br. In these ranges, the intercalated bromine molecules are not distributed homogeneously throughout the graphite crystal as dilute solid solution. The diffraction pattern of such samples shows the coexistence of regions of the virgin graphite and of regions invaded by considerable amounts of bromine. The regions invaded by bromine attain the structure of the 5th stage. At about the bromine vapor pressure of P/P, = 0.15, the structure becomes a homogeneous 5th stage structure of &Br. With further increase of bromine vapor
CARBON Table5.5th
00
1
00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
00 00 00 00 00 00 00 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
GRAPHITE stage
LAMELLAR
structure
of graphite 2%,,,. (de@
2O”bS (deg)
17.5 21.8 26.1 30.2 35.1 39.8 44.7 49.2 53.8 63.5 69.0 74.4 80.1 85.7 91.5 97.7 104.2 712.7 120.8 129.9 141.0
bromide
IW,,ioba
4.3 8.7 13.0 17.4 21.8 26.2 30.7 35.2 39.8 44.4 49.1 53.9 58.8 63.X 69.0 74.4 79.9 85.7 91.7 98.1 105%) 1124 120.6 129.9 141.6
-
COMPOUNDS C,,Br.
Wolearc 9
1 0
.5 14
100 3 9 6 14 29 63 9 9 14 22 54 7 7 7 9 14 37 9
1 1 14 2 1 10 0 3 10 14 17 23 64 2 1 1 14 1 7 22 53 1 9 13 16 20 42 5
I, = 20.41 A, Cu - Kcz Table 6. Expansion of carbon layer spacings with the intercalation of bromine Composition
Stage
CsBr
2
&Br &Br CzoBr
3 4 5
pressure, the structure of the graphite bromide follows the stages of higher bromine concentrations one by one, up to the 2nd stage structure of saturation, At intermediate bromine concentrations between the definite compositions corresponding to definite stages, the X-ray diffraction pattern shows a mixture of extra lines of both the higher and the lower stage structures. The strong
Interlayer
spacing(A) 7.03 7 *03 7.01 6.99
main lines, however, are not observed as distinct double peaks, but rather as broad single peaks. At higher bromine concentrations than &,Br, the main lines apparently shift continuously to the lower diffraction angles up to the position of CBBr, as if only a single phase was present in the course of the gradual change of the lattice constant. In the progressive debromination process
T. SASA, Y. TAKAHASHI
414
and T. MUKAIBO
of C&Br, the Srd, 4th and 5th stages are also observed in that order, however the equiiibrium vapor pressures are much Iower than those for the bromination process. After the vapor pressure of bromine is reduced to zero, a residue compound remains. The bromine residue compound obtained from Kish graphite was found to have a composition similar to C&Br, as was aIso reported by Eeles and TurnbufifZ]. But the structure of this compound is not ordered but rather strongly disordered, and the apparent lattice constant is found to be smaller than that of the 5th stage structure, 3.6 ~~t~~~t~ structures of graphite during the course of rapid bromination
2
_ k
*
f
I
2nd stage I
II
125 Diftraction
I‘
*
*
1
130
t
I.
135
angle
28’
Fig. 5. The change of profile with the increase of bromine concentration.
The changes in the layer structure of graphite during the course of reaction with the saturated vapor of bromine were investigated for Kish graphite. Figure 6 shows the changes of the diffraction pattern in the range of 28 from 50” to 90’. In the initial stages of the reaction, appearance of some new phase is observed whife the
graphite (006)
before
the
L bromination
I\
2nd stage
Diffraction
corner
I
angle
23’
Fig. 6. The change of profile during the course of rapid bromination.
GRAPHITE
LAMELLAR
virgin graphite remains unchanged. As the reaction proceeds, the lines of the new phase become sharper and extra lines characteristic of the 2nd stage appear. No other phase such as 3rd and 4th stage was seen during the reaction. Moreover, already about 100 min after the start of the reaction, the 2nd stage structure appeared, that is when the bromine content of the sample was still much lower than that corresponding to C8Br, possibly lower than half of that. Therefore the density of the bromine molecules in a layer of the second stage in this intermediate state seems to be less than half of that of C8Br. This phase corresponds to the metastable structure observed by Saunders et al. [S]. The phase C,,Br, which was observed by Eeles and Turnbull[2] during the course of the bromination is believed to be possibly also the same metastable phase. 4. DISCUSSION
4.1 Equilibrium lar compounds
phases of graphite-bromine
lamel-
It is not clearly known to what extent solid solution exists in a single phase, or to what extent definite stoichiometric phases are formed in graphite bromide. In the present studies, several intermediate phases of graphite bromide, which have characteristic compositions and structures, were found to exist under certain equilibrium vapor pressure of bromine. They were identified to have C,,Br composition and nth stage structure. The observed phase with a lowest bromine concentration was the 5th stage, and the phase of the primary solid solution of bromine with graphite could not be observed above the threshold vapor pressure of bromine. It is not clear whether stages of graphite bromide higher than the 5th stage exist or not. The situations may be different for different graphite samples, because the threshold vapor pressure for the intercalation is known to depend entirely on the type of the graphite sample used.
car
Vol. 9. No. 4-D
COMPOUNDS
415
Above the composition of &,Br, only four definite phases are found, namely, from the 5th to the 2nd stage. As the bromine concentration increases, a change of the structure of graphite bromide from one stage of lower concentration to another stage of higher concentration takes place. This requires some sort of rearrangements of intercalated bromine layers in graphite crystal. Nixon et al. [12] studied the progressive formation of graphite nitrate. They observed the successive appearance of structures from the 4th stage to the 1st stage. As the reaction proceeded, filling and emptying of certain intercalate layer positions were observed. Salzano and Aronson [13] measured the rates of bromine exchange between the vapor phase and the intercalation compound. The exchange is rapid and about 90 per cent of intercalated bromine was observed to exchange in an hour or so for small particles of graphite. It seems therefore that stripping and filling of various carbon layer spacings with bromine takes place in the course of the transformation. Daumas and Herold [14] proposed however a new mechanism for the successive appearance of higher stages during the decomposition of potassium graphite, C,K. According to their model, intercalated atoms are depleted from all layers. In each carbon layer spacing, certain zones continue to be occupied by the alkali metal layer, but the other zones are emptied entirely and the layer spacing shrinks to the normal spacing of graphite. Moreover, due to the repulsions between the intercalated alkali metal ions, the intercalated layers change their position in the carbon layer spacing so as to move away from the other intercalated ions in the neighboring layer spacings. This results in local ordered sequences of the intercalated metal atom layers along the c-axis direction, which are characteristic for the stages. Thus filling and emptying of whole layers is not needed for transformation of the structure from one stage to another. At present, the mechanism of the change of the structure of graphite bromide
416
T. SASA, Y. TAKAHASHI
from one stage to another is not yet elucidated satisfactorily. It is also an important problem to what extent the definite stoichiometry is realized at each stage. As seen from the curves Fig. 2, graphite bromide systems are not so distinctly stoichiometric as alkali metal-graphite compounds. At intermediate compositions between stages, the structure of graphite bromide seems to be some kind of mixture of both higher and lower stage structures. The problem of the structure of the intermediate compositions is certainly related to the preceding problem of the mechanism of the structural change from one stage to another. 4.2 Structure of bromine luyers The X-ray data shown in Table 1 for CsBr resemble that of graphite. This is because the bromine layer has a similar thickness as the carbon layer, and also the graphite bromine lamellar compound has a similar layer structure as graphite. Thus most of the diffraction lines in Table 1 are related closely to those of graphite. There are only four new lines that are observed and confirmed to be characteristic of graphite bromide. According to Eeles et al. [2], these (h&l) lines are attributed to the configuration of bromine molecules in bromine layers. It was found in this work that, although the content of graphite bromine bromide changed from about Br/C = 0.03 to the saturation composition, the diffraction patterns observed were quite the same as shown in Table 1, although slight shifts of the positions of the diffraction lines were noted. The four (hk0) lines, in particular, remain unaltered with change in bromine concentration. It is believed therefore that the density and the configuration of intercalated bromine molecules in a layer are quite the same for all the stable graphite bromides from the 5th to the 2nd stage. The atomic density of the bromine layer relative to the density of carbon atoms in an adjacent carbon layer is equal to
and T. MUKAIBO
Br/C = 3. This type of structure of bromine layer, which Eeles and Turnbull[2] reported for C8Br, may be the only form of bromine layer that appears in the normal graphite bromine lamellar compounds in equilibrium with vapor of bromine. Eeles et al. also reported another type of structure of bromine layer with lower bromine density, which has the atomic ratio of Br/C = l/7 relative to the density of an adjacent carbon layer. They found this type of structure in phases CzsBr and &Br. The configuration of bromine atoms in the layer was investigated in detail. Such structure is, however, believed to be a metastable one, appearing only in the course of bromination or debromination, or in nonequilibrium conditions.
REFERENCES 1. Riidorff W., Z. Anorg. Allg. Chem. 245, 383 (1941). 2. Eeles W. T. and Turnbull J. A., Proc. Roy. Sot. A 283,179 (1965). 3. Rayerson L. H., Wertz J. E., Weltner W., Jr. and Whitehurst H., J. Phys. Chem. 61, 1334 (1957). 4. Hooley J. G., Can.]. Chem. 37,899 (1959); Ibid. 4Q, 745 (1962). 5. Maire Jl and Mering J., Proc. 3rd Co@ Carbon 337 (1959). 6. Bach B., Bagouin M., Bloc F. and Herold A., C. R. Acud. Sci. Paris 257,681(1963). 7. Mukaibo T. and Takahashi Y., Bull. Chem. SOL. Japan 36,625 (1963). 8. Saunders G. A., Ubbelohde A. R. and Young D. A., Proc. Roy. Sot. A 271,499 (1963). 9. Hooley J. G. and Smee J. L., Carbon 2, 135 (1964). Hooley J. G., Garby W. P. and Valentin J., Carbon 3,7 (1965). 10. Sasa T., Takahashi Y. and Mukaibo T., Bull. Chem. Sot. Japan 43,34 (1970). 11. Takahashi Y., Sasa T. and Mukaibo T., Tanso (Carbon), 1969 (52), 199. 12. Nixon D. E., Parry G. S. and Ubbelohde A. R., Proc. Roy. Sot. A 291,324 (1965). 13. Salzano F. J. and Aronson S., J. Inorg. Nucl. Chem. 28,1343 (1966). 14. Daumas N. and Herold A., C. R. Acad. Sci. Paris, Ser. C, 268 (5), 373 (1969).
1971, \‘ol. !I. pp. Nl7-416.
Pergamon
Press.
Pnnted
an (Great Briuln
CRYSTAL STRUCTURE BROMINE LAMELLAR
OF GRAPHITE COMPOUNDS
T. SASA, Y. TARAHASHI and T. MURAIBO Department of Nuclear Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo,
Japan (Received 15 August 1970) Abstract-The isotherm of bromine on pyrolytic graphite was studied at 20°C by stepwise bromination, and several phases of graphite bromide were identified, which have compositions of C,Br, C,*Br, C16Br and C,,,Br. X-ray studies show that these possess the socalled 2nd, 3rd, 4th and 5th stage structures respectively. At bromine concentrations between these definite compositions, X-ray diffraction patterns show mixtures of the higher stage and the lower stage structures. The density of the intercalated bromine molecules in a layer is believed to stay unchanged with the change of the composition. During the course of the reaction with the saturated vapor of bromine, an intermediate metastable lamellar compound structure was found in which the bromine layers are intercalated into everv second laver soacing but for which the bromine concentration is believed to be much lo&er than that of’C,BrP 1. INTRODUCTION
Bromine is one of the substances which react with graphite to form lamellar compounds. The composition of the saturated graphite bromide is C8Br, and its structure was clarified by Riidorff [l] to be the second stage structure, where bromine molecules are intercalated into every second layer spacing of graphite, these carbon layer spacings being expanded from 3.35 A to 7.05 A. Eeles and Turnbull [2] more recently investigated the crystal structure of the saturated compound, and reported a crystal system of rhombohedral type with unit cell sides of a,, = 59.5 A, b,, = 34.5 A and co = 10.3 A, but the configuration of atoms within a bromine layer was not fully elucidated. The isotherms of graphite-bromine system have been studied by several investigators [3-51. Maire and Mering[5] concluded from their vapor pressure data and X-ray studies that above the composition of CIGBr, the second stage structure is established, and the bromine density of C&Br within a layer is half that of C,Br. Below the composition of C,,Br, bromine layers of lower density are
intercalated randomly among graphite layer spacings, and from C,,Br to C8Br, bromine layers gradually become more dense. Eeles and Turnbull[2] found a new phase CzsBr after decomposition of C8Br, and investigated its crystal structure in detail. Bromine molecules were found to be inserted into every fourth carbon layer spacing of graphite, and forming highly ordered chain structures in their own layers. They also reported formation of a Ci,Br phase during the course of bromination of graphite, in which bromine layers are intercalated into every second layer spacing of graphite. It was concluded that the structure of bromine layers takes two forms, one which occurs at concentrations in excess of 47 per cent of the saturation composition (C,,Br), the other at lower concentrations. Bach and Herold et al. [ 161 investigated the change in the composition and in the structure of graphite bromide with temperature increase, and reported mediate phases C,,Br the ordered structure fourth The
stages. mechanism
the existence of interand &Br, which take of the third and of the
of bromine
absorption
of
408
T. SASA,
Y. TAKAHASHI
graphite has been investigated by several workers [7-101. Saunders et al. [8] measured the dimensional changes of pyrolytic graphite with bromine absorption, and found a metastable structure which showed practically full expansion along the c-direction at only about one-third of the limiting uptake. Hooley et aZ.[9] also investigated the rate of absorption and the dimensional changes, and proposed a mechanism in which all layers are open to intercalation at a relatively initial stage of reaction. The present authors [lo] measured the changes in the electrical conductivity and in the X-ray diffraction pattern of graphite during the course of bromination, and reached similar conclusions. Graphite bromide goes through an almost continuous range of compositions up to the saturation compound CsBr. The information about the structure of these compounds is however confusing as has been shown above. In the present study, crystal structure of graphite-bromine lamellar compounds with different bromine concentrations is investigated, and the existence of some definite phases is confirmed. 2. EXPERIMENTAL 2.J Materials
The following three types of graphite were used in this work: (1) Natural graphite powder. (2) Pyrolytic graphite, 2 mm in thickness, deposited at 1800°C and heat-treated to 3000°C co = 6.712 & 0.002 A. Pyrolytic graphite was used as chipped flakes in most cases. (3) Kish Graphite flakes, a few mm across. GR grade bromine was distilled and passed through concentrated sulfuric acid before use. 2.2 Apparatus and procedures The apparatus used for the determination of the isotherms for graphite-bromine system has been described in another paper[lO]. A pyrolytic graphite block was chipped into flakes to quicken the attainment of the equilibration of reaction, the flakes were put into a
and T. MUKAIBO
quartz container and suspended by a quartz spring balance (sensitivity 20 mm/g). The weight of absorbed bromine was measured by means of a running microscope. The reaction tube was immersed in a water bath, the temperature of which was kept at 2O.O”Cby an electronic controller. The liquid bromine container was kept at a desired temperature by means of an alcohol bath (from -30°C to +2O”C) to control the vapor pressure of bromine. The vapor pressure was checked using a glass-spoon gauge. For the measurements of the macroscopic thickthe unchipped pyrolytic ness increase, graphite was used, making samples of definite suitable size. For the X-ray diffraction analysis using a Debye-Scherrer camera (11 a46 cm in diameter), natural graphite powder was loaded into a Lindemann glass tube of 0.5 mm in diameter, and allowed to react with bromine vapor of desired pressures. After the equilibrium was attained, the tube was sealed off. For the X-ray diffractometry by means of a G.M. counter with an automatic recorder, a specially designed sample holder system was utilised, which was described in another paper [ll]. It is shown in Fig. 1. The glass sample holder has a drilled hole of 1 mm diameter (Fig. 1) behind the graphite sample, and the bromine container is connected to the back surface of the glass plate. 0.2 mm thick Teflon sheet is placed over the front surface of the holder with “Daifloil’‘-wax (polymonochlorotrifluoroethylene), and the whole apparatus was made gas-tight. Pyrolytic graphite flakes or Kish graphite flakes were loaded into the holder, the whole system was evacuated, the bromine container being held at a desired temperature. The sample holder can be placed in the goniometer as it stands. 3. RESULTS 3.1 Isotherm of bromine on graphite
The isotherm of bromine on pyrolytic graphite at 2O.O”Cis shown in Fig. 2. For each datum point the attainment of sufficient
GRAPHITE
LAMELLAR
-to9
COMPOUNDS
[5] but actually several observed on the curve. spond approximately to C*Br, C,,Br, C&Br and of reducing the bromine steps are not sufficiently part of the curve. 3.2 Dimensional absorfition
Fig. 1. Arrangements of the sample holder 0 glass sample holder; 0 a hole of I mm diameter; 0 graphite sample; 0 “Teflon” sheet (0.2 mm thickness): 0 ground-glass contact; 6 Araldite sealing; 0 bromine; 8 water bath for bromine pressure control; 0 to bejointed to the vacuum system.
indistinct steps are These steps correthe compositions of C,,Br. In the course vapor pressure, such distinct on the lower
change of graphite with bromine
The relative increase in thickness along the c-axis of pyrolytic graphite due to bromine absorption is shown in Fig. 3. The dimensional change is nearly proportional to the bromine concentration, and is consistent with the result of Saunders et (~1.[$I. This observation suggests that the graphite layer spacings are either quite empty or are filled with bromine molecules, and the number of graphite layer spacings occupied fully by bromine increases with the increase in bromine concentration.
12 b x 10
The observed diffraction pattern of CsBr was given in Table 1, following the rhombohedral crystal system reported by Eeles and Turnbull[2]. The sides of the rhombohedral unit cell are taken as a,, = 59.5 A, b,, = 34.5 ,A and c0 = 20.6 A. The repeat distance along the c-axis must be taken at least twice as large
v
&a ;6 a
’
e ,-
4
s!
2
E
m
I
0
i
0
01 -A
’
I
I
I
I
0.2
0.4
0.6
0.8
1.0
Partial
pressure
of bromine
-l
a
I
P/P,
Fig, 2. Isotherm of bromine on pyrolytic graphite (‘WC) 0 on bromination; 0 on debromination. equilibrium conditions was necessary and took about 24 hr. Open circles represent the equilibrium states of graphite-bromine system with increasing bromine vapor pressure, and solid circles the state5 with decreasing bromine vapor pressure (Fig. 2). The isotherm curve is not composed of two stages as was reported by Maire and Mering
0
2 Bromine
4
6 uptake
8 (BrlC
Fig. 3. Thickness increase of pyrolytic sample by bromine absorption.
10
12
X IO21 graphite
410
T. SASA, Y. TAKAHASHI
and T. MUKAIBO
Table 1. X-ray diffraction pattern of graphite bromide CsBr. Rhombohedral system of a, = 59.5 A, b = 34.5 A and c,, = 20.6 A.
2@C,l, Intensity (deg)
(obs.)
20.8 21.7 26.0 31.2 33.0
20.6 21.5 25.9 31.1 33.4
m
m v. St m
42.6
42.6
W
44.6 53.5 59.4 77.8
44.4 53.3 59.2 77.6 84.2 83.3 99.9 148.0
st
(W 0 8 0 14 2 0 0 0 6 012 0 14 10 0 28 0 0 14 14 0 14 14 3 0 012 14 14 9 028 0 0 018 028 6 14 14 18 14 42 3
*Diffraction of CsBr.
83.6 101.1 148.7
line of natural
as that of Riidorff or Eeles et al. This may be understood when the displacements of carbon layers parallel to the layer planes are considered. Riidorff [l] reported that (001) lines were observed clearly, but (Ml) or (MO) lines were completely missing in the diffraction pattern of C8Br. However, the diffraction pattern of C,Br observed by us (Table 1) is quite different from that of Riidorff, in particular lines (Ml) and (MO) are actually observed. But the extra (001) lines characteristic of the second stage structure, observed by Riidorff, are not observed in our X-ray photograph. These lines are indispensable to the investigation of the distribution of bromine layers in the direction of the c-axis. 3.4 X-ray studies of bromide of well-oriented graphite with an automatic recording X-ray dajkxtometry When well-oriented graphite samples are used for the X-ray analysis, the diffraction pattern shows only (001) lines, which are very
Related line*
W
St
m-w m m W v. v. w
graphite
101 004 103 110 006 112 106 211
related with that
strong. For the purpose of detecting the (001) super-structure lines of graphite bromide such as were observed by Riidorff, Kish graphite was taken. Pyrolytic graphite flakes, which were used in the isotherm measurements, did not give sufficient intense patterns of diffraction after the reaction with bromine. The structural analyses were carried out with graphite bromides formed under various vapor pressures of bromine. In the isotherm study, the compositions of C,,Br with n = 2-5 only were found to be of importance. Certain ordered structures were observed at these compositions in the X-ray studies. The results of the structural analyses are shown in Tables 2-5. The observed structure factors are estimated from the intensities of the diffraction lines, with the corrections of the Lorentzpolarization factor, the absorption factor and the absorption by the Teflon sheet. Observed and calculated structure factors are compared on a relative scale. In addition to the confirmation of the second stage structure of C,Br, the structures of
GRAPHITE Table 2.2nd
LAMELLAR
stage structure
411
COMPOUNDS
of graphite
bromide
CsBr.
I
2&b, (degf
2&l,, (de@
Ifii~,loix
lWclca,c
00 1 00 2 00 3 00 4 00 5 00 6 00 7 00 8 00 9 0 0 10 00 11 0 0 12
9.0 17.0 25.7 34.5 43.6 52.8 62.5 72.8 83.7 95.7 109.3 125.7
8.5 17.1 25.7 34.5 436 52.8 62.5 728 83.7 95.7 109.3 125.7
23 23 100 19 3 1 77 14 23 54 6 13 35
17 2 1 100 15 27 68 14 25 5 1 1 1 22 42
00
I, = 10.39 A, Cu - Ka Table 3.3rd 00
1
001 00 2 00 3 00 4 00 5 00 6 00 7 00 8 00 9 0 0 10 00 11 0 0 12 0013 0 0 14 0 0 15 0 0 16 0 0 17
stage structure
of graphite
~~~
2&,,, idegf
12.9 19.2 25.8 32-7 39.4 46.1 53.3 60.8 68.3 76.2 84.5 103.7 114.6 127.5 145.5
6.4 12.9 19.4 25.9 32.7 39.4 46.2 53.3 60.7 68.3 76.4 84.6 93.7 103.5 114.6 127.5 145.1
bromide
ll;lFol0bs 13 23 100 10 18 ::: 8 1 :3 19 47 8 1 1 30 3
&Br.
lFiFotca,e 1 1 14 20 100 8 I 7 23 65 8 17 22 48 5 I 5 20 39 2
f, = 13.74 A, Cu - Ka C&Br, C,,Br and CzoBr were explained as the 3rd stage, the 4th stage and the 5th stage structures, respectively, where the bromine layers are intercalated into every 3rd, 4th and 5th layer spacing of graphite. This agrees with the results of Bach et al. [6]. As it is difficult to know exactly the stacking sequence of the carbon layers, the repeat distance along the c-axis direction, I, is taken for convenience
as that from the position of one bromine layer to the nearest neighbor bromine layer. Thus the indexings of (001) are not consistent with those in Table 1. For the structure of the nth stage, one bromine layer and then it carbon layers are taken repeatedly as shown in Fig. 4. Then I,/ (r$+ 1) has a similar value as the layer spacing of graphite. Thus the (OOn+ 1) lines and the
T. SASA, Y. TAKAHASHI
412
Table4.4th
00
stage structure
1
001 00 2 00 3 00 4 00 5 00 6 00 7 00 8 00 9 0 0 10 0 0 11 0 0 12 0 0 13 0 0 14 0 0 15 0016 0 0 17 0 0 18 0 0 19 0 0 20 0 0 21 I,=
of graphite
2;
2&i,, (deg)
10.6 15.6 20.6 26.1 31.6 37.0 42.3 47.8 53% 59.6 65.8 71.9 78.2 85.1 100.5 108.6 117.9 128.9 143.0
5.2 10.3 15.6 20.8 26.1 31.4 36.8 42.3 48.0 53.6 59.6 65.6 71.9 78.4 85.2 92.5 100.3 108.7 118.1 128.9 142.6
of them come out to be strong, and n super-structure lines are observed between them. The spacing of carbon layers that contain a bromine layer between them is shown in Table 6 for each stage. The observed layer spacings are a little smaller than the value 7.05 A, reported by Riidorff.
t
-----
t
-
-----
4
t
?P
0’
jf-
4_____ (b)
C,zBr
&
+
carbon
---
bromine
layer
-
$ ,-
::I,
_____Y-
-I(cl
-
-----
.d.
c-
--
lf
C,Br
t
-
g-
0, _____
(a)
-----
*a
-
x-
bromide
kWulot,s 15 16 28 100 13 17 20 20 73 4 8 20 29 64 10 1 1 18 45 7
CIGBr.
IF/F&a,c 10 10 13 18 100 4 12 16 2 1 64 4 12 16 20 46 2 10 14 18 37 2
17.08A,Cu-Ka
multiples
04
and T. MUKAIBO
_---C,,Br
(d)
C20Br
layer
Fig. 4. Structures of graphite bromine lamellar compounds (a) 2nd stage structure of CsBr; (b) 3rd stage structure of &Br; (c) 4th stage structure of CIGBr; (d) 5th stage structure of C,,Br.
3.5 The changes in the structure of graphite bromide with the change of bromine concentration Figure 5 shows the change of the profile of (008) diffraction line of graphite with increasing bromine content. When the bromine vapor pressure exceeds the threshold value for the intercalation, the reaction takes place. The concentration of absorbed bromine increases fast with the increase in bromine vapor pressure up to C,,)Br. In these ranges, the intercalated bromine molecules are not distributed homogeneously throughout the graphite crystal as dilute solid solution. The diffraction pattern of such samples shows the coexistence of regions of the virgin graphite and of regions invaded by considerable amounts of bromine. The regions invaded by bromine attain the structure of the 5th stage. At about the bromine vapor pressure of P/P, = 0.15, the structure becomes a homogeneous 5th stage structure of &Br. With further increase of bromine vapor
CARBON Table5.5th
00
1
00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
00 00 00 00 00 00 00 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
GRAPHITE stage
LAMELLAR
structure
of graphite 2%,,,. (de@
2O”bS (deg)
17.5 21.8 26.1 30.2 35.1 39.8 44.7 49.2 53.8 63.5 69.0 74.4 80.1 85.7 91.5 97.7 104.2 712.7 120.8 129.9 141.0
bromide
IW,,ioba
4.3 8.7 13.0 17.4 21.8 26.2 30.7 35.2 39.8 44.4 49.1 53.9 58.8 63.X 69.0 74.4 79.9 85.7 91.7 98.1 105%) 1124 120.6 129.9 141.6
-
COMPOUNDS C,,Br.
Wolearc 9
1 0
.5 14
100 3 9 6 14 29 63 9 9 14 22 54 7 7 7 9 14 37 9
1 1 14 2 1 10 0 3 10 14 17 23 64 2 1 1 14 1 7 22 53 1 9 13 16 20 42 5
I, = 20.41 A, Cu - Kcz Table 6. Expansion of carbon layer spacings with the intercalation of bromine Composition
Stage
CsBr
2
&Br &Br CzoBr
3 4 5
pressure, the structure of the graphite bromide follows the stages of higher bromine concentrations one by one, up to the 2nd stage structure of saturation, At intermediate bromine concentrations between the definite compositions corresponding to definite stages, the X-ray diffraction pattern shows a mixture of extra lines of both the higher and the lower stage structures. The strong
Interlayer
spacing(A) 7.03 7 *03 7.01 6.99
main lines, however, are not observed as distinct double peaks, but rather as broad single peaks. At higher bromine concentrations than &,Br, the main lines apparently shift continuously to the lower diffraction angles up to the position of CBBr, as if only a single phase was present in the course of the gradual change of the lattice constant. In the progressive debromination process
T. SASA, Y. TAKAHASHI
414
and T. MUKAIBO
of C&Br, the Srd, 4th and 5th stages are also observed in that order, however the equiiibrium vapor pressures are much Iower than those for the bromination process. After the vapor pressure of bromine is reduced to zero, a residue compound remains. The bromine residue compound obtained from Kish graphite was found to have a composition similar to C&Br, as was aIso reported by Eeles and TurnbufifZ]. But the structure of this compound is not ordered but rather strongly disordered, and the apparent lattice constant is found to be smaller than that of the 5th stage structure, 3.6 ~~t~~~t~ structures of graphite during the course of rapid bromination
2
_ k
*
f
I
2nd stage I
II
125 Diftraction
I‘
*
*
1
130
t
I.
135
angle
28’
Fig. 5. The change of profile with the increase of bromine concentration.
The changes in the layer structure of graphite during the course of reaction with the saturated vapor of bromine were investigated for Kish graphite. Figure 6 shows the changes of the diffraction pattern in the range of 28 from 50” to 90’. In the initial stages of the reaction, appearance of some new phase is observed whife the
graphite (006)
before
the
L bromination
I\
2nd stage
Diffraction
corner
I
angle
23’
Fig. 6. The change of profile during the course of rapid bromination.
GRAPHITE
LAMELLAR
virgin graphite remains unchanged. As the reaction proceeds, the lines of the new phase become sharper and extra lines characteristic of the 2nd stage appear. No other phase such as 3rd and 4th stage was seen during the reaction. Moreover, already about 100 min after the start of the reaction, the 2nd stage structure appeared, that is when the bromine content of the sample was still much lower than that corresponding to C8Br, possibly lower than half of that. Therefore the density of the bromine molecules in a layer of the second stage in this intermediate state seems to be less than half of that of C8Br. This phase corresponds to the metastable structure observed by Saunders et al. [S]. The phase C,,Br, which was observed by Eeles and Turnbull[2] during the course of the bromination is believed to be possibly also the same metastable phase. 4. DISCUSSION
4.1 Equilibrium lar compounds
phases of graphite-bromine
lamel-
It is not clearly known to what extent solid solution exists in a single phase, or to what extent definite stoichiometric phases are formed in graphite bromide. In the present studies, several intermediate phases of graphite bromide, which have characteristic compositions and structures, were found to exist under certain equilibrium vapor pressure of bromine. They were identified to have C,,Br composition and nth stage structure. The observed phase with a lowest bromine concentration was the 5th stage, and the phase of the primary solid solution of bromine with graphite could not be observed above the threshold vapor pressure of bromine. It is not clear whether stages of graphite bromide higher than the 5th stage exist or not. The situations may be different for different graphite samples, because the threshold vapor pressure for the intercalation is known to depend entirely on the type of the graphite sample used.
car
Vol. 9. No. 4-D
COMPOUNDS
415
Above the composition of &,Br, only four definite phases are found, namely, from the 5th to the 2nd stage. As the bromine concentration increases, a change of the structure of graphite bromide from one stage of lower concentration to another stage of higher concentration takes place. This requires some sort of rearrangements of intercalated bromine layers in graphite crystal. Nixon et al. [12] studied the progressive formation of graphite nitrate. They observed the successive appearance of structures from the 4th stage to the 1st stage. As the reaction proceeded, filling and emptying of certain intercalate layer positions were observed. Salzano and Aronson [13] measured the rates of bromine exchange between the vapor phase and the intercalation compound. The exchange is rapid and about 90 per cent of intercalated bromine was observed to exchange in an hour or so for small particles of graphite. It seems therefore that stripping and filling of various carbon layer spacings with bromine takes place in the course of the transformation. Daumas and Herold [14] proposed however a new mechanism for the successive appearance of higher stages during the decomposition of potassium graphite, C,K. According to their model, intercalated atoms are depleted from all layers. In each carbon layer spacing, certain zones continue to be occupied by the alkali metal layer, but the other zones are emptied entirely and the layer spacing shrinks to the normal spacing of graphite. Moreover, due to the repulsions between the intercalated alkali metal ions, the intercalated layers change their position in the carbon layer spacing so as to move away from the other intercalated ions in the neighboring layer spacings. This results in local ordered sequences of the intercalated metal atom layers along the c-axis direction, which are characteristic for the stages. Thus filling and emptying of whole layers is not needed for transformation of the structure from one stage to another. At present, the mechanism of the change of the structure of graphite bromide
416
T. SASA, Y. TAKAHASHI
from one stage to another is not yet elucidated satisfactorily. It is also an important problem to what extent the definite stoichiometry is realized at each stage. As seen from the curves Fig. 2, graphite bromide systems are not so distinctly stoichiometric as alkali metal-graphite compounds. At intermediate compositions between stages, the structure of graphite bromide seems to be some kind of mixture of both higher and lower stage structures. The problem of the structure of the intermediate compositions is certainly related to the preceding problem of the mechanism of the structural change from one stage to another. 4.2 Structure of bromine luyers The X-ray data shown in Table 1 for CsBr resemble that of graphite. This is because the bromine layer has a similar thickness as the carbon layer, and also the graphite bromine lamellar compound has a similar layer structure as graphite. Thus most of the diffraction lines in Table 1 are related closely to those of graphite. There are only four new lines that are observed and confirmed to be characteristic of graphite bromide. According to Eeles et al. [2], these (h&l) lines are attributed to the configuration of bromine molecules in bromine layers. It was found in this work that, although the content of graphite bromine bromide changed from about Br/C = 0.03 to the saturation composition, the diffraction patterns observed were quite the same as shown in Table 1, although slight shifts of the positions of the diffraction lines were noted. The four (hk0) lines, in particular, remain unaltered with change in bromine concentration. It is believed therefore that the density and the configuration of intercalated bromine molecules in a layer are quite the same for all the stable graphite bromides from the 5th to the 2nd stage. The atomic density of the bromine layer relative to the density of carbon atoms in an adjacent carbon layer is equal to
and T. MUKAIBO
Br/C = 3. This type of structure of bromine layer, which Eeles and Turnbull[2] reported for C8Br, may be the only form of bromine layer that appears in the normal graphite bromine lamellar compounds in equilibrium with vapor of bromine. Eeles et al. also reported another type of structure of bromine layer with lower bromine density, which has the atomic ratio of Br/C = l/7 relative to the density of an adjacent carbon layer. They found this type of structure in phases CzsBr and &Br. The configuration of bromine atoms in the layer was investigated in detail. Such structure is, however, believed to be a metastable one, appearing only in the course of bromination or debromination, or in nonequilibrium conditions.
REFERENCES 1. Riidorff W., Z. Anorg. Allg. Chem. 245, 383 (1941). 2. Eeles W. T. and Turnbull J. A., Proc. Roy. Sot. A 283,179 (1965). 3. Rayerson L. H., Wertz J. E., Weltner W., Jr. and Whitehurst H., J. Phys. Chem. 61, 1334 (1957). 4. Hooley J. G., Can.]. Chem. 37,899 (1959); Ibid. 4Q, 745 (1962). 5. Maire Jl and Mering J., Proc. 3rd Co@ Carbon 337 (1959). 6. Bach B., Bagouin M., Bloc F. and Herold A., C. R. Acud. Sci. Paris 257,681(1963). 7. Mukaibo T. and Takahashi Y., Bull. Chem. SOL. Japan 36,625 (1963). 8. Saunders G. A., Ubbelohde A. R. and Young D. A., Proc. Roy. Sot. A 271,499 (1963). 9. Hooley J. G. and Smee J. L., Carbon 2, 135 (1964). Hooley J. G., Garby W. P. and Valentin J., Carbon 3,7 (1965). 10. Sasa T., Takahashi Y. and Mukaibo T., Bull. Chem. Sot. Japan 43,34 (1970). 11. Takahashi Y., Sasa T. and Mukaibo T., Tanso (Carbon), 1969 (52), 199. 12. Nixon D. E., Parry G. S. and Ubbelohde A. R., Proc. Roy. Sot. A 291,324 (1965). 13. Salzano F. J. and Aronson S., J. Inorg. Nucl. Chem. 28,1343 (1966). 14. Daumas N. and Herold A., C. R. Acad. Sci. Paris, Ser. C, 268 (5), 373 (1969).