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Single-crystal growth of C70S8 – a new phase in the C70–sulphur system
Journal of Crystal Growth 213 (2000) 63}69
Single-crystal growth of C S } a new phase in the C }sulphur system A.V. Talyzin*, L.-E. Tergenius, U. Jansson Department of Inorganic Chemistry, Angstrom Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, SE, Sweden Received 18 January 2000; accepted 17 February 2000 Communicated by K. Sato
Abstract Large crystals (up to 2;1;0.5 mm) of a new C }sulphur compound were grown by evaporation of a benzene solution. The composition of this compound was determined by chemical analysis to C S . Single-crystal and X-ray powder di!raction experiments showed that the phase has orthorhombic structure with cell parameters a"30.18, b"30.41, and c"28.32 As and the space group Pbcn. The Raman spectrum from the new compound was very similar to those of pure C and sulphur with the exception of some changes in relative intensity and small shifts of a few peaks at wavenumbers below 500 cm\. The results suggest that the C }S interactions mainly consists of weak van der Waals bonds. Some irregularities in the peak shifts suggest that the distribution of S rings around the C molecules are less symmetrical that in the well-known C S phase. 2000 Elsevier Science B.V. All rights reserved. Keywords: C ; Sulphur; Fullerene; Solution growth
1. Introduction Compared with other groups of fullerene compounds, relatively little attention has been paid to compounds of fullerenes with sulphur. The properties of these compounds are determined by the strong tendency of sulphur to form S rings L (n"6}20). In most modi"cations, sulphur form S rings which are maintained also when sulphur is dissolved in organic solvents e.g. CS . S rings are also present in the vapour during sublimation and vapourisation of most sulphur modi"cations or of melts thereof. Hitherto, all known fullerene-sulphur * Corresponding author. Tel.: #46-18-471-3717; fax: #4618-5135-548. E-mail address: [email protected] (A.V. Talyzin).
compounds consist of fullerenes and S rings which are weakly bonded to each other with van der Waals bonds (see e.g. Refs. [1}3]). For example, Roth and Adelmann have synthesised and determined the structure of C S grown from a solu tion of C in a mixture of CCl and CS [1]. In contrast, they found that crystallisation from pure CS give a C S CS phase [2]. The C S and C S compounds also have been synthesised and the structures were determined [3}5]. Recently, the Raman studies of C S and C S were re ported [6]. The optical properties of a series of C compounds were investigated [7]. The study included C S and a new previously unknown phase in the C }sulphur system which was formed using a special method developed to grow crystals of fullerene compounds from organic solvents [8].
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 3 4 6 - 8
64
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
In Ref. [7], no detailed characterisation was carried out on the new C }sulphur compound and the chemical composition was given as C S which corresponds to the composition in the solution where the optimum conditions for crystallisation was found. The aim of the present study was to study the crystallisation process for the new compound in detail, to determine its chemical composition and to give more information about structural and bonding properties using X-ray di!raction (XRD) and Raman spectroscopy.
2. Experimental procedure C powder (99%, MER), sulphur (99.999%, CE RAC) and benzene (99.9%) were used in all growth experiments. The growth procedure appeared to be very sensitive to benzene impurities and the best results were achieved with freshly double-distilled solvent. The method used for crystal growth experiments has been published in Ref. [8]. Shortly, the procedure is as follows: the experiments are carried out in a closed system of the type shown in Fig. 1. The growth rate is regulated by the temperature gradient between the inner vessel (¹ ) and the outer vessel (¹ ) but depends on the shape and size of the outer vessel as well. In a simple but still
Fig. 1. Scheme of the crystallisation method: 1 } heater, 2 } inner vessel with solution, 3 } outer vessel.
rather e$cient set-up the ¹ is provided by room conditions while for ¹ any kind of conventional heater may be used. In a more advanced set-up the two independent water circulation thermostats were used, one as a heater (1) while the second forced circulation of water with temperature ¹ through the metallic pipes constructed as a shell around the outer vessel (3). The last set-up provided a precise temperature control (0.023C). After vapourisation from the inner vessel the solvate condenses on the walls of the outer vessel and #ows to the bottom of this vessel where it can be collected and used again. A condensation of solvent also took place on the walls of the inner vessel. A permanent #ow of liquid solvent along the walls of the inner vessel prevent nucleation on the walls. That is a great advantage because during evaporation from a vessel with a not completely closed cap, crystallisation occurs only on the walls of the vessel and the quality of the grown crystals became rather poor. Furthermore, the system shown in Fig. 1 allows us to carry out crystallisation experiments in a wide supersaturation range, use seed crystals and utilise other facilities common for growth of single crystals (e.g. controlled stirring of solution may be used with a help of rotating magnet placed below the heater (1) and metallic stirrer situated in solution). Small crystals of C S have previously been grown by evaporation of solution in CS [3,4]. In our study, however, we decided to use benzene as an alternative solvent. We have determined the solubility of C in benzene to around 1.1 mg/ml at 203C and found it to be almost temperature independent. In the crystal growth experiments, solutions of C and sulphur in benzene were mixed in di!erent proportions. A series of test experiments were performed with an initial volume of 20 ml of solution, but for growth of large crystals the initial volume of solution was 100}300 ml. Obtained crystals were washed by hexane to remove sulphur traces. The composition of the crystals was analysed by a commercial laboratory (MIKRO KEMI AB, Uppsala, Sweden). They used a #ush combustion gas chromatography method where the samples are completely oxidised into gaseous CO and SO . The gases are analysed by gas chromatogra phy and their concentration determined by a thermal conductivity detector (TCD). Single-crystal
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
data were collected with a Rigaku di!ractometer while X-ray di!raction powder data were collected with a Siemens 5000 di!ractometer using Cu K a radiation. Raman spectra were obtained by a Renishaw Raman 2000 spectrometer using a 780 nm excitation wavelength with a resolution of 2 cm\. Only low laser powers were used to avoid degradation of sample and photopolymerisation.
3. Results and discussion 3.1. Crystallisation Actually, the initial purpose of our experiments was to grow large crystals of C S . Following to the procedure described in a previous publication [1], we mixed C and sulphur solutions in a stoichiometry corresponding to the composition C S . Surprisingly, the result was di!erent from the experiments with CS . The initial saturated solution was almost black in colour and nontransparent but as crystallisation started, the solution became more and more transparent until it turned completely colourless. This remaining part of the solution contained only sulphur which suggests that the overall sulphur content in the crystals must be less than in C S . Furthermore, in the end of the process, crystals with two di!erent morphologies were formed. Most of the crystals were black, nontransparent rectangular platelets with a millimeter size. These crystals show very smooth shiny faces with spiral features on the surface which can be recognised under binocular even at low magni"cations. The other kind of crystals formed in the solution were very thin red transparent needles which are typical for C S . Chemical analysis on two samples (see experimental section) of each kind of crystals con"rmed that the red needles are of the well-known C S compound, while the rectangu lar black platelets exhibited a composition close to C S with slight excess of sulphur. The results suggest that the crystallisation occurs by the following way: initially, crystals of the new C S compound are formed. This phase contain less sulphur than the initial stoichiometry of the solution (C S ). As a result, the sulphur concentration in the solution will increase, and after a while, needles
65
of C S will start to form. However, since most of C is already deposited in the C S crystals some sulphur will remain in solution. Several experiments were carried out to investigate how the sulphur concentration in the solution in#uenced the crystallisation. A general observation was that no other phase than C S and C S were formed. Furthermore, the results showed that C S crystallise from a benzene solution only when the sulphur concentration is close to saturation for pure sulphur (i.e. much higher than the stoichiometric composition). An interesting observation was that C does not dissolve in a saturated sulphur solution. Adding a C solu tion to a saturated sulphur solution results in a fast formation of thin red platelets and needles of C S without any formation of the C S phase. This method is very useful for obtaining C S powder samples. For growth of the C S com pound we found that the best way to obtain large crystals was to use a solution with a sulphur content approximately twice higher than the stoichiometry of this compound. When a stoichiometric composition was used the crystals tend to grow as dendrites. This can be attributed to an insu$cient rate of sulphur di!usion to the surface of the growing crystals. One of the main problems with the growth of C S crystals from benzene is that very high supersaturations are required prior to the nucleation of this compound. For example, consider a starting solution saturated with C and with a sulphur content corresponding to C S (opti mum growth conditions for the C S phase). The volume of this solution has to be reduced with 70}90% by evaporation of the solvent prior to the nucleation of C S . As a result, the initially for med nuclei grew very fast and produced dendrites. As the supersaturation decreased each branch of such a dendrite continued to grow separately. This led to the formation of a polycrystalline sample consisting of up to 1}2 mm large crystals with approximately similar orientations. Nevertheless, large single crystals with sizes up to 2;1;0.5 mm were also obtained in some cases (Figs. 2a and b). Finally, it is clear that a new C S compound can be grown from a solution of C and sulphur in benzene. An obvious question then arises why this
66
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
phase has not previously been observed with CS as a solvent. A possible explanation is that the solubilities of sulphur and C in CS are much higher than in benzene (e.g. the solubility of C in CS at 203C is 10 mg/ml [10] but only 1.1 mg/ml in benzene). This leads to much lower di!usion rates of C and sulphur to the surface of the growing crystals which may favour the formation of the C S phase. Furthermore, in a previous study, C of a lower purity ((97%) was used to grow crystals using benzene as a solvent [8]. In this case only C S was formed and it is therefore possible that impurities in the C may favour the forma tion of this phase. It should be mentioned that the purity of C was not mentioned in Refs. [3,4]. 3.2. X-ray diwraction
Fig. 2. (a), (b) Crystals of C S (1 mm scale).
Attempts were made to solve the crystal structure of the C S compound. Single-crystal data collected at room temperature on a small 0.3;0.3 mm platelet gave us more than 20 000 re#ections. The data were consistent with an orthorhombic structure with the space group Pbcn and the cell parameters a"30.18, b"30.41 and c"28.32 As . However, we were unable to solve the structure using the SHELX software. We could locate S rings and parts of the C molecules
Fig. 3. Powder X-ray di!raction pattern of C S .
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
according to the suggested composition but failed to localise more than about half of the carbon atom positions. We attribute these problems to high degree of disorder at room temperature. Further studies at low temperatures will be carried out in a near future. Powder X-ray di!raction data from a crushed collection of small crystals are shown in Fig. 3. Special precautions were taken to crush the crystals gently since it is known that signi"cant degradation of the C structure can occur during the crushing procedure. As can be seen, the di!raction pattern shows a large number of re#ections, many of them heavily overlapped, which is typical for a large unit cell with low symmetry. The re#ections are consistent with an orthorhombic unit cell and structural parameters obtained with single crystal. Re#ections with I'14 are listed in Table 1. For the region 2H(153 re#ections with I'3 are listed because of their high importance. 3.3. Raman spectroscopy Raman spectra of pure sulphur, C , C S and the new C S compound are shown in Fig. 4. As can be seen, the spectra from C S and C S contain two types of peaks originating from C and S rings. No additional peaks or peak shifts can be seen in the spectra from C S and C S above 500 cm\. An interesting observation, however, was that the region below 500 cm\, which is dominated by C modes with more radial displacements [9], showed some discrepancies between spectra from C S and pure C . The most pronounced di!erences are additional peaks at 230 and 261 cm\ as well as some changes in the relative intensities (see Figs. 4a and b). A clear downshift of about 4 cm\ was also observed for peaks at e.g. 228, 263 and 456 cm\. The similarities between the spectrum of C S and the C #sul phur spectra in Figs. 4a and b suggest that the interaction between C and S rings is very weak. The observed discrepancies below 500 cm\, however, suggest some kind of C }S ring interaction. The C S compound contains six times less sulphur than C S and we should therefore ex pect weaker e!ects of the C }S interactions in the spectra as well as lower relative intensities of the
67
Table 1 X-ray powder di!raction data for C S d
hkl
I
15.22 15.11 10.30 10.71 8.570 7.589 6.878 6.758 6.095 5.919 5.264 5.164 5.064 5.010 4.887 4.867 4.776 4.521 4.437 4.392 4.221 4.115 3.952 3.795 3.724 3.417 3.339 3.243 3.295 3.284 3.243 3.159 3.131 3.059 3.017
020 200 202 220 222 040 104 420 422 431 441 404 060 442 611 532 6 2 0, 0 6 2 622 335 613 711 236 371 080 346 555 663 912 753 842 912 067 646 770 10 0 0
5 4 5 4 10 36 3 6 8 4 16 27 100 51 61 66 48 77 19 20 18 14 57 52 48 27 33 50 60 56 49 47 61 60 20
sulphur peaks. As can be seen in Fig. 4, the relative intensities of the sulphur peaks are about three times lower than in the C S spectrum. Further more, there are no additional peaks in the C S spectrum suggesting weaker C }S interactions in this phase than in C S . Fig. 4 shows, however, that the trends in the peak shifts are complicated in the new compound. For example, the peak originating from C at 456 cm\ is only downshifted with 1 cm\ in the C S spectrum compared to 4 cm\ in C S . This could be an indication of
68
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
Fig. 4. Raman spectra of C S compared with spectra of C S , pure C and sulphur, (a) in the range 100}1800 cm\, (b) in the range 100}600 cm\.
weaker C }S interactions. However, the situ ation is more complicated since the peak at 228 cm\ exhibits a similar downshift in both compounds and the 263 cm\ peak shows a stronger downshift for C S (6 cm\) than for C S (4 cm\). Consequently, each of these three peaks shows its own behaviour with respect to peak shifts in C S while all three peaks exhibit the same shift (4 cm\) in the C S spectrum. The result shows that the C }S interactions must be di!erent in
the two compounds. Finally, it should be noted that no peaks originating from the benzene solvent could be detected in the Raman spectra.
4. Conclusion A new method for crystal growth by evaporation of solutions in organic solvents was used to study crystallisation in the C }sulphur}benzene system.
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
The main advantages of this method stand on the controlled rate of solvent evaporation and less environmental pollution. Large crystals of a new compound were grown and the composition determined to C S by chemical analysis. The unit cell of C S was orthorhombic with the space group Pbcn. The crystal structure could not be determined at room temperature due to a strong disorder in the crystals. Raman spectroscopy showed shifts of some peaks below 500 cm\ and a weaker intensity of the sulphur peaks compared to a spectrum from C S . The results con"rm that the intramolecular interactions in C S consist of weak van der Waal bonds. However, di!erences in the peak shifts in the C S and C S spectra are consistent with a less symmetrical coordination of S rings around the C molecules in the new compound. To the knowledge of the authors, the crystals grown in this study are the largest ever reported for C compounds (including pure C ). We believe that this method o!ers unique possibilities to grow large crystals of a wide range of fullerene compounds, especially those which decompose at low temperatures. Acknowledgements The Swedish Natural Science Research Council (NFR), the GoK ran Gustavsson foundation are ac-
69
knowledged for "nancial support. Dr. J. Lindgren is acknowledged for placing the Raman spectrometer to our disposal. Some of the experiments were carried out in the A.F. Io!e Institute. Thanks to Prof. Lemanov V.V., Dr. Syrnikov P.P. and Bahurin V. for helpful discussions and technical support.
References [1] G. Roth, P. Adelmann, Appl. Phys. A 56 (1993) 169. [2] G. Roth, P. Adelmann, R. Knitter, Mater. Lett. 16 (1993) 357. [3] G. Roth, P. Adelmann, J. Phys. I France 2 (1992) 1541. [4] H.B. Burgi, P. Venugopalan, D. Schwarzenbach, F. Diederich, C. Thilgen, Helv. Chem. Acta 76 (1993) 2155. [5] R.H. Michel, M.M. Kappes, G. Roth, P. Adelmann, Angew. Chem. Int. Ed. Engl. 33 (1994) 1651. [6] A. Talyzin, U. Jansson, Thin Solid Films 350 (1999) 113. [7] V.N. Semkin, N.V. Drichko, A.V. Talyzin, A. Graja, S. Krol, D.N. Konarev, R.N. Lyubovskaya, Synth. Metals 93 (1998) 207. [8] A.V. Talyzin, V.V. Ratnicov, P.P. Syrnicov, Phys. Solid State 7 (1996) 1531. [9] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, J. Raman. Spectrosc. 27 (1996) 351. [10] T. Tomiyama, S. Uchiyama, H. Shinohara, Chem. Phys. Lett. 264 (1997) 143.
Single-crystal growth of C S } a new phase in the C }sulphur system A.V. Talyzin*, L.-E. Tergenius, U. Jansson Department of Inorganic Chemistry, Angstrom Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, SE, Sweden Received 18 January 2000; accepted 17 February 2000 Communicated by K. Sato
Abstract Large crystals (up to 2;1;0.5 mm) of a new C }sulphur compound were grown by evaporation of a benzene solution. The composition of this compound was determined by chemical analysis to C S . Single-crystal and X-ray powder di!raction experiments showed that the phase has orthorhombic structure with cell parameters a"30.18, b"30.41, and c"28.32 As and the space group Pbcn. The Raman spectrum from the new compound was very similar to those of pure C and sulphur with the exception of some changes in relative intensity and small shifts of a few peaks at wavenumbers below 500 cm\. The results suggest that the C }S interactions mainly consists of weak van der Waals bonds. Some irregularities in the peak shifts suggest that the distribution of S rings around the C molecules are less symmetrical that in the well-known C S phase. 2000 Elsevier Science B.V. All rights reserved. Keywords: C ; Sulphur; Fullerene; Solution growth
1. Introduction Compared with other groups of fullerene compounds, relatively little attention has been paid to compounds of fullerenes with sulphur. The properties of these compounds are determined by the strong tendency of sulphur to form S rings L (n"6}20). In most modi"cations, sulphur form S rings which are maintained also when sulphur is dissolved in organic solvents e.g. CS . S rings are also present in the vapour during sublimation and vapourisation of most sulphur modi"cations or of melts thereof. Hitherto, all known fullerene-sulphur * Corresponding author. Tel.: #46-18-471-3717; fax: #4618-5135-548. E-mail address: [email protected] (A.V. Talyzin).
compounds consist of fullerenes and S rings which are weakly bonded to each other with van der Waals bonds (see e.g. Refs. [1}3]). For example, Roth and Adelmann have synthesised and determined the structure of C S grown from a solu tion of C in a mixture of CCl and CS [1]. In contrast, they found that crystallisation from pure CS give a C S CS phase [2]. The C S and C S compounds also have been synthesised and the structures were determined [3}5]. Recently, the Raman studies of C S and C S were re ported [6]. The optical properties of a series of C compounds were investigated [7]. The study included C S and a new previously unknown phase in the C }sulphur system which was formed using a special method developed to grow crystals of fullerene compounds from organic solvents [8].
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 3 4 6 - 8
64
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
In Ref. [7], no detailed characterisation was carried out on the new C }sulphur compound and the chemical composition was given as C S which corresponds to the composition in the solution where the optimum conditions for crystallisation was found. The aim of the present study was to study the crystallisation process for the new compound in detail, to determine its chemical composition and to give more information about structural and bonding properties using X-ray di!raction (XRD) and Raman spectroscopy.
2. Experimental procedure C powder (99%, MER), sulphur (99.999%, CE RAC) and benzene (99.9%) were used in all growth experiments. The growth procedure appeared to be very sensitive to benzene impurities and the best results were achieved with freshly double-distilled solvent. The method used for crystal growth experiments has been published in Ref. [8]. Shortly, the procedure is as follows: the experiments are carried out in a closed system of the type shown in Fig. 1. The growth rate is regulated by the temperature gradient between the inner vessel (¹ ) and the outer vessel (¹ ) but depends on the shape and size of the outer vessel as well. In a simple but still
Fig. 1. Scheme of the crystallisation method: 1 } heater, 2 } inner vessel with solution, 3 } outer vessel.
rather e$cient set-up the ¹ is provided by room conditions while for ¹ any kind of conventional heater may be used. In a more advanced set-up the two independent water circulation thermostats were used, one as a heater (1) while the second forced circulation of water with temperature ¹ through the metallic pipes constructed as a shell around the outer vessel (3). The last set-up provided a precise temperature control (0.023C). After vapourisation from the inner vessel the solvate condenses on the walls of the outer vessel and #ows to the bottom of this vessel where it can be collected and used again. A condensation of solvent also took place on the walls of the inner vessel. A permanent #ow of liquid solvent along the walls of the inner vessel prevent nucleation on the walls. That is a great advantage because during evaporation from a vessel with a not completely closed cap, crystallisation occurs only on the walls of the vessel and the quality of the grown crystals became rather poor. Furthermore, the system shown in Fig. 1 allows us to carry out crystallisation experiments in a wide supersaturation range, use seed crystals and utilise other facilities common for growth of single crystals (e.g. controlled stirring of solution may be used with a help of rotating magnet placed below the heater (1) and metallic stirrer situated in solution). Small crystals of C S have previously been grown by evaporation of solution in CS [3,4]. In our study, however, we decided to use benzene as an alternative solvent. We have determined the solubility of C in benzene to around 1.1 mg/ml at 203C and found it to be almost temperature independent. In the crystal growth experiments, solutions of C and sulphur in benzene were mixed in di!erent proportions. A series of test experiments were performed with an initial volume of 20 ml of solution, but for growth of large crystals the initial volume of solution was 100}300 ml. Obtained crystals were washed by hexane to remove sulphur traces. The composition of the crystals was analysed by a commercial laboratory (MIKRO KEMI AB, Uppsala, Sweden). They used a #ush combustion gas chromatography method where the samples are completely oxidised into gaseous CO and SO . The gases are analysed by gas chromatogra phy and their concentration determined by a thermal conductivity detector (TCD). Single-crystal
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
data were collected with a Rigaku di!ractometer while X-ray di!raction powder data were collected with a Siemens 5000 di!ractometer using Cu K a radiation. Raman spectra were obtained by a Renishaw Raman 2000 spectrometer using a 780 nm excitation wavelength with a resolution of 2 cm\. Only low laser powers were used to avoid degradation of sample and photopolymerisation.
3. Results and discussion 3.1. Crystallisation Actually, the initial purpose of our experiments was to grow large crystals of C S . Following to the procedure described in a previous publication [1], we mixed C and sulphur solutions in a stoichiometry corresponding to the composition C S . Surprisingly, the result was di!erent from the experiments with CS . The initial saturated solution was almost black in colour and nontransparent but as crystallisation started, the solution became more and more transparent until it turned completely colourless. This remaining part of the solution contained only sulphur which suggests that the overall sulphur content in the crystals must be less than in C S . Furthermore, in the end of the process, crystals with two di!erent morphologies were formed. Most of the crystals were black, nontransparent rectangular platelets with a millimeter size. These crystals show very smooth shiny faces with spiral features on the surface which can be recognised under binocular even at low magni"cations. The other kind of crystals formed in the solution were very thin red transparent needles which are typical for C S . Chemical analysis on two samples (see experimental section) of each kind of crystals con"rmed that the red needles are of the well-known C S compound, while the rectangu lar black platelets exhibited a composition close to C S with slight excess of sulphur. The results suggest that the crystallisation occurs by the following way: initially, crystals of the new C S compound are formed. This phase contain less sulphur than the initial stoichiometry of the solution (C S ). As a result, the sulphur concentration in the solution will increase, and after a while, needles
65
of C S will start to form. However, since most of C is already deposited in the C S crystals some sulphur will remain in solution. Several experiments were carried out to investigate how the sulphur concentration in the solution in#uenced the crystallisation. A general observation was that no other phase than C S and C S were formed. Furthermore, the results showed that C S crystallise from a benzene solution only when the sulphur concentration is close to saturation for pure sulphur (i.e. much higher than the stoichiometric composition). An interesting observation was that C does not dissolve in a saturated sulphur solution. Adding a C solu tion to a saturated sulphur solution results in a fast formation of thin red platelets and needles of C S without any formation of the C S phase. This method is very useful for obtaining C S powder samples. For growth of the C S com pound we found that the best way to obtain large crystals was to use a solution with a sulphur content approximately twice higher than the stoichiometry of this compound. When a stoichiometric composition was used the crystals tend to grow as dendrites. This can be attributed to an insu$cient rate of sulphur di!usion to the surface of the growing crystals. One of the main problems with the growth of C S crystals from benzene is that very high supersaturations are required prior to the nucleation of this compound. For example, consider a starting solution saturated with C and with a sulphur content corresponding to C S (opti mum growth conditions for the C S phase). The volume of this solution has to be reduced with 70}90% by evaporation of the solvent prior to the nucleation of C S . As a result, the initially for med nuclei grew very fast and produced dendrites. As the supersaturation decreased each branch of such a dendrite continued to grow separately. This led to the formation of a polycrystalline sample consisting of up to 1}2 mm large crystals with approximately similar orientations. Nevertheless, large single crystals with sizes up to 2;1;0.5 mm were also obtained in some cases (Figs. 2a and b). Finally, it is clear that a new C S compound can be grown from a solution of C and sulphur in benzene. An obvious question then arises why this
66
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
phase has not previously been observed with CS as a solvent. A possible explanation is that the solubilities of sulphur and C in CS are much higher than in benzene (e.g. the solubility of C in CS at 203C is 10 mg/ml [10] but only 1.1 mg/ml in benzene). This leads to much lower di!usion rates of C and sulphur to the surface of the growing crystals which may favour the formation of the C S phase. Furthermore, in a previous study, C of a lower purity ((97%) was used to grow crystals using benzene as a solvent [8]. In this case only C S was formed and it is therefore possible that impurities in the C may favour the forma tion of this phase. It should be mentioned that the purity of C was not mentioned in Refs. [3,4]. 3.2. X-ray diwraction
Fig. 2. (a), (b) Crystals of C S (1 mm scale).
Attempts were made to solve the crystal structure of the C S compound. Single-crystal data collected at room temperature on a small 0.3;0.3 mm platelet gave us more than 20 000 re#ections. The data were consistent with an orthorhombic structure with the space group Pbcn and the cell parameters a"30.18, b"30.41 and c"28.32 As . However, we were unable to solve the structure using the SHELX software. We could locate S rings and parts of the C molecules
Fig. 3. Powder X-ray di!raction pattern of C S .
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
according to the suggested composition but failed to localise more than about half of the carbon atom positions. We attribute these problems to high degree of disorder at room temperature. Further studies at low temperatures will be carried out in a near future. Powder X-ray di!raction data from a crushed collection of small crystals are shown in Fig. 3. Special precautions were taken to crush the crystals gently since it is known that signi"cant degradation of the C structure can occur during the crushing procedure. As can be seen, the di!raction pattern shows a large number of re#ections, many of them heavily overlapped, which is typical for a large unit cell with low symmetry. The re#ections are consistent with an orthorhombic unit cell and structural parameters obtained with single crystal. Re#ections with I'14 are listed in Table 1. For the region 2H(153 re#ections with I'3 are listed because of their high importance. 3.3. Raman spectroscopy Raman spectra of pure sulphur, C , C S and the new C S compound are shown in Fig. 4. As can be seen, the spectra from C S and C S contain two types of peaks originating from C and S rings. No additional peaks or peak shifts can be seen in the spectra from C S and C S above 500 cm\. An interesting observation, however, was that the region below 500 cm\, which is dominated by C modes with more radial displacements [9], showed some discrepancies between spectra from C S and pure C . The most pronounced di!erences are additional peaks at 230 and 261 cm\ as well as some changes in the relative intensities (see Figs. 4a and b). A clear downshift of about 4 cm\ was also observed for peaks at e.g. 228, 263 and 456 cm\. The similarities between the spectrum of C S and the C #sul phur spectra in Figs. 4a and b suggest that the interaction between C and S rings is very weak. The observed discrepancies below 500 cm\, however, suggest some kind of C }S ring interaction. The C S compound contains six times less sulphur than C S and we should therefore ex pect weaker e!ects of the C }S interactions in the spectra as well as lower relative intensities of the
67
Table 1 X-ray powder di!raction data for C S d
hkl
I
15.22 15.11 10.30 10.71 8.570 7.589 6.878 6.758 6.095 5.919 5.264 5.164 5.064 5.010 4.887 4.867 4.776 4.521 4.437 4.392 4.221 4.115 3.952 3.795 3.724 3.417 3.339 3.243 3.295 3.284 3.243 3.159 3.131 3.059 3.017
020 200 202 220 222 040 104 420 422 431 441 404 060 442 611 532 6 2 0, 0 6 2 622 335 613 711 236 371 080 346 555 663 912 753 842 912 067 646 770 10 0 0
5 4 5 4 10 36 3 6 8 4 16 27 100 51 61 66 48 77 19 20 18 14 57 52 48 27 33 50 60 56 49 47 61 60 20
sulphur peaks. As can be seen in Fig. 4, the relative intensities of the sulphur peaks are about three times lower than in the C S spectrum. Further more, there are no additional peaks in the C S spectrum suggesting weaker C }S interactions in this phase than in C S . Fig. 4 shows, however, that the trends in the peak shifts are complicated in the new compound. For example, the peak originating from C at 456 cm\ is only downshifted with 1 cm\ in the C S spectrum compared to 4 cm\ in C S . This could be an indication of
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A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
Fig. 4. Raman spectra of C S compared with spectra of C S , pure C and sulphur, (a) in the range 100}1800 cm\, (b) in the range 100}600 cm\.
weaker C }S interactions. However, the situ ation is more complicated since the peak at 228 cm\ exhibits a similar downshift in both compounds and the 263 cm\ peak shows a stronger downshift for C S (6 cm\) than for C S (4 cm\). Consequently, each of these three peaks shows its own behaviour with respect to peak shifts in C S while all three peaks exhibit the same shift (4 cm\) in the C S spectrum. The result shows that the C }S interactions must be di!erent in
the two compounds. Finally, it should be noted that no peaks originating from the benzene solvent could be detected in the Raman spectra.
4. Conclusion A new method for crystal growth by evaporation of solutions in organic solvents was used to study crystallisation in the C }sulphur}benzene system.
A.V. Talyzin et al. / Journal of Crystal Growth 213 (2000) 63}69
The main advantages of this method stand on the controlled rate of solvent evaporation and less environmental pollution. Large crystals of a new compound were grown and the composition determined to C S by chemical analysis. The unit cell of C S was orthorhombic with the space group Pbcn. The crystal structure could not be determined at room temperature due to a strong disorder in the crystals. Raman spectroscopy showed shifts of some peaks below 500 cm\ and a weaker intensity of the sulphur peaks compared to a spectrum from C S . The results con"rm that the intramolecular interactions in C S consist of weak van der Waal bonds. However, di!erences in the peak shifts in the C S and C S spectra are consistent with a less symmetrical coordination of S rings around the C molecules in the new compound. To the knowledge of the authors, the crystals grown in this study are the largest ever reported for C compounds (including pure C ). We believe that this method o!ers unique possibilities to grow large crystals of a wide range of fullerene compounds, especially those which decompose at low temperatures. Acknowledgements The Swedish Natural Science Research Council (NFR), the GoK ran Gustavsson foundation are ac-
69
knowledged for "nancial support. Dr. J. Lindgren is acknowledged for placing the Raman spectrometer to our disposal. Some of the experiments were carried out in the A.F. Io!e Institute. Thanks to Prof. Lemanov V.V., Dr. Syrnikov P.P. and Bahurin V. for helpful discussions and technical support.
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