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Co-intercalation into graphite of lithium and sodium with an alkaline earth metal
Carbon 42 (2004) 1825–1831 www.elsevier.com/locate/carbon
Co-intercalation into graphite of lithium and sodium with an alkaline earth metal Sebastien Pruvost
a,1
, Claire Herold
a,*
, Albert Herold a, Philippe Lagrange
a,b
a
b
Laboratoire de Chimie du Solide Mineral––UMR CNRS UHP 7555, Universite Henri Poincare Nancy I, B.P. 239, 54506 Vandœuvre-les-Nancy Cedex, France Ecole Europeenne d’Ingenieurs en Genie des Materiaux, Institut National Polytechnique de Lorraine, 6 rue Bastien Lepage, B.P. 630, 54010 Nancy Cedex, France Received 1 December 2003; accepted 11 March 2004 Available online 15 April 2004
Abstract After a short overview on the co-intercalation into graphite of light alkali metals––lithium and sodium––with a third element, novel results concerning the co-intercalation of these alkali metals with an alkaline earth metal are given. The intercalation into graphite of lithium together with magnesium, calcium, strontium and barium is investigated, and the reaction products are described and characterised by X-ray diffraction. Ternary intercalation compounds were synthesised only in the graphite–lithium–calcium system. In the other systems, binary compounds were obtained. The co-intercalation of sodium and the alkaline earth metals calcium, strontium and barium is studied too. No ternary compounds were obtained. However, in the case of the sodium–barium system, a discussion on ternary graphite–sodium–barium compounds previously published is opened. Several data are in favour of a graphite–sodium–oxygen compound instead of a graphite–sodium–barium one. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Highly oriented graphite, Intercalation compounds; B. Intercalation; C. X-ray diffraction
1. Introduction The heavy alkali metals intercalate very easily into graphite leading to various binary compounds [1]. They are characterised by their stage that increases when the intercalated amount of metal decreases. Their intercalated sheets are systematically mono-layers and are commensurate with respect to the adjacent 2D graphene layers. The first stage compounds (KC8 , RbC8 and CsC8 ) are prepared by action of metal vapour on graphite samples at quite low temperature (100–200 °C). Their interplanar distances reach 535, 565 and 592 pm respectively. The intercalation into graphite of lithium and especially sodium is much more difficult. The reaction temperatures are appreciably higher. With sodium, it is * Corresponding author. Tel.: +33-383-684-884; fax: +33-383-684615. E-mail address: [email protected] (C. Herold). 1 Present address: Laboratoire d’Electrodynamique des Materiaux Avances––UMR-CNRS-CEA 6157, IUT de Blois, Universite de Tours, 3 Place Jean Jaures, C.S. 2903 41029 Blois Cedex, France.
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.03.014
strictly impossible to obtain compounds containing large quantities of metal, so that only high stage compounds (sixth, seventh and eighth stage) can be synthesised [2]. On the other hand, the first stage LiC6 compound [3] is well known and presents also mono-layers of the intercalated atoms, but the metal amount is especially high for this compound because of the small size of the lithium atoms. Nevertheless, its intercalated layers are also commensurate with respect to the adjacent graphene planes. In spite of a less pronounced electropositive character, the alkaline earth metals intercalate into graphite quite similar to lithium [4]. The first stage binaries are also well known (CaC6 , SrC6 and BaC6 ) and resemble LiC6 closely: the structure of the 2D intercalated layers is the same, only the c-axis stacking is different between lithium and alkaline earth metal compounds. Ternary graphite intercalation compounds containing a heavy alkali metal have been intensively studied in the past decades [5,6]. These ternaries are very numerous and the corresponding chemistry is extremely rich and interesting.
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On the other hand, an equivalent study with lithium and sodium had not been undertaken in the same manner. Only a few experiments are known; they have been carried out selectively and never systematically. In this paper, we have gathered firstly the main results of these limited studies. And secondly, we provide the novel results of our systematic study concerning the attempts of co-intercalation into graphite of lithium or sodium with an alkaline earth metal.
2. Previous results It has been well established that lithium is unable to co-intercalate into graphite with heavy alkali metals [7]. The results of all experiments show clearly that both metals intercalate separately, leading to two phases (LiC6 and MC8 ). On the other hand, sodium is able to co-intercalate into graphite with potassium, rubidium and caesium [8], leading to first stage ternary compounds, for which the intercalated sheets are mono-layered, sodium and second metal atoms being statistically distributed in the crystallographic sites of the intercalated layer. The formula of such a compound can be written Nax M1x C8 (M ¼ K, Rb or Cs), so that it appears as a ‘‘substitution ternary’’. If we exclude the very interesting results concerning the co-intercalation of lithium and calcium, that are related in the following part, all the other experiments concerning the intercalation of lithium associated with a second element have been negative [9]. But, in the case of sodium, several cases have led to positive results. Sodium indeed is able to intercalate into graphite associated with oxygen [10]. Liquid sodium containing a small amount of oxygen (about 1 at.%), intercalates very easily into graphite and leads to a blue second stage ternary compound, whose intercalated sheets are five-layered according to a [Na–O–Na–O–Na] c-axis sequence. Its repeat distance of 1080 pm corresponds to an interplanar distance of 745 pm. The intercalated oxygen appears as peroxide anions, whose O–O axis is parallel to the graphite c-axis, with an interatomic distance of 150 pm. The O2 2 anions are thus oriented perpendicular to the intercalated sodium layers. In the Na2 O2 binary peroxide, these anions are also perpendicular to the planes of sodium: planes of sodium atoms alternate with planes of oxygen atoms along the c-axis. It has been shown that it is possible to consider that the intercalated sheets are well cut (perpendicularly to its c-axis) sodium peroxide slices. The 2D unit cell is hexagonal, with a parameter of 367 pm, so that it is not commensurate with respect to the graphitic one. Sodium intercalates also into graphite in association with hydroxide anions [11]. A blue first stage compound
is synthesised by reaction at 350 °C of graphite with the mixture obtained by melting sodium with its hydroxide. This original compound contains Naþ and OH ions, with a small proportion of H anions. The intercalate forms double-layer sheets and leads to an interplanar distance of 921 pm. Both layers of these sheets are mixed, since they contain simultaneously Naþ and OH ions, so that highly ionic bonds arise in each layer as between both layers of a given sheet (this is a particular characteristic that is extremely rare). The 2D unit cell of the intercalate is square, with a parameter of 341 pm; it is of course not commensurate with the graphitic one. The double layer intercalated sheets exhibit an arrangement very close to that of the quadratic free sodium hydroxide. Ternary graphite intercalation compounds containing sodium associated with chloride, bromide and iodide anions [12] have been synthesised by heating graphite with liquid sodium in the presence of NaCl, NaBr and NaI between 450 and 1000 °C, during a time which varies from several hours to several weeks, according to the case. Liquid sodium plays a triple role: it reduces graphite, it intercalates into graphite and it is a transfer agent for the halide anions. The compounds belong to the stages 2, 3 and 4 and the stage decreases by increasing the temperature (this is an exception in graphite intercalation chemistry). The intercalated sheets are made of two sodium cation planes surrounding a halide anion layer and present some similarities with the corresponding (1 1 1) plane of the free halides. The interplanar distances reach 756 and 771 pm respectively for G–Na–Cl and G–Na–Br compounds. The 2D unit cell is hexagonal and commensurate with that of the graphene sheets, since the value of its parameter is 246 pm. On the other hand, the G–Na–I system is more complicated, since it leads to several interplanar distances, whose values range between 769 and 790 pm. In this system, more than 10 in-plane structures have been observed (commensurate or not, according to the case). Similar ternary compounds could not be obtained in the G–Na–F system. But, quaternary phases G–Na–I–F and G–Na–Cl–F have been synthesised by reaction at respectively 850 and 650 °C of graphite and liquid sodium associated with both NaF and NaI (or NaF and NaCl) halides. These quaternaries are very numerous and appear always as complex phases, so that they are not described here. Their complete description is given in a paper devoted to this topic [12].
3. Reactions of graphite with lithium and an alkaline earth metal (novel results) Numerous reactions were carried out with pyrographite platelets and liquid metallic alloys containing
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lithium and an alkaline earth metal. Magnesium, calcium, strontium and barium associated with lithium were studied. All the binary alloys are prepared in a glove box containing a very pure argon atmosphere. The reactor consists of a stainless steel tube, and the pyrographite sample is introduced into the molten metallic alloy. The reaction temperature depends strongly on the binary phase diagram of the lithium– alkaline earth metal system. However, it is also well known now that the alloy composition plays a major role in the obtaining of a binary or a ternary intercalation compound. Indeed, if the alloy is very rich in lithium, only a binary lithium–graphite compound will be obtained.
3.1. Graphite–lithium–magnesium system No definite compound exists in the binary Li–Mg phase diagram [13]. The liquidus temperature reaches values higher than 400 °C as soon as the magnesium concentration is greater than 30 at.%. Otherwise, the high reaction temperatures favour the formation of carbides [9]. Both elements are precisely weighed and introduced into a stainless steel tube in the glove box. The tube is closed and then transferred to an apparatus equipped with a furnace that can be used either under argon or under vacuum. This apparatus allows also to introduce the sample under argon. Before the introduction of the pyrographite platelet, the alloy is heated to a temperature higher than its melting point in order to homogenise it. The sample is then introduced into the molten alloy and the reaction is carried out under a slight argon overpressure. At the end of the reaction, the sample is extracted from the liquid alloy and after cooling, it is placed under an argon flow in a capillary tube for X-ray investigations. The composition of the first studied alloy is Li/ Mg ¼ 3, which is liquid at 350 °C. After 16 h in the liquid alloy at 450 °C, only lithium intercalates into graphite and the first stage binary LiC6 compound is obtained. The alloy is too rich in lithium so that its activity in the molten alloy is too high in order to permit the formation of a ternary compound. The second alloy contains less lithium since the Li/Mg ratio is equal to 3/2. The increase in the magnesium amount leads to an increase in the melting point of the alloy to 490 °C. After 12 h, as after 24 h of reaction at 540 °C, the pyrographite did not react. However, a small amount of lithium carbide is obtained after 48 h at 540 °C. Taking into account the binary Li–Mg phase diagram (high temperatures for the liquidus) and the experiences described above, the synthesis of a ternary graphite–lithium–magnesium compound seems to be very unlikely.
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3.2. Graphite–lithium–calcium system The Ca–Li binary phase diagram [14] strongly differs from the previous one. The liquidus becomes widely lower than 350 °C since the alloy contains less than 50 at.% of calcium. CaLi2 is the single definite compound of this system. Several alloys were studied with the following compositions: Li/Ca ¼ 1, 2, 3, 4. For these experiments, the alloy is heated a first time in order to homogenise it in the glove box and after cooling, the reactor is tightly closed with an hermetic top (Swagelokâ type) at one of the extremities, the other end was previously closed by welding in an electric arc under argon flow. The reaction is carried out outside the glove box. At the end of the reaction, the reactor is opened in a glove box containing a pure argon atmosphere, heated in order to extract the sample that is placed in a capillary tube for X-ray examination. For the Ca–Li alloys of composition 1:1 and 1:2, ternary graphite–lithium–calcium compounds were synthesised. A reaction of a pyrographite platelet and an equimolar Ca–Li alloy carried out at 350 °C for 10 days leads to a ternary compound called phase whose repeat distance is 776 pm. At the same temperature and with the same reaction time, but using a CaLi2 alloy, an other ternary compound noted phase was obtained. The repeat distance of the latter reaches 970 pm. When the alloy composition is included between Li/Ca ¼ 3 and Li/Ca ¼ 4, a c compound is synthesised whose repeat distance is 454 pm. Several techniques were implemented to determine the elemental composition of these three compounds. It turned out that nuclear microprobe was the most powerful, due to the presence of lithium. So, the chemical formulae of these compounds were established by nuclear microprobe measurements [15], and the classical methods as Castaing microprobe, SEM and TEM confirm the carbon and calcium amounts. These formulae are respectively: Li0:4 Ca2:7 C6 for the a compound Li3:1 Ca2:2 C6 for the b compound Li0:07 CaC6 for the c compound The a compound contains some in-depth concentration heterogeneities. The most usually observed one is characterised by a constant C/Ca ratio (equal to 2.2) and a Ca/Li ratio fluctuating from 2 to 6.5. This enrichment in lithium of phase domains corresponds to the increase of the amount of lithium in the intercalated sheets and probably contributes to the stability of the compound. Otherwise, one should emphasise that the c compound exhibits a very low concentration of lithium, so that it appears almost as a pseudo-binary compound, close to the well known CaC6 phase [4]. In contrast, the metal contents of the a and b compounds are especially considerable.
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The structural study [16] of these compounds has clearly shown that the intercalated sheets of both a and b compounds are poly-layered, according to the large metal amount. In the a-phase the intercalated sheet is five-layered with a [Li–Ca–Li–Ca–Li] sequence. This stacking can be also found in the structure of the CaLi2 binary solid alloy. CaLi2 appears in the Ca–Li phase diagram as the only definite compound whose structure, studied by Hellner and Laves [17] and later by Carfagno [18] is hexagonal and isomorphic with the ThMn2 -type Laves’s phase. Along the c-axis, the CaLi2 structural arrangement can be described as a perfect alternation of calcium and lithium layers, in accordance with the [Li– Ca–Li–Ca–Li] five-layered intercalated sheet. In the b compound that contains more metal, the intercalated sheets are seven-layered. Indeed, the middle lithium plane is split into three layers leading to a [Li–Ca–Li– Li–Li–Ca–Li] sequence. This stacking was established from the quantitative study of the 00l X-ray and neutron reflexions. The Fourier transform of the 00l structure factors allows to draw the experimental electronic density (or atomic density) profile along the c-axis that is compared with another one, calculated from a model (Fig. 1). A good agreement between them is observed for the seven-layered intercalated sheet. Moreover, the two-dimensional lattice of each compound was determined thanks to the hk0 reflexions obtained by Xray diffraction and electron microdiffraction as well as the charge transfer from the intercalated sheets to the graphene planes [16]. 3.3. Graphite–lithium–strontium system The binary Li–Sr phase diagram [19] strongly differs from the Li–Mg one and presents some analogies with that of the Ca–Li system. It contains two definite compounds: Li2 Sr3 and Li23 Sr6 . The liquidus temperature remains lower than 400 °C for all alloys containing less
than 75 at.% of strontium. Several experiments were carried out at 350 and 450 °C between a pyrographite platelet and a liquid alloy the composition of which was Li/Sr ¼ 2/3. At 450 °C, a second stage binary graphite lithium compound is obtained; the reaction is not complete even after 50 h since remained some unreacted graphite. At 350 °C, second and third stage graphite– lithium binary compounds are synthesised accompanied by a small amount of the first stage binary SrC6 compound and some unreacted graphite. The 00l X-ray diffraction diagram of this sample is presented Fig. 2. The repeat distance of SrC6 reaches 495 pm [4] whereas that of LiC6 is only 370 pm [3]. In all cases, no ternary phase was obtained. 3.4. Graphite–lithium–barium system In the binary Ba–Li phase diagram [20], there is only one definite compound: BaLi4 . And, as in the Li–Sr diagram, the liquidus temperatures are not very high. Two different alloy compositions were studied: Li/Ba ¼ 1 and Li/Ba ¼ 2/3. The reaction between a pyrographite platelet and the equimolar Ba–Li alloy at 350 °C during 3.5 days leads to the formation of the first stage binary LiC6 compound, the second stage lithium–graphite compound and a small amount of the first stage BaC6 compound. When the barium concentration increases in the reactive alloy, the results are different. Indeed, the reaction carried out of graphite and the liquid alloy (Li/ Ba ¼ 2/3) permits at 450 °C the synthesis of the pure binary BaC6 compound the repeat distance of which is 526 pm [4]. The 00l X-ray diffraction diagram of this compound is presented Fig. 3. In this system, we obtained binary compounds either with lithium, or with barium, depending on the concentration of the alloy. The great size difference between both metals added to the strong stability of the binary compounds does not
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Fig. 1. Electronic density profiles along the c-axis for the b-graphite– lithium–calcium ternary compound (black line: experiment; grey line: model).
Fig. 2. 00l X-ray diffraction diagram of a pyrographite sample after a reaction at 350 °C in a molten Li2 Sr3 alloy: (d) third stage graphite– lithium compound; () second stage graphite–lithium compound; () SrC6 compound; (M) graphite.
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Fig. 3. 00l X-ray diffraction diagram of the BaC6 compound, (d) prepared in a molten Li–Ba alloy (Li/Ba ¼ 1/3) at 450 °C. The broad peak at 2h ¼ 5° is due to the pyrex tube.
allow the formation of a ternary graphite–lithium–barium compound. 4. Reactions of graphite with sodium and an alkaline earth metal Experiments were carried out with graphite and Ca– Na, Sr–Na and Ba–Na alloys. No experiments were done using a Na–Mg alloy since the corresponding binary phase diagram [21] indicates that high reaction temperatures would be needed for obtaining a ternary graphite intercalation compound. For the three studied systems, the alloys are prepared in a glove box under a very pure argon atmosphere and the experimental method is that described for the graphite–lithium– magnesium system. 4.1. Graphite–sodium–calcium system The very high liquidus temperatures of the binary sodium–calcium phase diagram [22] and the absence of definite compounds create very bad conditions for obtaining a ternary graphite intercalation compound. Some experiments were realised with sodium–calcium alloys containing 2 and 10 at.% of calcium at 450 and 400 °C for 48 and 22 h, respectively. At the end of each reaction, the sample is grey and presents quite the same thickness as before the experiment. The repeat distance obtained from the 00l X-ray diffraction diagram reaches 2460 pm that corresponds to a binary seventh stage graphite–sodium compound [2]. The 00l X-ray diffraction diagram of this compound is shown Fig. 4. This result is not surprising taking into account the phase diagram observation. 4.2. Graphite–sodium–strontium system The binary sodium–strontium phase diagram [23] differs from the previous one since the liquidus temper-
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Fig. 4. 00l X-ray diffraction diagram of a seventh stage graphite–sodium compound obtained at 450 °C in a Na–Ca alloy containing 2 at.% of calcium. The insert shows better the small peaks (007 line is cut).
ature is lower than 500 °C as long as the sodium content in the alloy in higher than 50 at.%. The first reaction was carried out at 400 °C with a pyrographite sample and an alloy with Na/Sr ¼ 4/1. After 2 days of reaction, a binary sodium–graphite compound belonging to stage 8 was obtained. A similar result was observed using an equimolar alloy, at 500 °C. The obtaining of a ternary graphite intercalation compound in this system is unlikely too. 4.3. Graphite–sodium–barium system Among the sodium–alkaline earth metal systems, that sodium–barium seems to be the more interesting one in order to obtain a ternary graphite intercalation compound. Indeed, the binary phase diagram [24] shows that all the alloys containing a barium concentration lower than 73 at.% are liquid under 500 °C. Moreover, in this system, there is a definite compound: Na4 Ba. The reaction between graphite and sodium–barium alloys was studied 25 years ago [25]. The experiments were carried out in a stainless steel reactor with pyrographite or single crystal graphite samples and Na–Ba alloys whose composition was not specified. Blue and black compounds were obtained. Chemical analyses on blue compounds, that are the richest in metal, gave a C/ (Na + Ba) ratio between 7 and 8 and a Na/Ba ratio ranging from 1.6 to 6, but usually near 2. The 00l X-ray diffraction diagram of a blue compound has given a repeat distance of 1073 pm for a second stage compound with a proposed three-layered intercalated sheet corresponding to a [Na–Ba–Na] stacking. The in-plane structure was studied using X-ray diffraction and electronic diffraction. The hk0 reflexions of the intercalated sheet can be indexed in a hexagonal lattice: it is rotated by 30° with respect to the graphitic one and the a parameter is 368 pm. According to the stoichiometry, the second hypothesis is held. In conclusion, these
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compounds appear as ternary graphite–sodium–barium compounds containing metallic layers with a network parallel but incommensurate with that of graphite. The large fluctuation of the composition seems not to affect the structure. In 1997, El Gadi et al. [10] published a paper relative to ternary graphite–sodium–oxygen compounds. These compounds are prepared by immersing pyrographite or single crystal samples in liquid sodium slightly oxidised at 470 °C during a few days. After the reaction, the samples are blue and present a repeat distance of 1080 pm. As it is indicated above, the quantitative study of the 00l reflexions allowed to establish a 1D structural model along the c-axis. The compound belongs to stage 2 and its intercalated sheets are five-layered with a [Na– O–Na–O–Na] stacking. The study of the hk0 reflexions obtained from a single crystal shows that the intercalated sheet unit cell is hexagonal, incommensurate and rotated by almost 30° with respect to that of graphite. More precisely, there are two lattices rotated by 30.7° and 29.3° with respect to the [10] graphite direction. Moreover, the 1D structure along the c-axis and the 2D in-plane structure of the intercalated sheets present a great resemblance with that of sodium peroxide. Now, if we compare on the one hand the 00l X-ray diffraction diagrams and on the other hand the in-plane structures described in both publications, a lot of analogies appear. Concerning the 00l X-ray diffraction diagrams that are presented Fig. 5 for both ternary compounds (‘‘graphite–sodium–barium’’ and graphite–sodium– oxygen), the repeat distances are quite the same.
Moreover, in the graphite–sodium–oxygen system, two phases were observed, with very close repeat distances: 1077 and 1082 pm. The existence of several phases with quite similar stacking along the c-axis is a classic phenomenon exhibited by the ternary compounds [6]. The relative intensities of the 00l peaks are quite the same with the maximum for the 003 and an almost extinguished 008 reflection. The electronic microdiffraction pattern of the ‘‘graphite–sodium–barium’’ compound is very similar, for not saying identical to the flat-camera diagram and the electron microdiffraction of the graphite sodium peroxide compound [26]. Consequently one can suppose that the ternary compound interpreted as a ternary ‘‘graphite–sodium–barium’’ compound is undoubtedly a ternary graphite–sodium– oxygen compound. New experiments were carried out of pyrographite samples with sodium–barium alloys. The immersion of a pyrographite sample in the equimolar Na–Ba alloy at 400 °C for one day leads to a mixture of unreacted graphite, binary sodium–graphite compound of stage 8 and a small amount of the binary first stage BaC6 compound. The corresponding 00l X-ray diffraction diagram is presented Fig. 6. Other experiments were carried out with the Na4 Ba alloy. After a reaction at 250 °C during 1 day, graphite, binary graphite–sodium of stage 8 and BaC6 were obtained without ternary compound. With the same alloy concentration, at a higher temperature (400 °C) only unreacted graphite and a small amount of BaC6 were obtained. The objective analysis of all the results leads to the following conclusion: the ternary compound presented
Fig. 5. 00l X-ray diffraction diagrams (Mo, Ka1) of second stage ternary compounds: (a) graphite–sodium–barium compound described in [25]; (b) graphite–sodium–oxygen compound described in [10].
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Fig. 6. 00l X-ray diffraction diagram (Mo Ka1 anticathode) of a pyrographite sample after a reaction at 400 °C in an equimolar Na–Ba alloy: (d) BaC6 compound; (j) sodium–graphite compound of stage 8; (N) graphite.
in [25] as a ternary graphite–sodium–barium compound is certainly a graphite–sodium–oxygen compound. Indeed it is well known now that a very small oxygen amount (1 at.%) added to potassium or sodium is sufficient to obtain a ternary graphite–alkali metal–oxygen compound instead of the corresponding binary phases [6]. It is entirely conceivable that the sodium–barium alloy or the reaction atmosphere was not perfectly pure. It was then sufficient to prepare a ternary graphite sodium peroxide compound, but it was very difficult to come to such a conclusion considering the initial reagents and the misreading of the oxygen influence on the alkali metal intercalation. 5. Conclusion Contrary to the heavy alkali metals, lithium and sodium allow with difficulty the formation of ternary compounds. Among the alkaline earth metals as associated elements, only calcium is able to co-intercalate with lithium into graphite, leading to pure graphite– lithium–calcium compounds. Not any ternary compound was prepared by associating sodium with an alkaline earth metal. However, it was shown that the graphite–sodium–barium compound described in [25] was certainly a graphite–sodium–oxygen compound. References [1] Fredenhagen K, Cadenbach G. Die Bindung von Kalium durch Kolhenstoff. Z Anorg Allg Chem 1926;158:249–63. [2] Metrot A, Guerard D, Billaud D, Herold A. New results about the sodium–graphite system. Synth Met 1979/80;1:363–9. [3] Guerard D, Herold A. Intercalation of lithium into graphite and other carbons. Carbon 1975;13:337–45.
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[4] Guerard D, Chaabouni M, Lagrange P, El Makrini M, Herold A. Insertion de metaux alcalino-terreux dans le graphite. Carbon 1979;18:257–64. [5] Lagrange P. Chemical Physics on Intercalation II. New York and London: Plenum Press; 1993. p. 303–10. [6] Herold C, Herold A, Lagrange P. Ternary graphite intercalation compounds associating an alkali metal and a electronegative element or radical. Solid State Sci 2004;6(1):125–38. [7] Herold A, Billaud D, Guerard D, Lagrange P. Action competitive de deux metaux alcalins sur le graphite. Mater Sci Eng 1977;31:25–8. [8] Billaud D, Herold A. Recherches sur les composes ternaires graphite–sodium–alcalin lourd. Bull Soc Chim 1974;12:2715–8. [9] Pruvost S, Herold C, Herold A, Lagrange P. On the great difficulty of intercalating lithium with a second element into graphite. Carbon 2003;41:1281–9. [10] El Gadi M, Herold A, Herold C, Mar^eche JF, Lagrange P. Intercalation dans le graphite du sodium associe a des ions peroxyde. J Solid State Chem 1997;131:282–9. [11] Herold A, Lelaurain M, Mar^eche JF. Composes d’insertion du graphite contenant des ions sodium ou potassium associes a des ions hydroxyde. C R Acad Sci Paris 1991;312(Serie II):361–5. [12] Herold A, Mar^eche JF, Lelaurain M. Intercalation of sodium with its halides into graphite. Carbon 2000;38:1955–63. [13] Nayeb-Hashemi AA, Pelton AD, Clark JB. Lithium–magnesium phase diagram. In: Massalski TB, editor. Binary alloy phase diagrams. 2nd ed. USA: ASM International; 1990. p. 2444–5. [14] Bale CW, Pelton AD. The calcium–lithium system. Bull Alloys Phase Diagrams 1987;8(2):125–7. [15] Berger P, Pruvost S, Herold C, Lagrange P. PES and NRA nuclear microprobe characterization of ternary graphite–lithium– calcium intercalation compounds. Nucl. Instr. and Meth in Phys. Res. B, in press. [16] Pruvost S, Herold C, Herold A, Lagrange P. Structural study of novel graphite–lithium–calcium intercalation compounds. Eur. J. Inorg. Chem., in press. [17] Hellner E, Laves F. Die Kristallstrukturen von SrMg2 , BaMg2 und CaLi2 . Z Kristallogr 1943;105:135–43. [18] Carfagno DG. The calcium–lithium equilibrium system. PhD Thesis, Syracuse University, New York, 1966. [19] Bale CW, Pelton AD. Lithium–strontium phase diagram. In: Massalski TB, editor. Binary alloy phase diagrams. 2nd ed. USA: ASM International; 1990. p. 2471–2. [20] Pelton AD. Barium–lithium phase diagram. In: Massalski TB, editor. Binary alloy phase diagrams. 2nd ed. USA: ASM International; 1990. p. 587–9. [21] Pelton AD. Magnesium–sodium phase diagram. In: Massalski TB, editor. Binary alloy phase diagrams. 2nd ed. USA: ASM International; 1990. p. 2524–5. [22] Pelton AD. Calcium–sodium phase diagram. In: Massalski TB, editor. Binary alloy phase diagrams. 2nd ed. USA: ASM International; 1990. p. 932–4. [23] Pelton AD. Sodium–strontium phase diagram. In: Massalski TB, editor. Binary alloy phase diagrams. 2nd ed. USA: ASM International; 1990. p. 2733–5. [24] Pelton AD. Barium–sodium phase diagram. In: Massalski TB, editor. Binary alloy phase diagrams. 2nd ed. USA: ASM International; 1990. p. 595–6. [25] Billaud D, Herold A. Sur les composes graphite–sodium–baryum. Bull Soc Chim Fr 1978;3–4:131–4. [26] El Gadi M. Etude de l’insertion dans le graphite du potassium et du sodium en presence d’oxygene. Thesis Oujda University, Morocco, 1994:157–158.
Co-intercalation into graphite of lithium and sodium with an alkaline earth metal Sebastien Pruvost
a,1
, Claire Herold
a,*
, Albert Herold a, Philippe Lagrange
a,b
a
b
Laboratoire de Chimie du Solide Mineral––UMR CNRS UHP 7555, Universite Henri Poincare Nancy I, B.P. 239, 54506 Vandœuvre-les-Nancy Cedex, France Ecole Europeenne d’Ingenieurs en Genie des Materiaux, Institut National Polytechnique de Lorraine, 6 rue Bastien Lepage, B.P. 630, 54010 Nancy Cedex, France Received 1 December 2003; accepted 11 March 2004 Available online 15 April 2004
Abstract After a short overview on the co-intercalation into graphite of light alkali metals––lithium and sodium––with a third element, novel results concerning the co-intercalation of these alkali metals with an alkaline earth metal are given. The intercalation into graphite of lithium together with magnesium, calcium, strontium and barium is investigated, and the reaction products are described and characterised by X-ray diffraction. Ternary intercalation compounds were synthesised only in the graphite–lithium–calcium system. In the other systems, binary compounds were obtained. The co-intercalation of sodium and the alkaline earth metals calcium, strontium and barium is studied too. No ternary compounds were obtained. However, in the case of the sodium–barium system, a discussion on ternary graphite–sodium–barium compounds previously published is opened. Several data are in favour of a graphite–sodium–oxygen compound instead of a graphite–sodium–barium one. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Highly oriented graphite, Intercalation compounds; B. Intercalation; C. X-ray diffraction
1. Introduction The heavy alkali metals intercalate very easily into graphite leading to various binary compounds [1]. They are characterised by their stage that increases when the intercalated amount of metal decreases. Their intercalated sheets are systematically mono-layers and are commensurate with respect to the adjacent 2D graphene layers. The first stage compounds (KC8 , RbC8 and CsC8 ) are prepared by action of metal vapour on graphite samples at quite low temperature (100–200 °C). Their interplanar distances reach 535, 565 and 592 pm respectively. The intercalation into graphite of lithium and especially sodium is much more difficult. The reaction temperatures are appreciably higher. With sodium, it is * Corresponding author. Tel.: +33-383-684-884; fax: +33-383-684615. E-mail address: [email protected] (C. Herold). 1 Present address: Laboratoire d’Electrodynamique des Materiaux Avances––UMR-CNRS-CEA 6157, IUT de Blois, Universite de Tours, 3 Place Jean Jaures, C.S. 2903 41029 Blois Cedex, France.
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.03.014
strictly impossible to obtain compounds containing large quantities of metal, so that only high stage compounds (sixth, seventh and eighth stage) can be synthesised [2]. On the other hand, the first stage LiC6 compound [3] is well known and presents also mono-layers of the intercalated atoms, but the metal amount is especially high for this compound because of the small size of the lithium atoms. Nevertheless, its intercalated layers are also commensurate with respect to the adjacent graphene planes. In spite of a less pronounced electropositive character, the alkaline earth metals intercalate into graphite quite similar to lithium [4]. The first stage binaries are also well known (CaC6 , SrC6 and BaC6 ) and resemble LiC6 closely: the structure of the 2D intercalated layers is the same, only the c-axis stacking is different between lithium and alkaline earth metal compounds. Ternary graphite intercalation compounds containing a heavy alkali metal have been intensively studied in the past decades [5,6]. These ternaries are very numerous and the corresponding chemistry is extremely rich and interesting.
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On the other hand, an equivalent study with lithium and sodium had not been undertaken in the same manner. Only a few experiments are known; they have been carried out selectively and never systematically. In this paper, we have gathered firstly the main results of these limited studies. And secondly, we provide the novel results of our systematic study concerning the attempts of co-intercalation into graphite of lithium or sodium with an alkaline earth metal.
2. Previous results It has been well established that lithium is unable to co-intercalate into graphite with heavy alkali metals [7]. The results of all experiments show clearly that both metals intercalate separately, leading to two phases (LiC6 and MC8 ). On the other hand, sodium is able to co-intercalate into graphite with potassium, rubidium and caesium [8], leading to first stage ternary compounds, for which the intercalated sheets are mono-layered, sodium and second metal atoms being statistically distributed in the crystallographic sites of the intercalated layer. The formula of such a compound can be written Nax M1x C8 (M ¼ K, Rb or Cs), so that it appears as a ‘‘substitution ternary’’. If we exclude the very interesting results concerning the co-intercalation of lithium and calcium, that are related in the following part, all the other experiments concerning the intercalation of lithium associated with a second element have been negative [9]. But, in the case of sodium, several cases have led to positive results. Sodium indeed is able to intercalate into graphite associated with oxygen [10]. Liquid sodium containing a small amount of oxygen (about 1 at.%), intercalates very easily into graphite and leads to a blue second stage ternary compound, whose intercalated sheets are five-layered according to a [Na–O–Na–O–Na] c-axis sequence. Its repeat distance of 1080 pm corresponds to an interplanar distance of 745 pm. The intercalated oxygen appears as peroxide anions, whose O–O axis is parallel to the graphite c-axis, with an interatomic distance of 150 pm. The O2 2 anions are thus oriented perpendicular to the intercalated sodium layers. In the Na2 O2 binary peroxide, these anions are also perpendicular to the planes of sodium: planes of sodium atoms alternate with planes of oxygen atoms along the c-axis. It has been shown that it is possible to consider that the intercalated sheets are well cut (perpendicularly to its c-axis) sodium peroxide slices. The 2D unit cell is hexagonal, with a parameter of 367 pm, so that it is not commensurate with respect to the graphitic one. Sodium intercalates also into graphite in association with hydroxide anions [11]. A blue first stage compound
is synthesised by reaction at 350 °C of graphite with the mixture obtained by melting sodium with its hydroxide. This original compound contains Naþ and OH ions, with a small proportion of H anions. The intercalate forms double-layer sheets and leads to an interplanar distance of 921 pm. Both layers of these sheets are mixed, since they contain simultaneously Naþ and OH ions, so that highly ionic bonds arise in each layer as between both layers of a given sheet (this is a particular characteristic that is extremely rare). The 2D unit cell of the intercalate is square, with a parameter of 341 pm; it is of course not commensurate with the graphitic one. The double layer intercalated sheets exhibit an arrangement very close to that of the quadratic free sodium hydroxide. Ternary graphite intercalation compounds containing sodium associated with chloride, bromide and iodide anions [12] have been synthesised by heating graphite with liquid sodium in the presence of NaCl, NaBr and NaI between 450 and 1000 °C, during a time which varies from several hours to several weeks, according to the case. Liquid sodium plays a triple role: it reduces graphite, it intercalates into graphite and it is a transfer agent for the halide anions. The compounds belong to the stages 2, 3 and 4 and the stage decreases by increasing the temperature (this is an exception in graphite intercalation chemistry). The intercalated sheets are made of two sodium cation planes surrounding a halide anion layer and present some similarities with the corresponding (1 1 1) plane of the free halides. The interplanar distances reach 756 and 771 pm respectively for G–Na–Cl and G–Na–Br compounds. The 2D unit cell is hexagonal and commensurate with that of the graphene sheets, since the value of its parameter is 246 pm. On the other hand, the G–Na–I system is more complicated, since it leads to several interplanar distances, whose values range between 769 and 790 pm. In this system, more than 10 in-plane structures have been observed (commensurate or not, according to the case). Similar ternary compounds could not be obtained in the G–Na–F system. But, quaternary phases G–Na–I–F and G–Na–Cl–F have been synthesised by reaction at respectively 850 and 650 °C of graphite and liquid sodium associated with both NaF and NaI (or NaF and NaCl) halides. These quaternaries are very numerous and appear always as complex phases, so that they are not described here. Their complete description is given in a paper devoted to this topic [12].
3. Reactions of graphite with lithium and an alkaline earth metal (novel results) Numerous reactions were carried out with pyrographite platelets and liquid metallic alloys containing
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lithium and an alkaline earth metal. Magnesium, calcium, strontium and barium associated with lithium were studied. All the binary alloys are prepared in a glove box containing a very pure argon atmosphere. The reactor consists of a stainless steel tube, and the pyrographite sample is introduced into the molten metallic alloy. The reaction temperature depends strongly on the binary phase diagram of the lithium– alkaline earth metal system. However, it is also well known now that the alloy composition plays a major role in the obtaining of a binary or a ternary intercalation compound. Indeed, if the alloy is very rich in lithium, only a binary lithium–graphite compound will be obtained.
3.1. Graphite–lithium–magnesium system No definite compound exists in the binary Li–Mg phase diagram [13]. The liquidus temperature reaches values higher than 400 °C as soon as the magnesium concentration is greater than 30 at.%. Otherwise, the high reaction temperatures favour the formation of carbides [9]. Both elements are precisely weighed and introduced into a stainless steel tube in the glove box. The tube is closed and then transferred to an apparatus equipped with a furnace that can be used either under argon or under vacuum. This apparatus allows also to introduce the sample under argon. Before the introduction of the pyrographite platelet, the alloy is heated to a temperature higher than its melting point in order to homogenise it. The sample is then introduced into the molten alloy and the reaction is carried out under a slight argon overpressure. At the end of the reaction, the sample is extracted from the liquid alloy and after cooling, it is placed under an argon flow in a capillary tube for X-ray investigations. The composition of the first studied alloy is Li/ Mg ¼ 3, which is liquid at 350 °C. After 16 h in the liquid alloy at 450 °C, only lithium intercalates into graphite and the first stage binary LiC6 compound is obtained. The alloy is too rich in lithium so that its activity in the molten alloy is too high in order to permit the formation of a ternary compound. The second alloy contains less lithium since the Li/Mg ratio is equal to 3/2. The increase in the magnesium amount leads to an increase in the melting point of the alloy to 490 °C. After 12 h, as after 24 h of reaction at 540 °C, the pyrographite did not react. However, a small amount of lithium carbide is obtained after 48 h at 540 °C. Taking into account the binary Li–Mg phase diagram (high temperatures for the liquidus) and the experiences described above, the synthesis of a ternary graphite–lithium–magnesium compound seems to be very unlikely.
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3.2. Graphite–lithium–calcium system The Ca–Li binary phase diagram [14] strongly differs from the previous one. The liquidus becomes widely lower than 350 °C since the alloy contains less than 50 at.% of calcium. CaLi2 is the single definite compound of this system. Several alloys were studied with the following compositions: Li/Ca ¼ 1, 2, 3, 4. For these experiments, the alloy is heated a first time in order to homogenise it in the glove box and after cooling, the reactor is tightly closed with an hermetic top (Swagelokâ type) at one of the extremities, the other end was previously closed by welding in an electric arc under argon flow. The reaction is carried out outside the glove box. At the end of the reaction, the reactor is opened in a glove box containing a pure argon atmosphere, heated in order to extract the sample that is placed in a capillary tube for X-ray examination. For the Ca–Li alloys of composition 1:1 and 1:2, ternary graphite–lithium–calcium compounds were synthesised. A reaction of a pyrographite platelet and an equimolar Ca–Li alloy carried out at 350 °C for 10 days leads to a ternary compound called phase whose repeat distance is 776 pm. At the same temperature and with the same reaction time, but using a CaLi2 alloy, an other ternary compound noted phase was obtained. The repeat distance of the latter reaches 970 pm. When the alloy composition is included between Li/Ca ¼ 3 and Li/Ca ¼ 4, a c compound is synthesised whose repeat distance is 454 pm. Several techniques were implemented to determine the elemental composition of these three compounds. It turned out that nuclear microprobe was the most powerful, due to the presence of lithium. So, the chemical formulae of these compounds were established by nuclear microprobe measurements [15], and the classical methods as Castaing microprobe, SEM and TEM confirm the carbon and calcium amounts. These formulae are respectively: Li0:4 Ca2:7 C6 for the a compound Li3:1 Ca2:2 C6 for the b compound Li0:07 CaC6 for the c compound The a compound contains some in-depth concentration heterogeneities. The most usually observed one is characterised by a constant C/Ca ratio (equal to 2.2) and a Ca/Li ratio fluctuating from 2 to 6.5. This enrichment in lithium of phase domains corresponds to the increase of the amount of lithium in the intercalated sheets and probably contributes to the stability of the compound. Otherwise, one should emphasise that the c compound exhibits a very low concentration of lithium, so that it appears almost as a pseudo-binary compound, close to the well known CaC6 phase [4]. In contrast, the metal contents of the a and b compounds are especially considerable.
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The structural study [16] of these compounds has clearly shown that the intercalated sheets of both a and b compounds are poly-layered, according to the large metal amount. In the a-phase the intercalated sheet is five-layered with a [Li–Ca–Li–Ca–Li] sequence. This stacking can be also found in the structure of the CaLi2 binary solid alloy. CaLi2 appears in the Ca–Li phase diagram as the only definite compound whose structure, studied by Hellner and Laves [17] and later by Carfagno [18] is hexagonal and isomorphic with the ThMn2 -type Laves’s phase. Along the c-axis, the CaLi2 structural arrangement can be described as a perfect alternation of calcium and lithium layers, in accordance with the [Li– Ca–Li–Ca–Li] five-layered intercalated sheet. In the b compound that contains more metal, the intercalated sheets are seven-layered. Indeed, the middle lithium plane is split into three layers leading to a [Li–Ca–Li– Li–Li–Ca–Li] sequence. This stacking was established from the quantitative study of the 00l X-ray and neutron reflexions. The Fourier transform of the 00l structure factors allows to draw the experimental electronic density (or atomic density) profile along the c-axis that is compared with another one, calculated from a model (Fig. 1). A good agreement between them is observed for the seven-layered intercalated sheet. Moreover, the two-dimensional lattice of each compound was determined thanks to the hk0 reflexions obtained by Xray diffraction and electron microdiffraction as well as the charge transfer from the intercalated sheets to the graphene planes [16]. 3.3. Graphite–lithium–strontium system The binary Li–Sr phase diagram [19] strongly differs from the Li–Mg one and presents some analogies with that of the Ca–Li system. It contains two definite compounds: Li2 Sr3 and Li23 Sr6 . The liquidus temperature remains lower than 400 °C for all alloys containing less
than 75 at.% of strontium. Several experiments were carried out at 350 and 450 °C between a pyrographite platelet and a liquid alloy the composition of which was Li/Sr ¼ 2/3. At 450 °C, a second stage binary graphite lithium compound is obtained; the reaction is not complete even after 50 h since remained some unreacted graphite. At 350 °C, second and third stage graphite– lithium binary compounds are synthesised accompanied by a small amount of the first stage binary SrC6 compound and some unreacted graphite. The 00l X-ray diffraction diagram of this sample is presented Fig. 2. The repeat distance of SrC6 reaches 495 pm [4] whereas that of LiC6 is only 370 pm [3]. In all cases, no ternary phase was obtained. 3.4. Graphite–lithium–barium system In the binary Ba–Li phase diagram [20], there is only one definite compound: BaLi4 . And, as in the Li–Sr diagram, the liquidus temperatures are not very high. Two different alloy compositions were studied: Li/Ba ¼ 1 and Li/Ba ¼ 2/3. The reaction between a pyrographite platelet and the equimolar Ba–Li alloy at 350 °C during 3.5 days leads to the formation of the first stage binary LiC6 compound, the second stage lithium–graphite compound and a small amount of the first stage BaC6 compound. When the barium concentration increases in the reactive alloy, the results are different. Indeed, the reaction carried out of graphite and the liquid alloy (Li/ Ba ¼ 2/3) permits at 450 °C the synthesis of the pure binary BaC6 compound the repeat distance of which is 526 pm [4]. The 00l X-ray diffraction diagram of this compound is presented Fig. 3. In this system, we obtained binary compounds either with lithium, or with barium, depending on the concentration of the alloy. The great size difference between both metals added to the strong stability of the binary compounds does not
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Fig. 1. Electronic density profiles along the c-axis for the b-graphite– lithium–calcium ternary compound (black line: experiment; grey line: model).
Fig. 2. 00l X-ray diffraction diagram of a pyrographite sample after a reaction at 350 °C in a molten Li2 Sr3 alloy: (d) third stage graphite– lithium compound; () second stage graphite–lithium compound; () SrC6 compound; (M) graphite.
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Fig. 3. 00l X-ray diffraction diagram of the BaC6 compound, (d) prepared in a molten Li–Ba alloy (Li/Ba ¼ 1/3) at 450 °C. The broad peak at 2h ¼ 5° is due to the pyrex tube.
allow the formation of a ternary graphite–lithium–barium compound. 4. Reactions of graphite with sodium and an alkaline earth metal Experiments were carried out with graphite and Ca– Na, Sr–Na and Ba–Na alloys. No experiments were done using a Na–Mg alloy since the corresponding binary phase diagram [21] indicates that high reaction temperatures would be needed for obtaining a ternary graphite intercalation compound. For the three studied systems, the alloys are prepared in a glove box under a very pure argon atmosphere and the experimental method is that described for the graphite–lithium– magnesium system. 4.1. Graphite–sodium–calcium system The very high liquidus temperatures of the binary sodium–calcium phase diagram [22] and the absence of definite compounds create very bad conditions for obtaining a ternary graphite intercalation compound. Some experiments were realised with sodium–calcium alloys containing 2 and 10 at.% of calcium at 450 and 400 °C for 48 and 22 h, respectively. At the end of each reaction, the sample is grey and presents quite the same thickness as before the experiment. The repeat distance obtained from the 00l X-ray diffraction diagram reaches 2460 pm that corresponds to a binary seventh stage graphite–sodium compound [2]. The 00l X-ray diffraction diagram of this compound is shown Fig. 4. This result is not surprising taking into account the phase diagram observation. 4.2. Graphite–sodium–strontium system The binary sodium–strontium phase diagram [23] differs from the previous one since the liquidus temper-
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Fig. 4. 00l X-ray diffraction diagram of a seventh stage graphite–sodium compound obtained at 450 °C in a Na–Ca alloy containing 2 at.% of calcium. The insert shows better the small peaks (007 line is cut).
ature is lower than 500 °C as long as the sodium content in the alloy in higher than 50 at.%. The first reaction was carried out at 400 °C with a pyrographite sample and an alloy with Na/Sr ¼ 4/1. After 2 days of reaction, a binary sodium–graphite compound belonging to stage 8 was obtained. A similar result was observed using an equimolar alloy, at 500 °C. The obtaining of a ternary graphite intercalation compound in this system is unlikely too. 4.3. Graphite–sodium–barium system Among the sodium–alkaline earth metal systems, that sodium–barium seems to be the more interesting one in order to obtain a ternary graphite intercalation compound. Indeed, the binary phase diagram [24] shows that all the alloys containing a barium concentration lower than 73 at.% are liquid under 500 °C. Moreover, in this system, there is a definite compound: Na4 Ba. The reaction between graphite and sodium–barium alloys was studied 25 years ago [25]. The experiments were carried out in a stainless steel reactor with pyrographite or single crystal graphite samples and Na–Ba alloys whose composition was not specified. Blue and black compounds were obtained. Chemical analyses on blue compounds, that are the richest in metal, gave a C/ (Na + Ba) ratio between 7 and 8 and a Na/Ba ratio ranging from 1.6 to 6, but usually near 2. The 00l X-ray diffraction diagram of a blue compound has given a repeat distance of 1073 pm for a second stage compound with a proposed three-layered intercalated sheet corresponding to a [Na–Ba–Na] stacking. The in-plane structure was studied using X-ray diffraction and electronic diffraction. The hk0 reflexions of the intercalated sheet can be indexed in a hexagonal lattice: it is rotated by 30° with respect to the graphitic one and the a parameter is 368 pm. According to the stoichiometry, the second hypothesis is held. In conclusion, these
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compounds appear as ternary graphite–sodium–barium compounds containing metallic layers with a network parallel but incommensurate with that of graphite. The large fluctuation of the composition seems not to affect the structure. In 1997, El Gadi et al. [10] published a paper relative to ternary graphite–sodium–oxygen compounds. These compounds are prepared by immersing pyrographite or single crystal samples in liquid sodium slightly oxidised at 470 °C during a few days. After the reaction, the samples are blue and present a repeat distance of 1080 pm. As it is indicated above, the quantitative study of the 00l reflexions allowed to establish a 1D structural model along the c-axis. The compound belongs to stage 2 and its intercalated sheets are five-layered with a [Na– O–Na–O–Na] stacking. The study of the hk0 reflexions obtained from a single crystal shows that the intercalated sheet unit cell is hexagonal, incommensurate and rotated by almost 30° with respect to that of graphite. More precisely, there are two lattices rotated by 30.7° and 29.3° with respect to the [10] graphite direction. Moreover, the 1D structure along the c-axis and the 2D in-plane structure of the intercalated sheets present a great resemblance with that of sodium peroxide. Now, if we compare on the one hand the 00l X-ray diffraction diagrams and on the other hand the in-plane structures described in both publications, a lot of analogies appear. Concerning the 00l X-ray diffraction diagrams that are presented Fig. 5 for both ternary compounds (‘‘graphite–sodium–barium’’ and graphite–sodium– oxygen), the repeat distances are quite the same.
Moreover, in the graphite–sodium–oxygen system, two phases were observed, with very close repeat distances: 1077 and 1082 pm. The existence of several phases with quite similar stacking along the c-axis is a classic phenomenon exhibited by the ternary compounds [6]. The relative intensities of the 00l peaks are quite the same with the maximum for the 003 and an almost extinguished 008 reflection. The electronic microdiffraction pattern of the ‘‘graphite–sodium–barium’’ compound is very similar, for not saying identical to the flat-camera diagram and the electron microdiffraction of the graphite sodium peroxide compound [26]. Consequently one can suppose that the ternary compound interpreted as a ternary ‘‘graphite–sodium–barium’’ compound is undoubtedly a ternary graphite–sodium– oxygen compound. New experiments were carried out of pyrographite samples with sodium–barium alloys. The immersion of a pyrographite sample in the equimolar Na–Ba alloy at 400 °C for one day leads to a mixture of unreacted graphite, binary sodium–graphite compound of stage 8 and a small amount of the binary first stage BaC6 compound. The corresponding 00l X-ray diffraction diagram is presented Fig. 6. Other experiments were carried out with the Na4 Ba alloy. After a reaction at 250 °C during 1 day, graphite, binary graphite–sodium of stage 8 and BaC6 were obtained without ternary compound. With the same alloy concentration, at a higher temperature (400 °C) only unreacted graphite and a small amount of BaC6 were obtained. The objective analysis of all the results leads to the following conclusion: the ternary compound presented
Fig. 5. 00l X-ray diffraction diagrams (Mo, Ka1) of second stage ternary compounds: (a) graphite–sodium–barium compound described in [25]; (b) graphite–sodium–oxygen compound described in [10].
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Fig. 6. 00l X-ray diffraction diagram (Mo Ka1 anticathode) of a pyrographite sample after a reaction at 400 °C in an equimolar Na–Ba alloy: (d) BaC6 compound; (j) sodium–graphite compound of stage 8; (N) graphite.
in [25] as a ternary graphite–sodium–barium compound is certainly a graphite–sodium–oxygen compound. Indeed it is well known now that a very small oxygen amount (1 at.%) added to potassium or sodium is sufficient to obtain a ternary graphite–alkali metal–oxygen compound instead of the corresponding binary phases [6]. It is entirely conceivable that the sodium–barium alloy or the reaction atmosphere was not perfectly pure. It was then sufficient to prepare a ternary graphite sodium peroxide compound, but it was very difficult to come to such a conclusion considering the initial reagents and the misreading of the oxygen influence on the alkali metal intercalation. 5. Conclusion Contrary to the heavy alkali metals, lithium and sodium allow with difficulty the formation of ternary compounds. Among the alkaline earth metals as associated elements, only calcium is able to co-intercalate with lithium into graphite, leading to pure graphite– lithium–calcium compounds. Not any ternary compound was prepared by associating sodium with an alkaline earth metal. However, it was shown that the graphite–sodium–barium compound described in [25] was certainly a graphite–sodium–oxygen compound. References [1] Fredenhagen K, Cadenbach G. Die Bindung von Kalium durch Kolhenstoff. Z Anorg Allg Chem 1926;158:249–63. [2] Metrot A, Guerard D, Billaud D, Herold A. New results about the sodium–graphite system. Synth Met 1979/80;1:363–9. [3] Guerard D, Herold A. Intercalation of lithium into graphite and other carbons. Carbon 1975;13:337–45.
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