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Multi-phase superlattices and phase transitions in graphite intercalation compounds with Rb, Cs, K, Li
Physica 105B (1981) 272-276 North-Holland Publishing Company
M U L T I - P H A S E S U P E R L A T T I C E S AND PHASE T R A N S I T I O N S IN G R A P H I T E I N T E R C A L A T I O N C O M P O U N D S W I T H Rb, Cs, K, Li N. K A M B E * t , H. M A Z U R E K t , M.S. D R E S S E L H A U S * t and G. D R E S S E L H A U S * Massachusetts Institute of Technology, Cambridge, Massachusetts 021_79, USA
Recent electron diffraction and real space imaging experiments for alkali metal donor compounds are discussed and are shown to yield strong evidence for the coexistence of multiple phases for the in-plane intercalate structure. Island formation indicates the presence of coexisting multiphases. X-ray fluorescence and dark-field imaging measurements using the scanning transmission electron microscope indicate that the dark islands have a higher alkali metal concentration than the lighter background and that the superlattice diffraction pattern is associated with the islands.
1. Introduction In this p a p e r we summarize our recent electron diffraction and real space image results on graphite-alkali metal intercalation compounds [1--4]. These experiments give important information c o m p l e m e n t a r y to the X-ray [5] and neutron diffraction [6] studies reported on these same compounds. Electron microscopy, however, has the advantage of being able to investigate both real and reciprocal space under high resolution. A view of the bright-field image in real space identifies microstructural features of interest with a resolution of - 2 / ~ . The structure of these features can then be examined by taking electron diffraction patterns. Using the dark-field imaging technique, one can then identify regions in the sample giving rise to specific features in the electron diffraction patterns. With a conventional transmission electron microscope (I"EM), electron diffraction patterns can be m a d e from regions of - 1 / ~ m diameter, which is of comparable size to the single crystallite domains of highly oriented pyrolytic graphite ( H O P G ) . Thus, the electron b e a m views samples based on H O P G as single crystals, giving a spot pattern *Department of Electrical Engineering and Computer Sciences. t Center for Materials Science and Engineering. Francis Bitter National Magnet Laboratory, supported by the National Science Foundation.
for the graphite reflections rather than a ring pattern, as is observed with X-ray or neutron diffraction. While c o m p l e m e n t a r y to the X-ray and neutron diffraction techniques, electron microscopy offers a n u m b e r of attractive features that are not available with the other diffraction techniques. Electron microscopy is especially useful for the study of phase transitions in graphite intercalation compounds insofar as changes in both the diffraction pattern and the real space image at the transition t e m p e r a t u r e can be explored. Using a variety of t e m p e r a t u r e stages, it is possible to obtain both electron diffraction patterns and real space images in the temperature range 5 < T < 9 0 0 K, which covers all transition temperatures reported for phase transitions in alkali metal compounds. The real space images and electron diffraction patterns furthermore provide strong evidence for the coexistence of multiple phases in a wellstaged graphite-alkali metal sample. In establishing equilibrium between phases, a n u m b e r of important variables, such as temperature, pressure, and in-plane intercalate density, must be considered. It is only recently that the sensitivity of the in-plane phases to some of these parameters has been appreciated [7], although it had been previously reported that stage 1 graphite-alkali metal compounds with different stoichiometries (and hence in-plane densities)
0378-4363/81/0000-0000/$2.50 O North-Holland Publishing C o m p a n y and Y a m a d a Science Foundation
N. K a m b e et al./Multi-phase superlattices in G I C
could occur [8]. The present paper provides strong evidence on a microscopic level for the multiphase nature of donor graphite intercalation compounds. 2. Experimental details
Single-staged bulk compounds with the intercalants K, Rb and Cs were prepared in encapsulated pyrex ampoules by the two-zone method, while the compounds with Li were prepared by liquid intercalation methods. Thin samples (-1000~k thickness), appropriate for a TEM study, were cleaned and mounted in the sample holder within an Ar-filled glove box. The sample holder was then conveyed to the TEM column in petroleum ether or argon so as to minimize 02 and H20 contamination of the sample. Using the transmission geometry, (hkO) reflections from a single crystalline domain of a graphite intercalation compound based on a polycrystalline graphite (HOPG) host material were observed in the electron diffraction mode of a Philips EM300 transmission electron microscope iTEM) in the temperature range 5-< T < 900 K. Real space bright-field and dark-field images were taken in conjunction with the electron diffraction pictures [9]. Since only the area under direct observation is irradiated by the 100kV electron beam (-10 A diameter) in the scanning transmission electron microscope (STEM), room temperature studies can be made with minimal heating damage. The VG-HB5 scanning transmission electron microscope was used in the high resolution imaging, X-ray fluorescence, and electron energy loss spectroscopic modes. The X-ray fluorescence yielded the relative intercalant densities in different parts of the sample while the electron energy loss indicated no appreciable oxygen contamination. 3. Results
Evidence for the coexistence of multiple phases in alkali metal donor compounds has previously been given by analysis of electron diffraction patterns and the observation of island structure in the real space bright field images [2-4]. In the present work, further evidence
273
confirming the coexistence of multiple phases is presented, based on dark-field real space images, electron diffraction patterns, X-ray fluorescence measurements and X-ray integrated intensity (00/) measurements. A typical bright field electron micrograph for a stage 2 graphite-Rb sample, taken with the STEM at room temperature, shows island structure as in fig. la. On the corresponding electron diffraction pattern, shown in fig. lb, room temperature rings with superimposed spots are indexed with wave vectors of (1.68-+0.05) and (2.10-+ 0.05) ~-1. These are comparable to values of 1.78 and 2.18~ -1 obtained with neutron diffraction at lower temperatures [10]. The rings can thus be indexed with wave vectors close to those corresponding to the commensurate (X/3 × X/3) and (~/7 × X/7) phases. In addition to the rings, the diffraction pattern of fig. lb shows the characteristic graphite hexagonal spot pattern. By setting the objective aperture to the open circle indicated on the diffraction pattern and corresponding to wave vectors appropriate to the rings and superimposed spots, a dark-field electron micrograph was taken (fig. lc). Comparison between figs. la and lc shows that the dark islands of the bright-field image are also illuminated in the dark-field micrograph, thereby showing that the island regions predominately contribute to the rings and superimposed spot patterns. In contrast, a dark field micrograph taken when the objective aperture is set at the graphite diffraction spots shows the whole sample to be uniformly illuminated, so that all parts of the sample contribute to the graphite diffraction spots. Further evidence that the islands and background correspond to different phases comes from X-ray fluorescence measurements taken on the same region of the sample, as shown in fig. 1. These measurements show the darker island regions of fig. la to contain (10-+ 2)% more Rb than the lighter background regions, consistent with greater electron absorption by the film from regions of higher electron density. Further evidence for the coexistence of multiple phases is demonstrated by direct examination of the diffraction patterns from several
274
N. Kambe et al./Multi-phase superlattices in G I C
a
Fig. 2. Diffraction patterns for stage 1 graphite-Rb, showing two different interior regions of the sample as indicated by the superimposed real space image. The region (a) shows predominantly a (2 × 2) superlattice while region (b) shows coexisting regions with (2 x 2) and (X/3 × X/3) superlattice patterns.
Fig. 1. Real space and diffraction photographs of stage 2 graphite-Rb. Shown at the top (a) is the bright-field image under magnification 50000 x, where the scale is indicated. The diffraction pattern shown in (b) was used to place the aperture on the rings (circle) in order to produce the darkfield image (c). The illuminated regions in the dark field image are primarily responsible for the observed diffraction rings.
selected areas of a given sample. This is illustrated in figs. 2a and 2b, where the diffraction patterns for two central regions of a stage 1 graphite-Rb sample are shown. In each of these figures the diffraction pattern in reciprocal space is superimposed on a bright-field real space image, in which the (000) reflection indicates the origin of the superlattice. An equivalent diffraction pattern is found within 10/~m of the central beam (000) position in each case. Note that the intensity of superlattice reflections varies from
N. Kambe et al./Multi-phase superlattices in GIC
one picture to another. For the region shown in fig. 2a, the p(2 x 2) superlattice dominates, while for the region shown in fig. 2b, the diffraction pattern shows spots corresponding to both the p(~/3x V~) and p(2x 2) superlattices. In the case of stage 1 graphite-Li, the p(~/3 x ~/3)R30 ° superlattice is dominant, although the ot/3y intercalate layer stacking below 220 K causes the structure factor to vanish and the superlattice pattern to disappear [1]. For the higher stage alkali metal compounds with K, Rb and Cs, the coexistence of multiple phases has been identified from analysis of the observed electron diffraction patterns [3, 4]. For example, in the temperature range T~< T < T~, between the temperatures identified with the phase transitions T~ and Tu [11], the observed ring patterns are identified with an incommensurate intercalate ordering, unlocked from the graphite bounding layer, and the rings are indexed in terms of two incommensurate triangular lattices, as indicated in table I. For the lower temperature region, T < T~, the diffraction patterns show commensurate phases [3, 4], and for the case of stage 2 graphite-K it has been inferred [3] that for T < T~ the p(X/3 x X/3)R30 ° and the p(X/7 x X/7)R19.1 ° commensurate phases are in coexistence with disordered regions. In table I are listed the phases that have been directly Table I Superlattices observed by electron diffraction for stage 2 graphite-alkali metal intercalation compounds Intercalant
Temperature range T < TI" nb
0b
T , ' < T < Tu"
K
7
19.1
Rb
7
Cs
7
nb
0b
19.1
3.54"[ 5.05J 3.59] 5.24J'
0-360 0-360 0-360 0-360
19.1
3.41 "[ 5.33 J
0-360 0-360
a Values for the transition temperatures T~ and Tu are given in the text. b The notation for the superlattice is ( ~ / n × X/n)R0 °. Noninteger values of n indicate incommensurate superlattice structures. A rotation angle 0-360 indicates that a ring diffraction pattern was observed.
275
identified in the electron diffraction patterns of the alkali metal intercalation compounds. In this connection it should be noted that a p(~/3x X/3)R30 ° structure with a/3y intercalate interlayer stacking or a p(2x 2)R0 ° structure with ,~/378 intercalate interlayer stacking would not be observed because of the vanishing structure factor [1,2]. The non-integer values of n reported in table I for the range T > T~ imply an incommensurate intercalate superlattice. The 0 values listed as 0-360 in the temperature range Tz< T < To imply arbitrary orientation of the intercalate domains in real space or a ring diffraction pattern in reciprocal space. Above Tu the superlattice remains incommensurate but the pattern locks in to the graphite lattice with a rotation of about 30°. In the real space image, a slow increase in the island size is observed with increasing temperature for T < Tu, although a dramatic increase in island size (by a factor of - 3 ) and a pronounced coalescence of islands is found as T is increased above T~ [2]. It should be noted that the Tj values obtained by electron diffraction are in good agreement with values obtained from resistivity anomalies [11] and from X-ray and neutron diffraction studies [5, 6, 10]. In particular, values for T~ and Tu for stage 2 compounds with the intercalants K, Rb and Cs are respectively 86 and 130 K for K, 110 and 170 K for Rb, and 178 and 225 K for Cs. The observation if island formation in the real space images of the donor compounds and the temperature dependence of their size distribution is characteristic of a nucleated growth process where at high temperature the small particles "evaporate" and those particles have a critical size grow by diffusion. At low temperatures the critical size for stability of the islands is smaller (-40-50/~) so that a larger fraction of the intercalated material is found in the small island regions of the sample. Island formation has in fact been proposed as a step in the intercalation process [12], which proceeds at high temperature where there is no commensurate ordering between the intercalant and the adjacent graphite layers. There is thus no physical reason to expect an integer ratio between carbon and intercalate atoms in the
276
N. Kambe et al./Multi-phase superlattices in GIC
chemical formula of intercalation compounds. Evidence for island formation has in fact been given for an acceptor compound based on the measured stoichiometry C1L3 NiCl213 by Flandrois et al. [13] who argued that the stoichiometry was consistent with an island size of 100 ~ diameter and an intercalant occupation of - 7 0 % of the intercalate layer. Determinations of the in-plane intercalate densities have also been made for the alkali metal intercalation compounds based on integrated intensity measurements of (00/) X-ray diffraction peaks [14]. Results on stage 1 and 2 compounds with K, Rb and Cs indicate appreciable departures from CsX for the stage 1 compounds and from ~ 4 X for the stage 2 compounds. To yield commensurate structures, the measured in-plane densities indicate the presence of regions of more concentrated intercalant density and other regions that are less concentrated, such that the relative areas of each phase average to yield the measured in-plane density. 4. Conclusion In this paper we have given several arguments in support of coexistent multiphase regions in graphite donor intercalation compounds. A more direct determination of the coexistent phases is planned using the STEM in the microdiffraction mode to yield diffraction patterns from individual island structures in the real space images. Acknowledgements We gratefully acknowledge Dr. A. GarrattReed for technical assistance, Professor A.N.
Berker for valuable discussion, and NSF-DMR Grant 78-10858 for support of the research program. References [1] N. Kambe, M.S. Dresselhaus, G. Dresselhaus, S. Basu, A.R. McGhie and J.E. Fischer, Mater. Sci. Eng. 40 (1979) I. [2] N. Kambe, G. Dresselhaus and M.S. Dresselhaus, Phys. Rev. B21 (1980) 3491. [3] A.N. Berker, N. Kambe, G. Dresselhaus and M.S. Dresselhaus, Phys. Rev. I_att.45 (1980) 1452. [4] M.S. Dresselhaus, N. Kambe, A.N. Berker and G. Dresselhaus, SyntheticMetals 2 (1980) 121. [5] R. Clarke, N. Caswell and S.A. Solin, Phys. Rev. Left. 42 (1979) 61. R. Clarke, N. Caswell, S.A. Solin and P.M. Horn, Phys. Rev. Lett. 43 (1979) 2018. H. Zabel, S.C. Moss, N. Caswell and S.A. Solin, Phys. Rev. Lett. 43 (1979) 2022. H. Zabel, Y.M. Jan and S.C. Moss, Physica 99B (1980) 453. J.B. Hastings, W.D. Ellenson and J.E. Fischer, Phys. Rev. Lett. 42 (1979) 1552. [6] W.D. Ellenson, D. Semmingsen, D. Gu6rard, D.G. Onn and J.E. Fischer, Mater. Sci. Eng. 31 (1977) 137. [7] S. Ostlund and A.N. Berker, Phys. Rev. B21 (1980). [8] S. Aronson, F.J. Salzano and D. Bellafiore, J. Chem. Phys. 49 (1968) 434. [9] B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals (Buttersworths, London, 1965) ch. 6. [10] H. Suematsu, M. Suzuki, H. Ikeda and Y. Endoh, Synthetic Metals 2 (1980) 133. [11] D.G. Onn, G.M.T. Foley and J.E. Fischer, Phys. Rev. B19 (1979) 6474. [12] W. Metz and L. Siemsglfiss, Carbon 16 (1978) 225. [13] S. Flandrois, J.M. Masson, J.C. Rouillon, J. Gaultier and C. Hauw, Synthetic Metals, to be published. [14] S.Y. Leung, C. Underhill, G. Dresselhaus, T. Krapchev, R. Ogilvie and M.S. Dresselhaus, Solid State Commun. 32 (1979) 635.
M U L T I - P H A S E S U P E R L A T T I C E S AND PHASE T R A N S I T I O N S IN G R A P H I T E I N T E R C A L A T I O N C O M P O U N D S W I T H Rb, Cs, K, Li N. K A M B E * t , H. M A Z U R E K t , M.S. D R E S S E L H A U S * t and G. D R E S S E L H A U S * Massachusetts Institute of Technology, Cambridge, Massachusetts 021_79, USA
Recent electron diffraction and real space imaging experiments for alkali metal donor compounds are discussed and are shown to yield strong evidence for the coexistence of multiple phases for the in-plane intercalate structure. Island formation indicates the presence of coexisting multiphases. X-ray fluorescence and dark-field imaging measurements using the scanning transmission electron microscope indicate that the dark islands have a higher alkali metal concentration than the lighter background and that the superlattice diffraction pattern is associated with the islands.
1. Introduction In this p a p e r we summarize our recent electron diffraction and real space image results on graphite-alkali metal intercalation compounds [1--4]. These experiments give important information c o m p l e m e n t a r y to the X-ray [5] and neutron diffraction [6] studies reported on these same compounds. Electron microscopy, however, has the advantage of being able to investigate both real and reciprocal space under high resolution. A view of the bright-field image in real space identifies microstructural features of interest with a resolution of - 2 / ~ . The structure of these features can then be examined by taking electron diffraction patterns. Using the dark-field imaging technique, one can then identify regions in the sample giving rise to specific features in the electron diffraction patterns. With a conventional transmission electron microscope (I"EM), electron diffraction patterns can be m a d e from regions of - 1 / ~ m diameter, which is of comparable size to the single crystallite domains of highly oriented pyrolytic graphite ( H O P G ) . Thus, the electron b e a m views samples based on H O P G as single crystals, giving a spot pattern *Department of Electrical Engineering and Computer Sciences. t Center for Materials Science and Engineering. Francis Bitter National Magnet Laboratory, supported by the National Science Foundation.
for the graphite reflections rather than a ring pattern, as is observed with X-ray or neutron diffraction. While c o m p l e m e n t a r y to the X-ray and neutron diffraction techniques, electron microscopy offers a n u m b e r of attractive features that are not available with the other diffraction techniques. Electron microscopy is especially useful for the study of phase transitions in graphite intercalation compounds insofar as changes in both the diffraction pattern and the real space image at the transition t e m p e r a t u r e can be explored. Using a variety of t e m p e r a t u r e stages, it is possible to obtain both electron diffraction patterns and real space images in the temperature range 5 < T < 9 0 0 K, which covers all transition temperatures reported for phase transitions in alkali metal compounds. The real space images and electron diffraction patterns furthermore provide strong evidence for the coexistence of multiple phases in a wellstaged graphite-alkali metal sample. In establishing equilibrium between phases, a n u m b e r of important variables, such as temperature, pressure, and in-plane intercalate density, must be considered. It is only recently that the sensitivity of the in-plane phases to some of these parameters has been appreciated [7], although it had been previously reported that stage 1 graphite-alkali metal compounds with different stoichiometries (and hence in-plane densities)
0378-4363/81/0000-0000/$2.50 O North-Holland Publishing C o m p a n y and Y a m a d a Science Foundation
N. K a m b e et al./Multi-phase superlattices in G I C
could occur [8]. The present paper provides strong evidence on a microscopic level for the multiphase nature of donor graphite intercalation compounds. 2. Experimental details
Single-staged bulk compounds with the intercalants K, Rb and Cs were prepared in encapsulated pyrex ampoules by the two-zone method, while the compounds with Li were prepared by liquid intercalation methods. Thin samples (-1000~k thickness), appropriate for a TEM study, were cleaned and mounted in the sample holder within an Ar-filled glove box. The sample holder was then conveyed to the TEM column in petroleum ether or argon so as to minimize 02 and H20 contamination of the sample. Using the transmission geometry, (hkO) reflections from a single crystalline domain of a graphite intercalation compound based on a polycrystalline graphite (HOPG) host material were observed in the electron diffraction mode of a Philips EM300 transmission electron microscope iTEM) in the temperature range 5-< T < 900 K. Real space bright-field and dark-field images were taken in conjunction with the electron diffraction pictures [9]. Since only the area under direct observation is irradiated by the 100kV electron beam (-10 A diameter) in the scanning transmission electron microscope (STEM), room temperature studies can be made with minimal heating damage. The VG-HB5 scanning transmission electron microscope was used in the high resolution imaging, X-ray fluorescence, and electron energy loss spectroscopic modes. The X-ray fluorescence yielded the relative intercalant densities in different parts of the sample while the electron energy loss indicated no appreciable oxygen contamination. 3. Results
Evidence for the coexistence of multiple phases in alkali metal donor compounds has previously been given by analysis of electron diffraction patterns and the observation of island structure in the real space bright field images [2-4]. In the present work, further evidence
273
confirming the coexistence of multiple phases is presented, based on dark-field real space images, electron diffraction patterns, X-ray fluorescence measurements and X-ray integrated intensity (00/) measurements. A typical bright field electron micrograph for a stage 2 graphite-Rb sample, taken with the STEM at room temperature, shows island structure as in fig. la. On the corresponding electron diffraction pattern, shown in fig. lb, room temperature rings with superimposed spots are indexed with wave vectors of (1.68-+0.05) and (2.10-+ 0.05) ~-1. These are comparable to values of 1.78 and 2.18~ -1 obtained with neutron diffraction at lower temperatures [10]. The rings can thus be indexed with wave vectors close to those corresponding to the commensurate (X/3 × X/3) and (~/7 × X/7) phases. In addition to the rings, the diffraction pattern of fig. lb shows the characteristic graphite hexagonal spot pattern. By setting the objective aperture to the open circle indicated on the diffraction pattern and corresponding to wave vectors appropriate to the rings and superimposed spots, a dark-field electron micrograph was taken (fig. lc). Comparison between figs. la and lc shows that the dark islands of the bright-field image are also illuminated in the dark-field micrograph, thereby showing that the island regions predominately contribute to the rings and superimposed spot patterns. In contrast, a dark field micrograph taken when the objective aperture is set at the graphite diffraction spots shows the whole sample to be uniformly illuminated, so that all parts of the sample contribute to the graphite diffraction spots. Further evidence that the islands and background correspond to different phases comes from X-ray fluorescence measurements taken on the same region of the sample, as shown in fig. 1. These measurements show the darker island regions of fig. la to contain (10-+ 2)% more Rb than the lighter background regions, consistent with greater electron absorption by the film from regions of higher electron density. Further evidence for the coexistence of multiple phases is demonstrated by direct examination of the diffraction patterns from several
274
N. Kambe et al./Multi-phase superlattices in G I C
a
Fig. 2. Diffraction patterns for stage 1 graphite-Rb, showing two different interior regions of the sample as indicated by the superimposed real space image. The region (a) shows predominantly a (2 × 2) superlattice while region (b) shows coexisting regions with (2 x 2) and (X/3 × X/3) superlattice patterns.
Fig. 1. Real space and diffraction photographs of stage 2 graphite-Rb. Shown at the top (a) is the bright-field image under magnification 50000 x, where the scale is indicated. The diffraction pattern shown in (b) was used to place the aperture on the rings (circle) in order to produce the darkfield image (c). The illuminated regions in the dark field image are primarily responsible for the observed diffraction rings.
selected areas of a given sample. This is illustrated in figs. 2a and 2b, where the diffraction patterns for two central regions of a stage 1 graphite-Rb sample are shown. In each of these figures the diffraction pattern in reciprocal space is superimposed on a bright-field real space image, in which the (000) reflection indicates the origin of the superlattice. An equivalent diffraction pattern is found within 10/~m of the central beam (000) position in each case. Note that the intensity of superlattice reflections varies from
N. Kambe et al./Multi-phase superlattices in GIC
one picture to another. For the region shown in fig. 2a, the p(2 x 2) superlattice dominates, while for the region shown in fig. 2b, the diffraction pattern shows spots corresponding to both the p(~/3x V~) and p(2x 2) superlattices. In the case of stage 1 graphite-Li, the p(~/3 x ~/3)R30 ° superlattice is dominant, although the ot/3y intercalate layer stacking below 220 K causes the structure factor to vanish and the superlattice pattern to disappear [1]. For the higher stage alkali metal compounds with K, Rb and Cs, the coexistence of multiple phases has been identified from analysis of the observed electron diffraction patterns [3, 4]. For example, in the temperature range T~< T < T~, between the temperatures identified with the phase transitions T~ and Tu [11], the observed ring patterns are identified with an incommensurate intercalate ordering, unlocked from the graphite bounding layer, and the rings are indexed in terms of two incommensurate triangular lattices, as indicated in table I. For the lower temperature region, T < T~, the diffraction patterns show commensurate phases [3, 4], and for the case of stage 2 graphite-K it has been inferred [3] that for T < T~ the p(X/3 x X/3)R30 ° and the p(X/7 x X/7)R19.1 ° commensurate phases are in coexistence with disordered regions. In table I are listed the phases that have been directly Table I Superlattices observed by electron diffraction for stage 2 graphite-alkali metal intercalation compounds Intercalant
Temperature range T < TI" nb
0b
T , ' < T < Tu"
K
7
19.1
Rb
7
Cs
7
nb
0b
19.1
3.54"[ 5.05J 3.59] 5.24J'
0-360 0-360 0-360 0-360
19.1
3.41 "[ 5.33 J
0-360 0-360
a Values for the transition temperatures T~ and Tu are given in the text. b The notation for the superlattice is ( ~ / n × X/n)R0 °. Noninteger values of n indicate incommensurate superlattice structures. A rotation angle 0-360 indicates that a ring diffraction pattern was observed.
275
identified in the electron diffraction patterns of the alkali metal intercalation compounds. In this connection it should be noted that a p(~/3x X/3)R30 ° structure with a/3y intercalate interlayer stacking or a p(2x 2)R0 ° structure with ,~/378 intercalate interlayer stacking would not be observed because of the vanishing structure factor [1,2]. The non-integer values of n reported in table I for the range T > T~ imply an incommensurate intercalate superlattice. The 0 values listed as 0-360 in the temperature range Tz< T < To imply arbitrary orientation of the intercalate domains in real space or a ring diffraction pattern in reciprocal space. Above Tu the superlattice remains incommensurate but the pattern locks in to the graphite lattice with a rotation of about 30°. In the real space image, a slow increase in the island size is observed with increasing temperature for T < Tu, although a dramatic increase in island size (by a factor of - 3 ) and a pronounced coalescence of islands is found as T is increased above T~ [2]. It should be noted that the Tj values obtained by electron diffraction are in good agreement with values obtained from resistivity anomalies [11] and from X-ray and neutron diffraction studies [5, 6, 10]. In particular, values for T~ and Tu for stage 2 compounds with the intercalants K, Rb and Cs are respectively 86 and 130 K for K, 110 and 170 K for Rb, and 178 and 225 K for Cs. The observation if island formation in the real space images of the donor compounds and the temperature dependence of their size distribution is characteristic of a nucleated growth process where at high temperature the small particles "evaporate" and those particles have a critical size grow by diffusion. At low temperatures the critical size for stability of the islands is smaller (-40-50/~) so that a larger fraction of the intercalated material is found in the small island regions of the sample. Island formation has in fact been proposed as a step in the intercalation process [12], which proceeds at high temperature where there is no commensurate ordering between the intercalant and the adjacent graphite layers. There is thus no physical reason to expect an integer ratio between carbon and intercalate atoms in the
276
N. Kambe et al./Multi-phase superlattices in GIC
chemical formula of intercalation compounds. Evidence for island formation has in fact been given for an acceptor compound based on the measured stoichiometry C1L3 NiCl213 by Flandrois et al. [13] who argued that the stoichiometry was consistent with an island size of 100 ~ diameter and an intercalant occupation of - 7 0 % of the intercalate layer. Determinations of the in-plane intercalate densities have also been made for the alkali metal intercalation compounds based on integrated intensity measurements of (00/) X-ray diffraction peaks [14]. Results on stage 1 and 2 compounds with K, Rb and Cs indicate appreciable departures from CsX for the stage 1 compounds and from ~ 4 X for the stage 2 compounds. To yield commensurate structures, the measured in-plane densities indicate the presence of regions of more concentrated intercalant density and other regions that are less concentrated, such that the relative areas of each phase average to yield the measured in-plane density. 4. Conclusion In this paper we have given several arguments in support of coexistent multiphase regions in graphite donor intercalation compounds. A more direct determination of the coexistent phases is planned using the STEM in the microdiffraction mode to yield diffraction patterns from individual island structures in the real space images. Acknowledgements We gratefully acknowledge Dr. A. GarrattReed for technical assistance, Professor A.N.
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