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Structural and electronic study of KHg and RbHg intercalated graphite
Synthetic Metals, 2 (1980) 197 - 202 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
197
STRUCTURAL AND ELECTRONIC STUDY OF KHg AND RbHg INTERCALATED GRAPHITE
M. EL MAKRINI, G. FURDIN, P. LAGRANGE, J. F. MARECHE, E. McRAE and A. HEROLD Laboratoire de Chimie du Solide Mindral, associd au CNRS n ° 158, Service de Chimie Mindrale Appliqude, Universitd de Nancy I, CO 140 - - 5 4 0 3 7 - Nancy Cddex (France)
(Received June 7, 1980)
Summary We have shown that the potassium and rubidium amalgams, KHg and RbHg, intercalate into graphite forming very metal-rich ternary compounds of general formula MHgC4 (first stage) and MHgCs (second stage). The reaction provokes a very high -* " dilation of the host graphite: 203% for KHg c axis and 221% for RbHg in first stage products, attributed to the three-layer structure of the intercalant. Two planes of the alkali metal atoms are epitaxied on the carbon layers on either side of a mercury layer. These latter atoms occupy prismatic sites created by the strictly defined disposition of the former. A study of certain electronic properties has been undertaken. Room temperature values of electrical resistivity are higher than those of the corresponding alkali metal compounds: about 20 g~2 cm for first stage products and about 12 p~2 cm for second stage products. The variation with temperature down to 100 K has been measured yielding values smaller by a factor of about five than the ambient temperature values. The compounds possess a Pauli paramagnetic type of susceptibility, practically temperature independent from 4.2 to 300 K. The first stage compounds can give solid solutions, Kx Rbl-xHgC4 for which the average magnetic susceptibility shows a quite pronounced minimum as a function of composition.
1. Introduction
Mercury graphitides were first synthesized in this laboratory by Lagrange and El Makrini [1, 2]. These graphite intercalation compounds contain mercury and a heavy alkali metal in which the metal to carbon ratio is among the highest known for any of this class of layered materials. Stages one and two are presently' known, of formulae and b'axis repeat distance, respectively: MHgC4 (Ic = 10.16 A (K); 10.76 £ (Rb)) and MHgC8 ( I c = 13.51 £ (K); 14.11 )k (Rb)). M represents the intercalated potassium or
198 rubidium. The third stage c o m p o u n d has been seen but it has n o t yet proved possible to isolate it as a pure phase. The structure is as follows. Between two sheets of the host carbon are t w o layers of the alkali metal on either side of the central mercury. The alkali atoms are octally epitaxied on the carbon layers, one above the other, leaving prismatic sites for the mercury atoms. Homogeneity of the c o m p o u n d s has been studied by X-ray diffraction. The structural characteristics having been determined, we were interested in examining some of the galvanomagnetic properties of these compounds, particularly because of the highly organized nature of the intercalents, the very large value of interplanar distance, and the very low carbon to metal ratio, all important factors as regards charge transfer between intercalant and host. The dependence of the magnetic susceptibility and the electrical resistivity on composition and temperature was thus measured. Furthermore, aside from these ternary compounds, it is possible to synthesize solid solutions, of formula KxRbl_~HgC4, using powdered graphite as starting material. These have been characterized by X-ray diffraction, and room temperature measurements of magnetic susceptibility over the entire range of composition.
2. Preparation and measurement techniques Intercalated graphite powder was used for all of the susceptibility measurements except those of magnetic anisotropy of the first stage products; all resistivity measurements were based on Intercalated pyrographite. The initial powdered Ceylon graphite (125 - 200 pm) was carefully washed in hydrochloric acid and well outgassed. A sufficient quantity of each of the products was synthesized as detailed elsewhere [ 1, 2]. Figure 1 shows the evolution of the 001 X-ray diffraction diagrams for the quaternary system KxRbl_xHgC4. Careful manipulation in a glovebox was required to fill the sample holders for the magnetic measurements. These holders were small pyrex glass ampoules, sealed a short distance above the sample. It was essential that no trace of the c o m p o u n d remained on the walls of the pyrex, since any quantity of the intercalated material would decompose leaving free graphite. Given the low measured values of susceptibility and its very high compositional dependence, it was absolutely necessary to avoid this. The susceptibility was determined, using a highly sensitive apparatus based on the Faraday m e t h o d [3], in a field of 1.4 T with 100 mg of the compound. Following the initial temperature run, the sample holder was broken, the product removed, and the susceptibility of the former measured again over the same temperature range. Under these conditions, the overall resolution was 10 - s e.m.u.c.g.s./gram. The anisotropy was investigated using pyrographite as starting material: a thick disk for experiments with the magnetic field parallel to the carbon planes and a rectangular sample when the field was perpendicular.
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Fig. 1. X-ray diffraction patterns of KxRbl_xHgC4. Electrical resistivity measurements were effected using a contactless, variable temperature m e t h o d [4] allowing for in situ X-ray characterization with no further sample transfer.
3. Experimental results
(a) Ternary compounds: MHgC 4 and MHgCs The curves x(T} (Fig. 2) show that over most of the temperature range, these c o m p o u n d s possess a weak, practically temperature-independent paramagnetism, as do many of the other graphite-metal intercalation c o m p o u n d s [5]. Below 20 K, there is, in some cases, a rise in susceptibility. For the first stage samples on which anisotropy measurements were made, the parallel
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susceptibility is very low (× < 10 -7 e.m.u.c.g.s./gram within experimental error), whereas the perpendicular value is three times greater than that of the powdered products. The results of resistivity measurements as a function of temperature are given in Fig. 3. It is immediately apparent that room temperature values are greater than those of the alkali metal-intercalated binary compounds by a factor of about two [6], though the trend is similar in that there is a considerable decrease in going from first to second stage. Behaviour for the two second stage ternaries is similar, though the first stage potassium compound differs significantly from that containing rubidium. In fact, for this latter, there is a striking difference, even between two samples characterized as being the same from X-ray analysis and weight uptake: the upper curve of Fig. 3 shows almost linear behaviour, whereas the lower, as is the case for the three other curves, undergoes a slope change at about 200 K. Z3C n.m.r, measurements [7] show quite significant shifts of the 13C line. On assuming that the charge transfer from the intercalant to the graphite layer is proportional to this measured value, it is noted that the transfer is weaker for MHgC4 than for the corresponding MCs binaries. It may thus be hypothesized that the charge furnished by the alkali is shared in the mercury graphitides between the adjacent carbon layers and the central mercury layer. x (~6~muCGS/g)
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202
(b) Quaternary compounds: KxRbl-xHgC4 Figure 1 shows a few representative examples of X-ray diagrams obtained with these compounds. Two series of magnetic measurements were made over the range of compositions, evolution of the susceptibility being the same for both, the minimum value lying near x = 0.7. In this region, the curve of interplanar distance as a function of composition undergoes no sudden change (Fig. 4). The temperature dependence of magnetic susceptibility was investigated for the K0.sRb0.5HgC4 compound. Results given in Fig. 2. show that the behaviour is essentially the same as that of the corresponding ternaries.
4. Conclusion The mercury graphitides studied here present a pronounced metallic character, like the MC8 and MC24 binaries to which they are related. This is manifested by both the overall variation of resistivity with temperature and the temperature-independent Pauli paramagnetism.
References 1 P. Lagrange, M. E1 Makrini, D. Gu~rard and A. H6rold, Physica, 99 B (1980) 473. 2 M. E1 Makrini, P. Lagrange, D. Gu6rard and A. H$rold, C. R. Acad. Sci., Set. C, 288 (1979) 303. 3 G. Furdin and C. Zeller, C. R. Acad. Sci., Set., B, 275 (1972) 717. 4 E. McRae, J. F. Mar6ch~ and A. H~rold, J. Phys. E., 13 (1980) 241. 5 P. Delhaes, Mater. Sci. Eng., 31 (1977) 225. 6 E. McRae, D. Billaud, J. F. Mar6ch6 and A. H~rold, Physica, 99B (1980) 489. 7 J. Conard, H. Estrade, P. Lauginie, M. E1 Makrini, P. Lagrange and D. Gu6rard, Synth. Met., 2 (1980) 261.
197
STRUCTURAL AND ELECTRONIC STUDY OF KHg AND RbHg INTERCALATED GRAPHITE
M. EL MAKRINI, G. FURDIN, P. LAGRANGE, J. F. MARECHE, E. McRAE and A. HEROLD Laboratoire de Chimie du Solide Mindral, associd au CNRS n ° 158, Service de Chimie Mindrale Appliqude, Universitd de Nancy I, CO 140 - - 5 4 0 3 7 - Nancy Cddex (France)
(Received June 7, 1980)
Summary We have shown that the potassium and rubidium amalgams, KHg and RbHg, intercalate into graphite forming very metal-rich ternary compounds of general formula MHgC4 (first stage) and MHgCs (second stage). The reaction provokes a very high -* " dilation of the host graphite: 203% for KHg c axis and 221% for RbHg in first stage products, attributed to the three-layer structure of the intercalant. Two planes of the alkali metal atoms are epitaxied on the carbon layers on either side of a mercury layer. These latter atoms occupy prismatic sites created by the strictly defined disposition of the former. A study of certain electronic properties has been undertaken. Room temperature values of electrical resistivity are higher than those of the corresponding alkali metal compounds: about 20 g~2 cm for first stage products and about 12 p~2 cm for second stage products. The variation with temperature down to 100 K has been measured yielding values smaller by a factor of about five than the ambient temperature values. The compounds possess a Pauli paramagnetic type of susceptibility, practically temperature independent from 4.2 to 300 K. The first stage compounds can give solid solutions, Kx Rbl-xHgC4 for which the average magnetic susceptibility shows a quite pronounced minimum as a function of composition.
1. Introduction
Mercury graphitides were first synthesized in this laboratory by Lagrange and El Makrini [1, 2]. These graphite intercalation compounds contain mercury and a heavy alkali metal in which the metal to carbon ratio is among the highest known for any of this class of layered materials. Stages one and two are presently' known, of formulae and b'axis repeat distance, respectively: MHgC4 (Ic = 10.16 A (K); 10.76 £ (Rb)) and MHgC8 ( I c = 13.51 £ (K); 14.11 )k (Rb)). M represents the intercalated potassium or
198 rubidium. The third stage c o m p o u n d has been seen but it has n o t yet proved possible to isolate it as a pure phase. The structure is as follows. Between two sheets of the host carbon are t w o layers of the alkali metal on either side of the central mercury. The alkali atoms are octally epitaxied on the carbon layers, one above the other, leaving prismatic sites for the mercury atoms. Homogeneity of the c o m p o u n d s has been studied by X-ray diffraction. The structural characteristics having been determined, we were interested in examining some of the galvanomagnetic properties of these compounds, particularly because of the highly organized nature of the intercalents, the very large value of interplanar distance, and the very low carbon to metal ratio, all important factors as regards charge transfer between intercalant and host. The dependence of the magnetic susceptibility and the electrical resistivity on composition and temperature was thus measured. Furthermore, aside from these ternary compounds, it is possible to synthesize solid solutions, of formula KxRbl_~HgC4, using powdered graphite as starting material. These have been characterized by X-ray diffraction, and room temperature measurements of magnetic susceptibility over the entire range of composition.
2. Preparation and measurement techniques Intercalated graphite powder was used for all of the susceptibility measurements except those of magnetic anisotropy of the first stage products; all resistivity measurements were based on Intercalated pyrographite. The initial powdered Ceylon graphite (125 - 200 pm) was carefully washed in hydrochloric acid and well outgassed. A sufficient quantity of each of the products was synthesized as detailed elsewhere [ 1, 2]. Figure 1 shows the evolution of the 001 X-ray diffraction diagrams for the quaternary system KxRbl_xHgC4. Careful manipulation in a glovebox was required to fill the sample holders for the magnetic measurements. These holders were small pyrex glass ampoules, sealed a short distance above the sample. It was essential that no trace of the c o m p o u n d remained on the walls of the pyrex, since any quantity of the intercalated material would decompose leaving free graphite. Given the low measured values of susceptibility and its very high compositional dependence, it was absolutely necessary to avoid this. The susceptibility was determined, using a highly sensitive apparatus based on the Faraday m e t h o d [3], in a field of 1.4 T with 100 mg of the compound. Following the initial temperature run, the sample holder was broken, the product removed, and the susceptibility of the former measured again over the same temperature range. Under these conditions, the overall resolution was 10 - s e.m.u.c.g.s./gram. The anisotropy was investigated using pyrographite as starting material: a thick disk for experiments with the magnetic field parallel to the carbon planes and a rectangular sample when the field was perpendicular.
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20"
110"
Fig. 1. X-ray diffraction patterns of KxRbl_xHgC4. Electrical resistivity measurements were effected using a contactless, variable temperature m e t h o d [4] allowing for in situ X-ray characterization with no further sample transfer.
3. Experimental results
(a) Ternary compounds: MHgC 4 and MHgCs The curves x(T} (Fig. 2) show that over most of the temperature range, these c o m p o u n d s possess a weak, practically temperature-independent paramagnetism, as do many of the other graphite-metal intercalation c o m p o u n d s [5]. Below 20 K, there is, in some cases, a rise in susceptibility. For the first stage samples on which anisotropy measurements were made, the parallel
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susceptibility is very low (× < 10 -7 e.m.u.c.g.s./gram within experimental error), whereas the perpendicular value is three times greater than that of the powdered products. The results of resistivity measurements as a function of temperature are given in Fig. 3. It is immediately apparent that room temperature values are greater than those of the alkali metal-intercalated binary compounds by a factor of about two [6], though the trend is similar in that there is a considerable decrease in going from first to second stage. Behaviour for the two second stage ternaries is similar, though the first stage potassium compound differs significantly from that containing rubidium. In fact, for this latter, there is a striking difference, even between two samples characterized as being the same from X-ray analysis and weight uptake: the upper curve of Fig. 3 shows almost linear behaviour, whereas the lower, as is the case for the three other curves, undergoes a slope change at about 200 K. Z3C n.m.r, measurements [7] show quite significant shifts of the 13C line. On assuming that the charge transfer from the intercalant to the graphite layer is proportional to this measured value, it is noted that the transfer is weaker for MHgC4 than for the corresponding MCs binaries. It may thus be hypothesized that the charge furnished by the alkali is shared in the mercury graphitides between the adjacent carbon layers and the central mercury layer. x (~6~muCGS/g)
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Fig. 4. ~axis repeat distance, Ic, and room temperature magnetic susceptibility, X, for quaternary compounds K x R b l _ x H g C 4. (1) and (2) are for two different batches of products, showing different absolute values of X, but similar values of x for Xmin and Xmax.
202
(b) Quaternary compounds: KxRbl-xHgC4 Figure 1 shows a few representative examples of X-ray diagrams obtained with these compounds. Two series of magnetic measurements were made over the range of compositions, evolution of the susceptibility being the same for both, the minimum value lying near x = 0.7. In this region, the curve of interplanar distance as a function of composition undergoes no sudden change (Fig. 4). The temperature dependence of magnetic susceptibility was investigated for the K0.sRb0.5HgC4 compound. Results given in Fig. 2. show that the behaviour is essentially the same as that of the corresponding ternaries.
4. Conclusion The mercury graphitides studied here present a pronounced metallic character, like the MC8 and MC24 binaries to which they are related. This is manifested by both the overall variation of resistivity with temperature and the temperature-independent Pauli paramagnetism.
References 1 P. Lagrange, M. E1 Makrini, D. Gu~rard and A. H6rold, Physica, 99 B (1980) 473. 2 M. E1 Makrini, P. Lagrange, D. Gu6rard and A. H$rold, C. R. Acad. Sci., Set. C, 288 (1979) 303. 3 G. Furdin and C. Zeller, C. R. Acad. Sci., Set., B, 275 (1972) 717. 4 E. McRae, J. F. Mar6ch~ and A. H~rold, J. Phys. E., 13 (1980) 241. 5 P. Delhaes, Mater. Sci. Eng., 31 (1977) 225. 6 E. McRae, D. Billaud, J. F. Mar6ch6 and A. H~rold, Physica, 99B (1980) 489. 7 J. Conard, H. Estrade, P. Lauginie, M. E1 Makrini, P. Lagrange and D. Gu6rard, Synth. Met., 2 (1980) 261.