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Single crystal structure studies of C16AsF5
Synthetic Metals, 2 ( 1 9 8 0 ) 363 - 370
363
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
SINGLE CRYSTAL STRUCTURE STUDIES OF Cz6AsF5
R. S. M A R K I E W I C Z , J. S. K A S P E R a n d L. V. I N T E R R A N T E
General Electric Research and Development Center, Schenectady, N Y 12301 (U.S.A.) (Received J u n e 16, 1 9 8 0 )
Summary X-ray diffraction studies at 23 °C have been made for the second stage AsF 5 intercalated graphite of nominal composition Cz6AsF5. Single crystals of graphite, obtained by dissolving a marble matrix, were intercalated to stage 2 by exposure to AsF 5 vapor at a pressure of ~ 1 7 0 mm. The diffraction pattern consists of Bragg peaks corresponding to a hexagonal unit cell with a = 2.46 £ (as in graphite) and c = 11.50 A and a relatively uniform high background of diffuse scattering. The mosaic spread of Bragg peaks is essentially the same as in the single crystal o f graphite before intercalation. Integrated intensities (0-20 scans) were measured for the Bragg peaks with m o n o c h r o m a t e d Mo Ks radiation (0.710 £ ) on a single crystal diffractometer. The intercalant molecules are disordered in the intercalant layer with the consequence that except for 001 reflections the intensities of Bragg reflections are quantitatively accounted for by the arrangement of the graphite layers. An investigation into the nature of the intercalating species, and their location, has been made by least squares fitting of calculated to observed intensities for 19 001 reflections. Similar studies on an HOPG sample are also reported. The presence of both AsF5 and A s F ( seems to be needed for a satisfactory fit, the a m o u n t of AsF3 being indeterminate. The arrangement of graphite layers is in the same AB sequence as in ordinary graphite, although twinning is present. The C-C bond distance, the interlayer spacing, and the carbon thermal parameters are essentially the same as for pure graphite.
Introduction
We have made a room-temperature structure study of the second stage AsFs-intercalated graphite of nominal composition Cz6AsF5 by means of standard single crystal X-ray diffraction techniques. Single crystals of Ticonderoga graphite were obtained by dissolving a marble matrix with concentrated hydrochloric acid. These crystals were then intercalated to stage 2 by exposure to AsF 5 vapor at a pressure of ~ 1 7 0 mm. The crystals were contained in thin-walled glass capillaries.
364 In addition to precession photographs, quantitative intensity measurements were made on a single crystal diffractometer by means of 0-20 scans with m o n o c h r o m a t e d Mo Ks radiation (0.7107 A ). The quality of the intercalated crystals was about the same as that of the initial graphite crystals judged by the similarity in half-widths of diffraction peaks. The crystal that was most intensively studied was almost a regular hexagon with a mean distance between opposite edges of 0.69 mm and a thickness of 0.013 mm. The overall diffraction pattern consists of Bragg diffraction peaks (153 independent reflections for 20 ~< 75 °) and a uniform high background of diffuse scattering. It was n o t possible to assess the relative amounts of diffuse scattering due to disorder and to the glass capillary. The measured integrated intensities of Bragg reflections were corrected for absorption (~ = 21.62 cm -1).
The carbon arrangement The diffraction data correspond to a hexagonal unit cell with a = 2.456(2) A (as in graphite) and c = 11.50(1) •, with no systematic absences of reflections. The c dimension is somewhat larger than values reported previously for second stage material produced from HOPG graphite. The fact that the value of a is the same as in graphite is in itself evidence that there is no in-plane ordering of the intercalating species, which would require a basal cell at least four times as large in area. Intensity calculations also prove the lack of ordering and, furthermore, rule o u t the possibility of a lattice gas arrangement. All reflections other than 001 are due solely to the carbon layers. The layers are stacked in a regular stage 2 array, with a repeat unit consisting of one intercalant layer and t w o graphite layers. The structure of the intercalant layer will be discussed in the following subsection. The stacking arrangement of carbon layers is n o t immediately obvious. In many intercalated compounds, it is found that the usual AB stacking is disrupted by the intercalant, with an AA stacking across the intercalant layer. The lack of systematic absences and the apparent equality of F~z and F 2 r is compatible with many space groups, for all o f which the AA arrangement of superimposed layers is called for. On the other hand, the intensities of those reflections for which h -- k ¢ 0 (mod 3) are n o t at all in accord with an AA sequence. The intensities for reflections with h -- k = 0 (11l, 221, 30/, etc.), are independent of the type o f sequence as a result of the special x, y coordinates of the carbon atoms (00, 2/3 1/3 or 1/3 2/3). An AB sequence would seem to be ruled o u t since it would demand that F~z 4: F ~ r in general. The resolution of the dilemma resulted from the recognition that the crystals are twinned. A satisfactory intensity fit is obtained for a basic AB arrangement with frequent twinning across (001) planes to give a situation such as ABAB... AB/BABA... BA/AB . . . . In that case, with equal amounts of both twins, an average of the contributions from b o t h twins (1/2 [F2k~ + F ~ r ] ) is obtained for the intensity at both hkl and hk-{, resulting in the false sym-
365
metry. That the twinning is extensive is evidenced b y the occurrence of streaking that occurs along [001] through the series of reflections of the type Fhkl, h -- k ¢ 0 (mod 3). The best carbon parameters were obtained from a least squares fitting of observed to calculated structure factors for those reflections unaffected by the twinning, namely when h -- k = 0 ( m o d 3). The relevant parameters are the z parameter of C and its anisotropic thermal parameters. These are z = +0.3542(2), Ull = U22 = U12 = 0.0029{1) A 2, U ~ = 0.036(5) A 2, U13 = U23 = 0 after refinement for 38 reflections to give a value of R =
E {IFobs I -- IFca,c I} ~lFobs [
of 0.10. The resultant carbon arrangement is essentially the same as in graphite, as shown in Table 1. The only significant difference is in U3a, the mean square displacement in the z direction. The calculation o f 1/2(Fh2ki + F ~ r ) for those reflections affected by twinning was made with the carbon parameters listed above and assuming the AB sequence within a twin. The comparison with observed F 2 values results in R = 0.14.
The intercalant layer Only the (001)-reflections give information concerning the structure of the intercalant layer. These can be most conveniently analyzed by Fourier transforming the reflection amplitudes to find the charge density distribution (cdd) along the c-axis: C ( z ) = ~,Fooz cos (2nzl). I
(1)
The upper half of Fig. 1 shows the cdd observed in the intercalated single crystal, showing well-defined peaks attributable to As, F, and C. Shown also
TABLE 1 Carbon parameters
a (A) Interlayer distance (A) C-C (/~) Ull (A2 ) U33 (A2)
Graphite*
C16AsF 5
2.461(4)
2.456(2) 3.353(3) 1.418(1) 0.0029(1) 0.036(5)
3.353(2) 1.421(2) 0.0033(1) 0.0140(3)
*R. Chen, P. Trucano and R. F. Stewart, Acta Crystallogr., A 3 3 (1977) 823 - 828.
366
i >> ~~
-5-4-3-2-I
0 I 2
4 5
Fig. 1. Top. Observed charge density distribution (solid line) in C16AsF5, with fit (dashed line) to the C and As peaks. Bottom. Molecules which may be present in intercalate layer, drawn to scale (dotted lines = limits of single C-layer, 3.353 A across).
is a theoretical fit to the C and As peaks, assuming a stoichiometric ratio 16C:IAs. The parameters o f the fit are listed in Table 2. The F peak is considerably broader, possibly due to the presence o f several molecular species, as illustrated in Fig. 1. Bartlett e t al. [1] have proposed that the AsF5 transfers holes to the carbon layers v/a the disproportionation reaction: 3AsF5 + 2e- -~ 2AsF6- + AsF3.
(2)
Measurements of the hole Fermi surfaces [2] suggest that n o t all the AsF 5 is converted. An a t t e m p t was made to fit the fluorine charge distribution as a superposition from the various molecule~. The structures o f AsF 3 and AsF 5 were assumed to be the same as those determined b y electron diffraction [3] for isolated molecules in the gas phase. For AsF3, the A s - F bond length is 1.706(2) A and the F - A s - F angle is 96.2(2) ° for a pyramidal configuration. The AsF5 molecule is a trigonal bipyramid with 3 equatorial A s - F bond lengths of 1.656(4) A and 2 axial bond lengths of 1.711(5) A. For AsFs-, the structure reported [4] for this ion in KAsF8 was used. This is a distorted octahedron with A s - F bond length o f 1.85 A and an F - A s - F angle of 88 °. The C- and As-parameters were determined in a preliminary fit, and held fixed as the F-parameters were varied. The resultant fits to the cdd are shown in Fig. 2, with parameters listed in Table 2. Also listed are the parameters found for an HOPG sample. This sample, used in the Fermi surface studies
f612 0 f6/2 0
0.436 0.390 0.568 0
0.196 0.419 0 0.836
f5 0.300 0.349 0 0.992 58.5 ° 61.2 51.8
0 35.1 ° 25.3 48.9 0.038 0.038 0.057 -
U6 = U3 0.038/~2 0.038 0.068 0.025 0.025 0
U5 0.025 ~ 2 0.025 0.042
0.087 0.086 0.101 0.120
R 0.062 0.074 0.067 0.103
*Relative c o n c e n t r a t i o n s are labeled b y f 3 ( A s F 3 ) , f s ( A s F s ) and f 6 ( A s F 6 - ) , w i t h fo = f3 + f5 + f6. T h e y are n o r m a l i z e d in sueh a way t h a t fo = 1 c o r r e s p o n d s t o t h e s t o i c h i o m e t r i c As:C ratio o f 1:16. * * U n d e r l i n e d values were n o t a d j u s t e d in t h e fits.
f3 f6/2"* 0"* f6/2 0
f6 0.469 0.605 0.674 0
As p a r a m e t e r s
F parameters Single crystal (19 r e f l e c t i o n s ) Model 1 2 3 4 HOPG (14 reflections) 1 2 3 4
U33 U33
3.353 A 0.0225 A 2 0.0276 ~2 C o n c e n t r a t i o n * = f0 = f6 + f3 + f5
C-C distance
C parameters
Fits t o (00/)-reflections
TABLE 2
03
368
[2], had been stored under a N 2 atmosphere for several months prior to X-ray examination, and although it remained stage 2, it had lost a considerable amount of its initial AsFs, and the lattice parameter had shrunk b y a b o u t 0.2 A (11.25 - 11.35 )k in different samples). The fits show the AsF5 deficit, but otherwise the derived parameters are very similar. Four models were used to fit the F-cdd. These models differed in t h e presence or absence of various molecular species involved in the chemical reaction, eqn. (2). Model 1 assumed all three species were present. AsF3 was assumed absent in Model 2, and AsF 5 in Model 3 (reaction (2) far to the right), while only AsF5 was assumed in Model 4 (reaction (2) far to the left). For the single crystal, a good fit was obtained with either Model 1 (R = 0.062) or Model 3 (R = 0.067), although, in the latter case, the Debye-Waller factor is quite large. For the HOPG sample, Models 1 and 2 are favored (R ~ 0.087). This would argue against Model 3 for the single crystal, unless the process of intercalation is very different in the t w o samples. The best fits to the single crystal data have an As:C ratio very close to the stoichiometric ratio (1:16), but in the HOPG sample ~20% of the material has deintercaiated. The AsF5 trigonal bipyramid was found to be tilted within the interlayer space, as shown in Fig. 1. The orientation o f the AsF a molecule was n o t determined. The orientation in Fig. 1 was assumed in the fitting reported here, b u t similar fits have been found assuming the AsF8 tilted 90 °. The results found here agree with findings of other investigators. McCarron and Bartlett [5] showed that AsF3 evolved from p u m p e d samples, thereby ruling o u t Models 2 (for the single crystal) and 4. The observation of evolved AsF5 [5 - 7] would seem to argue against Model 3 as well (although this could be produced by the reversal of reaction (2)). Finally, the a m o u n t of charge transfer to the graphite layer -- assumed equal to f6, the AsFBfraction -- agrees well with electronic measurements of the charge transfer,
"/k
40
.}
a
~
20
zo
I0
I0 - , - -
b 0.18 ~
.: 7"7"7-.
OO 40
1.0 0.2 z O.3 Q4 0.5 0.0 .t c 40
r
i
0.1
.... .":7" 02Z.Q3
.
0.4
20
Jo
,o
0
d
0.0
02 z 03
0.1
-.~
"~ 0.2z0.3 0.4 0.5
//
HOPG
SINGLE CRYSTAL
!
"k,/ 0.4 0 . 5 0 0
•
IR
I
-I0 0.1
~
/ "" ~
oo,.
~,
-I0
1
\ 010
05
0.06~ 20
i
"'
..... \", -,> . . o
-,o . .[..,
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]
I
o.~ ~ , 0
J 0.1 0.2 03
O~ 05
0.6
7 0.8
f6 Fig. 2. Fits to fluorine peak in cdd. Solid line, o b s e r v e d ; d a s h e d , f i t ; d o t t e d , difference. Parameters of fit listed in Table 2. (a), Model 1; (b), Model 2; (c), Model 3; (d), Model 4. Fig. 3. Residual R as a f u n c t i o n of AsF 6 - c o n c e n t r a t i o n (f6), for b o t h single crystal and HOPG samples. Solid lines, Model 1, dashed line, Model 2.
369
f - 0.41 (discussed in ref. 2). For the single crystal f6 = 0.45 + 0.15 (Model 1), and for the HOPG f6 = 0.4 + 0.15 (Model 1 or 2). The large error brackets are due to the shallowness o f the minimum in R, Fig. 3. Within the accuracy of these measurements, the a m o u n t o f AsF6- is the same in the t w o samples, even though a considerable fraction o f the neutral molecules (~30%) have deintercalated from the HOPG sample. The same conclusion is reached in analyzing the Fermi surfaces [2] -- the area o f the surfaces changes b y a very small a m o u n t (it actually increases) as the samples age. The use of the cdd The construction o f the cdd has been of great utility in analyzing the (00/)-reflection spectrum. The well resolved peaks immediately give an indication of atomic positions in the unit cell, and can rule o u t certain structural models at a glance. For instance, n.m.r, studies show that the fluorine ions are in rapid motion at room temperature, b u t the presence of a well defined fluorine peak in Fig. 1 shows that any tumbling a b o u t an axis perpendicular to the c-axis is severely hindered. Also, the peak separation allows the parameters of each chemical species to be fitted independently. This same technique can also be applied to other chemical intercalants. Figure 4 shows some typical examples o f observed and fitted cdd's, using reflection intensities taken from the literature. Details of these studies will be published elsewhere.
0 20
20
I0 :~
I0
o! O0 15 -
0.1
0.2Z03
0.4 ¢
O: '~':;' L
0.5 (20 6OO
0.1
0.2zQ3
0.4 0.5
2OO
-5! QI 02 0.3 0.4 0.5 Z Fig. 4. Observed (solid line) and fit (dashed) cdd for a number of other intercalate systems. (a), stage 1 Rb [8] ; (b), stage 2 K [8] ; (c), stage 2 SbC15 [9] ; (d), stage 3 AsF 5 (present study). O0
0.1
02 z 0.3
0.4 0.5
0.0
Conclusion In conclusion, we have used X-ray diffraction to examine the room temperature structure o f Cz6AsFs. The carbon parameters, except for Uaa,
370
axe very little changed from their values in pure graphite, while the AsF~ is disordered in the plane. The (001)-reflections show a well defined orienting of intercalant along the c-axis. A good fit to the reflection intensities cannot be made unless it is assumed that an appreciable fraction of the AsF~ is converted to AsF6-. The best fit to the data suggests that about 45 + 15% of the As is present as AsF6-, in agreement with similar measurements on an HOPG sample. This chemical measure o f the fractional charge transfer agrees well with the electronic charge transfer found from Fermi surface measurements
[21. Acknowledgement We thank Ray Goehner for obtaining the HOPG X-ray data.
References 1 N. Bartlett, B. McQuillan and A. S. Robertson, Mater. Res. Bull., 13 (1978) 1259. 2 R.S. Markiewicz, H. R. Hart, Jr., L. V. Interrante and J. S. Kasper, Synth. Met., 2 (1980) 331. 3 F. B. Clippard and L. S. Bartell, Inorg. Chem., 9 (1970) 805 - 811. 4 R. B. Roof, Acta Crystallogr., 8 (1955) 739. 5 E. M. McCarron and N. Bartlett, J. Chem. Soc., Chem. Commun., (1980) 404. 6 H. Selig, M. J. Vasile, F. A. Stevie and W. H. Sunder, J. Fluor. Chem., 10 (1977) 299. 7 M.J. Moran, J. E. Fischer and W. R. Salaneck, BulL.Am. Phys. Soc., 25 (1980) 334. 8 S.Y. Leung, C. Underhill, G. Dresselhaus, T. Krapchev, R. Ogilvie and M. S. Dresselhaus, Solid State Commun., 32 (1979) 635. 9 J. Melin and A. H~rold, Carbon, 13 (1975) 357.
363
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
SINGLE CRYSTAL STRUCTURE STUDIES OF Cz6AsF5
R. S. M A R K I E W I C Z , J. S. K A S P E R a n d L. V. I N T E R R A N T E
General Electric Research and Development Center, Schenectady, N Y 12301 (U.S.A.) (Received J u n e 16, 1 9 8 0 )
Summary X-ray diffraction studies at 23 °C have been made for the second stage AsF 5 intercalated graphite of nominal composition Cz6AsF5. Single crystals of graphite, obtained by dissolving a marble matrix, were intercalated to stage 2 by exposure to AsF 5 vapor at a pressure of ~ 1 7 0 mm. The diffraction pattern consists of Bragg peaks corresponding to a hexagonal unit cell with a = 2.46 £ (as in graphite) and c = 11.50 A and a relatively uniform high background of diffuse scattering. The mosaic spread of Bragg peaks is essentially the same as in the single crystal o f graphite before intercalation. Integrated intensities (0-20 scans) were measured for the Bragg peaks with m o n o c h r o m a t e d Mo Ks radiation (0.710 £ ) on a single crystal diffractometer. The intercalant molecules are disordered in the intercalant layer with the consequence that except for 001 reflections the intensities of Bragg reflections are quantitatively accounted for by the arrangement of the graphite layers. An investigation into the nature of the intercalating species, and their location, has been made by least squares fitting of calculated to observed intensities for 19 001 reflections. Similar studies on an HOPG sample are also reported. The presence of both AsF5 and A s F ( seems to be needed for a satisfactory fit, the a m o u n t of AsF3 being indeterminate. The arrangement of graphite layers is in the same AB sequence as in ordinary graphite, although twinning is present. The C-C bond distance, the interlayer spacing, and the carbon thermal parameters are essentially the same as for pure graphite.
Introduction
We have made a room-temperature structure study of the second stage AsFs-intercalated graphite of nominal composition Cz6AsF5 by means of standard single crystal X-ray diffraction techniques. Single crystals of Ticonderoga graphite were obtained by dissolving a marble matrix with concentrated hydrochloric acid. These crystals were then intercalated to stage 2 by exposure to AsF 5 vapor at a pressure of ~ 1 7 0 mm. The crystals were contained in thin-walled glass capillaries.
364 In addition to precession photographs, quantitative intensity measurements were made on a single crystal diffractometer by means of 0-20 scans with m o n o c h r o m a t e d Mo Ks radiation (0.7107 A ). The quality of the intercalated crystals was about the same as that of the initial graphite crystals judged by the similarity in half-widths of diffraction peaks. The crystal that was most intensively studied was almost a regular hexagon with a mean distance between opposite edges of 0.69 mm and a thickness of 0.013 mm. The overall diffraction pattern consists of Bragg diffraction peaks (153 independent reflections for 20 ~< 75 °) and a uniform high background of diffuse scattering. It was n o t possible to assess the relative amounts of diffuse scattering due to disorder and to the glass capillary. The measured integrated intensities of Bragg reflections were corrected for absorption (~ = 21.62 cm -1).
The carbon arrangement The diffraction data correspond to a hexagonal unit cell with a = 2.456(2) A (as in graphite) and c = 11.50(1) •, with no systematic absences of reflections. The c dimension is somewhat larger than values reported previously for second stage material produced from HOPG graphite. The fact that the value of a is the same as in graphite is in itself evidence that there is no in-plane ordering of the intercalating species, which would require a basal cell at least four times as large in area. Intensity calculations also prove the lack of ordering and, furthermore, rule o u t the possibility of a lattice gas arrangement. All reflections other than 001 are due solely to the carbon layers. The layers are stacked in a regular stage 2 array, with a repeat unit consisting of one intercalant layer and t w o graphite layers. The structure of the intercalant layer will be discussed in the following subsection. The stacking arrangement of carbon layers is n o t immediately obvious. In many intercalated compounds, it is found that the usual AB stacking is disrupted by the intercalant, with an AA stacking across the intercalant layer. The lack of systematic absences and the apparent equality of F~z and F 2 r is compatible with many space groups, for all o f which the AA arrangement of superimposed layers is called for. On the other hand, the intensities of those reflections for which h -- k ¢ 0 (mod 3) are n o t at all in accord with an AA sequence. The intensities for reflections with h -- k = 0 (11l, 221, 30/, etc.), are independent of the type o f sequence as a result of the special x, y coordinates of the carbon atoms (00, 2/3 1/3 or 1/3 2/3). An AB sequence would seem to be ruled o u t since it would demand that F~z 4: F ~ r in general. The resolution of the dilemma resulted from the recognition that the crystals are twinned. A satisfactory intensity fit is obtained for a basic AB arrangement with frequent twinning across (001) planes to give a situation such as ABAB... AB/BABA... BA/AB . . . . In that case, with equal amounts of both twins, an average of the contributions from b o t h twins (1/2 [F2k~ + F ~ r ] ) is obtained for the intensity at both hkl and hk-{, resulting in the false sym-
365
metry. That the twinning is extensive is evidenced b y the occurrence of streaking that occurs along [001] through the series of reflections of the type Fhkl, h -- k ¢ 0 (mod 3). The best carbon parameters were obtained from a least squares fitting of observed to calculated structure factors for those reflections unaffected by the twinning, namely when h -- k = 0 ( m o d 3). The relevant parameters are the z parameter of C and its anisotropic thermal parameters. These are z = +0.3542(2), Ull = U22 = U12 = 0.0029{1) A 2, U ~ = 0.036(5) A 2, U13 = U23 = 0 after refinement for 38 reflections to give a value of R =
E {IFobs I -- IFca,c I} ~lFobs [
of 0.10. The resultant carbon arrangement is essentially the same as in graphite, as shown in Table 1. The only significant difference is in U3a, the mean square displacement in the z direction. The calculation o f 1/2(Fh2ki + F ~ r ) for those reflections affected by twinning was made with the carbon parameters listed above and assuming the AB sequence within a twin. The comparison with observed F 2 values results in R = 0.14.
The intercalant layer Only the (001)-reflections give information concerning the structure of the intercalant layer. These can be most conveniently analyzed by Fourier transforming the reflection amplitudes to find the charge density distribution (cdd) along the c-axis: C ( z ) = ~,Fooz cos (2nzl). I
(1)
The upper half of Fig. 1 shows the cdd observed in the intercalated single crystal, showing well-defined peaks attributable to As, F, and C. Shown also
TABLE 1 Carbon parameters
a (A) Interlayer distance (A) C-C (/~) Ull (A2 ) U33 (A2)
Graphite*
C16AsF 5
2.461(4)
2.456(2) 3.353(3) 1.418(1) 0.0029(1) 0.036(5)
3.353(2) 1.421(2) 0.0033(1) 0.0140(3)
*R. Chen, P. Trucano and R. F. Stewart, Acta Crystallogr., A 3 3 (1977) 823 - 828.
366
i >> ~~
-5-4-3-2-I
0 I 2
4 5
Fig. 1. Top. Observed charge density distribution (solid line) in C16AsF5, with fit (dashed line) to the C and As peaks. Bottom. Molecules which may be present in intercalate layer, drawn to scale (dotted lines = limits of single C-layer, 3.353 A across).
is a theoretical fit to the C and As peaks, assuming a stoichiometric ratio 16C:IAs. The parameters o f the fit are listed in Table 2. The F peak is considerably broader, possibly due to the presence o f several molecular species, as illustrated in Fig. 1. Bartlett e t al. [1] have proposed that the AsF5 transfers holes to the carbon layers v/a the disproportionation reaction: 3AsF5 + 2e- -~ 2AsF6- + AsF3.
(2)
Measurements of the hole Fermi surfaces [2] suggest that n o t all the AsF 5 is converted. An a t t e m p t was made to fit the fluorine charge distribution as a superposition from the various molecule~. The structures o f AsF 3 and AsF 5 were assumed to be the same as those determined b y electron diffraction [3] for isolated molecules in the gas phase. For AsF3, the A s - F bond length is 1.706(2) A and the F - A s - F angle is 96.2(2) ° for a pyramidal configuration. The AsF5 molecule is a trigonal bipyramid with 3 equatorial A s - F bond lengths of 1.656(4) A and 2 axial bond lengths of 1.711(5) A. For AsFs-, the structure reported [4] for this ion in KAsF8 was used. This is a distorted octahedron with A s - F bond length o f 1.85 A and an F - A s - F angle of 88 °. The C- and As-parameters were determined in a preliminary fit, and held fixed as the F-parameters were varied. The resultant fits to the cdd are shown in Fig. 2, with parameters listed in Table 2. Also listed are the parameters found for an HOPG sample. This sample, used in the Fermi surface studies
f612 0 f6/2 0
0.436 0.390 0.568 0
0.196 0.419 0 0.836
f5 0.300 0.349 0 0.992 58.5 ° 61.2 51.8
0 35.1 ° 25.3 48.9 0.038 0.038 0.057 -
U6 = U3 0.038/~2 0.038 0.068 0.025 0.025 0
U5 0.025 ~ 2 0.025 0.042
0.087 0.086 0.101 0.120
R 0.062 0.074 0.067 0.103
*Relative c o n c e n t r a t i o n s are labeled b y f 3 ( A s F 3 ) , f s ( A s F s ) and f 6 ( A s F 6 - ) , w i t h fo = f3 + f5 + f6. T h e y are n o r m a l i z e d in sueh a way t h a t fo = 1 c o r r e s p o n d s t o t h e s t o i c h i o m e t r i c As:C ratio o f 1:16. * * U n d e r l i n e d values were n o t a d j u s t e d in t h e fits.
f3 f6/2"* 0"* f6/2 0
f6 0.469 0.605 0.674 0
As p a r a m e t e r s
F parameters Single crystal (19 r e f l e c t i o n s ) Model 1 2 3 4 HOPG (14 reflections) 1 2 3 4
U33 U33
3.353 A 0.0225 A 2 0.0276 ~2 C o n c e n t r a t i o n * = f0 = f6 + f3 + f5
C-C distance
C parameters
Fits t o (00/)-reflections
TABLE 2
03
368
[2], had been stored under a N 2 atmosphere for several months prior to X-ray examination, and although it remained stage 2, it had lost a considerable amount of its initial AsFs, and the lattice parameter had shrunk b y a b o u t 0.2 A (11.25 - 11.35 )k in different samples). The fits show the AsF5 deficit, but otherwise the derived parameters are very similar. Four models were used to fit the F-cdd. These models differed in t h e presence or absence of various molecular species involved in the chemical reaction, eqn. (2). Model 1 assumed all three species were present. AsF3 was assumed absent in Model 2, and AsF 5 in Model 3 (reaction (2) far to the right), while only AsF5 was assumed in Model 4 (reaction (2) far to the left). For the single crystal, a good fit was obtained with either Model 1 (R = 0.062) or Model 3 (R = 0.067), although, in the latter case, the Debye-Waller factor is quite large. For the HOPG sample, Models 1 and 2 are favored (R ~ 0.087). This would argue against Model 3 for the single crystal, unless the process of intercalation is very different in the t w o samples. The best fits to the single crystal data have an As:C ratio very close to the stoichiometric ratio (1:16), but in the HOPG sample ~20% of the material has deintercaiated. The AsF5 trigonal bipyramid was found to be tilted within the interlayer space, as shown in Fig. 1. The orientation o f the AsF a molecule was n o t determined. The orientation in Fig. 1 was assumed in the fitting reported here, b u t similar fits have been found assuming the AsF8 tilted 90 °. The results found here agree with findings of other investigators. McCarron and Bartlett [5] showed that AsF3 evolved from p u m p e d samples, thereby ruling o u t Models 2 (for the single crystal) and 4. The observation of evolved AsF5 [5 - 7] would seem to argue against Model 3 as well (although this could be produced by the reversal of reaction (2)). Finally, the a m o u n t of charge transfer to the graphite layer -- assumed equal to f6, the AsFBfraction -- agrees well with electronic measurements of the charge transfer,
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369
f - 0.41 (discussed in ref. 2). For the single crystal f6 = 0.45 + 0.15 (Model 1), and for the HOPG f6 = 0.4 + 0.15 (Model 1 or 2). The large error brackets are due to the shallowness o f the minimum in R, Fig. 3. Within the accuracy of these measurements, the a m o u n t o f AsF6- is the same in the t w o samples, even though a considerable fraction o f the neutral molecules (~30%) have deintercalated from the HOPG sample. The same conclusion is reached in analyzing the Fermi surfaces [2] -- the area o f the surfaces changes b y a very small a m o u n t (it actually increases) as the samples age. The use of the cdd The construction o f the cdd has been of great utility in analyzing the (00/)-reflection spectrum. The well resolved peaks immediately give an indication of atomic positions in the unit cell, and can rule o u t certain structural models at a glance. For instance, n.m.r, studies show that the fluorine ions are in rapid motion at room temperature, b u t the presence of a well defined fluorine peak in Fig. 1 shows that any tumbling a b o u t an axis perpendicular to the c-axis is severely hindered. Also, the peak separation allows the parameters of each chemical species to be fitted independently. This same technique can also be applied to other chemical intercalants. Figure 4 shows some typical examples o f observed and fitted cdd's, using reflection intensities taken from the literature. Details of these studies will be published elsewhere.
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Conclusion In conclusion, we have used X-ray diffraction to examine the room temperature structure o f Cz6AsFs. The carbon parameters, except for Uaa,
370
axe very little changed from their values in pure graphite, while the AsF~ is disordered in the plane. The (001)-reflections show a well defined orienting of intercalant along the c-axis. A good fit to the reflection intensities cannot be made unless it is assumed that an appreciable fraction of the AsF~ is converted to AsF6-. The best fit to the data suggests that about 45 + 15% of the As is present as AsF6-, in agreement with similar measurements on an HOPG sample. This chemical measure o f the fractional charge transfer agrees well with the electronic charge transfer found from Fermi surface measurements
[21. Acknowledgement We thank Ray Goehner for obtaining the HOPG X-ray data.
References 1 N. Bartlett, B. McQuillan and A. S. Robertson, Mater. Res. Bull., 13 (1978) 1259. 2 R.S. Markiewicz, H. R. Hart, Jr., L. V. Interrante and J. S. Kasper, Synth. Met., 2 (1980) 331. 3 F. B. Clippard and L. S. Bartell, Inorg. Chem., 9 (1970) 805 - 811. 4 R. B. Roof, Acta Crystallogr., 8 (1955) 739. 5 E. M. McCarron and N. Bartlett, J. Chem. Soc., Chem. Commun., (1980) 404. 6 H. Selig, M. J. Vasile, F. A. Stevie and W. H. Sunder, J. Fluor. Chem., 10 (1977) 299. 7 M.J. Moran, J. E. Fischer and W. R. Salaneck, BulL.Am. Phys. Soc., 25 (1980) 334. 8 S.Y. Leung, C. Underhill, G. Dresselhaus, T. Krapchev, R. Ogilvie and M. S. Dresselhaus, Solid State Commun., 32 (1979) 635. 9 J. Melin and A. H~rold, Carbon, 13 (1975) 357.