Raman scattering in brominated (SN)x crystals

Solid State Communications, Vol. 25, pp. 409--413, 1978.

Pergamon Press.

Printed in Great Britain

RAMAN SCATTERING IN BROMINATED (SN)x CRYSTALS Z. Iqbal Energetic Materials Division, Army Research and Development Command, Dover, NJ 07801, U.S.A. and R.H. Baughman Materials Research Center, Allied Chemical Corporation Morristown, NJ 07960, U.S.A. and J. Kleppinger and A.G. MacDiarmid Department of Chemistry and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, U.S.A. (Received 29 July 1977 by A.A. Maradudin)

Detailed Raman scattering data on brominated (SN)x crystals ranging in composition from (SNBro.27)x to (SNBro~s)x, and preliminary Raman data on [SN(ICI)o.12s]x are discussed. The investigation involved a study under various conditions of temperature, pressure and levels of bromination, of the two primary Raman lines at 154 and 230 cm -1 in brominated (SN)~, which are polarized along the polymer axis direction. The analysis of the data is consistent with a model in which bromine enters the interfibrillar regions as Br~ and the (SN)x lattice as Br2. Infrared data is also presented showing an appreciable decrease in intensity of the SN modes at 995 and 670 cm -~ on bromination. 1. INTRODUCTION POLYMERIC sulfur nitride, (SN)x, has been extensively studied in recent years as the first example of a metallic and superconducting linear polymer [ 1]. Recently, Bernard et al.. [2] and Street etal. [3] independently observed that (SN)x reacts with bromine at room temperature to give a blue-black material. Compositions corresponding to (SNBroA)~ were established for brominated (SN)x by Street et al. [3] and Akhtar et al. [4]. Detailed optical reflectivity measurements on (SNBroA)~ [5, 6] at 295 K, and electrical conductivity measurements between 3 and 300 K for (SNBroA)x [5, 6] and a copper-colored composition (SNBro.2s)x [5], have also been reported. The optical reflectance data show a redshift of the plasma edge (observed with the incident photon electric vector polarized parallel to the chain direction) by ca. 1.0 eV compared with (SN)~. In contrast with normal (SN)~, however, the transverse reflectivity was found by Chiang et al. [5] to be nonmetallic, thus suggesting that (SNBroA)x is more nearly one-dimensional. The electrical conductivities of (SNBroA)x [5, 6] and (SNBro.2s)x [5] show metallic behaviour, with room temperature conductivity values which are approximately an order of magnitude higher than in normal (SN)x. From electron diffraction data Street et al. [3]. report that despite an approximately 50% volume 409

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Fig. 1. Raman spectra between 100 and 350 cm -~ at 295 K of (a) freshly brominated crystal of composition (SNBr~ss)x, (b) same crystal after pumping for 15 min, (c) same crystal after pumping for 1 hr, (d) coppercoloured (SNBroaT)x crystal. Spectra (a) to (c) were recorded at same instrumental settings and from the same spot on the crystal. Excitation used: 514.5 rim.

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RAMAN SCATTERING IN BROMINATED (SN)x CRYSTALS

expansion and major increase in density on bromination, the a and c lattice parameters remain unchanged and that the crystals decrease in perfection. No measureable change in the b axis dimension has been observed, but diffuse scattering corresponding to a chain direction spacing of about 2b has been reported [6]. In contrast with these results Bernard e t al. [2] report that only the chain axis spacing of (SN)x is unaffected by bromination. Baughman e t al. [7] have found that the unit cell parameters of heavily brominated (SN)x are significantly different from those of the unbrominated polymer. Three structural models, based on independent Raman scattering and diffraction measurements, have been proposed for brominated (SN)x. The first model, due to Street e t a l . [3] and Gill e t a l . [6], presumes that bromine enters the interfibrillar regions of (SN)x as Br2 molecules but that no bromine enters the (SN)x lattice. Iqbal e t al. [8], however, suggest that in addition to bromine in the interfibrillar regions, a small fraction of bromine enters the (SN)x lattice as Br2 to form an insertion compound. Finally, a third model due to Temkin e t al. [9] suggests that all the bromine enters the interfibrillar regions as Br~ ions. In this paper we report Raman scattering experiments on a variety of brominated (SN)x samples under different conditions. In addition, an in situ infrared absorption experiment in the 1100 to 600 cm -1 region and preliminary Raman data on [SN(IC1)oa2s] x, are discussed briefly. The structure of brominated~SN)x is analyzed based on these results. 2. EXPERIMENTAL Monoclinic (SN)x crystals were reacted with 60 torr of bromine at 295 K to obtain crystals of composition (SNBro.ss)x, which gave a blue-black composition (SNBroA)x on pumping for 30 min. On pumping (SNBroA)x crystals at 295 K for 45 hr or more the bromine content decreases and compositions of ca. (SNBroaa)x or lower are obtained. Copper-colored crystals of composition (SNBroas) x and (SNBro.27)x were obtained by annealing crystals of (SNBro,4)x at ca. 359 K with pumping for 30 and 15 hr respectively. All samples were analyzed for Br, S and N, and the sum of these figures added up to 100 +- 0.3%. [SN(IC1)oa2s]x crystals were prepared by reacting (SN)x with ca. 28 torr of IC1 vapor for 20 hr. The Raman scattering experiments were carried out with a Coderg T800 triple monochromator coupled to a thermoelectrically cooled $25 response EMI 9550 A photomultiplier tube and pulse-counting circuitry. No further processing of the data was performed. The spectra were excited with a stabilized argon ion laser operating at a single line power of ca. 30 mW and a

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Fig. 2. (a) Polarized Ranaan spectra of (SNBrg.4)x crystal at 15 K. Scattering angle is about 110 . Y is the chain direction and X is the incident photon direction. The weak scattering peaks for the Z Y and Y X tensor elements are due to the 1 l0 ° geometry used in our low temperature system to allow maximum collection of the scattered beam. Backscattering experiments at 295 K showed complete absence of peaks for the off-diagonal scattering tensor elements. Excitation used: 514.5 urn. (b) Raman spectrum between 300 and 1100 cm -1 with 488.0 nm excitation showing the series of lines discussed in the text. Arrows indicate positions of the main SN Raman lines in monodinic (SN)x. H e - N o laser operating at ca. 20 mW power. The low temperature data were obtained with a dosed cycle helium refrigerator system coupled to a proportional temperature controller (stability + 0.1 K). The absolute temperatures at the base of the cold f'mger were measured with a calibrated chromel-gold thermocouple (accuracy + 1.0 K). Variable high pressures were maintained to within 0.4 kbar with a l0 kbar hydrostatic pressure cell (with n-hexane as the the transmission fluid) described elsewhere [ lO]..The Roman data reported were obtained with a spectral band-width of 3 cm -1 or less, and calibrafed frequency accuracy of 1 cm -~ . To check for sample damage by laser heating, scans were repeated a number of times from the same spot in the crystal.

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RAMAN SCATTERING IN BROMINATED (SN)x CRYSTALS

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RarnonShift.,c m -1 Fig. 3. Raman spectra of (SNBroA)x crystal at various temperatures. Note that the spectra for 146, 98 and 15 K were run at factors of 1.4, 2.0 and 3.5 respectively lower instrumental sensitivity. Excitation used: 514.5 nm. 3. RESULTS AND DISCUSSION Three Raman peaks centered at 154, 230 and 305 cm -~ are observed in different crystals of brominated (SN)x at 295 K (in the following discussion the peaks will be identified by these frequency values). The peak at 305 cm -l is extremely broad (FWI-IM > 40era -l) at 295 K, whereas the 230 cm -1 line is rather asymmetric. The Raman spectra in the 100 to 400 cm -t region of a freshly brominated (SN),, crystal [corresponding to composition (SNBr0.ss)x ] after different periods of pumping and of a copper-colored (SNBro.27)x crystal, are shown in Fig. 1. From the spectra it is evident that the 154 and 230 cm -~ lines have approximately the same intensity in the freshly brominated sample, but with decreasing bromine content, the relative intensity of the 230 cm -1 line drops with respect to the line at 154 cm -t . Raman scattering from a [SN(ICl)oa2s]x crystal at 295 K, excited with 488.0 nm radiation, showed a relatively weak and broad line centered at ca. 100 cm -1 . The polarized Raman spectra of a (SNBro.4)x crystal at 15 K are shown in Fig. 2(a). As evident in the figure, the Raman peaks are observed exclusively for the Y Y scattering tensor element (where Y is the chain axis direction). The polarized spectra for crystals with varying levels of bromination show the same feature. The spectrum at 15 K of a (SNBro,4)x crystal, in the 250 to 1100 cm -~ frequency range, which was recorded at high instrument gain, and with the incident photon electric vector aligned along the Y axis, is shown in Fig. 2(b). A series of lines at 473,638 and 788 cm -1,

which decrease in intensity with increasing frequency shift and appear to be overtones of the 154 and 305 cm -1 peaks is observed. The positions of the Raman lines at 464,660 and 1004 cm -1 observed in monoclinic (SN)x [I 1, 12] are indicated in Fig. 2(b). From our data it is evident that except for a weak and broad feature near 700 cm -~ , lines assignable to SN modes are absent or extremely weak in (SNBro.4)x. However, it was observed that even in unbrominated (SN)x it is extremely difficult to obtain Raman spectra. The infrared spectra between 600 and 1100 cm -1 of a (SN)x film on KBr recorded in a vacuum cell with NaC1 windows, after increasing levels of bromination in situ, showed the near disappearance of the strong SN infrared line at 995 cm -1 and the appreciable weakening of the very strong infrared absorption at 670cm -l of monoclinic (SN)x [13], to a broad shoulder on the NaC1 window cut-off absorption. Also, the lines at 670 and 995 cm -1 in (SN)x were not observed in the infrared spectra o f crushed brominated (SN)x crystals in KBr. The relative intensities and peak frequencies of the Raman lines in brominated (SN)= crystals did not vary with laser excitation wavelength in the 457.9 to 514.2 nm range. However, with 632.8 nm excitation the Raman lines decreased tremendously in scattering intensity, thus indicating that resonance-enhancement of the Raman spectra occurs wi.th excitation at shorter wavelengths. The observed Y Y polarization of the spectra is consistent with a coupling of the observed vibrational modes with the backbone charge density. With decreasing temperature the Raman spectra of

RAMAN SCATTERING IN BROMINATED (SN)x CRYSTALS

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Fig. 4(a) Temperature dependence of the integrated intensities of the 154 cm -1 (open circles) and 305 cm -1 (closed circles) lines for a (SNBroA)x crystal, and of the 154 cm -1 line (crosses) for a (SNBro.27)x crystal. Dotted curve is drawn through the data points obtained for the (SNBro~7)x crystal. (b) Temperature dependence of the line.width at half maximum ( r ) for the 154 cm -1 (open circles) and 305 cm -1 (closed circles) lines. The data for a (SNBroaT)x crystal are shown by crosses. Full line is a linear fit to the data points for the 154 cm -I line above 100 K. (SNBro.4)x and (SNBro.27)= crystals show rather interesting changes, as evident from the spectra for (SNBroA)x displayed in Fig. 3. The integrated intensities of the 154 and 305 cm -~ lines increase appreciably below 200 K [Fig. 4(a)] together with a substantial narrowing of the 305 cm -1 line [Fig. 4(b)]. The 230cm -l line, however, decreases somewhat in integrated intensity with decreasing temperature with no accompanying change in linewidth. The 305 and 154 cm -~ lines increase in ferquency to values of 316 and 159 cm -~ respectively at 15 K, whereas the 230 cm -~ line decreases in peak frequency

Vol. 25, No. 6

to a value of about 219cm -1 at 15 K. The drop in peak frequency of the 230 cm -1 line at low temperature is anomalous since vibrational modes normalty increase in frequency with decreasing temperature. The line.width at half maximum ( r ) of the 154 cm -1 line is a linear function of temperature above ca. 100 K [Fig. 4(b)] consistent with anharmonic lattice dynamics [14], whereas the data for the 305 cm -1 line show substantial non-linearity of r with temperature above 100 K [Fig. 4(b)]. Isotopic splitting due to the 1 : 1 natural abundance of ~9Br and aSBr in the brominated polymer (expected to be spread over a 3.6 cm -I interval for Br2), is not evident in the relatively narrow 154 cm -~ line, which was scanned with an instrumental resolution of ca. 0.5 cm -~ at 15 K. The Raman spectra up to 10kbar at 295 K showed very little change in line-shape, intensity and peak position of the 154 cm -1 line. In contrast, the 230 cm -~ line showed an anomalous 0.8 cm -l per kbar decrease in frequency with increasing pressure. The rather low frequency of the 154 cm -1 line compared to that of free [15] and intercalated [16] bromine, and the absence of isotopic splitting, suggest that it is not associated with a purely Br2 stretching mode. In accordance with Temkin e t al. [9] this line can be assigned to the symmetric stretching (vl)mode of the linear Br~ ions which can form in the interfibrillar regions of the polymer, ul is located at ca. 162 cm -l in a number of Br~ salts [17], while the antisymmetric stretching (u3) mode, which is Ramanactive for asymmetric Br~ ions, is located at 193 cm -~ for solid Bu4NBr3 and at 196 cm -1 for Me4NBra in (CH2C1)2 [17]. The 230 cm -l line frequency in brominated (SN)x ,,therefore, appears to be somewhat high for assignment as the v3 mode of Br~ ions. Since its frequency is close to that of the 240 cm -t line of Br: in bromine intercalated graphite [16], it is possible that it is instead associated with Br2 inserted in the (SN)x lattice, in terms of the model due to Iqbal e t al. [8]. The anomalous high pressure and low temperature induced decrease in frequency of the 230 cm -~ line can then be considered to be due to increased intercalate to lattice interaction with increasing lattice compression or contraction. In contrast with bromine intercalated graphite, the 230 cm -l line in brominated (SN)x consists of a lower frequency asymmetric component which may be associated with disorder-induced scattering from k ~ 0 modes. The u3 mode of Br~ could be a part of the asymmetric wing of the 230 cm -~ line or it could be Raman-inactive due to centre-symmetry [18], The 154 and 305 cm -~ lines occur in an approximately 1 : 2 frequency ratio, which suggests that the latter peak is a harmonic of the much stronger 154 cm -~ line. The non-linear temperature dependence of the line-

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RAMAN SCATTERING IN BROMINATED (SN)x CRYSTALS

width of the 305 cm -~ peak is also in agreement with this assignment. The weak series of lines in brominated (SN) x at 473,638 and 788 cm -1 would then correspond to the third, fourth and fifth harmonics respectively of the 154cm -~ line. The assignment of the 154 and 230 cm -~ lines to different bromine sites is consistent with the variation in the relative intensities of these lines with varying bromine content. Also, the dramatic increase of the 154cm -l line with decreasing temperature (probably due to the selective increase in resonance-enhancement of the Br3 spectra at low temperatures) and the absence of a temperature effect on the intensity of the 230 cm -~ line, support the above arguments in favor of a two-site model. The weakening of the SN infrared-active intensities in brominated (SN)x relative to those in

413

normal (SN)x , could be associated with the depletion of charge density likely to occur with the probable formation of Br~ ions in the interfibriUar regions. It is interesting to note that in [SN(IC1)o.12s] x a Raman line at 100 cm -~ corresponding to the line at 103 cm -~ in 1.8 mole % IC1 intercalated graphite [16] was observed, but no evidence of spectra corresponding to the IC12 species was obtained. Acknowledgements - The authors wish to thank Dr. A.J. Heeger, Mrs J. Milliken and Mr M.J. Moran (University of Pennsylvania) for helpful discussions concerning the synthesis and properties of brominated (SN)x and the samples used in this study. The authors also wish to thank Drs H. Temkin and B.D. Fitchen (Cornell University) and Dr G.B. Street (IBM, San Jose) for interesting discussions.

1.

GESERICH H-P. & PINTSCHOVIUS L., in Advances in Solid State Physics, Vol. XVI (Edited by TREUSCH J.), p. 65, Vieweg, Braunschweig (1976); GREENE R.L. & STREET G.B., in Chemistry andPhysics of One-climensionalMetals, (KELLER HJ. Editor), Plenum, New York (1977); HSU C. & LABES M.M., J. Chem. Phys. 61,4640 (1974); MACDIARMID A.G., MIKULSKI C.M., SARAN M.S., RUSSO P.J., COHEN M.J., BRIGHT A.A., GARITO A.F. & HEEGER A.J., Advances in Chemistry Series, No. 150 (Edited by KING R.B.), 1976; BAUGHMAN R.H., APGAR P.A., CHANCE R.R., MACDIARMID A.G. & GARITO A.F., J. Chem. Phys. 66, 401 (1977).

2.

BERNARD C., HEROLD A., LELAURAIN M. & ROBERT G., C.R. Acad, Sc. Paris, C283, 625 (1976).

3.

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4.

AKHTAR M., KLEPPINGER J,, MACDIARMID A.G.. MILLIKEN J., MORAN M.J., CHIANG C.K., COHEN M.J., HEEGER A.J. & PEEBLES D.L., Chem. Comm. 473 (1977).

5.

CHIANG C.K., COHEN M.J., PEEBLES D.L., HEEGER A.J., AKHTAR M., KLEPPINGER J. & MACDIARMID A.G., Solid State Commun. 23,607 (1977).

6.

GILL W.D., BLUDAU W., GEISS R.H., GRANT P.M., GREENE R.L., MAYERLE J.J. & STREET G.B., Phys. Rev. Lett. 38, 1305 (1977).

7.

BAUGHMAN R.H., KIM H.G., IQBAL Z., KLEPPINGER J. & MACDIARMID A.G. (to be published).

8.

IQBAL Z., BAUGHMAN R.H., KLEPPINGER J. & MACDIARMID A.G., Prec. Conf. on Synthesis and Properties of Low Dimensional Materials, NY Acad Sciences, (1977) (to be published).

9.

TEMKIN H., FITCHEN D.B. & STREET G.B., Prec. Conf. on Synthesis and Properties of Low Dimensional Materials, NY Acad Sciences, (1977) to be published.

10.

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11.

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12.

STOLZ H.J., WENDEL H., OTTO A., PINTSCHOVIUS L. & KAHLERT H., Phys. Status Solidi (b)78,277 (1976).

13.

IQBAL Z. & DOWNS D.S., J. Chem. Phys. (to be published).

14.

COCHRAN W., The Dynamics of Atoms in Crystals, Chap. 8, Edward Arnold Publishers, London (1973).

15.

HERZBERG G., Spectra of Diatomic Molecules, D. Van Nostrand Inc. New York (1950).

16.

SONG J.J., SUNG D.D.L., EKLUND P.C. & DRESSELHAUS M.S., Solid State Commun. 20, 1111 (1976).

17.

PERSON W.B., ANDERSON G.R., FORDEMWALT J.N., STAMMREICH H. & FORNERIS R.,J. Chem. Phys. 35,908 (1961).

18.

For review see: DOWNS A.J. & ADAMS C.J. in Comprehensive Inorganic Chemistry (Edited by BAILAR J.C.), Vol. 2, p. 1534, Pergamon Press, Oxford (1973).