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Preparation and properties of graphite hexafluoroarsenates CxAsF6
Journal of Fluorine Chemistry 105 (2000) 239±248
Preparation and properties of graphite hexa¯uoroarsenates CxAsF6 Fujio Okino* Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan
Abstract Preparation, structures, reactions and adsorption properties of graphite hexa¯uoroarsenates CxAsF6 are described in connection with those of graphite intercalation compounds CxAsF5, CxSbCl5, CxSbF5 and CxSbF6. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Graphite intercalation compounds; Hexa¯uoroarsenate; Nestling; Stacking sequences; Adsorption; Nano-space; Micropores; Arsenic penta¯uoride
1. Introduction Graphite hexa¯uoroarsenates CxAsF6 [1±6] are a class of graphite intercalation compounds (GICs), and called as such when the nature of the guest species is to be speci®ed, for they are salts of hexa¯uoroarsenate. They are related to but different from CxAsF5 GICs prepared by the reaction of graphite with AsF5. CxAsF5 GICs [7] have drawn much attention particularly because of their high electrical conductivities [8,9]. Falardeau et al. [10], using X-ray diffraction (XRD), gravimetry and c-axis thickness measurements on highly oriented pyrolytic graphite (HOPG) intercalated samples, characterized graphite-AsF5 intercalation compounds by the formula C8nAsF5 with the c-axis repeat distance Ic of Ê , which transforms to 8.10 3.35 (n ÿ 1) A Ê, 4.75 3.35n A where n is the stage. There has been much controversy about the nature of the guest species in CxAsF5. X-ray arsenic preabsorption edge studies by Bartlett et al. [1,2], established that the intercalation of graphite by AsF5 was accompanied by electron oxidation of the graphite according to the equation: 3 2 AsF5
eÿ ! AsF6 ÿ 12 AsF3
2. Preparation of CxAsF6 CxAsF6 can be prepared via either of the following three schemes:
Scheme 1
Scheme 2
(1)
The implications of this for the preparation of CxAsF5 GICs, and thus for their physical and chemical properties, had not been fully comprehended. The author has been preparing a series of CxAsF6 GICs, the existence of which had been implied by Eq. (1). In the following sections, preparation, structures, reactions and adsorption properties of graphite hexa¯uoroarse* Tel.: 81-268-21-5393; fax: 81-268-21-5391. E-mail address: [email protected] (F. Okino).
nates CxAsF6 are described in connection with those of CxAsF5, CxSbCl5, CxSbF5 and CxSbF6 GICs.
Scheme 3
In Scheme 1, the reaction according to Eq. (3) is not followed stoichiometrically, for some loss of AsF5 always accompanies the loss of AsF3. This point is addressed in the following section. In all cases including Scheme 1, the vacuum stable products contain solely AsF6ÿ as guest species.
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 7 1 - 7
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F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
The ready loss of AsF5 in comparison with AsF3 can be explained as follows: the AsF5 can form As2F11ÿ with AsF6ÿ [13] and an AsF5 molecule can, thereby, be generated in a new location without actual translational movement of the molecule according to the following equation where two arsenic atoms are differentiated with and without an asterisk.
2.1. Preparation of CxAsF6 by the reaction of graphite with AsF5 alone When graphite is reacted with an excess amount of AsF5 (C : AsF5 < 8), the reaction product indeed shows a composition of ca. C8AsF5 [10]. Although compositions of GICs are usually formulated as CmnM, where n is the stage and m or CmM indicates the in-plane density of guest species M, there may not be a compelling reason for m to be an integer, unless the guest species are somehow registered to the host graphite lattice. The situation of non-integral m is termed incommensurate or nonstoichiometric. Graphite-AsF5 compounds may fall into this category, but they have added complexity of the formation of AsF6ÿ and AsF3 according to Eq. (1). The conversion of AsF5 into AsF6ÿ and AsF3 according to Eq. (3) is not complete in C8AsF5, and it readily loses AsF5 mainly at the beginning and then AsF3 slowly under vacuum [4,5]. The composition of the solid product, after the initial removal of the gases, is usually close to C10AsF5d (d ! 1). Materials made in this way have been usually reported as C10AsF5 by many workers. But the F : As ratio must be larger than 5, since evolution of AsF3 has occurred. In one of the cases, after 200 h of evacuation, the carbon content of the sample became 55.2%, which, assuming the remaining guest species to be AsF6ÿ, corresponds to C19.4AsF6. The composition of this sample by chemical analysis was C20.28AsF6.01 as given in Table 1 supporting that AsF6ÿ is the sole guest species [5]. The composition implies that the ionization extent of a single treatment of graphite with AsF5 is ca. C20 [11,12]. When graphite is repeatedly treated with AsF5 followed by evacuation, the composition after six cycles became C15.94AsF5.98 as given in Table 1 [5]. The results show that interaction of graphite with AsF5 alone is an effective route to the production of CxAsF6 salts, so long as care is taken to remove the volatiles completely. After several cycles the Stage-1 phase was in much greater concentration than the Stage-2 phase, whereas the reverse was true for one cycle. However, the repeated treatment of graphite with AsF5 alone, followed by evacuation, never leads to pure Stage-1 products owing to its low oxidizing ability (see Section 2.4).
AsF5 AsF6 ÿ ! AsF5 ÿ F ÿ AsF5 ÿ ! AsF6 ÿ AsF5 (6)
On the other hand, AsF3, being a much inferior ¯uoride ion acceptor than AsF5, does not have such a mechanism for effective transfer available to it. Jostling of the AsF3 molecules through a relatively close packed environment is evidently slow. 2.2. Preparation of CxAsF6 by the reaction of graphite with AsF5 and F2 CxAsF6 can be formed by the reaction of graphite with AsF5 F2 [3,5]. The composition of a representative sample was C13.05AsF6.01 as given in Table 1 [5]. Although this route seems to be a straightforward way to the formation of CxAsF6 salts, the exact ratio of F : As 6 and the control of the ratio of C : AsF6ÿ are dif®cult to attain owing to the high reactivity of AsF5 F2; not only AsF5 F2 slowly attacks the inner wall of the vacuum line, but also F2 is adsorbed by CxAsF5 [4] or CxAsF6 [14], or may react with low-stage CxAsF6 as it is formed to yield a CxF phase [15]. 2.3. Preparation of CxAsF6 by the reaction of graphite with O2AsF6 Originally the reaction of graphite with O2AsF6 was run in SO2ClF solvent [1,2,4], but the solvent SO2ClF was found to be incorporated along with AsF6ÿ [14]. It was later found that the direct solid±solid reaction of O2AsF6 with graphite yields CxAsF6 but the reaction was thought to be very slow [5]. Compositions of the samples after several days of mixing of graphite with O2AsF6 were C14.01AsF6.01 and C16.96AsF6.00 as given in Table 1 [5]. Nonetheless, the results indicated that this is also an effective route to the production of CxAsF6 salts.
Table 1 Analytical data for the vacuum-stable solid products obtained from (1) C AsF5, (2) C AsF5 F2, and (3) C O2AsF6 [5] Reactants
Methoda
Composition
C
H
As
F
N
O
C As F
17.26 19.63 21.56 20.91 19.05 22.23
26.30 29.79 32.85 31.85 28.98 33.78
<0.001 0.003 0.010 0.023 0.001 0.001
<0.5 ± <0.5 <0.3 <0.3 <0.5
99.68 99.57 99.50 99.72 99.82 99.43
b
Analysis (%) (1) AsF5 (2) AsF5 F2 (3) O2AsF6
one cycle six cycles gas mixture solid-phase reaction
(30 ) O2AsF6
thermally decomposed
a b
C20.28AsF6.01 C15.94AsF5.98 C13.05AsF6.01 C14.01AsF6.01 C16.96AsF6.00 C12.81AsF5.99
56.12 50.15 45.09 46.96 51.79 43.42
See text for detailed methods. Experimental values determined by Galbraith Laboratories, Inc.
0.70 0.05 0.09 0.21 <0.01 0.11
F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
Fig. 1. Progress of the reaction of graphite with O2AsF6 for the formation of nestled C14AsF6 monitored by pressure increase (reproduced with permission from [16]).
This reaction, however, was found to be much faster than it had been thought; the reaction proceeds essentially within a matter of 10 s when an effective mixing of the solids (ca. 100 mg) is applied. In this reaction, as the weights of graphite powder and O2AsF6 are weighed separately, the ratio of C : AsF6ÿ in the product is easily controlled. Fig. 1 shows the reaction progress for the formation of nestled C14AsF6 monitored by oxygen pressure increase [16]. The pressure rises according to Eq. (5). Initially the cell was ®lled with nitrogen and the pressure was measured every second. At t 40 s, O2AsF6 was transferred onto graphite. The reaction immediately started as the mixture was perturbed in an effort to transfer O2AsF6 thoroughly. When a violent shaking started at t 200 s, the pressure increased drastically but stopped increasing within 10 s, accompanied with the change of the sample color from black±grey to metallic blue, indicating a very fast solid phase reaction.
241
AsF5 to do likewise, can therefore be attributed to the greater oxidizing power of O2AsF6 and AsF5 F2. That this inferiority in oxidizing capability of AsF5 is indeed thermodynamically and not kinetically based, is indicated by the product of the interaction of AsF3 with Stage-1 salts ca. C14AsF6 which had been prepared using either AsF5 F2, or O2AsF6 [3±5]. When the volatiles are removed from C14AsF61/2AsF3 after AsF3 is incorporated within the gallery, they include AsF5. The vacuum stable residual solid is a mixture of Stage-1 C14AsF6 and Stage-2 C28AsF6 salts, comparable to the mixture derived from the interaction of graphite with AsF5 after an extended removal of volatiles. Clearly, in forming the Stage-2 phase within the bulk, AsF3 acts as a reducing agent towards C14AsF6ÿ: 2C14 AsF6 12 AsF3 ! C28 AsF6 32 AsF5
(9)
3. Structures of nestled C14nAsF6 and un-nestled CxAsFy 3.1. Structures of Stage-1 nestled C14AsF6 and un-nestled CxAsFy (8 x < 14, 5 y 6) Signi®cant differences are observed between the room temperature (RT) XRD patterns for the Stage-1 graphite Ê ) [5], and for the hexa¯uoroarsenate, C14AsF6 (Ic 7.6 A Stage-1 graphite-AsF5 intercalation compound, ca. C10AsF5 Ê ). Typical diffraction patterns are shown in [8,10] (Ic 8.0 A Fig. 2 [6]. The C14AsF6 was made by the reaction of graphite with O2AsF6 and the C10AsF5 with AsF5. The XRD pattern Ê ), for C14AsF6 is characterized by its very small Ic (7.6 A and peculiar features; the appearance of a low angle halo
2.4. The extent of intercalation and the oxidizing power of the guest precursors The inability of AsF5 alone with graphite to generate pure Stage-1 salts stands in marked contrast with the ready formation of such salts by AsF5 F2, and O2AsF6. The basis for these differences appears to be thermodynamic. Free energy changes for related oxidizing half reactions are evaluated as follows [5]: 3 2 AsF5 g
125
eÿ ! AsF6 ÿ g 12 AsF3 g DG298 ÿ125 kcal molÿ1
(1)
AsF5 g 12 F2 g eÿ ! AsF6 ÿ g 170
DG298 ÿ170 kcal molÿ1
(7)
O2 AsF6 c eÿ ! AsF6 ÿ g O2 g 155
DG298 ÿ155 kcal molÿ1
(8)
The ready formation of pure Stage-1 salt C14AsF6 by the oxidizers O2AsF6 and AsF5-F2 mixture, but the failure of
Ê ), (b) ca. Fig. 2. XRD patterns (Cu Ka) of (a) ca. C14AsF6 (Ic 7.6 A Ê ), and (c) quartz capillary background (reproduced C10AsF5 (Ic 8.0 A with permission from [6]).
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Fig. 3. In-plane H7 H7 arrangement of AsF6ÿ for the nestled C14AsF6 (reproduced with permission from [5]).
[halo A in pattern (a) in Fig. 2] and two peaks (B and C), and the absence of (1 0 0) and (1 0 l) re¯ections. These features suggest that the ¯uorine ligands of AsF6ÿ are nestled in contiguous three-fold sets of carbon atom hexagons of the graphite. The nestling of AsF6ÿ is geometrically justi®ed as Ê follows. The As±F distance in the AsF6ÿ ion is ca. H3 A [17,18]. Then, assuming that the ion is a regular octahedron Ê , the neighboring F±F distance is with d (As±F) H3 A Ê H6 A, which is essentially the same as the lattice constant, a0, of graphite. The nestling of AsF6ÿ requires a staggering of the enclosing carbon layers. Adjacent pairs of carbonatom sheets contain assemblies of AsF6ÿ of short range order. Ideally, the most dense arrangement of nestled AsF6ÿ has the composition C14AsF6 shown in Fig. 3 [5]. This is because the two ¯uorine ligands belonging to two different AsF6ÿ cannot nestle in two neighboring hexagons of graphite network owing to the `large' size of the ¯uorine ligands. The (1 0 0) re¯ection of this superlattice unit cell [a0 H7a0(graphite)] falls at the center of halo A as indicated by the dotted line above it in Fig. 4 [5]. Aside from the restriction imposed by AsF6ÿ nestling, the carbon layers are randomly stacked. A model has been proposed in which staggered carbon layers are randomly stacked, e.g. A|B|A|C|B|A| where | denotes the intercalate layer, with short-range ordering or domain structure of nestled AsF6ÿ anions in the gallery. All the peculiar features of the pattern Ê are explained by the for the C14AsF6 sample with Ic 7.6 A nestling of AsF6ÿ. The mathematical treatment of the model shows that diffuse scatterings are observed for (h k l) with h ÿ k 6 3n and usual crystal re¯ections for (h k l) with h ÿ k 3n [5]. In the model, the ¯uorine atoms are placed at the center of the carbon hexagons in the ab-projection and Ê above and below the central arsenic atom along the c1.0 A axis direction. The calculated patterns based on the nestled model for C14AsF6 as well as the observed pattern are shown in Fig. 4 [5]. In the ®gure, pattern (c) is the calculated (1 0 l)
Fig. 4. (a) Observed XRD patterns (Cu Ka) of the nestled ca. C14AsF6, (b) quartz capillary background, (c) calculated (1 0 l) diffuse scattering, and (d) calculated pattern with (0 0 l) and (1 1 l) crystal-reflections, and (1 0 l) diffuse scattering (reproduced with permission from [5]).
diffuse scattering and pattern (d) consists of the calculated (0 0 l) and (1 1 l) crystal-re¯ections and (1 0 l) diffuse scattering. In cases where the graphite galleries are richer in AsF6ÿ, i.e. C<14AsF6, or AsF5 and AsF3 are among the guest species, i.e. CxAsF5ÿ6, comfortable nestling of AsF6ÿ is no longer possible. Consequently, the Ic-distances are larger Ê ) and the enclosing carbon layers are eclipsed. (ca. 8.0 A These conclusions are unambiguously drawn from the XRD pattern for ca. C10AsF5 given in Fig. 2 (b). The pattern is simpler than that for C14AsF6 (Fig. 2(a)) and all re¯ections are accounted for by the unit cell with a0 a0(graphite) and c0 Ic. Since the staggering of carbon layers requires c0 to be a multiple of Ic, and an ordered arrangement of the AsFy species in the gallery requires a0 to be larger than a0(graphite), the carbon layers are eclipsed and the guest species are randomly placed. The calculated pattern based on this model for C10AsF5 as well as the observed pattern are shown in Fig. 5 [5]. The observed low angle halo in this case is solely attributable to the quartz capillary shown by pattern (c). Qualitatively, the same pattern occurs for Stage-1 unnestled compounds with a wide range of stoichiometries covered by the formula CxAsFy (8 x < 14, 5 y 6). Here the conditions for x and y should not be taken rigorously. For example, C13AsF5 will be a mixture of Stages-1 and -2, and C8AsF6 may not exist. 3.2. Structure of Stage-1 nestled C16AsF6 When the temperature is lowered, superlattice re¯ections appear for ca. C14AsF6, indicating an order±disorder transition in the AsF6ÿ arrangement and, consequently, in the carbon layer stacking sequence at ca. 170 K. The re¯ection pattern at lower temperature has been satisfactorily simu-
F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
243
Fig. 5. (a) Observed XRD patterns (Cu Ka) for the un-nestled material of composition ca. C10AsF5, (b) pattern, calculated (c) quartz capillary background (reproduced with permission from [5]).
lated by a nestled C16AsF6 model. The observed and calculated diffraction patterns at 90 K are shown in Fig. 6, and the in-plane arrangement of AsF6ÿ for the model is shown in Fig. 7(a) [6]. The pseudo face-centered orthorhombic arrangement of AsF6ÿ requires the carbon-layer stacking sequence to be A|B|A|B|A|B| (see Fig. 7(b)). Apparently, strong Coulomb repulsive interaction among AsF6ÿ in the same and different galleries controls the structure of C16AsF6. The electrostatic interaction probably renders the C14AsF6 structure less stable, since (i) a torque is exerted on each AsF6ÿ by the electric ®eld produced by the other AsF6ÿ ions in the same gallery in C14AsF6 (see Fig. 3), and (ii) the repulsion between AsF6ÿ ions are stronger in C14AsF6 with the most dense arrangement of nestled AsF6ÿ ions, although the total attraction between C14 and AsF6ÿ in C14AsF6 is stronger than that between C16 and AsF6ÿ in C16AsF6.
Fig. 6. (a) Observed and (b) calculated XRD patterns (Cu Ka) for C16AsF6 at 90 K (reproduced with permission from [6]).
Fig. 7. Structures of C16AsF6: (a) in-plane structure of C16AsF6 (nestling of AsF6ÿ is shown) and (b) positions of arsenic atoms in different galleries (reproduced with permission from [6]).
3.3. Structures of Stage-2 nestled C28AsF6 and un-nestled CxAsFy (16 x < 28, 5 y 6) The structures, especially the carbon-stacking sequences, of the Stage-2 nestled C28AsF6 and un-nestled CxAsFy have recently been clari®ed [19]. XRD patterns for ca. C28AsF6 and ca. C25AsF6 are shown in Fig. 8. There are clear
Fig. 8. XRD patterns (Cu Ka) of Stage-2 CxAsF6 for (a) nestled ca. C28AsF6 and (b) un-nestled ca. C25AsF6.
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F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
differences between them besides in the positions of (0 0 l) arising from the Ic-distance difference; pattern (a) has a low angle halo indicated by H and their (1 0) or (1 0 l) bands at 2y 42±508 are different from each other. Although it is tempting to assume, as an extension of the Stage-1 GICs, that halo H and the (1 0) band in pattern (a) are caused by superlattice arrangement of nestled AsF6ÿ and random carbon-layer stacking, respectively, and that the re¯ections in pattern (b) are indexed by a small unit cell with a0 a0(graphite) and c0 m Ic where m is an integer, it is only half-true; the (1 0) band of pattern (b) is also caused by random stacking of carbon layers. The stacking sequences in the nestled and un-nestled Stage-2 CxAsF6 are both random but in a different sense, and are expressed by e.g. A|BC|AC|BA|C| and A|AB|BA|AC|C| , respectively, as a consequence of the different arrangements of carbon layers across the intercalate layer, i.e. staggered and eclipsed. The observed and calculated XRD patterns for (1 0) bands are shown in Figs. 9 and 10 for nestled C28AsF6 and un-nestled C25AsF6, respectively. Qualitatively, the same pattern is expected to occur for Stage-2 un-nestled compounds with a wide range of stoichiometries covered by the formula CxAsFy (16 x < 28, 5 y 6). Here the same remark on the conditions for x and y applies as in Section 3.1. 3.4. Nestling phenomena and carbon-layer stacking sequences Carbon-layer stacking sequence in GICs is an interesting issue. In pristine graphite the most stable stacking sequence is well known to be ABAB . Apparently, there are signi®cant interactions between non-adjacent layers. For Stage1 GICs if the requirement is solely such that the two enclosing carbon layers be staggered, the stacking sequence will be random, e.g. A|B|A|C|B|A| . This is the case with the nestled C14AsF6. When the requirement is such that two adjacent layers be eclipsed, the sequence will be unequivocally A|A|A| , although B|B|B| or C|C|C| notation is alternatively possible. This is the case with Stage-1 unnestled CxAsFy and Stage-1 alkali metal GICs among others.
Fig. 9. Observed and calculated XRD (1 0)-band patterns (Cu Ka) for nestled Stage-2 C28AsF6.
Fig. 10. Observed and calculated XRD (1 0)-band patterns (Cu Ka) for un-nestled Stage-2 C25AsF6.
Spherical alkali metal ions nestle between carbon layers requiring an eclipsed arrangement for the enclosing carbon layers. For Stage-2 and higher-stage GICs, as the adjacent carbon layers without an interleaved guest layer are likely to be staggered, there is always a possibility of the stacking sequence to be random regardless of the arrangement (staggered or eclipsed) of the carbon layers across the intercalate layer. Possible stacking sequences of Stages-1 and -2 GICs are summarized in Table 2. In the single-crystal structure study of a Stage-2 C16AsF5 material, Markiewicz et al. [20] concluded that the carbon-
Table 2 Carbon-layer stacking sequences in Stages-1 and -2 GICs Enclosing layersa
Stage
Ordered or randomb
Examplesc
Compounds
Eclipsed
1 2
ordered ordered
un-nestled CxAsFy (8 x < 14, 5 y 6)
Staggered
1
random ordered
2
random ordered
A|A|A|A|A|A| A|AB|BA|AB|B A|AB|BC|CA|A| A|AB|BA|AC|C A|B|A|B|A|B| A|B|C|A|B|C| A|B|A|C|B|A| A|BA|BA|BA|B A|BC|AB|CA|B A|BC|AC|BA|C
random a
Arrangement of carbon layers across the intercalate layer. Stacking sequences. c Some examples are shown. Other sequences are possible except for the case of eclipsed Stage-1. b
un-nestled CxAsFy (16 x < 28, 5 y 6) nestled C16AsF6 nestled C14AsF6 nestled C28AsF6
F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
245
Table 3 Compositions, Stage, Ic and adsorption properties of CxMF6 (M As, Sb) Sample composition
Stage
Ê) Ic (A
Adsorbed N2 at P/P0 0.05 (cm3/g)
Number of N2 molecule per CxMF6
Specific surface area (m2/g)
Pristine graphite C9.8AsF6 (un-nestled) C11.9AsF6 (un-nestled) C13.6AsF6 (nestled) C15.1AsF6 (nestled) C11.7SbF6 (un-nestled) C26.0SbF6 (un-nestled) C36.5SbF6 (un-nestled)
± 1 1 1 1 1 2 3
3.35 7.88 7.79 7.60 7.60 8.21 11.53 14.94
0.5 4.8 5.3 22.5 30.6 0.6 0.8 1.9
± 0.07 0.08 0.35 0.51 0.01 0.02 0.06
2 17 19 80 109 3 5 10
layer stacking sequence is AB with frequent twinning to give a situation such as ABAB. . .AB/BABA. . .BA/AB. . ., where / denotes the occurrence of twinning. Their lattice Ê and c0 11.50 A Ê . The constants were given as a0 2.46 A preparational and structural studies on nestled C14nAsF6 and un-nestled CxAsFy described in Sections 2 and 3 suggest that, because the sample was not evacuated, it was rich in AsF5 and AsF3 neutrals. The Ic-distance was large (the c0 Ê corresponds to an Ic-distance of 8.15 A Ê of value of 11.50 A the Stage-1 phase), therefore, the guest species were unlikely to be nestled, and the carbon-layers enclosing the guest species tend to be eclipsed. Such a tendency is likely to be the cause of the frequent twinning. Although they did not specify the intercalate layers, the above sequence could be written as AB|AB AB|BA|BA BA|AB denoting the intercalate layers by |. The observed occurrence of streaking along [0 0 1] for the series of re¯ections (h k l) with h ÿ k 6 3n may have been caused not only by the twinning, but also by the participation of the third type of the carbon layer, C (among A, B and C), leading to a random stacking of the A, B and C carbon layers. Homma and Clarke [21] also drew the same conclusion on SbCl5 GICs, i.e., the graphite stacking sequence for all stages is ABAB with stacking faults in the same sense as the `twinning' used by Markiewicz et al. [20]. Although they were able to ®t the data for Stages 3ÿ5, no good agreement was obtained for Stage-1. It is possible that the room temperature Stage-1 C-SbCl5 has an intermediate structure of the nestled C14AsF6 and C16AsF6, and un-nestled CxAsFy, provided that SbCl6ÿ plays an active role and can nestle. Vaknin and Fischer [22] concluded that their Stage-2 C16AsF5 has an AB|CA|BC stacking sequence without the twinning mentioned by Markiewicz et al. It is to be cautioned, however, that different results may be obtained depending on the host graphite materials used for experiment. In the above three cases it is not certain whether the AB stacking sequence is a consequence of the nestling of MX6ÿ species. In graphite-SbCl5 system the carbon layers enclosing guest species are found to be staggered and domains of (H7 H7) R (19.118) is observed [21]. Furthermore, the small Ic is explained in terms of registry of Cl-ligands on a graphite hexagon. Since SbCl6ÿ as well as SbCl3 are found within the gallery, it is possible that SbCl6ÿ anions nestle as
does AsF6ÿ. However, in the case of SbCl5 GIC the interpretation for nestling and H7 H7 structure is made based on the close-packing of SbCl6ÿ species themselves irrespective of graphite layers and the coincidence of the superlattice size to H7 H7 structure. It is noted that, compared with AsF6ÿ, SbCl6ÿ is large enough to make a close packing of such a size. However, XRD patterns of CxSbF6 prepared by the reaction of graphite with O2SbF6 have not indicated nestling of SbF6ÿ probably because of its larger size compared with AsF6ÿ [23]. Compositions, stage and Ic of CxSbF6 are given in Table 3. The results suggest the formula C12nSbF6 with Ê , and the in-plane density of Ic 8.2 3.35(n ÿ 1) A C12SbF6 is too high for SbF6ÿ to nestle. The Ic value of 8.2 is signi®cantly smaller than that reported for Stage-1 Ê [24]). This is because of the less compact CxSbF5 (8.44 A structure of SbF5 and coexistence of SbF3, SbF5 and SbF6ÿ species in the gallery of CxSbF5 according to the equation: 3SbF5 2eÿ ! 2SbF6 ÿ SbF3 ÿ
(10)
The height of SbF6 along the three-fold axis is smaller than the height of SbF5. A similar difference is observed for Ê [10]) and un-nestled CxAsF6 (ca. Stage-1 CxAsF5 (8.10 A ÿ Ê 8.0 A). As SbF6 ions have not been found to nestle so far, much larger SbCl6ÿ ions are unlikely to nestle, although partial nestling of some ¯uorine or chlorine ligands in SbF5 and SbCl5 GICs can occur, which may be the cause of the observation of H7 H7 structure in SbCl5 GICs. The nestling of AsF6ÿ suggests that many other (pseudo) Oh or Td ¯uoro-, oxo-, or oxo¯uoro-ligand guest species will be found to nestle in the graphite layers, if the guest concentration is suf®ciently low to permit it. Diffraction data [25] on C8SO3Fÿ [1] indicate that the SO3Fÿ are nestled. A small Ic-distance is a strong indicator of such nestling but the impact upon the XRD pattern, of both the staggering of the enclosing carbon sheets, and their random stacking, should also signal occurrence of nestling. Clearly neutral guest species, because they are not strongly attracted to the carbon sheets, will be less likely to nestle than anions. Also when the guest concentration exceeds a critical value, nestling will no longer be possible. In that situation the most commodious arrangement of the enclosing carbon sheets appears to be the eclipsed one.
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4. Reactions of CxAsF6 4.1. Reactions of nestled C14AsF6 with graphite The nestled Stage-2 C28AsF6 is obtained, not only via Schemes 1 and 2 (with dif®culty), and via Scheme 3, but also by the reaction of nestled Stage-1 C14AsF6 with graphite: 14 C C14 AsF6 ! C28 AsF6
(11)
The reaction of Eq. (11) is carried out by mixing graphite and C14AsF6 in the same manner as for graphite and O2AsF6 [26]. It is to be noted that this reaction proceeds easily at room temperature indicating a high mobility of AsF6ÿ anions within the graphite gallery and across the crystallite interfaces. The formation of nestled Stage-2 C28AsF6 from nestled Stage-1 C14AsF6 clearly indicates that the guest species in C14nAsF6 is indeed AsF6ÿ. It is expected that higher-stage C14nAsF6 (n 3) GICs of pure phase can be formed easily in the same manner: 14 n ÿ 1C C14 AsF6 ! C14n AsF6
(12)
Of course, reaction of any CxAsF6 with graphite is possible, but the staging formula C14nAsF6 was emphasized here to demonstrate the easy control of stoichiometry and staging by the reaction according to Eqs. (11) and (12). Direct reactions between two GICs of donor- and acceptor-types, e.g. between LiC6 or KC8 and C14AsF6 may also proceed relatively easily: LiC6 C14 AsF6 ! 20 C LiAsF6
(13)
KC8 C14 AsF6 ! 22 C KAsF6
(14)
These reactions would be regarded as an acid-base neutralization reaction taking the amphoteric nature of graphite into consideration. They could also be run electrochemically. 4.2. Reactions of CxAsF6 with F2 in HF CxAsF5 adsorbs or reacts with ¯uorine as expected from Eq. (4) [4], and so does Stage-1 ca. C14AsF6 to yield compounds with composition ca. C14AsF6yF (y 1 ÿ 3) [14]. When CxAsF6 salts higher than Stage-1 are treated with F2 in liquid hydrogen ¯uoride (AHF) at ambient temperature, a graphite ¯uoride, CxF is produced together with a Stage-1 ¯uoroarsenate salt [15]. The oxidatively inert CxF phase can be isolated by destructive oxidation of the (graphite)AsF6ÿ with perchloric acid at 1508C. C1.3F represents the highest ¯uorine content of the product. The reaction scheme can be given as follows: Scheme 4 Ê and The unit cell parameters of C1.3F are a0 2.478 A Ê . The IR and XPS spectra indicate that the c0 6.40 A oxidatively inert, black, C1.3F and its relatives are based on an sp2 carbon atom network like that of the parent graphite, to which the ¯uorine atoms are bound semi-ionically.
Scheme 4
4.3. Reaction of CxAsF6 with fluorobases A Stage-1 GIC of a strong ¯uoroacid, C10AsF6, was found to react with ¯uorobases such as KF in AHF to give a Stage2 compound [27]. The reaction proceeds as: 8 C10 AsF6 5 KF ! 3 C20 AsF6 5 C4 F 5 KAsF6
(17)
XRD and XPS analyses revealed that the compound is covered with planar-sheet graphite ¯uoride, CxF. C10AsF6 also reacts with the solvent hydrogen ¯uoride, liberating AsF5 to give a mixture of Stages-1 and -2 compounds. It is to be noted that the ¯uoroacidity of the Stage-1 C10AsF6 is so high that HF acts as a ¯uorobase towards it. The reaction can be expressed by the following equation: 20 ÿ x 10 ÿ x C10 AsF6 2HF ! C20 AsF6 Cx F 10 10 (18) H2 F AsF6 ÿ The weight gain of the GICs exposed to air is reduced by the reaction with ¯uorobases. This is attributed to the surface coverage of the material with graphite ¯uoride protects the bulk graphite ¯uoroarsenates from the attack of moisture in air. A schematic illustration of the reaction product of C10AsF6 with KF is given in Fig. 11 [27]. 4.4. Other reactions of CxAsF6 Many other reactions are possible for CxAsF6, or more in general, for many GICs. Reactions of GICs, however, are usually dif®cult to elucidate because of the less crystalline nature of graphite materials and the less stoichiometric nature of GIC products. As explained in Section 2.4, AsF3 acts as a reducing agent towards C14AsF6 after being incorporated within the gallery to form ca. C14AsF61/2AsF3 [3±5]. The incorporation of AsF3 can be regarded as adsorption of neutral molecules to form a ternary GICs. When solvents are used for the formation of GICs, they are often found to be co-intercalated. As explained in Section 2.4, when the reaction of graphite with O2AsF6 is run in SO2ClF solvent [1,2,4], the solvent is incorporated along with AsF6ÿ [14]. The maximum amount of incorporated SO2ClF corresponds to ca. C14AsF60.4SO2ClF, and SO2ClF can be quasi-reversibly removed or added.
F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
247
Fig. 11. A schematic illustration of the reaction product of C10AsF6 with KF (reproduced with permission from [27]).
It is true that adsorption of nitrogen can also be included in the previous section, but this theme is treated separately since its aim is to probe the pore structure in nestled CxAsF6. Studies of adsorption of nitrogen and other gases by donortype GICs are not rare, but such investigations have seldom been carried out on acceptor-type GICs. Adsorption isotherms of nitrogen at 77 K for (a) graphite, (b) un-nestled C9.8AsF6, (c) un-nestled C11.9AsF6, (d) nestled C13.6AsF6, and (e) nestled C15.1AsF6 are shown Fig. 12 [16]. Their BET surface areas are 2, 17, 19, 80 and 109 m2/g, respectively. Sample (b) was made with AsF5 F2, and the others with O2AsF6. The results are summarized in Table 3. As can be seen from the ®gure and the table, the amounts of N2 adsorbed by the nestled CxAsF6 are signi®cantly larger than those by the un-nestled CxAsF6. For the nestled C15.1AsF6, the isotherm is of Type I, and the initial sharp rise indicates a large amount of micropores within the material. The amount of adsorbed N2 corresponds
approximately to the formula C15.1AsF61/2N2. This is because that, in the nestled C14AsF6, the in-plane density of AsF6ÿ is relatively low as shown in Fig. 3, and AsF6ÿ ions do not `touch' each other. The value of 14 in the Stage-1 C14AsF6 is to be compared with 8 in the Stage-1 C8AsF5 [10]. Without nestling, C14AsF6 would be a mixture of Stages-1 and -2 with a higher in-plane density of guest species, which would hinder the incorporation of N2. As explained in Section 3.1, the composition C14AsF6 corresponds to the highest density of nestled AsF6ÿ. For unnestled CxAsF6, the composition of ca. C10AsF6 corresponds to the highest density of AsF6ÿ. This allows us to estimate the space available for N2 adsorption in the nestled C14AsF6 as follows. C14AsF6 can be formulated as C4C10AsF6. The size of AsF6ÿ in the ab-projection is larger than three ¯uorine ligands which roughly corresponds to the size of (3/2) N2. As a result, the formula C4(N2)0.6C10AsF6 is obtained for N2 adsorption. This can further be approximately reformulated to C14AsF61/2N2. For comparison, adsorption isotherms of nitrogen at 77 K for Stages-1, -2, and -3 CxSbF6, are shown in Fig. 13 [28]
Fig. 12. Adsorption isotherms of nitrogen at 77 K for (a) graphite, (b) C9.8AsF6, (c) C11.9AsF6, (d) nestled C13.6AsF6, and (e) nestled C15.1AsF6 (reproduced with permission from [16]).
Fig. 13. Adsorption isotherms of nitrogen at 77 K for (a) graphite, (b) Stage-1 C11.7SbF6, (c) Stage-2 C26.0SbF6, and (d) Stage-3 C36.5SbF6.
5. Adsorption properties and nano-space of nestled CxAsF6
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and the results are summarized in Table 3. The amounts of N2 adsorbed by CxSbF6 are signi®cantly smaller than those by the nestled CxAsF6. This is probably because that SbF6ÿ ions do not nestle between graphite layers. It is not unambiguously clear, however, whether N2 molecules are adsorbed in the graphite gallery. The high speci®c surface area for the nestled C14AsF6 might have been caused via the reaction of graphite with O2AsF6, which might have served as surface activation of the graphite, and it may not be associated with the interlayer gallery. However, the difference in adsorption capacity for nestled C14AsF6 and un-nestled CxAsF6, both of which have experienced the possible surface-activation reaction, support the idea of the interlayer gallery of the nestled C14AsF6 being an active site for N2 incorporation. The nano-spaces of the C14AsF6 reside in the multi-decked vast nano-galleries or -halls with ¯oors and ceilings of graphite layers supported by bold pillars of nestled AsF6ÿ. Acknowledgements This work was supported by the Research for the Future Program of the Japan Society for the Promotion of Science (JSPS-RFTF96R11701). References [1] N. Bartlett, R.N. Biagioni, B.W. McQuillan, A.S. Robertson, A.C. Thompson, J. Chem. Soc., Chem. Commun. (1978) 200. [2] N. Bartlett, B. McQuillan, A.S. Robertson, Mat. Res. Bull. 13 (1978) 1259. [3] E.M. McCarron, N. Bartlett, J. Chem. Soc., Chem. Commun. (1980) 404.
[4] T.E. Thompson, E.M. McCarron, N. Bartlett, Synthetic Metals 3 (1981) 255. [5] F. Okino, N. Bartlett, J. Chem. Soc., Dalton Trans. (1993) 2081. [6] F. Okino, Y. Sugiura, H. Touhara, A. Simon, J. Chem. Soc., Chem. Commun. (1993) 562. [7] Lin Chun-Hsu, H. Selig, M. Rabinovits, I. Agranat, S. Sarig, Inorg., Nucl., Chem., Lett. 11 (1975) 601. [8] E.R. Falardeau, G.M.T. Foley, C. Zeller, F.L. Vogel, J. Chem. Soc., Chem. Commun. (1977) 389. [9] G.M.T. Foley, C. Zeller, E.R. Falardeau, F.L. Vogel, Solid State Commun. 24 (1977) 371. [10] E.R. Falardeau, L.R. Hanlon, T.E. Thompson, Inorg. Chem. 17 (1978) 301. [11] J.E. Fischer, J. Chem. Soc. Chem. Commun. (1978) 544. [12] M.J. Moran, J.E. Fischer, W.R. Salaneck, J. Chem. Phys. 73 (1980) 629. [13] P.A.W. Dean, R.J. Gillespie, R. Hulme, J. Chem. Soc. Chem. Commun. (1969) 990. [14] F. Okino, Ph.D. Thesis, University of California, Berkeley, 1984. [15] R. Hagiwara, M. Lerner and N. Bartlett, J. Chem. Soc. Chem. Commun. (1989) 573. [16] F. Okino, S. Kawasaki, H. Touhara, Mol. Cryst. Liq. Cryst., in press. [17] N. Bartlett, B.G. DeBoer, F.J. Hollander, F.O. Sladky, D.H. Templeton, A. Zalkin, Inorg. Chem. 13 (1974) 780. [18] J.A. Ibers, Acta Crystallogr. 9 (1956) 967. [19] F. Okino et al., in press. [20] R.S. Markiewicz, J.S. Kasper, L.V. Interrante, Synthetic Metals 2 (1980) 363. [21] H. Homma, R. Clarke, Phys. Rev. B 31 (1985) 5865. [22] D. Vaknin, J.E. Fischer, Synthetic Metals 23 (1988) 101. [23] F. Okino, K. Maruyama, M. Kawawaki, S. Kawasaki, H. Touhara, Extended Abstracts, in: International Symposium on Carbon, 1998, Tokyo, I10±08, 208. [24] T.E. Thompson, E.R. Falardeau, L.R. Hanlon, Carbon 15 (1977) 39. [25] R.N. Biagioni, Ph.D. Thesis, University of California, Berkeley, 1980. [26] F. Okino et. al., in press. [27] R. Hagiwara, K. Tozawa, Y. Ito, J. Fluorine Chem. 88 (1998) 201. [28] F. Okino, K. Maruyama, M. Kawawaki, S. Kawasaki, H. Touhara, Extended Abstracts, in: 14th Biennial Conference on Carbon, Charleston, 1999, 160.
Preparation and properties of graphite hexa¯uoroarsenates CxAsF6 Fujio Okino* Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan
Abstract Preparation, structures, reactions and adsorption properties of graphite hexa¯uoroarsenates CxAsF6 are described in connection with those of graphite intercalation compounds CxAsF5, CxSbCl5, CxSbF5 and CxSbF6. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Graphite intercalation compounds; Hexa¯uoroarsenate; Nestling; Stacking sequences; Adsorption; Nano-space; Micropores; Arsenic penta¯uoride
1. Introduction Graphite hexa¯uoroarsenates CxAsF6 [1±6] are a class of graphite intercalation compounds (GICs), and called as such when the nature of the guest species is to be speci®ed, for they are salts of hexa¯uoroarsenate. They are related to but different from CxAsF5 GICs prepared by the reaction of graphite with AsF5. CxAsF5 GICs [7] have drawn much attention particularly because of their high electrical conductivities [8,9]. Falardeau et al. [10], using X-ray diffraction (XRD), gravimetry and c-axis thickness measurements on highly oriented pyrolytic graphite (HOPG) intercalated samples, characterized graphite-AsF5 intercalation compounds by the formula C8nAsF5 with the c-axis repeat distance Ic of Ê , which transforms to 8.10 3.35 (n ÿ 1) A Ê, 4.75 3.35n A where n is the stage. There has been much controversy about the nature of the guest species in CxAsF5. X-ray arsenic preabsorption edge studies by Bartlett et al. [1,2], established that the intercalation of graphite by AsF5 was accompanied by electron oxidation of the graphite according to the equation: 3 2 AsF5
eÿ ! AsF6 ÿ 12 AsF3
2. Preparation of CxAsF6 CxAsF6 can be prepared via either of the following three schemes:
Scheme 1
Scheme 2
(1)
The implications of this for the preparation of CxAsF5 GICs, and thus for their physical and chemical properties, had not been fully comprehended. The author has been preparing a series of CxAsF6 GICs, the existence of which had been implied by Eq. (1). In the following sections, preparation, structures, reactions and adsorption properties of graphite hexa¯uoroarse* Tel.: 81-268-21-5393; fax: 81-268-21-5391. E-mail address: [email protected] (F. Okino).
nates CxAsF6 are described in connection with those of CxAsF5, CxSbCl5, CxSbF5 and CxSbF6 GICs.
Scheme 3
In Scheme 1, the reaction according to Eq. (3) is not followed stoichiometrically, for some loss of AsF5 always accompanies the loss of AsF3. This point is addressed in the following section. In all cases including Scheme 1, the vacuum stable products contain solely AsF6ÿ as guest species.
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 7 1 - 7
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F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
The ready loss of AsF5 in comparison with AsF3 can be explained as follows: the AsF5 can form As2F11ÿ with AsF6ÿ [13] and an AsF5 molecule can, thereby, be generated in a new location without actual translational movement of the molecule according to the following equation where two arsenic atoms are differentiated with and without an asterisk.
2.1. Preparation of CxAsF6 by the reaction of graphite with AsF5 alone When graphite is reacted with an excess amount of AsF5 (C : AsF5 < 8), the reaction product indeed shows a composition of ca. C8AsF5 [10]. Although compositions of GICs are usually formulated as CmnM, where n is the stage and m or CmM indicates the in-plane density of guest species M, there may not be a compelling reason for m to be an integer, unless the guest species are somehow registered to the host graphite lattice. The situation of non-integral m is termed incommensurate or nonstoichiometric. Graphite-AsF5 compounds may fall into this category, but they have added complexity of the formation of AsF6ÿ and AsF3 according to Eq. (1). The conversion of AsF5 into AsF6ÿ and AsF3 according to Eq. (3) is not complete in C8AsF5, and it readily loses AsF5 mainly at the beginning and then AsF3 slowly under vacuum [4,5]. The composition of the solid product, after the initial removal of the gases, is usually close to C10AsF5d (d ! 1). Materials made in this way have been usually reported as C10AsF5 by many workers. But the F : As ratio must be larger than 5, since evolution of AsF3 has occurred. In one of the cases, after 200 h of evacuation, the carbon content of the sample became 55.2%, which, assuming the remaining guest species to be AsF6ÿ, corresponds to C19.4AsF6. The composition of this sample by chemical analysis was C20.28AsF6.01 as given in Table 1 supporting that AsF6ÿ is the sole guest species [5]. The composition implies that the ionization extent of a single treatment of graphite with AsF5 is ca. C20 [11,12]. When graphite is repeatedly treated with AsF5 followed by evacuation, the composition after six cycles became C15.94AsF5.98 as given in Table 1 [5]. The results show that interaction of graphite with AsF5 alone is an effective route to the production of CxAsF6 salts, so long as care is taken to remove the volatiles completely. After several cycles the Stage-1 phase was in much greater concentration than the Stage-2 phase, whereas the reverse was true for one cycle. However, the repeated treatment of graphite with AsF5 alone, followed by evacuation, never leads to pure Stage-1 products owing to its low oxidizing ability (see Section 2.4).
AsF5 AsF6 ÿ ! AsF5 ÿ F ÿ AsF5 ÿ ! AsF6 ÿ AsF5 (6)
On the other hand, AsF3, being a much inferior ¯uoride ion acceptor than AsF5, does not have such a mechanism for effective transfer available to it. Jostling of the AsF3 molecules through a relatively close packed environment is evidently slow. 2.2. Preparation of CxAsF6 by the reaction of graphite with AsF5 and F2 CxAsF6 can be formed by the reaction of graphite with AsF5 F2 [3,5]. The composition of a representative sample was C13.05AsF6.01 as given in Table 1 [5]. Although this route seems to be a straightforward way to the formation of CxAsF6 salts, the exact ratio of F : As 6 and the control of the ratio of C : AsF6ÿ are dif®cult to attain owing to the high reactivity of AsF5 F2; not only AsF5 F2 slowly attacks the inner wall of the vacuum line, but also F2 is adsorbed by CxAsF5 [4] or CxAsF6 [14], or may react with low-stage CxAsF6 as it is formed to yield a CxF phase [15]. 2.3. Preparation of CxAsF6 by the reaction of graphite with O2AsF6 Originally the reaction of graphite with O2AsF6 was run in SO2ClF solvent [1,2,4], but the solvent SO2ClF was found to be incorporated along with AsF6ÿ [14]. It was later found that the direct solid±solid reaction of O2AsF6 with graphite yields CxAsF6 but the reaction was thought to be very slow [5]. Compositions of the samples after several days of mixing of graphite with O2AsF6 were C14.01AsF6.01 and C16.96AsF6.00 as given in Table 1 [5]. Nonetheless, the results indicated that this is also an effective route to the production of CxAsF6 salts.
Table 1 Analytical data for the vacuum-stable solid products obtained from (1) C AsF5, (2) C AsF5 F2, and (3) C O2AsF6 [5] Reactants
Methoda
Composition
C
H
As
F
N
O
C As F
17.26 19.63 21.56 20.91 19.05 22.23
26.30 29.79 32.85 31.85 28.98 33.78
<0.001 0.003 0.010 0.023 0.001 0.001
<0.5 ± <0.5 <0.3 <0.3 <0.5
99.68 99.57 99.50 99.72 99.82 99.43
b
Analysis (%) (1) AsF5 (2) AsF5 F2 (3) O2AsF6
one cycle six cycles gas mixture solid-phase reaction
(30 ) O2AsF6
thermally decomposed
a b
C20.28AsF6.01 C15.94AsF5.98 C13.05AsF6.01 C14.01AsF6.01 C16.96AsF6.00 C12.81AsF5.99
56.12 50.15 45.09 46.96 51.79 43.42
See text for detailed methods. Experimental values determined by Galbraith Laboratories, Inc.
0.70 0.05 0.09 0.21 <0.01 0.11
F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
Fig. 1. Progress of the reaction of graphite with O2AsF6 for the formation of nestled C14AsF6 monitored by pressure increase (reproduced with permission from [16]).
This reaction, however, was found to be much faster than it had been thought; the reaction proceeds essentially within a matter of 10 s when an effective mixing of the solids (ca. 100 mg) is applied. In this reaction, as the weights of graphite powder and O2AsF6 are weighed separately, the ratio of C : AsF6ÿ in the product is easily controlled. Fig. 1 shows the reaction progress for the formation of nestled C14AsF6 monitored by oxygen pressure increase [16]. The pressure rises according to Eq. (5). Initially the cell was ®lled with nitrogen and the pressure was measured every second. At t 40 s, O2AsF6 was transferred onto graphite. The reaction immediately started as the mixture was perturbed in an effort to transfer O2AsF6 thoroughly. When a violent shaking started at t 200 s, the pressure increased drastically but stopped increasing within 10 s, accompanied with the change of the sample color from black±grey to metallic blue, indicating a very fast solid phase reaction.
241
AsF5 to do likewise, can therefore be attributed to the greater oxidizing power of O2AsF6 and AsF5 F2. That this inferiority in oxidizing capability of AsF5 is indeed thermodynamically and not kinetically based, is indicated by the product of the interaction of AsF3 with Stage-1 salts ca. C14AsF6 which had been prepared using either AsF5 F2, or O2AsF6 [3±5]. When the volatiles are removed from C14AsF61/2AsF3 after AsF3 is incorporated within the gallery, they include AsF5. The vacuum stable residual solid is a mixture of Stage-1 C14AsF6 and Stage-2 C28AsF6 salts, comparable to the mixture derived from the interaction of graphite with AsF5 after an extended removal of volatiles. Clearly, in forming the Stage-2 phase within the bulk, AsF3 acts as a reducing agent towards C14AsF6ÿ: 2C14 AsF6 12 AsF3 ! C28 AsF6 32 AsF5
(9)
3. Structures of nestled C14nAsF6 and un-nestled CxAsFy 3.1. Structures of Stage-1 nestled C14AsF6 and un-nestled CxAsFy (8 x < 14, 5 y 6) Signi®cant differences are observed between the room temperature (RT) XRD patterns for the Stage-1 graphite Ê ) [5], and for the hexa¯uoroarsenate, C14AsF6 (Ic 7.6 A Stage-1 graphite-AsF5 intercalation compound, ca. C10AsF5 Ê ). Typical diffraction patterns are shown in [8,10] (Ic 8.0 A Fig. 2 [6]. The C14AsF6 was made by the reaction of graphite with O2AsF6 and the C10AsF5 with AsF5. The XRD pattern Ê ), for C14AsF6 is characterized by its very small Ic (7.6 A and peculiar features; the appearance of a low angle halo
2.4. The extent of intercalation and the oxidizing power of the guest precursors The inability of AsF5 alone with graphite to generate pure Stage-1 salts stands in marked contrast with the ready formation of such salts by AsF5 F2, and O2AsF6. The basis for these differences appears to be thermodynamic. Free energy changes for related oxidizing half reactions are evaluated as follows [5]: 3 2 AsF5 g
125
eÿ ! AsF6 ÿ g 12 AsF3 g DG298 ÿ125 kcal molÿ1
(1)
AsF5 g 12 F2 g eÿ ! AsF6 ÿ g 170
DG298 ÿ170 kcal molÿ1
(7)
O2 AsF6 c eÿ ! AsF6 ÿ g O2 g 155
DG298 ÿ155 kcal molÿ1
(8)
The ready formation of pure Stage-1 salt C14AsF6 by the oxidizers O2AsF6 and AsF5-F2 mixture, but the failure of
Ê ), (b) ca. Fig. 2. XRD patterns (Cu Ka) of (a) ca. C14AsF6 (Ic 7.6 A Ê ), and (c) quartz capillary background (reproduced C10AsF5 (Ic 8.0 A with permission from [6]).
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Fig. 3. In-plane H7 H7 arrangement of AsF6ÿ for the nestled C14AsF6 (reproduced with permission from [5]).
[halo A in pattern (a) in Fig. 2] and two peaks (B and C), and the absence of (1 0 0) and (1 0 l) re¯ections. These features suggest that the ¯uorine ligands of AsF6ÿ are nestled in contiguous three-fold sets of carbon atom hexagons of the graphite. The nestling of AsF6ÿ is geometrically justi®ed as Ê follows. The As±F distance in the AsF6ÿ ion is ca. H3 A [17,18]. Then, assuming that the ion is a regular octahedron Ê , the neighboring F±F distance is with d (As±F) H3 A Ê H6 A, which is essentially the same as the lattice constant, a0, of graphite. The nestling of AsF6ÿ requires a staggering of the enclosing carbon layers. Adjacent pairs of carbonatom sheets contain assemblies of AsF6ÿ of short range order. Ideally, the most dense arrangement of nestled AsF6ÿ has the composition C14AsF6 shown in Fig. 3 [5]. This is because the two ¯uorine ligands belonging to two different AsF6ÿ cannot nestle in two neighboring hexagons of graphite network owing to the `large' size of the ¯uorine ligands. The (1 0 0) re¯ection of this superlattice unit cell [a0 H7a0(graphite)] falls at the center of halo A as indicated by the dotted line above it in Fig. 4 [5]. Aside from the restriction imposed by AsF6ÿ nestling, the carbon layers are randomly stacked. A model has been proposed in which staggered carbon layers are randomly stacked, e.g. A|B|A|C|B|A| where | denotes the intercalate layer, with short-range ordering or domain structure of nestled AsF6ÿ anions in the gallery. All the peculiar features of the pattern Ê are explained by the for the C14AsF6 sample with Ic 7.6 A nestling of AsF6ÿ. The mathematical treatment of the model shows that diffuse scatterings are observed for (h k l) with h ÿ k 6 3n and usual crystal re¯ections for (h k l) with h ÿ k 3n [5]. In the model, the ¯uorine atoms are placed at the center of the carbon hexagons in the ab-projection and Ê above and below the central arsenic atom along the c1.0 A axis direction. The calculated patterns based on the nestled model for C14AsF6 as well as the observed pattern are shown in Fig. 4 [5]. In the ®gure, pattern (c) is the calculated (1 0 l)
Fig. 4. (a) Observed XRD patterns (Cu Ka) of the nestled ca. C14AsF6, (b) quartz capillary background, (c) calculated (1 0 l) diffuse scattering, and (d) calculated pattern with (0 0 l) and (1 1 l) crystal-reflections, and (1 0 l) diffuse scattering (reproduced with permission from [5]).
diffuse scattering and pattern (d) consists of the calculated (0 0 l) and (1 1 l) crystal-re¯ections and (1 0 l) diffuse scattering. In cases where the graphite galleries are richer in AsF6ÿ, i.e. C<14AsF6, or AsF5 and AsF3 are among the guest species, i.e. CxAsF5ÿ6, comfortable nestling of AsF6ÿ is no longer possible. Consequently, the Ic-distances are larger Ê ) and the enclosing carbon layers are eclipsed. (ca. 8.0 A These conclusions are unambiguously drawn from the XRD pattern for ca. C10AsF5 given in Fig. 2 (b). The pattern is simpler than that for C14AsF6 (Fig. 2(a)) and all re¯ections are accounted for by the unit cell with a0 a0(graphite) and c0 Ic. Since the staggering of carbon layers requires c0 to be a multiple of Ic, and an ordered arrangement of the AsFy species in the gallery requires a0 to be larger than a0(graphite), the carbon layers are eclipsed and the guest species are randomly placed. The calculated pattern based on this model for C10AsF5 as well as the observed pattern are shown in Fig. 5 [5]. The observed low angle halo in this case is solely attributable to the quartz capillary shown by pattern (c). Qualitatively, the same pattern occurs for Stage-1 unnestled compounds with a wide range of stoichiometries covered by the formula CxAsFy (8 x < 14, 5 y 6). Here the conditions for x and y should not be taken rigorously. For example, C13AsF5 will be a mixture of Stages-1 and -2, and C8AsF6 may not exist. 3.2. Structure of Stage-1 nestled C16AsF6 When the temperature is lowered, superlattice re¯ections appear for ca. C14AsF6, indicating an order±disorder transition in the AsF6ÿ arrangement and, consequently, in the carbon layer stacking sequence at ca. 170 K. The re¯ection pattern at lower temperature has been satisfactorily simu-
F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
243
Fig. 5. (a) Observed XRD patterns (Cu Ka) for the un-nestled material of composition ca. C10AsF5, (b) pattern, calculated (c) quartz capillary background (reproduced with permission from [5]).
lated by a nestled C16AsF6 model. The observed and calculated diffraction patterns at 90 K are shown in Fig. 6, and the in-plane arrangement of AsF6ÿ for the model is shown in Fig. 7(a) [6]. The pseudo face-centered orthorhombic arrangement of AsF6ÿ requires the carbon-layer stacking sequence to be A|B|A|B|A|B| (see Fig. 7(b)). Apparently, strong Coulomb repulsive interaction among AsF6ÿ in the same and different galleries controls the structure of C16AsF6. The electrostatic interaction probably renders the C14AsF6 structure less stable, since (i) a torque is exerted on each AsF6ÿ by the electric ®eld produced by the other AsF6ÿ ions in the same gallery in C14AsF6 (see Fig. 3), and (ii) the repulsion between AsF6ÿ ions are stronger in C14AsF6 with the most dense arrangement of nestled AsF6ÿ ions, although the total attraction between C14 and AsF6ÿ in C14AsF6 is stronger than that between C16 and AsF6ÿ in C16AsF6.
Fig. 6. (a) Observed and (b) calculated XRD patterns (Cu Ka) for C16AsF6 at 90 K (reproduced with permission from [6]).
Fig. 7. Structures of C16AsF6: (a) in-plane structure of C16AsF6 (nestling of AsF6ÿ is shown) and (b) positions of arsenic atoms in different galleries (reproduced with permission from [6]).
3.3. Structures of Stage-2 nestled C28AsF6 and un-nestled CxAsFy (16 x < 28, 5 y 6) The structures, especially the carbon-stacking sequences, of the Stage-2 nestled C28AsF6 and un-nestled CxAsFy have recently been clari®ed [19]. XRD patterns for ca. C28AsF6 and ca. C25AsF6 are shown in Fig. 8. There are clear
Fig. 8. XRD patterns (Cu Ka) of Stage-2 CxAsF6 for (a) nestled ca. C28AsF6 and (b) un-nestled ca. C25AsF6.
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F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
differences between them besides in the positions of (0 0 l) arising from the Ic-distance difference; pattern (a) has a low angle halo indicated by H and their (1 0) or (1 0 l) bands at 2y 42±508 are different from each other. Although it is tempting to assume, as an extension of the Stage-1 GICs, that halo H and the (1 0) band in pattern (a) are caused by superlattice arrangement of nestled AsF6ÿ and random carbon-layer stacking, respectively, and that the re¯ections in pattern (b) are indexed by a small unit cell with a0 a0(graphite) and c0 m Ic where m is an integer, it is only half-true; the (1 0) band of pattern (b) is also caused by random stacking of carbon layers. The stacking sequences in the nestled and un-nestled Stage-2 CxAsF6 are both random but in a different sense, and are expressed by e.g. A|BC|AC|BA|C| and A|AB|BA|AC|C| , respectively, as a consequence of the different arrangements of carbon layers across the intercalate layer, i.e. staggered and eclipsed. The observed and calculated XRD patterns for (1 0) bands are shown in Figs. 9 and 10 for nestled C28AsF6 and un-nestled C25AsF6, respectively. Qualitatively, the same pattern is expected to occur for Stage-2 un-nestled compounds with a wide range of stoichiometries covered by the formula CxAsFy (16 x < 28, 5 y 6). Here the same remark on the conditions for x and y applies as in Section 3.1. 3.4. Nestling phenomena and carbon-layer stacking sequences Carbon-layer stacking sequence in GICs is an interesting issue. In pristine graphite the most stable stacking sequence is well known to be ABAB . Apparently, there are signi®cant interactions between non-adjacent layers. For Stage1 GICs if the requirement is solely such that the two enclosing carbon layers be staggered, the stacking sequence will be random, e.g. A|B|A|C|B|A| . This is the case with the nestled C14AsF6. When the requirement is such that two adjacent layers be eclipsed, the sequence will be unequivocally A|A|A| , although B|B|B| or C|C|C| notation is alternatively possible. This is the case with Stage-1 unnestled CxAsFy and Stage-1 alkali metal GICs among others.
Fig. 9. Observed and calculated XRD (1 0)-band patterns (Cu Ka) for nestled Stage-2 C28AsF6.
Fig. 10. Observed and calculated XRD (1 0)-band patterns (Cu Ka) for un-nestled Stage-2 C25AsF6.
Spherical alkali metal ions nestle between carbon layers requiring an eclipsed arrangement for the enclosing carbon layers. For Stage-2 and higher-stage GICs, as the adjacent carbon layers without an interleaved guest layer are likely to be staggered, there is always a possibility of the stacking sequence to be random regardless of the arrangement (staggered or eclipsed) of the carbon layers across the intercalate layer. Possible stacking sequences of Stages-1 and -2 GICs are summarized in Table 2. In the single-crystal structure study of a Stage-2 C16AsF5 material, Markiewicz et al. [20] concluded that the carbon-
Table 2 Carbon-layer stacking sequences in Stages-1 and -2 GICs Enclosing layersa
Stage
Ordered or randomb
Examplesc
Compounds
Eclipsed
1 2
ordered ordered
un-nestled CxAsFy (8 x < 14, 5 y 6)
Staggered
1
random ordered
2
random ordered
A|A|A|A|A|A| A|AB|BA|AB|B A|AB|BC|CA|A| A|AB|BA|AC|C A|B|A|B|A|B| A|B|C|A|B|C| A|B|A|C|B|A| A|BA|BA|BA|B A|BC|AB|CA|B A|BC|AC|BA|C
random a
Arrangement of carbon layers across the intercalate layer. Stacking sequences. c Some examples are shown. Other sequences are possible except for the case of eclipsed Stage-1. b
un-nestled CxAsFy (16 x < 28, 5 y 6) nestled C16AsF6 nestled C14AsF6 nestled C28AsF6
F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
245
Table 3 Compositions, Stage, Ic and adsorption properties of CxMF6 (M As, Sb) Sample composition
Stage
Ê) Ic (A
Adsorbed N2 at P/P0 0.05 (cm3/g)
Number of N2 molecule per CxMF6
Specific surface area (m2/g)
Pristine graphite C9.8AsF6 (un-nestled) C11.9AsF6 (un-nestled) C13.6AsF6 (nestled) C15.1AsF6 (nestled) C11.7SbF6 (un-nestled) C26.0SbF6 (un-nestled) C36.5SbF6 (un-nestled)
± 1 1 1 1 1 2 3
3.35 7.88 7.79 7.60 7.60 8.21 11.53 14.94
0.5 4.8 5.3 22.5 30.6 0.6 0.8 1.9
± 0.07 0.08 0.35 0.51 0.01 0.02 0.06
2 17 19 80 109 3 5 10
layer stacking sequence is AB with frequent twinning to give a situation such as ABAB. . .AB/BABA. . .BA/AB. . ., where / denotes the occurrence of twinning. Their lattice Ê and c0 11.50 A Ê . The constants were given as a0 2.46 A preparational and structural studies on nestled C14nAsF6 and un-nestled CxAsFy described in Sections 2 and 3 suggest that, because the sample was not evacuated, it was rich in AsF5 and AsF3 neutrals. The Ic-distance was large (the c0 Ê corresponds to an Ic-distance of 8.15 A Ê of value of 11.50 A the Stage-1 phase), therefore, the guest species were unlikely to be nestled, and the carbon-layers enclosing the guest species tend to be eclipsed. Such a tendency is likely to be the cause of the frequent twinning. Although they did not specify the intercalate layers, the above sequence could be written as AB|AB AB|BA|BA BA|AB denoting the intercalate layers by |. The observed occurrence of streaking along [0 0 1] for the series of re¯ections (h k l) with h ÿ k 6 3n may have been caused not only by the twinning, but also by the participation of the third type of the carbon layer, C (among A, B and C), leading to a random stacking of the A, B and C carbon layers. Homma and Clarke [21] also drew the same conclusion on SbCl5 GICs, i.e., the graphite stacking sequence for all stages is ABAB with stacking faults in the same sense as the `twinning' used by Markiewicz et al. [20]. Although they were able to ®t the data for Stages 3ÿ5, no good agreement was obtained for Stage-1. It is possible that the room temperature Stage-1 C-SbCl5 has an intermediate structure of the nestled C14AsF6 and C16AsF6, and un-nestled CxAsFy, provided that SbCl6ÿ plays an active role and can nestle. Vaknin and Fischer [22] concluded that their Stage-2 C16AsF5 has an AB|CA|BC stacking sequence without the twinning mentioned by Markiewicz et al. It is to be cautioned, however, that different results may be obtained depending on the host graphite materials used for experiment. In the above three cases it is not certain whether the AB stacking sequence is a consequence of the nestling of MX6ÿ species. In graphite-SbCl5 system the carbon layers enclosing guest species are found to be staggered and domains of (H7 H7) R (19.118) is observed [21]. Furthermore, the small Ic is explained in terms of registry of Cl-ligands on a graphite hexagon. Since SbCl6ÿ as well as SbCl3 are found within the gallery, it is possible that SbCl6ÿ anions nestle as
does AsF6ÿ. However, in the case of SbCl5 GIC the interpretation for nestling and H7 H7 structure is made based on the close-packing of SbCl6ÿ species themselves irrespective of graphite layers and the coincidence of the superlattice size to H7 H7 structure. It is noted that, compared with AsF6ÿ, SbCl6ÿ is large enough to make a close packing of such a size. However, XRD patterns of CxSbF6 prepared by the reaction of graphite with O2SbF6 have not indicated nestling of SbF6ÿ probably because of its larger size compared with AsF6ÿ [23]. Compositions, stage and Ic of CxSbF6 are given in Table 3. The results suggest the formula C12nSbF6 with Ê , and the in-plane density of Ic 8.2 3.35(n ÿ 1) A C12SbF6 is too high for SbF6ÿ to nestle. The Ic value of 8.2 is signi®cantly smaller than that reported for Stage-1 Ê [24]). This is because of the less compact CxSbF5 (8.44 A structure of SbF5 and coexistence of SbF3, SbF5 and SbF6ÿ species in the gallery of CxSbF5 according to the equation: 3SbF5 2eÿ ! 2SbF6 ÿ SbF3 ÿ
(10)
The height of SbF6 along the three-fold axis is smaller than the height of SbF5. A similar difference is observed for Ê [10]) and un-nestled CxAsF6 (ca. Stage-1 CxAsF5 (8.10 A ÿ Ê 8.0 A). As SbF6 ions have not been found to nestle so far, much larger SbCl6ÿ ions are unlikely to nestle, although partial nestling of some ¯uorine or chlorine ligands in SbF5 and SbCl5 GICs can occur, which may be the cause of the observation of H7 H7 structure in SbCl5 GICs. The nestling of AsF6ÿ suggests that many other (pseudo) Oh or Td ¯uoro-, oxo-, or oxo¯uoro-ligand guest species will be found to nestle in the graphite layers, if the guest concentration is suf®ciently low to permit it. Diffraction data [25] on C8SO3Fÿ [1] indicate that the SO3Fÿ are nestled. A small Ic-distance is a strong indicator of such nestling but the impact upon the XRD pattern, of both the staggering of the enclosing carbon sheets, and their random stacking, should also signal occurrence of nestling. Clearly neutral guest species, because they are not strongly attracted to the carbon sheets, will be less likely to nestle than anions. Also when the guest concentration exceeds a critical value, nestling will no longer be possible. In that situation the most commodious arrangement of the enclosing carbon sheets appears to be the eclipsed one.
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4. Reactions of CxAsF6 4.1. Reactions of nestled C14AsF6 with graphite The nestled Stage-2 C28AsF6 is obtained, not only via Schemes 1 and 2 (with dif®culty), and via Scheme 3, but also by the reaction of nestled Stage-1 C14AsF6 with graphite: 14 C C14 AsF6 ! C28 AsF6
(11)
The reaction of Eq. (11) is carried out by mixing graphite and C14AsF6 in the same manner as for graphite and O2AsF6 [26]. It is to be noted that this reaction proceeds easily at room temperature indicating a high mobility of AsF6ÿ anions within the graphite gallery and across the crystallite interfaces. The formation of nestled Stage-2 C28AsF6 from nestled Stage-1 C14AsF6 clearly indicates that the guest species in C14nAsF6 is indeed AsF6ÿ. It is expected that higher-stage C14nAsF6 (n 3) GICs of pure phase can be formed easily in the same manner: 14 n ÿ 1C C14 AsF6 ! C14n AsF6
(12)
Of course, reaction of any CxAsF6 with graphite is possible, but the staging formula C14nAsF6 was emphasized here to demonstrate the easy control of stoichiometry and staging by the reaction according to Eqs. (11) and (12). Direct reactions between two GICs of donor- and acceptor-types, e.g. between LiC6 or KC8 and C14AsF6 may also proceed relatively easily: LiC6 C14 AsF6 ! 20 C LiAsF6
(13)
KC8 C14 AsF6 ! 22 C KAsF6
(14)
These reactions would be regarded as an acid-base neutralization reaction taking the amphoteric nature of graphite into consideration. They could also be run electrochemically. 4.2. Reactions of CxAsF6 with F2 in HF CxAsF5 adsorbs or reacts with ¯uorine as expected from Eq. (4) [4], and so does Stage-1 ca. C14AsF6 to yield compounds with composition ca. C14AsF6yF (y 1 ÿ 3) [14]. When CxAsF6 salts higher than Stage-1 are treated with F2 in liquid hydrogen ¯uoride (AHF) at ambient temperature, a graphite ¯uoride, CxF is produced together with a Stage-1 ¯uoroarsenate salt [15]. The oxidatively inert CxF phase can be isolated by destructive oxidation of the (graphite)AsF6ÿ with perchloric acid at 1508C. C1.3F represents the highest ¯uorine content of the product. The reaction scheme can be given as follows: Scheme 4 Ê and The unit cell parameters of C1.3F are a0 2.478 A Ê . The IR and XPS spectra indicate that the c0 6.40 A oxidatively inert, black, C1.3F and its relatives are based on an sp2 carbon atom network like that of the parent graphite, to which the ¯uorine atoms are bound semi-ionically.
Scheme 4
4.3. Reaction of CxAsF6 with fluorobases A Stage-1 GIC of a strong ¯uoroacid, C10AsF6, was found to react with ¯uorobases such as KF in AHF to give a Stage2 compound [27]. The reaction proceeds as: 8 C10 AsF6 5 KF ! 3 C20 AsF6 5 C4 F 5 KAsF6
(17)
XRD and XPS analyses revealed that the compound is covered with planar-sheet graphite ¯uoride, CxF. C10AsF6 also reacts with the solvent hydrogen ¯uoride, liberating AsF5 to give a mixture of Stages-1 and -2 compounds. It is to be noted that the ¯uoroacidity of the Stage-1 C10AsF6 is so high that HF acts as a ¯uorobase towards it. The reaction can be expressed by the following equation: 20 ÿ x 10 ÿ x C10 AsF6 2HF ! C20 AsF6 Cx F 10 10 (18) H2 F AsF6 ÿ The weight gain of the GICs exposed to air is reduced by the reaction with ¯uorobases. This is attributed to the surface coverage of the material with graphite ¯uoride protects the bulk graphite ¯uoroarsenates from the attack of moisture in air. A schematic illustration of the reaction product of C10AsF6 with KF is given in Fig. 11 [27]. 4.4. Other reactions of CxAsF6 Many other reactions are possible for CxAsF6, or more in general, for many GICs. Reactions of GICs, however, are usually dif®cult to elucidate because of the less crystalline nature of graphite materials and the less stoichiometric nature of GIC products. As explained in Section 2.4, AsF3 acts as a reducing agent towards C14AsF6 after being incorporated within the gallery to form ca. C14AsF61/2AsF3 [3±5]. The incorporation of AsF3 can be regarded as adsorption of neutral molecules to form a ternary GICs. When solvents are used for the formation of GICs, they are often found to be co-intercalated. As explained in Section 2.4, when the reaction of graphite with O2AsF6 is run in SO2ClF solvent [1,2,4], the solvent is incorporated along with AsF6ÿ [14]. The maximum amount of incorporated SO2ClF corresponds to ca. C14AsF60.4SO2ClF, and SO2ClF can be quasi-reversibly removed or added.
F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
247
Fig. 11. A schematic illustration of the reaction product of C10AsF6 with KF (reproduced with permission from [27]).
It is true that adsorption of nitrogen can also be included in the previous section, but this theme is treated separately since its aim is to probe the pore structure in nestled CxAsF6. Studies of adsorption of nitrogen and other gases by donortype GICs are not rare, but such investigations have seldom been carried out on acceptor-type GICs. Adsorption isotherms of nitrogen at 77 K for (a) graphite, (b) un-nestled C9.8AsF6, (c) un-nestled C11.9AsF6, (d) nestled C13.6AsF6, and (e) nestled C15.1AsF6 are shown Fig. 12 [16]. Their BET surface areas are 2, 17, 19, 80 and 109 m2/g, respectively. Sample (b) was made with AsF5 F2, and the others with O2AsF6. The results are summarized in Table 3. As can be seen from the ®gure and the table, the amounts of N2 adsorbed by the nestled CxAsF6 are signi®cantly larger than those by the un-nestled CxAsF6. For the nestled C15.1AsF6, the isotherm is of Type I, and the initial sharp rise indicates a large amount of micropores within the material. The amount of adsorbed N2 corresponds
approximately to the formula C15.1AsF61/2N2. This is because that, in the nestled C14AsF6, the in-plane density of AsF6ÿ is relatively low as shown in Fig. 3, and AsF6ÿ ions do not `touch' each other. The value of 14 in the Stage-1 C14AsF6 is to be compared with 8 in the Stage-1 C8AsF5 [10]. Without nestling, C14AsF6 would be a mixture of Stages-1 and -2 with a higher in-plane density of guest species, which would hinder the incorporation of N2. As explained in Section 3.1, the composition C14AsF6 corresponds to the highest density of nestled AsF6ÿ. For unnestled CxAsF6, the composition of ca. C10AsF6 corresponds to the highest density of AsF6ÿ. This allows us to estimate the space available for N2 adsorption in the nestled C14AsF6 as follows. C14AsF6 can be formulated as C4C10AsF6. The size of AsF6ÿ in the ab-projection is larger than three ¯uorine ligands which roughly corresponds to the size of (3/2) N2. As a result, the formula C4(N2)0.6C10AsF6 is obtained for N2 adsorption. This can further be approximately reformulated to C14AsF61/2N2. For comparison, adsorption isotherms of nitrogen at 77 K for Stages-1, -2, and -3 CxSbF6, are shown in Fig. 13 [28]
Fig. 12. Adsorption isotherms of nitrogen at 77 K for (a) graphite, (b) C9.8AsF6, (c) C11.9AsF6, (d) nestled C13.6AsF6, and (e) nestled C15.1AsF6 (reproduced with permission from [16]).
Fig. 13. Adsorption isotherms of nitrogen at 77 K for (a) graphite, (b) Stage-1 C11.7SbF6, (c) Stage-2 C26.0SbF6, and (d) Stage-3 C36.5SbF6.
5. Adsorption properties and nano-space of nestled CxAsF6
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F. Okino / Journal of Fluorine Chemistry 105 (2000) 239±248
and the results are summarized in Table 3. The amounts of N2 adsorbed by CxSbF6 are signi®cantly smaller than those by the nestled CxAsF6. This is probably because that SbF6ÿ ions do not nestle between graphite layers. It is not unambiguously clear, however, whether N2 molecules are adsorbed in the graphite gallery. The high speci®c surface area for the nestled C14AsF6 might have been caused via the reaction of graphite with O2AsF6, which might have served as surface activation of the graphite, and it may not be associated with the interlayer gallery. However, the difference in adsorption capacity for nestled C14AsF6 and un-nestled CxAsF6, both of which have experienced the possible surface-activation reaction, support the idea of the interlayer gallery of the nestled C14AsF6 being an active site for N2 incorporation. The nano-spaces of the C14AsF6 reside in the multi-decked vast nano-galleries or -halls with ¯oors and ceilings of graphite layers supported by bold pillars of nestled AsF6ÿ. Acknowledgements This work was supported by the Research for the Future Program of the Japan Society for the Promotion of Science (JSPS-RFTF96R11701). References [1] N. Bartlett, R.N. Biagioni, B.W. McQuillan, A.S. Robertson, A.C. Thompson, J. Chem. Soc., Chem. Commun. (1978) 200. [2] N. Bartlett, B. McQuillan, A.S. Robertson, Mat. Res. Bull. 13 (1978) 1259. [3] E.M. McCarron, N. Bartlett, J. Chem. Soc., Chem. Commun. (1980) 404.
[4] T.E. Thompson, E.M. McCarron, N. Bartlett, Synthetic Metals 3 (1981) 255. [5] F. Okino, N. Bartlett, J. Chem. Soc., Dalton Trans. (1993) 2081. [6] F. Okino, Y. Sugiura, H. Touhara, A. Simon, J. Chem. Soc., Chem. Commun. (1993) 562. [7] Lin Chun-Hsu, H. Selig, M. Rabinovits, I. Agranat, S. Sarig, Inorg., Nucl., Chem., Lett. 11 (1975) 601. [8] E.R. Falardeau, G.M.T. Foley, C. Zeller, F.L. Vogel, J. Chem. Soc., Chem. Commun. (1977) 389. [9] G.M.T. Foley, C. Zeller, E.R. Falardeau, F.L. Vogel, Solid State Commun. 24 (1977) 371. [10] E.R. Falardeau, L.R. Hanlon, T.E. Thompson, Inorg. Chem. 17 (1978) 301. [11] J.E. Fischer, J. Chem. Soc. Chem. Commun. (1978) 544. [12] M.J. Moran, J.E. Fischer, W.R. Salaneck, J. Chem. Phys. 73 (1980) 629. [13] P.A.W. Dean, R.J. Gillespie, R. Hulme, J. Chem. Soc. Chem. Commun. (1969) 990. [14] F. Okino, Ph.D. Thesis, University of California, Berkeley, 1984. [15] R. Hagiwara, M. Lerner and N. Bartlett, J. Chem. Soc. Chem. Commun. (1989) 573. [16] F. Okino, S. Kawasaki, H. Touhara, Mol. Cryst. Liq. Cryst., in press. [17] N. Bartlett, B.G. DeBoer, F.J. Hollander, F.O. Sladky, D.H. Templeton, A. Zalkin, Inorg. Chem. 13 (1974) 780. [18] J.A. Ibers, Acta Crystallogr. 9 (1956) 967. [19] F. Okino et al., in press. [20] R.S. Markiewicz, J.S. Kasper, L.V. Interrante, Synthetic Metals 2 (1980) 363. [21] H. Homma, R. Clarke, Phys. Rev. B 31 (1985) 5865. [22] D. Vaknin, J.E. Fischer, Synthetic Metals 23 (1988) 101. [23] F. Okino, K. Maruyama, M. Kawawaki, S. Kawasaki, H. Touhara, Extended Abstracts, in: International Symposium on Carbon, 1998, Tokyo, I10±08, 208. [24] T.E. Thompson, E.R. Falardeau, L.R. Hanlon, Carbon 15 (1977) 39. [25] R.N. Biagioni, Ph.D. Thesis, University of California, Berkeley, 1980. [26] F. Okino et. al., in press. [27] R. Hagiwara, K. Tozawa, Y. Ito, J. Fluorine Chem. 88 (1998) 201. [28] F. Okino, K. Maruyama, M. Kawawaki, S. Kawasaki, H. Touhara, Extended Abstracts, in: 14th Biennial Conference on Carbon, Charleston, 1999, 160.