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Syntheses of tin and lead fluoride graphite intercalation compounds and the phase transition of the tin fluoride compound
ELSEVIER
SyntheticMetals 74 ( 1995) 89-93
Syntheses of tin and lead fluoride graphite intercalation compounds and the phase transition of the tin fluoride compound Yoshiyuki Hattori, Masayuki Kurihara, Shinji Kawasaki, Fuji0 Okino, Hidekazu Touhara * Department of Chemistry, Faculty of Textile Science and Technology, Shinshu Universi@, Ueda 386, Japan
Received 18 April 1995;accepted 18 April 1995
Abstract The reaction of tin tetrafluoride with graphite in liquid anhydrous hydrogen fluoride ( AHF) in the presence of fluorine yields a stage-2 graphite intercalation compound (GIG) with an identity period I, = 11.53 A, whereas no intercalation reaction occurs without fluorine. With lead tetrafluoride, however, the reaction proceeds without fluorine yielding a stage-2 compound. X-ray diffraction and differential scanning calorimetry measurements on the SnF,-based GICs confirmed a two-dimensional phase transition over a wide temperature range from 120 to 170 K. The optical reflectivity in the UV-Vis region on the SnF,-based GICs has also been studied as a function of stage number. The experimental results were analyzed using the Blinowski-Rigaux model. It was found that the reflectivity minimum shifts towards lower energy with the increase of stage number. This behavior is attributed to the increase of carrier density. The charge transfer per carbon atom in CXSnF6does not differ greatly from those in C6F and C,,AsF,. The derived electrical conductivity of C,SnF, is not as high as that of Ct,AsF, owing to the shorter mean free path of the carriers in the former as a consequence of lattice defects generated in the course of the intercalation in the AHF solutions. Keywords: Graphite
intercalation compounds;Fluorides; Synthesis; Phasetransition
1. Introduction Fluoride-based graphite intercalation compounds (GICs) have been mostly prepared by the direct reaction of volatile fluorides, e.g. SbFs [ 1 ] and AsFs [ 21, with graphite. The presence of F2 gas, however, is required for less oxidizing
fluorides, e.g. GeF, [ 31 and SiF, [4], for the formation of GICs. Recently, a reaction method of GICs of involatile fluorides has been developed, where graphite is immersed in anhydrous hydrogen fluoride (AI-IF) solution of metal fluoride, and F2 gas is flowed through the solution [ $61. Using this method, intercalation reactions of graphite have been extended to involatile fluorides, e.g. SnF,, PbF,, CrF, and RhF,. SnF,-based GICs have been characterized by X-ray diffraction (XRD) , NMR, Mijssbauer spectra and in-plane electrical conductivity measurements, and the existence of a phase transition has been suggested [ 51. This paper deals with the ability of SnF, and PbF, each alone to form GICs in AI-IF solution. The intercalation mechanism in AI-IF solution saturated with the involatile fluorides with or without elemental F, is discussed. A significant importance of the small amount of F2, which is left in the * Corresponding
author.
0379-6779/95/$09.50 0 1995 SSDIO379-6779(95)03346-L
Elsevier Science S.A. All rights reserved
system after the purification of AI-IF, is addressed. We have also studied phase transition of C&F, by means of XRD and differential scanning calorimetry (DSC) . Furthermore, the optical reflectivity has been measured for the stage-2 C,SnFe and analyzed in terms of the two-dimensional electronic bands model for acceptor compounds proposed by Blinowski and Rigaux [ 71. Using this model, we have evaluated the change in Fermi energy level due to the charge transfer between fluorine and carbon layers and the related physical properties are compared with those of other GICs.
2. Experimental 2.1. Preparation HOPG chips (5 X 5 mm, 0.5-l mm thick) heat-treated at 600 “C in vacua were used as the host material. Commercially
available SnF, and PbF, were treated by FZ at 50 “C for 3 days prior to use. The F2 gas and liquid AI-IF used in this work were supplied by Daikin Industries, Ltd. and their purities were better than 99.7 and 99%, respectively. The liquid AI-IF purified by flowing F, gas at 10 “C for 20 min was introduced into the FEPreaction tube containing HOPG chips
Y. Hatrori et al. /Synthetic Metals 74 (1995) 89-93
90
Table 1 Reactivity of metal
fluorides under
Fluoride
various reaction conditions
F2 flow
Without F2 flow HF purified by Fz Residual F2
a
SnF,
cm&F4
PbF,
C,,-,sPbP6a
Ldnh
HF not purified Fz removed
a
no reaction Cz3PbFe
C,-,6PbF6 a
no reaction C,_2sPbF,P
a The range in carbon content signifies that more than one reaction was carried out under the same conditions.
and metal fluoride. The reaction tube was then allowed to stand for prescribed reaction days at 10 “C. After the reaction, the liquid AHF was transferred to an empty reaction tube and the remaining AHF in the reaction tube was removed by evaporation using a flow of pure Ar. The samples were handled in the Ar atmosphere of a glove box. 2.2. Characterization of GlCs The characterization of GICs has been carried out using XRD and X-ray photoelectron spectroscopy (XPS). XRD and DSC measurements in the temperature range 80-270 K were carried out for tin-fluoride-intercalated GICs. The optical reflectivity was measured at room temperature using a spectrophotometer in the UV-Vis region, and the experimental data were fitted by the Blinowski-Rigaux model using the change in Fermi energy level A EF and relaxation time r as the fitting parameters. In-plane electrical conductivities were measured by the contactless Wien-Bridge method at room temperature.
3. Results and discussion 3.1. Intercalation reactions
3. I. 1. Reactions of SnF, with graphite When F2 gas was flowed through the AHF solution containing HOPG chips and SnF,, a stage-2 compound with Z,= 11.43 A was formed. The intercalation reaction occurred irrespective of the prior purification of liquid AHF by FZ. When liquid AHF was purified by F2 flow prior to use, the reaction of graphite in the AHF solution saturated with SnF, 2.0
wJ3) (004)
1.5 v)
81.0
f 1 0.5 o I 0
’ ,, I :I / JA’u_JI(0117)m I 10
I
20
30
(Or-’ I
40 50 2 6ldeg
,
60
70
Fig. 1. XRD pattern of stage-2 C&SnF~.
80
without Fz flow yielded a metallic-blue lustrous GIC after 15 days reaction duration. The XRD pattern of this product shown in Fig. 1 indicates the formation of a single-phase stage-2 compound. The identity period Z, is 11.48 8, and the average thickness of the intercalate di is 4.78 A. The XPS spectra of the product show that the ratio of integrated intensity of Sn( 3d5,*) to that of F( 1s) is about 1:6, indicating that the intercalated species is S~LF,~-. The composition determined by gravimetry is C,,SnF,. The di value corresponds to the thickness of an octahedral arrangement of six fluorine atoms with the C, axis perpendicular to the ab plane. Initially these results led us to believe that SnF, had reacted with graphite in AHF solution without F, flow to form a GIC. However, as the following careful experiments indicated, this system was found to contain residual F2 gas used for the purification of liquid AHF. When neither liquid AHF was purified by F,, nor F2 was flowed through the solution during the reaction, SnF, did not react with graphite in liquid AHF. In order to find out whether the reaction proceeds without the residual F2 left in the AHF solution after its purification, the following manipulation was made. As the purified AI-IF was being transferred to the FEP reaction tube containing HOPG chips and SnF,, the reaction tube was kept at dry icemethanol temperature ( 195 K),and the gaseous AHF being transferred was exposed to a static vacuum of a large volume containing activated alumina, A1203.Owing to the difference in boiling points of HF and F2, the residual fluorine was removed and most of AHF was condensed in the reaction tube. No intercalation was observed in this system after 29 days reaction duration. XRD data of the solid product showed only a crystalline graphite pattern identical to that of the starting material. The reactivity of tin fluoride with graphite under various conditions is summarized in Table 1. The results indicate that the intercalation of tin fluoride occurs only in the presence of F,, which either is flowed through or remains after the AI-IF purification. Consequently, the reaction mechanism can be expressed by the following scheme: xC++F,+HFSnF, + 4HF a
C,I-IF2 SnFe2- + 2H2F+
C,( I-IF,) 2+ SnFG2- +
C,SnF, + 2HF,-
Y. Hattori et al. /Synthetic
Metals 74 (199s) 89-93
91
In this system SnF, cannot act as an oxidizer for the formation of GICs. 3.1.2. Reactions of PbF, with graphite When PbF, was used, the reaction proceeded in AHF solution with F2 flow or residual F,. When the liquid AI-IF was purified by F, flow prior to use and was introduced into the reaction tube under a static vacuum as described above, PbF, reacted with graphite. The XRD pattern of the product after 35 days reaction duration suggests that the product is a stage2 compound with Z, = 11.55 8, and di = 4.85 %, (Fig. 2)) and the XPS spectra of the product show that the ratio of the integrated intensity of Pb(4f,,,) to that of F( Is) is about 1:6, indicating that the intercalate is PbF6*-. The composition determined by gravimetry is C23PbFs. Even if liquid AHF is not purified by F, flow prior to use, the reaction occurs without F2 flow. 0.5
Fig. 4. Temperature dependences of d, for (a) stage-2 C$hF6 stage-2 C,,SnF,.
and (b)
The reactivity of PbF, with graphite under various conditions is summarized in Table 1. From the results and discussion presented above, it is concluded that reaction in this system proceeds in the absence of F2 and the following reaction mechanism can be proposed:
0.4 rl
10
0
20
30
40 2
50
60
70
60
@/(de&
xC+PbF,+2HFPbF, + 4HF +
C#-IF2),+PbF2 PbF6*- + 2H2F+
Fig. 2. XRD pattern of stage-2 Cz3PbFs.
C,(HF,),+PbF,*-
*
CJbF, + 2HF2 -
In this system part of the PbF, acts as an oxidizing agent for the formation of a GIC.
I
3.2. Two-dimensional phase transition in CJnF,
.o -
B 5 0.5 -
0 32
31
33
2 8/deg
.,
13
75
77
2 9ldeg Fig. 3. Temperature dependences of peak positions for (004) and (009) diffraction lines of (a) stage-2 C&nF, and (b) stage-2 C,SnF,.
Fig. 3 shows the temperature dependence of the peak positions for (004) and (009) diffraction lines of the stage-2 C,,SnF, and C2$nFs, respectively. Though subtle, their temperature dependences are not linear. However, the temperature dependence of the lattice constant co of pristine graphite is known to be linear in the temperature range 80-270 K. Therefore, the di values of C,SnF6 at various temperatures can be calculated by subtracting the carbon-layer thickness of the pristine graphite. Fig. 4 shows the temperature dependence of the di values between 80 and 260 K. The second derivatives of the curves for C2+nF6 and C,,SnF, change sign in the temperature ranges 120-140 and 135-160 K, respectively. These temperature ranges are in accord with the temperature ranges for the appearance of the drastic changes in the ‘9 NMR [ 51 and i19Sn Mossbauer spectra [8] of C~nF5. These results are attributable to the phase transition of SnF,*- in C,SnF,. It is possible that the F ligands of SnF,*- are nestled in contiguous three-fold sets of C-atom hexagons of a carbon layer at lower temperatures [ 9, lo]. Because of the increase of translational and rotational motion with the rise in temperature, the di values increase.
Y. Hattori et al. /Synthetic Metals 74 (1995) 89-93
92
Table 2 AE,, T, cr+,& and&, from optical reflectivity measurements for stage-2 GICs and electrical conductivity data from the contactless Wien-Bridge method Sample
CssSnF, G&F, ’ CPb
A-G
7
(W
( x lo-‘4s)
1.04 1.02 1.01
1.75 5.0 1.7
fc
gw
fm
gobs (XlO’Scm-‘)
(X10’S cm-‘) 0.024 0.023 0.022
0.37 1.05 0.45
0.83 0.36 0.13
0.94 = 1.4-2.3 ’ 0.2-1.8 =
‘FromRef. [ll]. ‘FromRef. [12]. c C&&. dC IO-,rAsFs. e f&F.
3.3. Optical rejectivity in UV-Vis region of CJnF, (a)
(b)
LI
O 120
I
I I 140
I
I
I
160 Temp./K
I
180
200
Fig. 5. DSC thermograms of (a) HOFG, (b) stage-2 C17SnF,and (c) stage2 c*7snF_$.
0.5
1.0
1.5
2.0
2.5
Photon energy/eV Fig. 6. The optical reflectivity spectra of HOF’Gand C$nF,. The solid lines are observed spectra and the filled circles ate calculated spectra by the Blmowski-Rigaux model.
Fig. 5 shows DSC measurements on HOPG, stage-2 C17SnFs and stage-2 C,,SnF, in the temperature range 120200 K. Neither an endothermal nor exothermal transition is observed for pristine HOPG. However, an endothermal transition is observed in Ci7SnF~ and C2,SnF6 in the approximate temperature ranges 120-135 and 160-170 K, respectively. Since these temperature ranges are consistent with those observed in other temperature dependence measurements, the above results imply that this endothermal transition is associated with the phase transition of SnF,2-. The results of these experiments indicate that the endothermic two-dimensional phase transition exists over a wide temperature range 120-170 K.
Fig. 6 shows W-Vis reflectivity spectra of HOPG and C,SnF,. The W-Vis reflectivity spectra of HOPG show no minimum, but those of the GICs show the plasmaedges which are characteristic to metal. The plasma edge shifts towards higher energy side with increase of stage number. This is because the charge carrier density increases as the intercalate concentration increases. The optical reflectivity data of stage2 C,,SnF, have been fitted by the Blinowski-Rigaux model. The best fit was obtained when T= 1.75 X lo- l4 s and A EF = 1.04 eV. Table 2 summarizes the values of the relaxation time T, the change in Fermi energy AE,, optical electrical conductivity (~,r~,the charge transfer per carbon atom f, and the charge transfer per fluorine atom f, of stage-2 C,,SnF, with the reported values of several GICs [ 11,121. For comparison, observed electrical conductivities cobs of stage-2 compounds are also given in Table 2. The values of f_r,rt are ahOSt comparable With those Of (T,bs. T and (T,,~~ values of C,,SnF, are nearly equal to those of C,F, but are much smaller than those of C,,&F,. These results suggest that the scattering frequency is greater in C,,SnF, than in C1,&FS, because T is proportional to the reciprocal of the scattering frequency of inter&ate. This may be because that lattice defects can be more easily formed at the crystallite boundaries when GICs are synthesized in AHF solution.
Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific Research (C), No. 05650840, supported by the Ministry of Education, Science and Culture, Japan. The authors thank Messrs Y. Sugiura and H. Saito for carrying out some of the optical reflectance and electrical conductivity measurements.
References [ 11 J.M. Lalancette and J. Lafontaine, J. Chem. Sot., Chem. Commun., (1973) 815.
Y. Hattori et al. /Synthetic
121 Lin Chun-Hsu, H. Selig, M. Rabinovitz, I. Agranat and S. Sarig,lnorg. Nucl. Chem. Len., II (1975) 601. [ 31 E.M. McCarron, Y.J. Grannec and N. Bartlett, J. Chem. Sot., C/tern. Commun., (1980) 890. 141 G.L. Rosenthal, T.E. Mallouk and N. Bartlett, Synth. Met., 9 (1984) A,?
-tJJ.
151 H. Touhara, K. Kadono, H. Imoto, N. Watanabe, A. Tressaud and J. Grannec, Synth. Met.. 18 ( 1987) 549. [61 K. Amine, A. Tressaud, P. Hagenmuller, H. Imoto, H. Touhara and T. Nakajima, Mater. Rex Bull., 25 (1990) 1219.
Metals 74 (1995) 89-93
93
[7] J. Blinowski, Nguyen Hy Hau, C. Rigaux and J.P. Vieren, J. Phys. Paris, 41 (1980) 47. [8] L. Fournes, T. Roisnel, J. Graunec, A. Tressaud, P. Hagenmuller, H. Imoto and H. Touhata, Mater. Res. Bull., 25 ( 1990) 79. [9] F. Okino, Y. Sugiura, H. Touhara and A. Simon, J. Chem. Sot., Chem. Commun., (1993) 562.
[ 101 F. Okino and N. Bartlett, J. Chem. Sot.. Dalton Trans., (1993) 2081. [ 1l] M. Saint Jean, Nguyen Hy Hau and C. Rigaux, SolidState Commun., 46 (1983) 55. [ 121 I. O&a, I. Palchan, Y. Yacoby, D. Davidov and H. Selig, Phys. Rev. B, 38 (1988) 12627.
SyntheticMetals 74 ( 1995) 89-93
Syntheses of tin and lead fluoride graphite intercalation compounds and the phase transition of the tin fluoride compound Yoshiyuki Hattori, Masayuki Kurihara, Shinji Kawasaki, Fuji0 Okino, Hidekazu Touhara * Department of Chemistry, Faculty of Textile Science and Technology, Shinshu Universi@, Ueda 386, Japan
Received 18 April 1995;accepted 18 April 1995
Abstract The reaction of tin tetrafluoride with graphite in liquid anhydrous hydrogen fluoride ( AHF) in the presence of fluorine yields a stage-2 graphite intercalation compound (GIG) with an identity period I, = 11.53 A, whereas no intercalation reaction occurs without fluorine. With lead tetrafluoride, however, the reaction proceeds without fluorine yielding a stage-2 compound. X-ray diffraction and differential scanning calorimetry measurements on the SnF,-based GICs confirmed a two-dimensional phase transition over a wide temperature range from 120 to 170 K. The optical reflectivity in the UV-Vis region on the SnF,-based GICs has also been studied as a function of stage number. The experimental results were analyzed using the Blinowski-Rigaux model. It was found that the reflectivity minimum shifts towards lower energy with the increase of stage number. This behavior is attributed to the increase of carrier density. The charge transfer per carbon atom in CXSnF6does not differ greatly from those in C6F and C,,AsF,. The derived electrical conductivity of C,SnF, is not as high as that of Ct,AsF, owing to the shorter mean free path of the carriers in the former as a consequence of lattice defects generated in the course of the intercalation in the AHF solutions. Keywords: Graphite
intercalation compounds;Fluorides; Synthesis; Phasetransition
1. Introduction Fluoride-based graphite intercalation compounds (GICs) have been mostly prepared by the direct reaction of volatile fluorides, e.g. SbFs [ 1 ] and AsFs [ 21, with graphite. The presence of F2 gas, however, is required for less oxidizing
fluorides, e.g. GeF, [ 31 and SiF, [4], for the formation of GICs. Recently, a reaction method of GICs of involatile fluorides has been developed, where graphite is immersed in anhydrous hydrogen fluoride (AI-IF) solution of metal fluoride, and F2 gas is flowed through the solution [ $61. Using this method, intercalation reactions of graphite have been extended to involatile fluorides, e.g. SnF,, PbF,, CrF, and RhF,. SnF,-based GICs have been characterized by X-ray diffraction (XRD) , NMR, Mijssbauer spectra and in-plane electrical conductivity measurements, and the existence of a phase transition has been suggested [ 51. This paper deals with the ability of SnF, and PbF, each alone to form GICs in AI-IF solution. The intercalation mechanism in AI-IF solution saturated with the involatile fluorides with or without elemental F, is discussed. A significant importance of the small amount of F2, which is left in the * Corresponding
author.
0379-6779/95/$09.50 0 1995 SSDIO379-6779(95)03346-L
Elsevier Science S.A. All rights reserved
system after the purification of AI-IF, is addressed. We have also studied phase transition of C&F, by means of XRD and differential scanning calorimetry (DSC) . Furthermore, the optical reflectivity has been measured for the stage-2 C,SnFe and analyzed in terms of the two-dimensional electronic bands model for acceptor compounds proposed by Blinowski and Rigaux [ 71. Using this model, we have evaluated the change in Fermi energy level due to the charge transfer between fluorine and carbon layers and the related physical properties are compared with those of other GICs.
2. Experimental 2.1. Preparation HOPG chips (5 X 5 mm, 0.5-l mm thick) heat-treated at 600 “C in vacua were used as the host material. Commercially
available SnF, and PbF, were treated by FZ at 50 “C for 3 days prior to use. The F2 gas and liquid AI-IF used in this work were supplied by Daikin Industries, Ltd. and their purities were better than 99.7 and 99%, respectively. The liquid AI-IF purified by flowing F, gas at 10 “C for 20 min was introduced into the FEPreaction tube containing HOPG chips
Y. Hatrori et al. /Synthetic Metals 74 (1995) 89-93
90
Table 1 Reactivity of metal
fluorides under
Fluoride
various reaction conditions
F2 flow
Without F2 flow HF purified by Fz Residual F2
a
SnF,
cm&F4
PbF,
C,,-,sPbP6a
Ldnh
HF not purified Fz removed
a
no reaction Cz3PbFe
C,-,6PbF6 a
no reaction C,_2sPbF,P
a The range in carbon content signifies that more than one reaction was carried out under the same conditions.
and metal fluoride. The reaction tube was then allowed to stand for prescribed reaction days at 10 “C. After the reaction, the liquid AHF was transferred to an empty reaction tube and the remaining AHF in the reaction tube was removed by evaporation using a flow of pure Ar. The samples were handled in the Ar atmosphere of a glove box. 2.2. Characterization of GlCs The characterization of GICs has been carried out using XRD and X-ray photoelectron spectroscopy (XPS). XRD and DSC measurements in the temperature range 80-270 K were carried out for tin-fluoride-intercalated GICs. The optical reflectivity was measured at room temperature using a spectrophotometer in the UV-Vis region, and the experimental data were fitted by the Blinowski-Rigaux model using the change in Fermi energy level A EF and relaxation time r as the fitting parameters. In-plane electrical conductivities were measured by the contactless Wien-Bridge method at room temperature.
3. Results and discussion 3.1. Intercalation reactions
3. I. 1. Reactions of SnF, with graphite When F2 gas was flowed through the AHF solution containing HOPG chips and SnF,, a stage-2 compound with Z,= 11.43 A was formed. The intercalation reaction occurred irrespective of the prior purification of liquid AHF by FZ. When liquid AHF was purified by F2 flow prior to use, the reaction of graphite in the AHF solution saturated with SnF, 2.0
wJ3) (004)
1.5 v)
81.0
f 1 0.5 o I 0
’ ,, I :I / JA’u_JI(0117)m I 10
I
20
30
(Or-’ I
40 50 2 6ldeg
,
60
70
Fig. 1. XRD pattern of stage-2 C&SnF~.
80
without Fz flow yielded a metallic-blue lustrous GIC after 15 days reaction duration. The XRD pattern of this product shown in Fig. 1 indicates the formation of a single-phase stage-2 compound. The identity period Z, is 11.48 8, and the average thickness of the intercalate di is 4.78 A. The XPS spectra of the product show that the ratio of integrated intensity of Sn( 3d5,*) to that of F( 1s) is about 1:6, indicating that the intercalated species is S~LF,~-. The composition determined by gravimetry is C,,SnF,. The di value corresponds to the thickness of an octahedral arrangement of six fluorine atoms with the C, axis perpendicular to the ab plane. Initially these results led us to believe that SnF, had reacted with graphite in AHF solution without F, flow to form a GIC. However, as the following careful experiments indicated, this system was found to contain residual F2 gas used for the purification of liquid AHF. When neither liquid AHF was purified by F,, nor F2 was flowed through the solution during the reaction, SnF, did not react with graphite in liquid AHF. In order to find out whether the reaction proceeds without the residual F2 left in the AHF solution after its purification, the following manipulation was made. As the purified AI-IF was being transferred to the FEP reaction tube containing HOPG chips and SnF,, the reaction tube was kept at dry icemethanol temperature ( 195 K),and the gaseous AHF being transferred was exposed to a static vacuum of a large volume containing activated alumina, A1203.Owing to the difference in boiling points of HF and F2, the residual fluorine was removed and most of AHF was condensed in the reaction tube. No intercalation was observed in this system after 29 days reaction duration. XRD data of the solid product showed only a crystalline graphite pattern identical to that of the starting material. The reactivity of tin fluoride with graphite under various conditions is summarized in Table 1. The results indicate that the intercalation of tin fluoride occurs only in the presence of F,, which either is flowed through or remains after the AI-IF purification. Consequently, the reaction mechanism can be expressed by the following scheme: xC++F,+HFSnF, + 4HF a
C,I-IF2 SnFe2- + 2H2F+
C,( I-IF,) 2+ SnFG2- +
C,SnF, + 2HF,-
Y. Hattori et al. /Synthetic
Metals 74 (199s) 89-93
91
In this system SnF, cannot act as an oxidizer for the formation of GICs. 3.1.2. Reactions of PbF, with graphite When PbF, was used, the reaction proceeded in AHF solution with F2 flow or residual F,. When the liquid AI-IF was purified by F, flow prior to use and was introduced into the reaction tube under a static vacuum as described above, PbF, reacted with graphite. The XRD pattern of the product after 35 days reaction duration suggests that the product is a stage2 compound with Z, = 11.55 8, and di = 4.85 %, (Fig. 2)) and the XPS spectra of the product show that the ratio of the integrated intensity of Pb(4f,,,) to that of F( Is) is about 1:6, indicating that the intercalate is PbF6*-. The composition determined by gravimetry is C23PbFs. Even if liquid AHF is not purified by F, flow prior to use, the reaction occurs without F2 flow. 0.5
Fig. 4. Temperature dependences of d, for (a) stage-2 C$hF6 stage-2 C,,SnF,.
and (b)
The reactivity of PbF, with graphite under various conditions is summarized in Table 1. From the results and discussion presented above, it is concluded that reaction in this system proceeds in the absence of F2 and the following reaction mechanism can be proposed:
0.4 rl
10
0
20
30
40 2
50
60
70
60
@/(de&
xC+PbF,+2HFPbF, + 4HF +
C#-IF2),+PbF2 PbF6*- + 2H2F+
Fig. 2. XRD pattern of stage-2 Cz3PbFs.
C,(HF,),+PbF,*-
*
CJbF, + 2HF2 -
In this system part of the PbF, acts as an oxidizing agent for the formation of a GIC.
I
3.2. Two-dimensional phase transition in CJnF,
.o -
B 5 0.5 -
0 32
31
33
2 8/deg
.,
13
75
77
2 9ldeg Fig. 3. Temperature dependences of peak positions for (004) and (009) diffraction lines of (a) stage-2 C&nF, and (b) stage-2 C,SnF,.
Fig. 3 shows the temperature dependence of the peak positions for (004) and (009) diffraction lines of the stage-2 C,,SnF, and C2$nFs, respectively. Though subtle, their temperature dependences are not linear. However, the temperature dependence of the lattice constant co of pristine graphite is known to be linear in the temperature range 80-270 K. Therefore, the di values of C,SnF6 at various temperatures can be calculated by subtracting the carbon-layer thickness of the pristine graphite. Fig. 4 shows the temperature dependence of the di values between 80 and 260 K. The second derivatives of the curves for C2+nF6 and C,,SnF, change sign in the temperature ranges 120-140 and 135-160 K, respectively. These temperature ranges are in accord with the temperature ranges for the appearance of the drastic changes in the ‘9 NMR [ 51 and i19Sn Mossbauer spectra [8] of C~nF5. These results are attributable to the phase transition of SnF,*- in C,SnF,. It is possible that the F ligands of SnF,*- are nestled in contiguous three-fold sets of C-atom hexagons of a carbon layer at lower temperatures [ 9, lo]. Because of the increase of translational and rotational motion with the rise in temperature, the di values increase.
Y. Hattori et al. /Synthetic Metals 74 (1995) 89-93
92
Table 2 AE,, T, cr+,& and&, from optical reflectivity measurements for stage-2 GICs and electrical conductivity data from the contactless Wien-Bridge method Sample
CssSnF, G&F, ’ CPb
A-G
7
(W
( x lo-‘4s)
1.04 1.02 1.01
1.75 5.0 1.7
fc
gw
fm
gobs (XlO’Scm-‘)
(X10’S cm-‘) 0.024 0.023 0.022
0.37 1.05 0.45
0.83 0.36 0.13
0.94 = 1.4-2.3 ’ 0.2-1.8 =
‘FromRef. [ll]. ‘FromRef. [12]. c C&&. dC IO-,rAsFs. e f&F.
3.3. Optical rejectivity in UV-Vis region of CJnF, (a)
(b)
LI
O 120
I
I I 140
I
I
I
160 Temp./K
I
180
200
Fig. 5. DSC thermograms of (a) HOFG, (b) stage-2 C17SnF,and (c) stage2 c*7snF_$.
0.5
1.0
1.5
2.0
2.5
Photon energy/eV Fig. 6. The optical reflectivity spectra of HOF’Gand C$nF,. The solid lines are observed spectra and the filled circles ate calculated spectra by the Blmowski-Rigaux model.
Fig. 5 shows DSC measurements on HOPG, stage-2 C17SnFs and stage-2 C,,SnF, in the temperature range 120200 K. Neither an endothermal nor exothermal transition is observed for pristine HOPG. However, an endothermal transition is observed in Ci7SnF~ and C2,SnF6 in the approximate temperature ranges 120-135 and 160-170 K, respectively. Since these temperature ranges are consistent with those observed in other temperature dependence measurements, the above results imply that this endothermal transition is associated with the phase transition of SnF,2-. The results of these experiments indicate that the endothermic two-dimensional phase transition exists over a wide temperature range 120-170 K.
Fig. 6 shows W-Vis reflectivity spectra of HOPG and C,SnF,. The W-Vis reflectivity spectra of HOPG show no minimum, but those of the GICs show the plasmaedges which are characteristic to metal. The plasma edge shifts towards higher energy side with increase of stage number. This is because the charge carrier density increases as the intercalate concentration increases. The optical reflectivity data of stage2 C,,SnF, have been fitted by the Blinowski-Rigaux model. The best fit was obtained when T= 1.75 X lo- l4 s and A EF = 1.04 eV. Table 2 summarizes the values of the relaxation time T, the change in Fermi energy AE,, optical electrical conductivity (~,r~,the charge transfer per carbon atom f, and the charge transfer per fluorine atom f, of stage-2 C,,SnF, with the reported values of several GICs [ 11,121. For comparison, observed electrical conductivities cobs of stage-2 compounds are also given in Table 2. The values of f_r,rt are ahOSt comparable With those Of (T,bs. T and (T,,~~ values of C,,SnF, are nearly equal to those of C,F, but are much smaller than those of C,,&F,. These results suggest that the scattering frequency is greater in C,,SnF, than in C1,&FS, because T is proportional to the reciprocal of the scattering frequency of inter&ate. This may be because that lattice defects can be more easily formed at the crystallite boundaries when GICs are synthesized in AHF solution.
Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific Research (C), No. 05650840, supported by the Ministry of Education, Science and Culture, Japan. The authors thank Messrs Y. Sugiura and H. Saito for carrying out some of the optical reflectance and electrical conductivity measurements.
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