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Scanning transmission electron microscopy of multiphases in graphite-alkali metal intercalation compounds
Carbon Voi 20. No 4. pp 297-301, 1982 Printed in Great Britain.
SCANNING TRANSMISSION ELECTRON MICROSCOPY OF MULTIPHASES IN GRAPHITE-ALKALI METAL INTERCALATION COMPOUNDS H. MAZUREK,~$M. S. DREssELHAUstff and C. DRESSELHAUSq Massachusetts Institute of Technology, C~bridge~ MA 02139,U.S.A. (Received 30 October 1981) Abstract-Structural and micro-analytical evidence is presented for the presence of multiphase regions in graphiteRb intercalation compounds for stages n 2 2. The intercalate layers are composedof islands of alkali metal, ordered incommensuratelywith respect to the adjacent graphite layers and embedded in a background of disordered rubidium in the intercalate layer. The results confirm the non-integral stoichiometry of graphite alkali metal intercalation compounds for stages n 2 2. 1. INTRODUCTION
The transmission electron microscope (TEM) is an especially versatile instrument which can be used to investigate both the real space (imaging) and reciprocal space (diffraction) structure of graphite intercalation compounds. The electron diffraction studies done with the TEM are in fact complementary to X-ray[l-31 and neutron[rl, 51 diffraction studies. Recent high resolution bright field images (with a resolution of 3 8, or better), formed by the transmitted electron beam, have shown evidence of possible Daumas-H~roId domains~6] in FeCl, intercalated graphite[7]. TEM has also been widely used to study the in-plane structure of graphite alkali metal intercalation compounds [8-l 11. In these studies the real space bright field micrographs give direct evidence for ordered alkali metal islands in the intercalate layers for compounds with stages n 2 2. Because of this island structure, bulk X-ray and neutron diffraction measurements yield diffraction vectors and structure factors which can only be interpreted in terms of averaged in-plane densities and in-plane intercalant order[12-141. Thus the in-plane intercalate density, as determined by analysis of integrated intensity X-ray profiIes~l5, 161,does not agree with the accepted integral stoichiometry, such as LX for alkali metal donor compounds with stage> 2. These X-ray measurements of the average in-plane intercalate density[l5, 161 are consistent with the results of the TEM studies which show that the in-plane island structure consists of regions with different in-plane densities[9-I I]. Such multiphase structure is expected to give rise to non-integral average stoichiometries. The bright field TEM micrographs, obtained for the intercalant in the alkali metal donor compounds for stage L 2,
VZenter for Materials Science and Engineering. SPresent address, ARC0 Chemical Comoanv. Research and Engineering Center, Newtown Square, PA 1607j, U.S.A. PDepartment of Electrical Engineering and Computer Science. TFrancis Bitter National Magnet Laboratory, supported by NSF.
show dark alkali-rich “island” regions on a lighter background[lll]. Stereo micrographs clearly show that the islands are not only associated with the surface[14] but are distributed throughout the bulk. Although the electron diffraction superlattice patterns obtained by TEM yield valuable information on the structure of the multiphase system, the conventional TEM lacks the capability of giving quantitative information on the distribution of intercalant species throughout the graphite host. In addition to conventional di~raction and bright and dark field (vi& injra) real space imaging, the scanning transmission electron microscope (STEM} can provide in situ quantitative, micro-analytical X-ray fluorescence data, characteristic of the intercalant, from areas as small as 15 x IS A’. Similarly, electron energy loss spectroscopy may be performed in situ, providing complementary quantitative elemental analysis. The scanned beam minimizes radiation damage; such radiation damage is normally a problem with the use of transmission electron microscopy for the study of graphite intercalation compounds at ambient temperatures. At high beam currents, damage can be detected by the time variation of the image. The STEM enables one to carry out simuItaneous microstructural and quantitative chemical study of graphite intercalation compounds. The systematic structural and micro-analytical study of various stages of graphite-rubidium intercalation compounds described in this paper was undertaken to study the stage dependence of the island formation and the intercalate concentration in the islands. 2. EXPERIMENTAL PROCEDURE
The samples were prepared by intercalating Rb into highly oriented pyrolytic graphite (HOPG) using the conventional two-zone vapor transport technique~l71. After intercalation, the compounds were characterized for stage index and stage fidelity by their (001) X-ray diffractograms. The large bulk samples were transferred to an argon-filled glove box, where thin specimens were prepared and transferred to the sample holder of the
H. MAZUREKet a/
298
STEM[18]. An anaerobic “air lock” technique was used to transfer the sample holder to the STEM[18]. No resulting surface contamination could be detected by energy loss spectroscopy. The experiment was performed on a VG-HBS STEM which could be used for in situ X-ray fluorescence (energy dispersive) and electron energy loss spectroscopy, bright and dark field imaging, and electron diffraction measurements. Since the samples used were sensitive to electron irradiation at room temperature, the exposure time and radiation damage were minimized using the computer enhancement of the STEM image. During the fluorescence experiment, the area under investigation (- 1.5A dia.) was exposed for only 30 set to minimize radiation damage. During the course of the experiment, no evidence of oxygen contamination was detected on the basis of electron energy loss measurements. The samples did not desorb in the high vacuum required by the STEM, since rubidium desorption predominantly occurs in the presence of oxygen. Measurements were made on samples with stage indices 2, 4 and 8. 3. RESULTSANDDISCUSSION
Typical micrographs taken during a STEM experiment are shown in Fig. 1. Firstly, a bright field micrograph is taken of the sample. Figure l(a) shows a bright field micrograph taken on a stage 2 Rb sample, and exhibits the typical dark island domains seen in transmission electron microscopy of graphite intercalation compounds[8-111.
The islands in this figure range in size from 50 to 4000 8, dia. The selected area diffraction pattern shown in Fig. l(c) was taken for the same area of the sample, as is illustrated in Fig. l(a). The diffraction pattern shows both the hexagonal symmetry of the graphite and a superlattice diffraction pattern identified with the intercalate and consisting of two diffuse rings with spots superimposed upon them. These room temperature rings are indexed with wave vectors of 1.6820.05 and 2.10~0.05 A-‘. These values are in agreement with the wave vectors 1.79 and 2.13 A-’ measured with the TEM technique for stage 2 graphite-Rb at 170K [ 181.These values are also in good agreement with the values of 1.78 and 2.18&’ obtained with neutron diffraction at T < 165K[4]. To identify the region of the sample associated with the superlattice pattern, a dark field micrograph was taken, In this case, the dark field micrograph was generated by placing the objective aperture on the diffuse rings, and the results are shown in Fig. l(b). This dark field image was obtained in transmission and was formed by all diffracted beams with wave vectors between 4 = 1.68 and 2.1OA-‘. The dark field image was independent of where on the ring the aperture was placed, or whether the aperture coincided with a spot or not. This was established by direct experiment. The light regions in Fig. l(b) give rise to the superlattice diffraction ring. The light regions of the dark field micrograph are in close correspondence with the dark regions in the bright field micrograph, clearly indicating that only the island domains give rise to the superlattice reflections. Because
(a) Fig. l.(a) Bright field micrograph of a stage 2 graphite-lb sample. (b) Dark field micrograph generated by placing the aperture on the superlattice rings (see text). Note the correspondence between the dark island domains seen in the bright field image with the light island domains seen in the dark field image. (c) Selected area diffraction pattern from the area illustrated in (a) and (b).
Scanningtransmissionelectron microscopyof multiphases
of instrumental limitations, it was not possible to generate dark field micrographs from each ring separately. The micrograph obtained by placing the objective aperture on the graphitic spots indicated that the entire sample, as expected, contributed to the graphite spots, from which we conclude that the graphite Iayers retain
299
their in-plane order upon intercalation. No reflections other than the superlattice reflections seen in Fig. l(c) and higher order graphite reflections were found in the di~raction pattern. Similar diffraction results were obtained at room temperature for the stages 4 and 8 graphite-Rb samples.
Fig. 3. A ma~ified view of the island domains from a stage 8 graphite-Rb sample. Note the inhomogeneous intercalant density within the island domains. X-Ray fluorescence data indicate that the darker outer region contains a higher concentration of Rb than the lighter interior. Fig. 2. Bright field micro~aph of a (a) stage 4 and (b) stage 8 graphite-Br intercalation compound.
To study the compositions variation in the intercalate layer, bright field micrographs were obtained for all the samples (Fig. 2), and X-ray fluorescence measurements of selected areas on each micrograph were taken. While both the dark island domains and the lighter shaded background contain intercalant material, the islands, which correspond to an ordered phase, contain a higher concentration of Rb than the background. This, coupled with the lack of superlattice reflections associated with the background as seen in the dark field micrographs, leads to the conclusion that the Rb in the background areas of this micrograph is disordered. (Our inability to accurately determine the sample thicknesses precludes any determination of absolute intercalate concentration in the islands.) The difference in the Rb concentration in the island domains relative to the background is found to be stage dependent, with a difference of 1022% for stage 2, 25 2 2% for stage 4 and 55 rf:2% for stage 8. Also the fraction of sample area consisting of island domains appears to be stage dependent with values of 20, 10 and 5% for stages 2, 4 and 8, respectively. These fractional sampies areas are consistent with the intercalate concentration being inversely proportional to the stage. A magnified view of the bright field micrograph from the stage 8 sample (Fig. 3) ctearly indicates that the Rb intercalant is not uniformly distributed within each of the island domains. In light of the limitations imposed on the dark field experiment (the objective aperture was too large to generate dark field images from each ring
separately), the X-ray fluorescence probe was used to determine the Rb concentrations in the lighter interior portion and the darker “atoll” ring of an island. Both were found to have a higher concentration of Rb than the background, the darker “atoll” region showing 25% higher Rb concentration than the lighter interior regions. The electron diffraction wave vectors, 1.68 and 2.10 A-‘, may be assigned to an expanded t/3 x L/3 and a contracted t’7 x t/7 structure, respectively. Berker et ~1.19, 11, 181 have postulated that the nearest Rb-Rb neighbor distances for the expanded -\/3 x d3 and contracted ~‘7 x d/7 phases are 4.62 and 5.52 A, respectively. The experimentally determined difference in intercalate concentration between the “atoll” and interior region, 25%, is consistent with the theoretical concentration difference, 30%, based on these two superlattice structures. Thus, the darker “atoll” region may perhaps be associated with an incommensurate, expanded 2/3 x t/3 structure, and the lighter interior region with an incommensurate contracted t/7 x 2/7 struct~e, connected by a discommensurationIi91 region. In this paper we have presented both structural and micro-analytical evidence for the existence of multiphase regions in graphite donor intercalation compounds. At room temperature, these multiphase regions are observed as island domains with a short range, locally ordered arrangement of alkali metal ions, incommensurate with respect to the adjacent graphite layers. These multiphase ordered regions are randomly distributed in a background of disordered alkali metal. These observations of the intercalate structure and distribution provide microscopic evidence in support of previous X-ray
Scanning transmission electron microscopy of multiphases
observationsfl51 that no integral stoichiomet~c relation (such as C,,,X) is found for alkali donor compounds with stage 3 2. Acknowledgements-We
gratefully acknowledge Dr. A. W. Moore of Union Carbide for donating the HOPG, Dr. A. GarrattReed for technical assistance, Prof. A. N. Berker, and Dr. N. Kambe for valuable discussions, and support from AFOSR Contract No. F4%2@-81-C-0006.
REFERENCES I.
2. 3. 4. 5, 6.
J. B. Hastings, W. D. Ellenson and J. E. Fischer, Phys, Reu. Wt. 42, 1552(1979). H. Zabel, Y. M. Jan and S. C. Moss, Physica 99B, 453 (1980). R. Clarke, N. Caswell, S. A. Solin and P. M. Horn, Phys. Rev. Lelt. 43, 2018(1979). H. Suematsu, M. Suzuki, H. Ikeda and Y. Endoh, Syn. Net. 2, 133(1980). W. D. Ellenson, D. Semmingsen, D. Guerard, D. G. Onn and J. E. Fischer, h&fer. Sci Engttg31, $37(1977). N. Daumas and A, H&old, C. R. Acad. Sci. CX8,373 (1969).
301
7. J. M. Thomas, C. R. Millward, R. F. Schfogl and H. P. Boehm, Muf. Res. Bull. 15, 671 (1980). 8. N. Kambe, C. Dresselhaus and M. S. Dresselhaus, Phys. Rev. B21,3491 (1980). 9. M. S. Dresselhaus, N. Kambe, A. N. Berker and G. Dresselhaus, Synrh. Metals 2, 121(1980). 10. N. Kambe. H. Mazurek. M. S. Dresselhaus and G. Dressethaus, Physics IflsB, 27i (1981). 1I. A. N. Berker, N. Kambe, G. Dresselhaus and M. S, Dresselhaus, Phys. Reu. Letf. 45, 1452(1980). 12. D. E. Nixon and G. S. Parry, J; Phys. C2, 1732(1969). 13. G. S. Parry, Mater. Sci. Engng 31, 99 (1977). 14. H. Mazurek and M. S. Dresselhaus, Unpublished observations. 15. S. Y. Leung, C. Underhill, G. Dresselhaus, T. Krapchev, R. Ogilvie and M. S. Dressefhaus, Solid State Commun. 32, 635 (1979). 16. S. Y. Leung, C. Underhill, G. Dresselhaus, T. Krapchev, R. Ogilvie and-M. S. Dresselhaus, Phys. Left. 76A, 89(1980). 17. D. E. Nixon and G. S. Parrv. J. Phvs. Dl. 291 (1968).
18. N. Kambe, Ph.D. Thesis, MIT, (1981)(unpublished): 19. Joining two ordered regions, each incommensurate with the adjacent graphite layers but distinct from each other, is a discommensurat~on region.
SCANNING TRANSMISSION ELECTRON MICROSCOPY OF MULTIPHASES IN GRAPHITE-ALKALI METAL INTERCALATION COMPOUNDS H. MAZUREK,~$M. S. DREssELHAUstff and C. DRESSELHAUSq Massachusetts Institute of Technology, C~bridge~ MA 02139,U.S.A. (Received 30 October 1981) Abstract-Structural and micro-analytical evidence is presented for the presence of multiphase regions in graphiteRb intercalation compounds for stages n 2 2. The intercalate layers are composedof islands of alkali metal, ordered incommensuratelywith respect to the adjacent graphite layers and embedded in a background of disordered rubidium in the intercalate layer. The results confirm the non-integral stoichiometry of graphite alkali metal intercalation compounds for stages n 2 2. 1. INTRODUCTION
The transmission electron microscope (TEM) is an especially versatile instrument which can be used to investigate both the real space (imaging) and reciprocal space (diffraction) structure of graphite intercalation compounds. The electron diffraction studies done with the TEM are in fact complementary to X-ray[l-31 and neutron[rl, 51 diffraction studies. Recent high resolution bright field images (with a resolution of 3 8, or better), formed by the transmitted electron beam, have shown evidence of possible Daumas-H~roId domains~6] in FeCl, intercalated graphite[7]. TEM has also been widely used to study the in-plane structure of graphite alkali metal intercalation compounds [8-l 11. In these studies the real space bright field micrographs give direct evidence for ordered alkali metal islands in the intercalate layers for compounds with stages n 2 2. Because of this island structure, bulk X-ray and neutron diffraction measurements yield diffraction vectors and structure factors which can only be interpreted in terms of averaged in-plane densities and in-plane intercalant order[12-141. Thus the in-plane intercalate density, as determined by analysis of integrated intensity X-ray profiIes~l5, 161,does not agree with the accepted integral stoichiometry, such as LX for alkali metal donor compounds with stage> 2. These X-ray measurements of the average in-plane intercalate density[l5, 161 are consistent with the results of the TEM studies which show that the in-plane island structure consists of regions with different in-plane densities[9-I I]. Such multiphase structure is expected to give rise to non-integral average stoichiometries. The bright field TEM micrographs, obtained for the intercalant in the alkali metal donor compounds for stage L 2,
VZenter for Materials Science and Engineering. SPresent address, ARC0 Chemical Comoanv. Research and Engineering Center, Newtown Square, PA 1607j, U.S.A. PDepartment of Electrical Engineering and Computer Science. TFrancis Bitter National Magnet Laboratory, supported by NSF.
show dark alkali-rich “island” regions on a lighter background[lll]. Stereo micrographs clearly show that the islands are not only associated with the surface[14] but are distributed throughout the bulk. Although the electron diffraction superlattice patterns obtained by TEM yield valuable information on the structure of the multiphase system, the conventional TEM lacks the capability of giving quantitative information on the distribution of intercalant species throughout the graphite host. In addition to conventional di~raction and bright and dark field (vi& injra) real space imaging, the scanning transmission electron microscope (STEM} can provide in situ quantitative, micro-analytical X-ray fluorescence data, characteristic of the intercalant, from areas as small as 15 x IS A’. Similarly, electron energy loss spectroscopy may be performed in situ, providing complementary quantitative elemental analysis. The scanned beam minimizes radiation damage; such radiation damage is normally a problem with the use of transmission electron microscopy for the study of graphite intercalation compounds at ambient temperatures. At high beam currents, damage can be detected by the time variation of the image. The STEM enables one to carry out simuItaneous microstructural and quantitative chemical study of graphite intercalation compounds. The systematic structural and micro-analytical study of various stages of graphite-rubidium intercalation compounds described in this paper was undertaken to study the stage dependence of the island formation and the intercalate concentration in the islands. 2. EXPERIMENTAL PROCEDURE
The samples were prepared by intercalating Rb into highly oriented pyrolytic graphite (HOPG) using the conventional two-zone vapor transport technique~l71. After intercalation, the compounds were characterized for stage index and stage fidelity by their (001) X-ray diffractograms. The large bulk samples were transferred to an argon-filled glove box, where thin specimens were prepared and transferred to the sample holder of the
H. MAZUREKet a/
298
STEM[18]. An anaerobic “air lock” technique was used to transfer the sample holder to the STEM[18]. No resulting surface contamination could be detected by energy loss spectroscopy. The experiment was performed on a VG-HBS STEM which could be used for in situ X-ray fluorescence (energy dispersive) and electron energy loss spectroscopy, bright and dark field imaging, and electron diffraction measurements. Since the samples used were sensitive to electron irradiation at room temperature, the exposure time and radiation damage were minimized using the computer enhancement of the STEM image. During the fluorescence experiment, the area under investigation (- 1.5A dia.) was exposed for only 30 set to minimize radiation damage. During the course of the experiment, no evidence of oxygen contamination was detected on the basis of electron energy loss measurements. The samples did not desorb in the high vacuum required by the STEM, since rubidium desorption predominantly occurs in the presence of oxygen. Measurements were made on samples with stage indices 2, 4 and 8. 3. RESULTSANDDISCUSSION
Typical micrographs taken during a STEM experiment are shown in Fig. 1. Firstly, a bright field micrograph is taken of the sample. Figure l(a) shows a bright field micrograph taken on a stage 2 Rb sample, and exhibits the typical dark island domains seen in transmission electron microscopy of graphite intercalation compounds[8-111.
The islands in this figure range in size from 50 to 4000 8, dia. The selected area diffraction pattern shown in Fig. l(c) was taken for the same area of the sample, as is illustrated in Fig. l(a). The diffraction pattern shows both the hexagonal symmetry of the graphite and a superlattice diffraction pattern identified with the intercalate and consisting of two diffuse rings with spots superimposed upon them. These room temperature rings are indexed with wave vectors of 1.6820.05 and 2.10~0.05 A-‘. These values are in agreement with the wave vectors 1.79 and 2.13 A-’ measured with the TEM technique for stage 2 graphite-Rb at 170K [ 181.These values are also in good agreement with the values of 1.78 and 2.18&’ obtained with neutron diffraction at T < 165K[4]. To identify the region of the sample associated with the superlattice pattern, a dark field micrograph was taken, In this case, the dark field micrograph was generated by placing the objective aperture on the diffuse rings, and the results are shown in Fig. l(b). This dark field image was obtained in transmission and was formed by all diffracted beams with wave vectors between 4 = 1.68 and 2.1OA-‘. The dark field image was independent of where on the ring the aperture was placed, or whether the aperture coincided with a spot or not. This was established by direct experiment. The light regions in Fig. l(b) give rise to the superlattice diffraction ring. The light regions of the dark field micrograph are in close correspondence with the dark regions in the bright field micrograph, clearly indicating that only the island domains give rise to the superlattice reflections. Because
(a) Fig. l.(a) Bright field micrograph of a stage 2 graphite-lb sample. (b) Dark field micrograph generated by placing the aperture on the superlattice rings (see text). Note the correspondence between the dark island domains seen in the bright field image with the light island domains seen in the dark field image. (c) Selected area diffraction pattern from the area illustrated in (a) and (b).
Scanningtransmissionelectron microscopyof multiphases
of instrumental limitations, it was not possible to generate dark field micrographs from each ring separately. The micrograph obtained by placing the objective aperture on the graphitic spots indicated that the entire sample, as expected, contributed to the graphite spots, from which we conclude that the graphite Iayers retain
299
their in-plane order upon intercalation. No reflections other than the superlattice reflections seen in Fig. l(c) and higher order graphite reflections were found in the di~raction pattern. Similar diffraction results were obtained at room temperature for the stages 4 and 8 graphite-Rb samples.
Fig. 3. A ma~ified view of the island domains from a stage 8 graphite-Rb sample. Note the inhomogeneous intercalant density within the island domains. X-Ray fluorescence data indicate that the darker outer region contains a higher concentration of Rb than the lighter interior. Fig. 2. Bright field micro~aph of a (a) stage 4 and (b) stage 8 graphite-Br intercalation compound.
To study the compositions variation in the intercalate layer, bright field micrographs were obtained for all the samples (Fig. 2), and X-ray fluorescence measurements of selected areas on each micrograph were taken. While both the dark island domains and the lighter shaded background contain intercalant material, the islands, which correspond to an ordered phase, contain a higher concentration of Rb than the background. This, coupled with the lack of superlattice reflections associated with the background as seen in the dark field micrographs, leads to the conclusion that the Rb in the background areas of this micrograph is disordered. (Our inability to accurately determine the sample thicknesses precludes any determination of absolute intercalate concentration in the islands.) The difference in the Rb concentration in the island domains relative to the background is found to be stage dependent, with a difference of 1022% for stage 2, 25 2 2% for stage 4 and 55 rf:2% for stage 8. Also the fraction of sample area consisting of island domains appears to be stage dependent with values of 20, 10 and 5% for stages 2, 4 and 8, respectively. These fractional sampies areas are consistent with the intercalate concentration being inversely proportional to the stage. A magnified view of the bright field micrograph from the stage 8 sample (Fig. 3) ctearly indicates that the Rb intercalant is not uniformly distributed within each of the island domains. In light of the limitations imposed on the dark field experiment (the objective aperture was too large to generate dark field images from each ring
separately), the X-ray fluorescence probe was used to determine the Rb concentrations in the lighter interior portion and the darker “atoll” ring of an island. Both were found to have a higher concentration of Rb than the background, the darker “atoll” region showing 25% higher Rb concentration than the lighter interior regions. The electron diffraction wave vectors, 1.68 and 2.10 A-‘, may be assigned to an expanded t/3 x L/3 and a contracted t’7 x t/7 structure, respectively. Berker et ~1.19, 11, 181 have postulated that the nearest Rb-Rb neighbor distances for the expanded -\/3 x d3 and contracted ~‘7 x d/7 phases are 4.62 and 5.52 A, respectively. The experimentally determined difference in intercalate concentration between the “atoll” and interior region, 25%, is consistent with the theoretical concentration difference, 30%, based on these two superlattice structures. Thus, the darker “atoll” region may perhaps be associated with an incommensurate, expanded 2/3 x t/3 structure, and the lighter interior region with an incommensurate contracted t/7 x 2/7 struct~e, connected by a discommensurationIi91 region. In this paper we have presented both structural and micro-analytical evidence for the existence of multiphase regions in graphite donor intercalation compounds. At room temperature, these multiphase regions are observed as island domains with a short range, locally ordered arrangement of alkali metal ions, incommensurate with respect to the adjacent graphite layers. These multiphase ordered regions are randomly distributed in a background of disordered alkali metal. These observations of the intercalate structure and distribution provide microscopic evidence in support of previous X-ray
Scanning transmission electron microscopy of multiphases
observationsfl51 that no integral stoichiomet~c relation (such as C,,,X) is found for alkali donor compounds with stage 3 2. Acknowledgements-We
gratefully acknowledge Dr. A. W. Moore of Union Carbide for donating the HOPG, Dr. A. GarrattReed for technical assistance, Prof. A. N. Berker, and Dr. N. Kambe for valuable discussions, and support from AFOSR Contract No. F4%2@-81-C-0006.
REFERENCES I.
2. 3. 4. 5, 6.
J. B. Hastings, W. D. Ellenson and J. E. Fischer, Phys, Reu. Wt. 42, 1552(1979). H. Zabel, Y. M. Jan and S. C. Moss, Physica 99B, 453 (1980). R. Clarke, N. Caswell, S. A. Solin and P. M. Horn, Phys. Rev. Lelt. 43, 2018(1979). H. Suematsu, M. Suzuki, H. Ikeda and Y. Endoh, Syn. Net. 2, 133(1980). W. D. Ellenson, D. Semmingsen, D. Guerard, D. G. Onn and J. E. Fischer, h&fer. Sci Engttg31, $37(1977). N. Daumas and A, H&old, C. R. Acad. Sci. CX8,373 (1969).
301
7. J. M. Thomas, C. R. Millward, R. F. Schfogl and H. P. Boehm, Muf. Res. Bull. 15, 671 (1980). 8. N. Kambe, C. Dresselhaus and M. S. Dresselhaus, Phys. Rev. B21,3491 (1980). 9. M. S. Dresselhaus, N. Kambe, A. N. Berker and G. Dresselhaus, Synrh. Metals 2, 121(1980). 10. N. Kambe. H. Mazurek. M. S. Dresselhaus and G. Dressethaus, Physics IflsB, 27i (1981). 1I. A. N. Berker, N. Kambe, G. Dresselhaus and M. S, Dresselhaus, Phys. Reu. Letf. 45, 1452(1980). 12. D. E. Nixon and G. S. Parry, J; Phys. C2, 1732(1969). 13. G. S. Parry, Mater. Sci. Engng 31, 99 (1977). 14. H. Mazurek and M. S. Dresselhaus, Unpublished observations. 15. S. Y. Leung, C. Underhill, G. Dresselhaus, T. Krapchev, R. Ogilvie and M. S. Dressefhaus, Solid State Commun. 32, 635 (1979). 16. S. Y. Leung, C. Underhill, G. Dresselhaus, T. Krapchev, R. Ogilvie and-M. S. Dresselhaus, Phys. Left. 76A, 89(1980). 17. D. E. Nixon and G. S. Parrv. J. Phvs. Dl. 291 (1968).
18. N. Kambe, Ph.D. Thesis, MIT, (1981)(unpublished): 19. Joining two ordered regions, each incommensurate with the adjacent graphite layers but distinct from each other, is a discommensurat~on region.