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Liquid phase reactions in the system graphite-potassium-caesium-mercury
Journal of Alloys and Compounds, 204 (1994) 21-25 JALCOM 862
21
Liquid phase reactions in the system graphite-potassiumcaesium-mercury I.A. U d o d
and H.B. Orman
M.V. Lomonosov Moscow State University, Department of Chemistry, 119899 Moscow (Russian Federation) (Received June 18, 1993)
Abstract The intercalation of metals and alloys in different second-stage donor graphite intercalation compounds (GICs) was investigated. The proposed schemes of the segregation reaction in the graphite-potassium-caesium-mercury system can explain why the quantity of donor-donor bi-intercalated GICs is essentially restricted in contrast with acceptor-acceptor bi-intercalated GICs.
1. Introduction The existence of the typical "stage structure" (the ordered sequence of occupied and vacant interlayer spacings) [1] of graphite intercalation compounds (GICs) is a consequence of the electrostatic repulsion of intercalated layers [2] and the elastic properties of the graphite matrix [3]. The tensions on the domain bonds within the same crystal will be at a minimum according to the Daumas-Herold model [4] (Fig. 1). Previously [5, 6] we investigated why, on intercalation of lithium into vacant interlayer spacings of the secondstage GIC CsKHg, a segregation reaction takes place and a mixture of the first-stage GICs C6Li and C4KHg is formed. The formation of C4KHg demonstrates the intercrystalline redistribution of intercalates from one domain to another. Alternatively, an interaction between melted lithium and KHg must take place. Thus the existence of the GICs C6Li and C4KHg within the limit of the same crystal assumes large tensions on the graphite networks (Fig. 2). The tensions on the graphite networks will be essentially lower if a bi-intercalated GIC (bi-GIC) (GIC with alternating layers of intercalates) is formed (Fig.
E 0000(3/ ~,
/0 000 0 ] /0 O000
Fig. 1. Structure of the third stage according to the Daumas-Herold model.
0925-8388/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0 9 2 5 - 8 3 8 8 ( 9 3 ) 0 0 8 6 2 - S
ooggoQ~ 000000 ~ o@0ooo oo0ooo 00000 00000
0 0
C4KHg Fig. 2. Coexistence of the first-stage GICs C4KHg and C6Li within the same crystal.
~:~:z:z~:z~:~ ~ ~ 0 0 00
ooo Fig. 3. Structure of a bi-intercalated GIC.
3). In general, all the known bi-GICs can be classified according to the type of intercalate, e.g. donor-donor, donor-acceptor and acceptor-acceptor compounds. In this work, we attempt to clarify the mechanism of intercalation in high stage donor GICs.
2. Experimental details Highly oriented pyrolytic graphite (disorientation angle along the c-axis of about 1°) in the form of quasisingle-crystal plates was used. Intercalation on a graphite powder proceeds too rapidly and does not allow the
22
I.A. Udod, H.B. Orman / Liquid phase reactions in graphite-K-Cs-Hg
formation of intermediate compounds to be observed. When plates are used, the reaction step can be controlled by a time factor. The reactions studied proceed not only within the limits of the same graphite single-crystal plate, but within the limits of the same crystal. Therefore methods of chemical microanalysis are not acceptable. Thus the only technique which can be used for the identification of synthesized compounds is X-ray analysis, which yields the identity period Ic of the GICs obtained. The identity period Ic is the individual characteristic of a GIC, whereas the interplanar distance d~ of the occupied layer characterizes the intercalate and does not depend on the stage number (Fig. 1). The identity period is related to the interplanar distance and the stage number by the correlation Ic =d~ + 3.35 × (n - 1)
(1)
As the interplanar distance is independent of the stage number, the identity period of a bi-intercalated compound (Fig. 3) is equal to Ic = d,, + d, 2
(2)
where d~, and di= are the interplanar distances of the intercalates in binary compounds proper. In the majority of cases the experimental value of the identity period lc is close to that calculated (if the intitial intercalate layer is unchanged).
3. Results and discussion
Ternary GICs with multilayer packages can be formed by heavy alkali metals (K, Rb, Cs) and d 1° metals (Hg, TI, Bi, Sb, As) and alkali metal hydrides [7]. Lagrange [8] noted that compounds of this type are formed in systems in which the attainment of the thermodynamic equilibrium three-dimensional alloy ,
' alloy in GIC
(3)
is possible. For the graphite-caesium-mercury system, the equilibrium is displaced to the formation of a binary caesium GIC, because caesium has a stronger affinity for graphite than does mercury [8]. The composition of the binary alloy changes on intercalation, which leads to the displacement of the equilibrium, and so the reaction should be carried out with an excess of binary alloy. The stage number of the ternary multilayer GIC depends on the initial concentration of the binary alloy, whereas the composition of the intercalate is independent of the stage number. The increase in the stage number of the ternary GIC with a decrease in alkali metal content in the binary alloy is caused by the increase in the affinity of the alkali metal for the alloy [8]. Moreover, an increase in the stage number of the
ternary GIC as a function of time is observed in the graphite-caesium-antimony and graphite--caesiumarsenic systems at certain compositions of the binary alloy [9]. This may be a consequence of the different mobility of caesium atoms and d 1° heavy metal atoms along the grain boundaries. The intercrystalline space can be considered as an intergraphite layer spacing with a larger interplanar distance than 3.35 /~. In this case caesium-rich alloys will be preferable for insertion into the intercrystalline field. Thus during the interaction between graphite and alloy, a successive displacement within the series of equilibria [5] occur three-dimensional alloy ~ alloy in intercrystalline space ,
~ alloy in GIC
(4)
The attainment of thermodynamic equilibrium is necessary for the formation of ternary GICs of this type [3], which is possible during liquid phase interaction. The formation of pure C, KHg (first stage) on intercalation of the KHg alloy from the gas phase into the graphite phase does not take place [10]. On liquid phase interaction of potassium and rubidium with CsKHg (second stage), the composition of the intercrystalline alloy is displaced to the region of high alkali metal content and, according to ref. 8, results in the formation of a GIC with one intercalated layer. In the case of potassium C8K is formed, and in the case of rubidium C8KxRb4_x is formed. Bi-GICs do not form because of the decomposition of the KHg package. Let us consider in more detail cases where the KHg package does not decompose immediately. For instance, on interaction of CsKHg with caesium in soft conditions (less than 100 °C), the segregation reaction takes place within 1 h CsKHg + Cs stage II 13.55/~
~ C8Cs + C4KHg stage I 5.95/~
(5)
stage I 10.20/~
Caesium, which forms alloys with potassium and with mercury, will also be mixable with potassium-mercury alloys. Moreover, caesium has a stronger affinity for graphite than does the potassium-mercury package. For caesium interaction with CsKHg, the intercalation of caesium in unoccupied interlayers and the partial substitution of the potassium-mercury package by caesium are possible. The complete substitution of the KHg layer does not occur immediately because of dimensional difficulties. The caesium layer tightens on itself in the graphite layers due to its smaller interplanar distance and the stronger affinity for graphite. Thus the effect of a clutched "cherry stone" is observed. Since the interlayer space on the other edge of the potassium-mercury package is unoccupied, the possibility to
1.4. Udod, H.B. Orman / Liquid phase reactions in graphite-K-Cs-Hg
displace KHg layers, as shown in Fig. 4, is present. Thus a mixture of two first-stage GICs is formed, the ternary potassium-mercury GIC being isolated from the melt. The reaction must be performed at higher temperatures (250-300 °C) for several days to yield complete substitution of the KHg package. Vacancies between crystals should be formed with this arrangement of domains. These vacancies are occupied by metallic phases, which partially stabilize the strained state. Therefore the processes described above should result in significant swelling of the graphite single-crystal plate. The increase in plate thickness during caesium interaction with CsKHg is not 5.95/3.35 = 1.8, but 3-4 times larger. Equilibrium (3) is not displaced to the region of binary GIC formation if the intercalation of RbHg alloy into C4KHg is performed. The intercalation of such alloys into pristine graphite occurs in two steps: alkali metal is intercalated in the first step with the formation of a first-stage binary GIC (CsM); this is followed by simultaneous post-intercalation of the rest of the alkali metal and mercury [11]. In this case the process consists of the intercalation of alkali metal, but without the decomposition of the KHg package. The 00l lines are observed in the order CsRb (first stage) + C4KHg (first stage), followed by C4RbHg (first stage)+ C4KHg. The transformation of CsRb into C4RbHg can occur only if the binary GIC is in intimate contact with the intercrystalline alloy. This condition correlates well with our understanding of the segregation process (Fig. 4). Let us consider, using literature data, how segregation can be avoided. If the post-intercalated phase is allowed to soften or dissolve in the intercalate of the initial second-stage GIC and then substitutes for it due to the stronger affinity for the graphite network, intercalation will occur in the unoccupied interlayer space. The interaction of heavy alkali metals with the ternary second-stage potassium-hydrogen GIC C8KH2~3results in the formation of a donor-donor bi-GIC [12] 2C8KH2/3 + M
where M ~ K, Rb, Cs. The bi-intercalated compounds are formed due to the inertness of the hydride phase to the alkali metal. Direct interaction between two phases can be avoided using gas phase reaction under a low-pressure saturated stream of the intercalate. However, in this case, the intercalation to the second-stage donor GIC does not proceed due to the weak acceptor power of the carbon networks. Second-stage acceptor GICs have essentially higher acceptor power of the carbon networks, such that Herold et al. [13] succeeded in intercalating heavy alkali metals and even sodium into second-stage CdC12GIC and donor-acceptor bi-GICs were thus synthesized. An increase in alkali metal stream pressure or liquid phase intercalation results in the interaction of alkali metal with intercalated cadmium chloride and, as a consequence, to partial substitution. According to the mechanism described above (Fig. 4) this interaction produces a mixture of first-stage GICs [14]. Partial substitution on intercalation is demonstrated by the formation of reduction products [13]. Bi-GICs are produced if the post-intercalated substance has a lower affinity for graphite than does the intercalated substance. For instance, on interaction of potassium melt with the second-stage GIC C24Cs, the perfect heterostructure C16KCs is formed, according to the following reaction [15, 16]
(6)
, C]6K2H4/6M
0 O0
C24Cs + K
, C]6KCs + CsK
CsKHg
CsKHg C4KHg
Fig. 4. Scheme of caesium intercalation into CsKHg.
(7)
The contrasting C 2 4 K + C s reaction does not give the bi-intercalated compound, because caesium possesses a stronger affinity for graphite than does potassium [171. We performed the analogous reaction of C_~Cs with KHg alloy. The transformation of the X-ray diffraction pattern as a function of time is given in Fig. 5. As can be seen, potassium intercalation in the first step leads to the heterostructure C16KCs (Ic = 5.40 + 5.95 = 11.35 /~) and C24K (lc = 8.75 ~). The presence of CsCs and C4KHg as the final products demonstrates the instability of the potassium-caesium heterostructure to the influence of the mercury-rich alloy. Let us try to understand
0 0 ° CsCs
23
24
1.4. Udod, H.B. Orman Liquid phase reactions in graphite-K-Cs-Hg "Cz4Cs (11"istooe) T¢: 9"40~'
L
v C~sKCs(bi-GZC) Ic:11.35~ x CBK (I stage) Ic=5.40~
t=O
v
v
20 ~o
6o
5o
go
2'0
v~o ]
v CI6KCs(bi-GIC) Ic =11.35~
,o
7o
~o
~o
,~o
so
(at
30 rnin
• CsKHg(11"stage) I¢= 13.55~ zxCeCs(I stage) I¢ =5.95~
zb 70
~o
~b
4b
go
+ C~_,K('ITetage) Ie =8.70~
v*
e
2~ 7'o
b
6o
~'o
A
8
~b . 3'0
,b
~o
*
~o
a CsCs(T stage)I~= 5.95~
2"0 ~'o
o'o
;0
.c 28 70 (b)
;o I day
.?
~b
2'0
t
60
J
5'0
40
s'o
20
10
Fig. 6. 001 diffraction patterns after potassium (a) and mercury (b) intercalations according to eqn. (8) (see text).
,b
0~
(~ 0 f~)(..~C)(_)('~c)",.3 ro r , D - - ~ ~
000
Fig. 5. Evolution of the 001diffraction patterns for the Ca4Cs+ KHg reaction as a function of time. o
the mechanism of the reaction. According to the conditions of C4KHg formation [8], we can assume that caesium layers are displaced deep within the crystal. However, as mentioned above, caesium has a higher affinity for graphite than do potassium or KHg. Thus the mechanism of displacement of caesium layers should differ from that described above (Fig. 4). If the intercalation of the second-stage caesium GIC by potassium and mercury is carried out not simull taneously but in consecutive order, the following series of transformations takes place (Fig. 6). C24C s +K~ CI6KCs + C8K +Hg ~ C8KHg + C8Cs
stage II 9.40 ]k
bi-GIC 11.35 ]k
stage I 5.40/~
stage II 13.55/~
(8)
stage I 5.95/~
The intercalation of mercury in the second reaction should be performed from the gaseous phase using a strictly equimolar quantity of mercury, so as to avoid the attainment of equilibrium [3]. With a large excess of mercury in the intercrystalline alloy, the three-layer package is unstable and high stage binary potassium GICs are produced [9]. As shown in ref. 9, first-stage binary potassium GIC CsK, reacting with an equimolar quantity of mercury in the gas phase, produces the second-stage GIC CsKHg. Subtracting the last reaction from reaction (8), we obtain
--.
CI6KCs
CeCs
CsKHg
CI~KCs
\~x~ \~J
o
v
v
oo
CsKHg
Fig. 7. Scheme of mercury intercalation into CI6KCs.
+ Hg
C16KCs
) CsKHg + CaCs
(9)
To obtain the KHg package, two potassium layers are required in the octal epitaxy adjacent to the graphite layer [18]. The mechanism of the last reaction is shown in Fig. 7. As can be seen, external compression of the graphite networks can result in the displacement of caesium layers relative to each other. Returning to the previous reaction (C24Cs+KHg), the formation of C4KHg can easily be explained as the post-intercalation of KHg alloy into CsKHg. The interaction of caesium with C4KHg under hard conditions (250-300 °C) results in the complete substitution of the KHg package by the caesium layer. The sample is disordered due to the movement of a large amount of alloy to the intercrystalline space. However, it should be noted that, in addition to the presence of the 00l lines of the first-stage caesium GIC, the 004 line (strongest) in the C16KCs heterostructure is present (Fig. 7). This proves that decomposition of the KHg package can occur before substitution.
I.A. Udod, H.B. Orman / Liquid phase reactions in graphite-K-Cs-Hg
4. Conclusions Previously [5, 6] we considered the chemical reactions which occur in the graphite-potassium-lithium-mercury and graphite-potassium-sodium-mercury systems. As a result of the isomorphism of the structures of the ternary potassium-mercury and rubidium-mercury GICs, the chemical transformations in the graphitepotassium-rubidium-mercury system are equivalent to the transformations in the ternary graphitepotassium-mercury and graphite-rubidiummercury systems. The ease of decomposition of the KHg package on contact with the potassium (rubidium) melt was noted. The two-dimensional melting point of the KHg package in C4KHg is 408 °C [19]. However, in our case, no more than 30 min is needed to decompose the KHg package as soon as free potassium has melted [20]. We can only suppose that this proceeds via diffusion of potassium (rubidium) between the decomposed mercury layers (similar to sodium intercalation into the KHg package [5]). The decomposition in this case can occur fairly rapidly since the five-layer package K-Hg-K(Rb)-Hg-K is unstable at the free metal melting point; this package was not observed for any concentration of potassium-mercury alloy on interaction with graphite. The K-Hg-Na-Hg-K package can be synthesized in equilibrium conditions [5]. In this paper, the chemical reactions in the graphite-potassium-caesium-mercury system were investigated in detail. The proposed schemes of transformation within the crystal explain why the quantity of donor-donor bi-GlCs is essentially restricted in contrast with bi-GICs of the acceptor-acceptor type.
25
References 1 A. Herold, in F. Levy (ed.), Physics and Chemistry of Materials with Layered Structures, Intercalated Materials, Vol. 6, Riedel, Dordrecht, 1979, p. 323. 2 S.A. Safran and D.R. Hamann, Physica B, 99 (1980) 494. 3 S.E. Ullos and G. Kirgenow, Comments Cond. Mater. Phys., 12 (1986) 181. 4 N. Daumas and A. Herold, C.R. Acad. Sci. Ser. C, 268 (1969) 373. 5 I.A. Udod, M.V. G n a t e n k o and S.P. Sirotinkin, J. Alloys Comp., 186 (1992) 89. 6 I.A. Udod, M.V. G n a t e n k o and H.B. Orman, J. Alloys Comp., 196 (1993) 41. 7 D. Guerard, P. Lagrange, A. Herold and F. Rosseaux, Synth Met., 23 (1988) 421. 8 P. Lagrange, J. Mater. Res., 2 (6) (1987) 839. 9 P. Lagrange, A. Essaddek and J. Assoik, Synth. Met., 34 (1989) 8. 10 H.J. Kim, H. Mertwoy, T. Koch, J.E. Fischer, D.B. McWhan and J.D. Axe, Phys. Rev. B, 29 (1984) 5947. 11 M. El Makrini, P. Lagrange, D. Guerard and A. Herold, Carbon, 18 (3) (1980) 211. 12 D. Guerard, N.E. Elalem, S. El Hadigui, L. Ansari, P. Lagrange, F. Rousseaux, H. Estrade Szwarckopf, J. Conard and P. Lauginie, Y. Less-Common Met., 131 (1987) 173. 13 A. Herold, G. Furdin, D. Guerard, L. Hachim, M. Lelaurain, N.-E. Nadi and R. Vangelisti, Synth. Met., 12 (1985) 11. 14 M. Suzuki, e.C. Chow and H. Zabel, Phys. Rev. B, 32 (10) (1985) 6800. 15 B.R. York, S.K Hark and S.A. Solin, Phys. Rev. Lett., 50 (1983) 1470. 16 S.K. Hark, B.R. York and S.A. Solin, 3". Chem. Phys., 82 (1985) 921. 17 A. Oufkir and D. Guerard, Proc. 20th Biennial Conf. on
Carbon, University of California, June 24-28, 1991, American Carbon Society, Santa Barbara, p. 664. 18 P. Lagrange, M. El Makrini and A. Herold, Rev. Chim. Miner., 20 (1983) 229. 19 P. Lagrange, H. Fuzellier and M. El Makrini, Carbon, 22 (1984) 626. 20 I.A. Udod, V.A. Nalimova, V.V. Avdeev and K.N. Semenenko, Synth. Met., 52 (1992) 257.
21
Liquid phase reactions in the system graphite-potassiumcaesium-mercury I.A. U d o d
and H.B. Orman
M.V. Lomonosov Moscow State University, Department of Chemistry, 119899 Moscow (Russian Federation) (Received June 18, 1993)
Abstract The intercalation of metals and alloys in different second-stage donor graphite intercalation compounds (GICs) was investigated. The proposed schemes of the segregation reaction in the graphite-potassium-caesium-mercury system can explain why the quantity of donor-donor bi-intercalated GICs is essentially restricted in contrast with acceptor-acceptor bi-intercalated GICs.
1. Introduction The existence of the typical "stage structure" (the ordered sequence of occupied and vacant interlayer spacings) [1] of graphite intercalation compounds (GICs) is a consequence of the electrostatic repulsion of intercalated layers [2] and the elastic properties of the graphite matrix [3]. The tensions on the domain bonds within the same crystal will be at a minimum according to the Daumas-Herold model [4] (Fig. 1). Previously [5, 6] we investigated why, on intercalation of lithium into vacant interlayer spacings of the secondstage GIC CsKHg, a segregation reaction takes place and a mixture of the first-stage GICs C6Li and C4KHg is formed. The formation of C4KHg demonstrates the intercrystalline redistribution of intercalates from one domain to another. Alternatively, an interaction between melted lithium and KHg must take place. Thus the existence of the GICs C6Li and C4KHg within the limit of the same crystal assumes large tensions on the graphite networks (Fig. 2). The tensions on the graphite networks will be essentially lower if a bi-intercalated GIC (bi-GIC) (GIC with alternating layers of intercalates) is formed (Fig.
E 0000(3/ ~,
/0 000 0 ] /0 O000
Fig. 1. Structure of the third stage according to the Daumas-Herold model.
0925-8388/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0 9 2 5 - 8 3 8 8 ( 9 3 ) 0 0 8 6 2 - S
ooggoQ~ 000000 ~ o@0ooo oo0ooo 00000 00000
0 0
C4KHg Fig. 2. Coexistence of the first-stage GICs C4KHg and C6Li within the same crystal.
~:~:z:z~:z~:~ ~ ~ 0 0 00
ooo Fig. 3. Structure of a bi-intercalated GIC.
3). In general, all the known bi-GICs can be classified according to the type of intercalate, e.g. donor-donor, donor-acceptor and acceptor-acceptor compounds. In this work, we attempt to clarify the mechanism of intercalation in high stage donor GICs.
2. Experimental details Highly oriented pyrolytic graphite (disorientation angle along the c-axis of about 1°) in the form of quasisingle-crystal plates was used. Intercalation on a graphite powder proceeds too rapidly and does not allow the
22
I.A. Udod, H.B. Orman / Liquid phase reactions in graphite-K-Cs-Hg
formation of intermediate compounds to be observed. When plates are used, the reaction step can be controlled by a time factor. The reactions studied proceed not only within the limits of the same graphite single-crystal plate, but within the limits of the same crystal. Therefore methods of chemical microanalysis are not acceptable. Thus the only technique which can be used for the identification of synthesized compounds is X-ray analysis, which yields the identity period Ic of the GICs obtained. The identity period Ic is the individual characteristic of a GIC, whereas the interplanar distance d~ of the occupied layer characterizes the intercalate and does not depend on the stage number (Fig. 1). The identity period is related to the interplanar distance and the stage number by the correlation Ic =d~ + 3.35 × (n - 1)
(1)
As the interplanar distance is independent of the stage number, the identity period of a bi-intercalated compound (Fig. 3) is equal to Ic = d,, + d, 2
(2)
where d~, and di= are the interplanar distances of the intercalates in binary compounds proper. In the majority of cases the experimental value of the identity period lc is close to that calculated (if the intitial intercalate layer is unchanged).
3. Results and discussion
Ternary GICs with multilayer packages can be formed by heavy alkali metals (K, Rb, Cs) and d 1° metals (Hg, TI, Bi, Sb, As) and alkali metal hydrides [7]. Lagrange [8] noted that compounds of this type are formed in systems in which the attainment of the thermodynamic equilibrium three-dimensional alloy ,
' alloy in GIC
(3)
is possible. For the graphite-caesium-mercury system, the equilibrium is displaced to the formation of a binary caesium GIC, because caesium has a stronger affinity for graphite than does mercury [8]. The composition of the binary alloy changes on intercalation, which leads to the displacement of the equilibrium, and so the reaction should be carried out with an excess of binary alloy. The stage number of the ternary multilayer GIC depends on the initial concentration of the binary alloy, whereas the composition of the intercalate is independent of the stage number. The increase in the stage number of the ternary GIC with a decrease in alkali metal content in the binary alloy is caused by the increase in the affinity of the alkali metal for the alloy [8]. Moreover, an increase in the stage number of the
ternary GIC as a function of time is observed in the graphite-caesium-antimony and graphite--caesiumarsenic systems at certain compositions of the binary alloy [9]. This may be a consequence of the different mobility of caesium atoms and d 1° heavy metal atoms along the grain boundaries. The intercrystalline space can be considered as an intergraphite layer spacing with a larger interplanar distance than 3.35 /~. In this case caesium-rich alloys will be preferable for insertion into the intercrystalline field. Thus during the interaction between graphite and alloy, a successive displacement within the series of equilibria [5] occur three-dimensional alloy ~ alloy in intercrystalline space ,
~ alloy in GIC
(4)
The attainment of thermodynamic equilibrium is necessary for the formation of ternary GICs of this type [3], which is possible during liquid phase interaction. The formation of pure C, KHg (first stage) on intercalation of the KHg alloy from the gas phase into the graphite phase does not take place [10]. On liquid phase interaction of potassium and rubidium with CsKHg (second stage), the composition of the intercrystalline alloy is displaced to the region of high alkali metal content and, according to ref. 8, results in the formation of a GIC with one intercalated layer. In the case of potassium C8K is formed, and in the case of rubidium C8KxRb4_x is formed. Bi-GICs do not form because of the decomposition of the KHg package. Let us consider in more detail cases where the KHg package does not decompose immediately. For instance, on interaction of CsKHg with caesium in soft conditions (less than 100 °C), the segregation reaction takes place within 1 h CsKHg + Cs stage II 13.55/~
~ C8Cs + C4KHg stage I 5.95/~
(5)
stage I 10.20/~
Caesium, which forms alloys with potassium and with mercury, will also be mixable with potassium-mercury alloys. Moreover, caesium has a stronger affinity for graphite than does the potassium-mercury package. For caesium interaction with CsKHg, the intercalation of caesium in unoccupied interlayers and the partial substitution of the potassium-mercury package by caesium are possible. The complete substitution of the KHg layer does not occur immediately because of dimensional difficulties. The caesium layer tightens on itself in the graphite layers due to its smaller interplanar distance and the stronger affinity for graphite. Thus the effect of a clutched "cherry stone" is observed. Since the interlayer space on the other edge of the potassium-mercury package is unoccupied, the possibility to
1.4. Udod, H.B. Orman / Liquid phase reactions in graphite-K-Cs-Hg
displace KHg layers, as shown in Fig. 4, is present. Thus a mixture of two first-stage GICs is formed, the ternary potassium-mercury GIC being isolated from the melt. The reaction must be performed at higher temperatures (250-300 °C) for several days to yield complete substitution of the KHg package. Vacancies between crystals should be formed with this arrangement of domains. These vacancies are occupied by metallic phases, which partially stabilize the strained state. Therefore the processes described above should result in significant swelling of the graphite single-crystal plate. The increase in plate thickness during caesium interaction with CsKHg is not 5.95/3.35 = 1.8, but 3-4 times larger. Equilibrium (3) is not displaced to the region of binary GIC formation if the intercalation of RbHg alloy into C4KHg is performed. The intercalation of such alloys into pristine graphite occurs in two steps: alkali metal is intercalated in the first step with the formation of a first-stage binary GIC (CsM); this is followed by simultaneous post-intercalation of the rest of the alkali metal and mercury [11]. In this case the process consists of the intercalation of alkali metal, but without the decomposition of the KHg package. The 00l lines are observed in the order CsRb (first stage) + C4KHg (first stage), followed by C4RbHg (first stage)+ C4KHg. The transformation of CsRb into C4RbHg can occur only if the binary GIC is in intimate contact with the intercrystalline alloy. This condition correlates well with our understanding of the segregation process (Fig. 4). Let us consider, using literature data, how segregation can be avoided. If the post-intercalated phase is allowed to soften or dissolve in the intercalate of the initial second-stage GIC and then substitutes for it due to the stronger affinity for the graphite network, intercalation will occur in the unoccupied interlayer space. The interaction of heavy alkali metals with the ternary second-stage potassium-hydrogen GIC C8KH2~3results in the formation of a donor-donor bi-GIC [12] 2C8KH2/3 + M
where M ~ K, Rb, Cs. The bi-intercalated compounds are formed due to the inertness of the hydride phase to the alkali metal. Direct interaction between two phases can be avoided using gas phase reaction under a low-pressure saturated stream of the intercalate. However, in this case, the intercalation to the second-stage donor GIC does not proceed due to the weak acceptor power of the carbon networks. Second-stage acceptor GICs have essentially higher acceptor power of the carbon networks, such that Herold et al. [13] succeeded in intercalating heavy alkali metals and even sodium into second-stage CdC12GIC and donor-acceptor bi-GICs were thus synthesized. An increase in alkali metal stream pressure or liquid phase intercalation results in the interaction of alkali metal with intercalated cadmium chloride and, as a consequence, to partial substitution. According to the mechanism described above (Fig. 4) this interaction produces a mixture of first-stage GICs [14]. Partial substitution on intercalation is demonstrated by the formation of reduction products [13]. Bi-GICs are produced if the post-intercalated substance has a lower affinity for graphite than does the intercalated substance. For instance, on interaction of potassium melt with the second-stage GIC C24Cs, the perfect heterostructure C16KCs is formed, according to the following reaction [15, 16]
(6)
, C]6K2H4/6M
0 O0
C24Cs + K
, C]6KCs + CsK
CsKHg
CsKHg C4KHg
Fig. 4. Scheme of caesium intercalation into CsKHg.
(7)
The contrasting C 2 4 K + C s reaction does not give the bi-intercalated compound, because caesium possesses a stronger affinity for graphite than does potassium [171. We performed the analogous reaction of C_~Cs with KHg alloy. The transformation of the X-ray diffraction pattern as a function of time is given in Fig. 5. As can be seen, potassium intercalation in the first step leads to the heterostructure C16KCs (Ic = 5.40 + 5.95 = 11.35 /~) and C24K (lc = 8.75 ~). The presence of CsCs and C4KHg as the final products demonstrates the instability of the potassium-caesium heterostructure to the influence of the mercury-rich alloy. Let us try to understand
0 0 ° CsCs
23
24
1.4. Udod, H.B. Orman Liquid phase reactions in graphite-K-Cs-Hg "Cz4Cs (11"istooe) T¢: 9"40~'
L
v C~sKCs(bi-GZC) Ic:11.35~ x CBK (I stage) Ic=5.40~
t=O
v
v
20 ~o
6o
5o
go
2'0
v~o ]
v CI6KCs(bi-GIC) Ic =11.35~
,o
7o
~o
~o
,~o
so
(at
30 rnin
• CsKHg(11"stage) I¢= 13.55~ zxCeCs(I stage) I¢ =5.95~
zb 70
~o
~b
4b
go
+ C~_,K('ITetage) Ie =8.70~
v*
e
2~ 7'o
b
6o
~'o
A
8
~b . 3'0
,b
~o
*
~o
a CsCs(T stage)I~= 5.95~
2"0 ~'o
o'o
;0
.c 28 70 (b)
;o I day
.?
~b
2'0
t
60
J
5'0
40
s'o
20
10
Fig. 6. 001 diffraction patterns after potassium (a) and mercury (b) intercalations according to eqn. (8) (see text).
,b
0~
(~ 0 f~)(..~C)(_)('~c)",.3 ro r , D - - ~ ~
000
Fig. 5. Evolution of the 001diffraction patterns for the Ca4Cs+ KHg reaction as a function of time. o
the mechanism of the reaction. According to the conditions of C4KHg formation [8], we can assume that caesium layers are displaced deep within the crystal. However, as mentioned above, caesium has a higher affinity for graphite than do potassium or KHg. Thus the mechanism of displacement of caesium layers should differ from that described above (Fig. 4). If the intercalation of the second-stage caesium GIC by potassium and mercury is carried out not simull taneously but in consecutive order, the following series of transformations takes place (Fig. 6). C24C s +K~ CI6KCs + C8K +Hg ~ C8KHg + C8Cs
stage II 9.40 ]k
bi-GIC 11.35 ]k
stage I 5.40/~
stage II 13.55/~
(8)
stage I 5.95/~
The intercalation of mercury in the second reaction should be performed from the gaseous phase using a strictly equimolar quantity of mercury, so as to avoid the attainment of equilibrium [3]. With a large excess of mercury in the intercrystalline alloy, the three-layer package is unstable and high stage binary potassium GICs are produced [9]. As shown in ref. 9, first-stage binary potassium GIC CsK, reacting with an equimolar quantity of mercury in the gas phase, produces the second-stage GIC CsKHg. Subtracting the last reaction from reaction (8), we obtain
--.
CI6KCs
CeCs
CsKHg
CI~KCs
\~x~ \~J
o
v
v
oo
CsKHg
Fig. 7. Scheme of mercury intercalation into CI6KCs.
+ Hg
C16KCs
) CsKHg + CaCs
(9)
To obtain the KHg package, two potassium layers are required in the octal epitaxy adjacent to the graphite layer [18]. The mechanism of the last reaction is shown in Fig. 7. As can be seen, external compression of the graphite networks can result in the displacement of caesium layers relative to each other. Returning to the previous reaction (C24Cs+KHg), the formation of C4KHg can easily be explained as the post-intercalation of KHg alloy into CsKHg. The interaction of caesium with C4KHg under hard conditions (250-300 °C) results in the complete substitution of the KHg package by the caesium layer. The sample is disordered due to the movement of a large amount of alloy to the intercrystalline space. However, it should be noted that, in addition to the presence of the 00l lines of the first-stage caesium GIC, the 004 line (strongest) in the C16KCs heterostructure is present (Fig. 7). This proves that decomposition of the KHg package can occur before substitution.
I.A. Udod, H.B. Orman / Liquid phase reactions in graphite-K-Cs-Hg
4. Conclusions Previously [5, 6] we considered the chemical reactions which occur in the graphite-potassium-lithium-mercury and graphite-potassium-sodium-mercury systems. As a result of the isomorphism of the structures of the ternary potassium-mercury and rubidium-mercury GICs, the chemical transformations in the graphitepotassium-rubidium-mercury system are equivalent to the transformations in the ternary graphitepotassium-mercury and graphite-rubidiummercury systems. The ease of decomposition of the KHg package on contact with the potassium (rubidium) melt was noted. The two-dimensional melting point of the KHg package in C4KHg is 408 °C [19]. However, in our case, no more than 30 min is needed to decompose the KHg package as soon as free potassium has melted [20]. We can only suppose that this proceeds via diffusion of potassium (rubidium) between the decomposed mercury layers (similar to sodium intercalation into the KHg package [5]). The decomposition in this case can occur fairly rapidly since the five-layer package K-Hg-K(Rb)-Hg-K is unstable at the free metal melting point; this package was not observed for any concentration of potassium-mercury alloy on interaction with graphite. The K-Hg-Na-Hg-K package can be synthesized in equilibrium conditions [5]. In this paper, the chemical reactions in the graphite-potassium-caesium-mercury system were investigated in detail. The proposed schemes of transformation within the crystal explain why the quantity of donor-donor bi-GlCs is essentially restricted in contrast with bi-GICs of the acceptor-acceptor type.
25
References 1 A. Herold, in F. Levy (ed.), Physics and Chemistry of Materials with Layered Structures, Intercalated Materials, Vol. 6, Riedel, Dordrecht, 1979, p. 323. 2 S.A. Safran and D.R. Hamann, Physica B, 99 (1980) 494. 3 S.E. Ullos and G. Kirgenow, Comments Cond. Mater. Phys., 12 (1986) 181. 4 N. Daumas and A. Herold, C.R. Acad. Sci. Ser. C, 268 (1969) 373. 5 I.A. Udod, M.V. G n a t e n k o and S.P. Sirotinkin, J. Alloys Comp., 186 (1992) 89. 6 I.A. Udod, M.V. G n a t e n k o and H.B. Orman, J. Alloys Comp., 196 (1993) 41. 7 D. Guerard, P. Lagrange, A. Herold and F. Rosseaux, Synth Met., 23 (1988) 421. 8 P. Lagrange, J. Mater. Res., 2 (6) (1987) 839. 9 P. Lagrange, A. Essaddek and J. Assoik, Synth. Met., 34 (1989) 8. 10 H.J. Kim, H. Mertwoy, T. Koch, J.E. Fischer, D.B. McWhan and J.D. Axe, Phys. Rev. B, 29 (1984) 5947. 11 M. El Makrini, P. Lagrange, D. Guerard and A. Herold, Carbon, 18 (3) (1980) 211. 12 D. Guerard, N.E. Elalem, S. El Hadigui, L. Ansari, P. Lagrange, F. Rousseaux, H. Estrade Szwarckopf, J. Conard and P. Lauginie, Y. Less-Common Met., 131 (1987) 173. 13 A. Herold, G. Furdin, D. Guerard, L. Hachim, M. Lelaurain, N.-E. Nadi and R. Vangelisti, Synth. Met., 12 (1985) 11. 14 M. Suzuki, e.C. Chow and H. Zabel, Phys. Rev. B, 32 (10) (1985) 6800. 15 B.R. York, S.K Hark and S.A. Solin, Phys. Rev. Lett., 50 (1983) 1470. 16 S.K. Hark, B.R. York and S.A. Solin, 3". Chem. Phys., 82 (1985) 921. 17 A. Oufkir and D. Guerard, Proc. 20th Biennial Conf. on
Carbon, University of California, June 24-28, 1991, American Carbon Society, Santa Barbara, p. 664. 18 P. Lagrange, M. El Makrini and A. Herold, Rev. Chim. Miner., 20 (1983) 229. 19 P. Lagrange, H. Fuzellier and M. El Makrini, Carbon, 22 (1984) 626. 20 I.A. Udod, V.A. Nalimova, V.V. Avdeev and K.N. Semenenko, Synth. Met., 52 (1992) 257.