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Vibrational studies of lithium-intercalated SnS2
SOLID STATE IONICS
Solid State Ionics 89 (1996) 337-343
ELSEVIER
Vibrational
studies of lithium-intercalated
SnS 2
C. Julien*, C. PCrez-Vicente Laboratoire de Physique des Solides, Universite’ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
Received 20 December
1995; accepted
20 January
1996
Abstract We report on the vibrational properties of lithium-intercalated SnS, single crystal. Raman scattering and infrared absorption spectra have been recorded as a function of temperature in the frequency range 50-600 cm _ ’. The new bands are interpreted such as modes due to vibrations of lithium atoms located in the van der Waals gap against the nearest neighbouring chalcogen atoms. Results are analyzed in terms of stretching vibrations of Li-S, and Li-S, entities. From a comparison of the spectra, it is concluded that lithium ions are more likely to occupy tetrahedral sites in the lithium-rich phase. Using a simple model of lattice dynamics, the coupling force constants are determined and compared with those of the pristine material. Keywords: Lithium intercalation; Tin disulfide; Raman scattering; Infrared spectra
1. Introduction The lattice dynamics and electronic properties of intercalated layered materials are of interest because of their two-dimensional nature and because of their applications such as solid solutions in solid state batteries [ 11. Recently, some infrared (IR) absorption and Raman scattering (RS) experiments have been performed on layered compounds intercalated by alkali metals for studying the intercalation mechanism and evaluating the interaction between host lattice and intercalants [2-71. Barj et al. [2] have studied IR absorption in lithium-intercalated NiPS, and they found new absorption bands attributed to vibrations of lithium atoms located in the octahedral sites in the van der Waals gaps. In order to explain new peaks in the Raman spectrum of MoS, after *Corresponding author. 0167-2738/96/$15.00 01996 Elsevier Science B.V. All rights reserved PII SO167-2738(96)00270-6
intercalation with lithium, the lattice dynamics of Li,MoS, has been calculated by applying a central force model [5]. It was found that they can be attributed to new vibrational modes in which the intercalant atoms strongly vibrate against the host lattice. Julien et al. [8] have demonstrated that from optical and electrical-transport measurements, the intercalation mechanism of Li in Li,TiS, is consistent with the assumption of an increased d-band filling and a high degree of charge transfer. The structural and electronic changes occurring in the lithiated phases Li,NbS, have been reported by Sourisseau et al. [6]. The RS spectra of various Li,NbS, (0 5 x 5 3) intercalated compounds are very sensitive to the strong interactions between lithium to sulphur; these studies indicate that most of the lithium ions are accommodated in various LiS, environments upon the degree of intercalation. SnS, crystal occurs in the CdI,-type of layered
338
C. Julien, C. P&z-Vicente
I Solid State Ionics 89 (1996) 337-343
structure. The suitability of SnS, such as host for lithium intercalation makes it an interesting material for understanding the general problem of the localization and mobility of the alkali ions in the gaps. Some structural and thermodynamical properties of lithiated phases Li,SnS, have been reported [9-l 11. Chemical lithiation of SnS, has been carried out by treating the host material with lithium solutions in liquid ammonia [9] and also in aqueous media [lo] where lithium atoms were solvated by NH, and H,O, respectively. Recently, Morales et al. [ 1 l] have studied the Li intercalation in 2H-SnS, using a chemical process in n-butyl lithium and an electrochemical titration technique. A new phase showing a staging effect has been identified. Thus, the nature of the lithium site occupancy is still controversial in SnX, compounds, vibrational spectroscopy studies can give some information about the position of the intercalant species in the host structure [12]. In this report, we present Raman scattering and infrared absorption spectra of lithium-intercalated SnS, crystal in order to better understand the nature of the intercalation process in this system. We attempt to explain the experimental results of vibrational spectroscopy through the use of lattice dynamical models such as a simplified linear-chain model and a simple local-symmetry approach. By using vibrational spectroscopy, the question of the insertion sites of the lithium in SnS, is addressed.
2. Experimental Samples of SnS, were grown by direct synthesis from their constituent elements. The stoichiometric mixture of the elements was sealed in an evacuated fused quartz ampoule. The temperature was slowly increased to reach 500°C in 2 h. The ampoule was kept for one day at this temperature and then cooled down to room temperature in gradual steps. After grounding and washing with CS,, the same heating procedure was used to obtain the final products. Lithium intercalation in host material was accomplished at room temperature by immersion of the sample in n-butyl lithium in hexane solution as reported by Whittingham and Dines [ 131. The sample preparation was carried out in a purified argon atmosphere containing less than 1 ppm of water.
After lithiation, the samples were washed in hexane and stored in a closed tube in the glove-box over 7-day period. The lithiated samples were inserted into a vacuum cryostat model Oxford 1104, which is temperature variable, inside the glove-box, so that the intercalated crystals never came into contact with air. Absorption spectra were recorded in a Bruker IFS 113 vacuum Fourier transform infrared (IT-IR) interferometer [14]. In the far-infrared region (50600 cm-‘), this apparatus is equipped with a 3.5 pm-thick beam splitter, a globar source and a liquid helium cooled bolometer. Each spectrum was the average of 64 scans and was taken with a spectral resolution of 2 cm-‘. Powdered samples were dispersed in solid paraffin which does not exhibit any infrared absorption peaks in the studied wavelength range (13-200 ,um). We have always checked that the samples were water or oxygen free by the absence of infrared bands in the 3400, 1600 cm-’ or 1100-800 cm-’ regions.
3. Results and discussion Fig. 1 shows the Raman scattering spectra of pure (curve a) and Li-intercalated SnS, (curve b) recorded at room temperature. The IT-IR spectra of SnS, and its lithiated phase recorded at room temperature in the range 50-600 cm ’ are shown in Fig. 2. In order to investigate the effects of alkali metal intercalation in SnS, samples, first we briefly analyse the lattice dynamics of pure material. This compound is a layer-type semiconductor having the CdI, crystal structure whose symmetry corresponds to the D3,, space group. The individual tightly bonded layer in SnS, is an S-Sn-S sandwich in which the Sn atoms are octahedrally coordinated to six nearest-neighbor sulphur atoms. The three-atom basis of the elementary unit cell gives rise to nine vibrational modes; three doubly degenerate E modes in which the atomic motions are parallel to the layer planes and three nondegenerate A modes in which the atomic motions are perpendicular to the layers. Four optical modes are active at the center of the Brillouin zone. There are two Raman-active modes, A Ig + E, and two IR-active modes, A,, + E,. These active modes are clearly observed in the spectra (Figs. 1 and 2).
C. Julien, C. Pbez-Vicente
I Solid State Ionics 89 (1996) 337-343
It has been demonstrated that the long-wavelength lattice vibrations of a layered compound can be described by a linear-chain model that includes both intralayer (K) and interlayer (k) force constants [15]. The crystal symmetries of SnS, imply that the vector displacements of the atomic planes in all the normal modes are either parallel or perpendicular to the c axis. The dynamical equations for the normal vibrations of the atomic planes can be written as
Li,SnSZ
- Mnm2u, =c SnS2
Frequency
km-’
339
m
K,,(u,
- u”),
(1)
where M, is the mass of the atom lying in the nth plane of the primitive cell, w2 is the angular frequency, u, and u, are the normal displacements of the planes n and m, and K,, is the shear or compressional force constant between the planes n and m. Considerations of crystal symmetry lead the following relations giving the modes frequencies
1
Fig. 1. Raman scattering spectra recorded at room temperature SnS, crystal (curve a) and LiSnS, (curve b) sample.
w(Azu, E,) = Wt.d’2,
(24
w(A ,g, E,) = [(K + 2k)lM,]“2,
(2b)
of
Here ,U is the reduced mass ,u =M,M,l(M, + 2M,) where M, and M, are the tin and sulphur masses. The frequencies of the 2H-SnS, Raman and infrared modes determined experimentally are the following
loo
200
300 FlI?qwq
COO km-‘)
!iOO
Fig. 2. FT-IR spectra recorded at room temperature crystal (curve a) and LiSnS, (curve b) sample.
6 of SnS,
w(A,,)
= 348 cm-‘,
w(E,) = 207 cm-‘,
w(A,,)
= 316 cm-‘,
w(E,) = 20.5 cm-‘.
These mode frequencies are in good agreement with those reported in the literature [ 16-191. It is worth noting that (a) the frequencies of the A,, and E, modes are not very far from each other, (b) the E, mode gives a very broad band with full-width at half maximum of the order of 140 cm-‘, (c) the strength of the A,, mode is roughly 20 times less than that of the E, mode which is a consequence of the preferential orientation of the sample along the layer plane. The effect of lithium intercalation on the vibrational spectra of SnS, is twofold: (1) the normal modes of the host material remain almost unchanged; only a frequency shift (few wavenumbers) of the lattice modes of SnS, occurs; (2) new broad bands are observed which are located in the high frequency
C. Mien,
340
C. Pbez-Vicente
I Solid State Ionics
region at 126 and 383 cm-’ in Raman and at 146, 420 and 444 cm-’ in IR at room temperature. The corresponding band wavenumbers and assignments are reported in Table 1. The general trend in the spectra of lithiated phase (Figs. 1 and 2) is a small shift of the SnS,-like modes. We observed an increase in the frequency of the E,, A ,g and E, modes, whereas the frequency of The the A,, mode decreases upon intercalation. frequency shift of the E, mode indicates an increase in the restoring force for this type of vibration which is probably due to a decrease in the intralayer distance, a consequence of the tin reduction which creates a negative charge in the intralayer spacing upon lithium intercalation. Thus the net effect of the Li intercalation corresponds to a change of the polarizability in the intralayer bonding which produces an increase of the normal mode frequencies. For the lithiated products, the phase that is isostructural with SnS, shows minor changes in the unit cell parameters with the lithium content [ 111. A decrease of the basal spacing has been observed, from c =5.884 A in SnS, to c=5.864 A in Li,.,,SnS,, which is attributed to the strong attractions due to the presence of lithium cations between negatively charged layers. The small changes in the normal modes frequencies are in good agreement with the results of Morales et al. [ 111. The increase of the compressional optical mode E, corresponds to an increase of the intralayer force constant, K, a consequence of the c-axis modification. On the other hand, we observed a small decrease of the frequency of the shear mode, A 2u, of about 5 cm- ’ . This is attributed to the change in the two-dimensional
Table 1 Frequency LiSnS, Raman
(in cm-‘)
and assignment
of the vibrational
IR
Attribution
146
v>(E) v,(Fz )
210
=% E”
126 209 319
Al, 343
A
420 444
v,k) v,(F,) v#z)
383
modes of
89 (1996)
337-343
character of LiSnS,. By considering the frequency of the A,, mode (Eq. (2a)), if the change in the interlayer force constant, k, is more important than that of K, we can expect a shift of this mode towards lower frequencies. We attempt to explain the experimental results of vibrational spectroscopy in Li,SnS, through the use of lattice dynamical models such as a simple localsymmetry approach and a simplified linear-chain model. From the above results it appears that the lithium intercalated phases, whatever the reaction mechanism, give rise to intense infrared new intercalation modes which are derived in part from the stretching vibrations (v,) in LiS, or LiS, entities. This allows us to distinguish the accommodation of Li+ ions in tetrahedral or octahedral sites. As a matter of fact, LiS, surroundings with Li-S distances ranging from 2.35 to 2.44 A have been evidenced in Li,S [20], Li,FeS, [21], Li,MoS, [22] and Li,NbS, [6] compounds giving rise to v,-infrared modes located at 465, 390-420, 390-450 and 430-460 cm-‘, respectively. These wavenumbers can be well compared to the 11 l-420 cm-’ infrared band in Li,SnS,. All these results suggest that similar Li-S distances and bondings are occurring. On the other hand, the octahedral lithium accommodation was encountered in the wavenumbers range 320-350 cm-’ [21]. These wavenumbers compare favourably with those at 328 cm-’ in Li,SnSe, intercalates [23]. Considering a simple local-symmetry model, the tetrahedral LiS, entities have a Td symmetry. Fig. 3 shows the displacement of atoms in such a symmetry. The four normal modes of vibration of a tetrahedral XY, molecule are all Raman active, whereas only vj and vq (with vq
C. Julien, C. Pe’rez-Vicente I Solid State tonics 89 (1996) 337-343
v1
(Al) R
Fig. 3. Representation
of the vibrational
v2
~4 (F2)
03 R
modes considering
~3 W2) ir,R
ir, R a simple local model in the Td symmetry
for the tetrahedral
attribution of the peaks in the region 400-450 cm-‘. Spectra recorded between 5 and 300 K are displayed in Fig. 4. These results show that the v3(F2) mode is split in two components located at 420 and 444 cm-’ which correspond to a site effect in LiSnS,. The high-frequency peak at 444 cm-’ is well resolved and dominates the IR spectrum at low temperature. A simplified linear-chain model has been applied to SnS, intercalated with lithium as shown by the one-dimensional representation in Fig. 5. Considering one displacement direction, either longitudinal or transverse, in this figure the chain direction corresponds to the direction perpendicular to the layered plane [9]. We assumed that there is a diatomic molecule in the host unit cell which interacts with the nearest-neighbour molecules by a weak interlayer coupling. Moreover, we inserted a tetrahedrally accommodated intercalant between the layers and a repulsive interaction with a force constant, t, exists between two nearest-neighbouring intercalants. The host atoms are positioned equidistantly and the intercalants are located on the tetrahedral holes. Using the dynamical Eq. (l), we obtain the three frequencies given by w,, = 0,
(3a)
WOP= {(K + k)l/_&}“*,
(3b)
win = {(K + k + 2t)lm}“2.
(3c)
Of course, the frequency wac corresponds to that of the /acoustic mode. The frequency of the optical (intermolecular) mode woP shifts to a slightly higher
LiS, entities.
200 K\
100
2@@
300
400
500
6
Frequency (cm-l) Fig. 4. The temperature dependence of the m-IR spectrum of LiSnS,. These results show that the v,(F,) mode is well resolved at low temperature with a site effect for the LiSnS, sample.
C. Julien, C. P&ez-Vicente
342
km
K
km
X3"-1
'h-3
X3ll
! Solid State tonics 89 (1996) 337-343
K
'3n+2 '3n+3
Fig. 5. The linear-chain model of lattice dynamics in the intercalated layered materials where M and m are the masses of the SnS2 molecule and of the lithium atom, respectively.
frequency. Using this simple model of lattice dynamics, the coupling force constants are determined (Table 2) and can be compared with those of the pristine material. The new mode win appears in the intercalated materials. Therefore, we can easily guess that the frequency of the intercalation mode is almost independent of the concentration of intercalant. This result is consistent with the experimental data in many intercalated layered compounds [2-41. Using the frequency 4, =444 cm-’ measured by infrared absorption, one obtains the interlayer force constant K + k=68.2 N/m and the Li-Li repulsive force constant t= -7.5 N/m. Furthermore, it can be remarked that: (a) the frequency of the intercalation mode is always higher than woP even if t is close to zero and (b) if the tetrahedral sites have quite different occupation, the linear chain model does not give good agreement with experimental data. These results are in good agreement with the recent data obtained from the solid state NMR studies of 6Li and ‘Li in Li,SnS, [24]. An analysis of the NMR spectra revealed the existence of at least two different environments of the lithium which is consistent with an occupation of the vacant sites in the SnS, structure, i.e., both tetrahedral and octahedTable 2 The coupling
force constants
(in N/m) in SnS, and LiSnS,
Force
SnS,
LiSnS,
constant
Ilc
Ic
Ilc
Ic
K k
148.7 20.1
52.6 14.2
144.5 24.0
54.1 14.2
ral sites can be occupied simultaneously. It has been also reported that the two tetrahedral sites are not equivalent for intercalants but one of them is preferently occupied. Our spectroscopic results show that bands due to vibration of LiS, entities are observed. By other side, the IR-active modes of LiSnS, at ca. 420 and 444 cm-’ can be ascribed to the two different LiS, sites being dominant. Thus, from the Raman and IR experiments, we can conclude that there are two tetrahedral sites occupied by Li ions in SnS, and that the dominant occupation corresponds to the tetrahedral site with an active mode located at 444 cm-‘.
4. Conclusion We have studied lattice dynamics in lithium intercalated SnS, crystal. The effect of lithium intercalation on the vibrational spectra of these compounds is twofold. First, the normal modes of the host material remain almost unchanged; only a frequency shift (few wavenumbers) of the lattice modes of the SnS, occurs. Secondly, new broad bands are observed which are located in the high frequency region at around 400 cm-‘. Using lattice dynamical models such as a simple local-symmetry approach and a simplified linear-chain model we found that the lithium intercaled atoms have a tetrahedral environment in the van der Waals gap of the layered host structure. We conclude that the two types of tetrahedral sites are occupied with one of them which is a preferential position giving rise to an infrared vibrational mode located at high frequency.
References [I] C. Juhen and GA. Nazri, Solid State Batteries: Materials Design and Optimization (Kluwer, Boston, 1994). [2] M. Barj, C. Sourisseau, G. Ouvrard and R. Brec, Solid State Ionics 11 (1983) 179. [3] T. Sekine, M. Jouanne, C. Julien and M. Balkanski, Mater. Sci. Eng. B 3 (1989) 91. [4] T. Sekine, C. Julien, I. Samaras, M. Jouanne and M. Balkanski, Mater. Sci. Eng. B 3 (1989) 153. [5] T. Sekine, C. Juhen, M. Jouanne, M. Weber, I. Samaras and M. Balkanski, Phys. Status Solidi, B 168 (1991) 103.
C. Mien,
C. Pkrez-Vicente
I Solid State Ionics 89 (1996) 337-343
[6] C. Sourisseau, N. Allah and M. Danot, Eur. J. Solid State Inorg. Chem. 29 (1992) 1 Il. [7] C. Julien, Mat. Res. Sot. Symp. Proc. 293 (1993) 411. [8] C. Julien, I. Samaras, 0. Gorochov and A. Ghorayeb, Phys. Rev. B 45 (1992) 13390. [9] M. Danot, A. LcBlanc and J. Rouxel, Bull. Sot. Chim. Fr. 8 ( 1969) 2670. [IO] A. Lerf and R. Schollhorn, Inorg. Chem. 16 (1977) 2950. [ 1 l] J. Morales, C. Perez-Vicente and J.L. Tirado, Solid State Ionics 51 (1992) 133. [12] T. Sekine and M. Balkanski, Mater. Sci. Eng. B 1 (1989) 155. [ 131 M.S. Whittingham and M.B. Dines, J. Electrochem. Sot. 124 (1977) 1387. [14] C. Julien, in: Microionics, Solid State Integrable Batteries, ed. M. Balkanski (North-Holland, Amsterdam, 1991) p. 197. [ 151 T.J. Wieting, Solid State Commun. 12 (1973) 93 I. [16] G. Lucovsky, J.C. Mikkelsen Jr., W.Y. Liang, R.M. White and R.M. Martin, Phys. Rev. B 14 (1976) 1663.
343
[ 171 H. Kohler and C.R. Becker, Phys. Status Solidi, B 76 (1976) K15. [I81 A.K. Garg, J. Phys. C 19 (1986) 3949. [I91 C. Julien, H.S. Mavi, K.P. Jain, M. Balkanski, C. PerezVicente and J. Morales, Mater. Sci. Eng. B 23 (1994) 98. [20] R.R. Chianelh and M.B. Dines, Inorg. Chem. 14 (1975) 2417. [21] P. Gard, C. Sourisseau, G. Ouvrard and R. Brec, Solid State Ionics 20 (1986) 231. [22] C. Sourisseau, 0. Gorochov and D.M. Schleich, Mater. Sci. Eng. B 3 (1989) 113. [23] C. Perez-Vicente and C. Julien, in: Proc. 2nd Euroconf. on Solid State Ionics, ed. W. Weppner (Inst. for Ionics, Stuttgart, 1995). [24] T. Pietrass, M. Henry, F. Taulelle, P. Lavela, J. OlivierFourcade and J.C. Jumas, in: Ext. Abstr. 5th Eur. Conf. on Solid State Chemistry (Montpellier, 1995) p. 617.
Solid State Ionics 89 (1996) 337-343
ELSEVIER
Vibrational
studies of lithium-intercalated
SnS 2
C. Julien*, C. PCrez-Vicente Laboratoire de Physique des Solides, Universite’ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
Received 20 December
1995; accepted
20 January
1996
Abstract We report on the vibrational properties of lithium-intercalated SnS, single crystal. Raman scattering and infrared absorption spectra have been recorded as a function of temperature in the frequency range 50-600 cm _ ’. The new bands are interpreted such as modes due to vibrations of lithium atoms located in the van der Waals gap against the nearest neighbouring chalcogen atoms. Results are analyzed in terms of stretching vibrations of Li-S, and Li-S, entities. From a comparison of the spectra, it is concluded that lithium ions are more likely to occupy tetrahedral sites in the lithium-rich phase. Using a simple model of lattice dynamics, the coupling force constants are determined and compared with those of the pristine material. Keywords: Lithium intercalation; Tin disulfide; Raman scattering; Infrared spectra
1. Introduction The lattice dynamics and electronic properties of intercalated layered materials are of interest because of their two-dimensional nature and because of their applications such as solid solutions in solid state batteries [ 11. Recently, some infrared (IR) absorption and Raman scattering (RS) experiments have been performed on layered compounds intercalated by alkali metals for studying the intercalation mechanism and evaluating the interaction between host lattice and intercalants [2-71. Barj et al. [2] have studied IR absorption in lithium-intercalated NiPS, and they found new absorption bands attributed to vibrations of lithium atoms located in the octahedral sites in the van der Waals gaps. In order to explain new peaks in the Raman spectrum of MoS, after *Corresponding author. 0167-2738/96/$15.00 01996 Elsevier Science B.V. All rights reserved PII SO167-2738(96)00270-6
intercalation with lithium, the lattice dynamics of Li,MoS, has been calculated by applying a central force model [5]. It was found that they can be attributed to new vibrational modes in which the intercalant atoms strongly vibrate against the host lattice. Julien et al. [8] have demonstrated that from optical and electrical-transport measurements, the intercalation mechanism of Li in Li,TiS, is consistent with the assumption of an increased d-band filling and a high degree of charge transfer. The structural and electronic changes occurring in the lithiated phases Li,NbS, have been reported by Sourisseau et al. [6]. The RS spectra of various Li,NbS, (0 5 x 5 3) intercalated compounds are very sensitive to the strong interactions between lithium to sulphur; these studies indicate that most of the lithium ions are accommodated in various LiS, environments upon the degree of intercalation. SnS, crystal occurs in the CdI,-type of layered
338
C. Julien, C. P&z-Vicente
I Solid State Ionics 89 (1996) 337-343
structure. The suitability of SnS, such as host for lithium intercalation makes it an interesting material for understanding the general problem of the localization and mobility of the alkali ions in the gaps. Some structural and thermodynamical properties of lithiated phases Li,SnS, have been reported [9-l 11. Chemical lithiation of SnS, has been carried out by treating the host material with lithium solutions in liquid ammonia [9] and also in aqueous media [lo] where lithium atoms were solvated by NH, and H,O, respectively. Recently, Morales et al. [ 1 l] have studied the Li intercalation in 2H-SnS, using a chemical process in n-butyl lithium and an electrochemical titration technique. A new phase showing a staging effect has been identified. Thus, the nature of the lithium site occupancy is still controversial in SnX, compounds, vibrational spectroscopy studies can give some information about the position of the intercalant species in the host structure [12]. In this report, we present Raman scattering and infrared absorption spectra of lithium-intercalated SnS, crystal in order to better understand the nature of the intercalation process in this system. We attempt to explain the experimental results of vibrational spectroscopy through the use of lattice dynamical models such as a simplified linear-chain model and a simple local-symmetry approach. By using vibrational spectroscopy, the question of the insertion sites of the lithium in SnS, is addressed.
2. Experimental Samples of SnS, were grown by direct synthesis from their constituent elements. The stoichiometric mixture of the elements was sealed in an evacuated fused quartz ampoule. The temperature was slowly increased to reach 500°C in 2 h. The ampoule was kept for one day at this temperature and then cooled down to room temperature in gradual steps. After grounding and washing with CS,, the same heating procedure was used to obtain the final products. Lithium intercalation in host material was accomplished at room temperature by immersion of the sample in n-butyl lithium in hexane solution as reported by Whittingham and Dines [ 131. The sample preparation was carried out in a purified argon atmosphere containing less than 1 ppm of water.
After lithiation, the samples were washed in hexane and stored in a closed tube in the glove-box over 7-day period. The lithiated samples were inserted into a vacuum cryostat model Oxford 1104, which is temperature variable, inside the glove-box, so that the intercalated crystals never came into contact with air. Absorption spectra were recorded in a Bruker IFS 113 vacuum Fourier transform infrared (IT-IR) interferometer [14]. In the far-infrared region (50600 cm-‘), this apparatus is equipped with a 3.5 pm-thick beam splitter, a globar source and a liquid helium cooled bolometer. Each spectrum was the average of 64 scans and was taken with a spectral resolution of 2 cm-‘. Powdered samples were dispersed in solid paraffin which does not exhibit any infrared absorption peaks in the studied wavelength range (13-200 ,um). We have always checked that the samples were water or oxygen free by the absence of infrared bands in the 3400, 1600 cm-’ or 1100-800 cm-’ regions.
3. Results and discussion Fig. 1 shows the Raman scattering spectra of pure (curve a) and Li-intercalated SnS, (curve b) recorded at room temperature. The IT-IR spectra of SnS, and its lithiated phase recorded at room temperature in the range 50-600 cm ’ are shown in Fig. 2. In order to investigate the effects of alkali metal intercalation in SnS, samples, first we briefly analyse the lattice dynamics of pure material. This compound is a layer-type semiconductor having the CdI, crystal structure whose symmetry corresponds to the D3,, space group. The individual tightly bonded layer in SnS, is an S-Sn-S sandwich in which the Sn atoms are octahedrally coordinated to six nearest-neighbor sulphur atoms. The three-atom basis of the elementary unit cell gives rise to nine vibrational modes; three doubly degenerate E modes in which the atomic motions are parallel to the layer planes and three nondegenerate A modes in which the atomic motions are perpendicular to the layers. Four optical modes are active at the center of the Brillouin zone. There are two Raman-active modes, A Ig + E, and two IR-active modes, A,, + E,. These active modes are clearly observed in the spectra (Figs. 1 and 2).
C. Julien, C. Pbez-Vicente
I Solid State Ionics 89 (1996) 337-343
It has been demonstrated that the long-wavelength lattice vibrations of a layered compound can be described by a linear-chain model that includes both intralayer (K) and interlayer (k) force constants [15]. The crystal symmetries of SnS, imply that the vector displacements of the atomic planes in all the normal modes are either parallel or perpendicular to the c axis. The dynamical equations for the normal vibrations of the atomic planes can be written as
Li,SnSZ
- Mnm2u, =c SnS2
Frequency
km-’
339
m
K,,(u,
- u”),
(1)
where M, is the mass of the atom lying in the nth plane of the primitive cell, w2 is the angular frequency, u, and u, are the normal displacements of the planes n and m, and K,, is the shear or compressional force constant between the planes n and m. Considerations of crystal symmetry lead the following relations giving the modes frequencies
1
Fig. 1. Raman scattering spectra recorded at room temperature SnS, crystal (curve a) and LiSnS, (curve b) sample.
w(Azu, E,) = Wt.d’2,
(24
w(A ,g, E,) = [(K + 2k)lM,]“2,
(2b)
of
Here ,U is the reduced mass ,u =M,M,l(M, + 2M,) where M, and M, are the tin and sulphur masses. The frequencies of the 2H-SnS, Raman and infrared modes determined experimentally are the following
loo
200
300 FlI?qwq
COO km-‘)
!iOO
Fig. 2. FT-IR spectra recorded at room temperature crystal (curve a) and LiSnS, (curve b) sample.
6 of SnS,
w(A,,)
= 348 cm-‘,
w(E,) = 207 cm-‘,
w(A,,)
= 316 cm-‘,
w(E,) = 20.5 cm-‘.
These mode frequencies are in good agreement with those reported in the literature [ 16-191. It is worth noting that (a) the frequencies of the A,, and E, modes are not very far from each other, (b) the E, mode gives a very broad band with full-width at half maximum of the order of 140 cm-‘, (c) the strength of the A,, mode is roughly 20 times less than that of the E, mode which is a consequence of the preferential orientation of the sample along the layer plane. The effect of lithium intercalation on the vibrational spectra of SnS, is twofold: (1) the normal modes of the host material remain almost unchanged; only a frequency shift (few wavenumbers) of the lattice modes of SnS, occurs; (2) new broad bands are observed which are located in the high frequency
C. Mien,
340
C. Pbez-Vicente
I Solid State Ionics
region at 126 and 383 cm-’ in Raman and at 146, 420 and 444 cm-’ in IR at room temperature. The corresponding band wavenumbers and assignments are reported in Table 1. The general trend in the spectra of lithiated phase (Figs. 1 and 2) is a small shift of the SnS,-like modes. We observed an increase in the frequency of the E,, A ,g and E, modes, whereas the frequency of The the A,, mode decreases upon intercalation. frequency shift of the E, mode indicates an increase in the restoring force for this type of vibration which is probably due to a decrease in the intralayer distance, a consequence of the tin reduction which creates a negative charge in the intralayer spacing upon lithium intercalation. Thus the net effect of the Li intercalation corresponds to a change of the polarizability in the intralayer bonding which produces an increase of the normal mode frequencies. For the lithiated products, the phase that is isostructural with SnS, shows minor changes in the unit cell parameters with the lithium content [ 111. A decrease of the basal spacing has been observed, from c =5.884 A in SnS, to c=5.864 A in Li,.,,SnS,, which is attributed to the strong attractions due to the presence of lithium cations between negatively charged layers. The small changes in the normal modes frequencies are in good agreement with the results of Morales et al. [ 111. The increase of the compressional optical mode E, corresponds to an increase of the intralayer force constant, K, a consequence of the c-axis modification. On the other hand, we observed a small decrease of the frequency of the shear mode, A 2u, of about 5 cm- ’ . This is attributed to the change in the two-dimensional
Table 1 Frequency LiSnS, Raman
(in cm-‘)
and assignment
of the vibrational
IR
Attribution
146
v>(E) v,(Fz )
210
=% E”
126 209 319
Al, 343
A
420 444
v,k) v,(F,) v#z)
383
modes of
89 (1996)
337-343
character of LiSnS,. By considering the frequency of the A,, mode (Eq. (2a)), if the change in the interlayer force constant, k, is more important than that of K, we can expect a shift of this mode towards lower frequencies. We attempt to explain the experimental results of vibrational spectroscopy in Li,SnS, through the use of lattice dynamical models such as a simple localsymmetry approach and a simplified linear-chain model. From the above results it appears that the lithium intercalated phases, whatever the reaction mechanism, give rise to intense infrared new intercalation modes which are derived in part from the stretching vibrations (v,) in LiS, or LiS, entities. This allows us to distinguish the accommodation of Li+ ions in tetrahedral or octahedral sites. As a matter of fact, LiS, surroundings with Li-S distances ranging from 2.35 to 2.44 A have been evidenced in Li,S [20], Li,FeS, [21], Li,MoS, [22] and Li,NbS, [6] compounds giving rise to v,-infrared modes located at 465, 390-420, 390-450 and 430-460 cm-‘, respectively. These wavenumbers can be well compared to the 11 l-420 cm-’ infrared band in Li,SnS,. All these results suggest that similar Li-S distances and bondings are occurring. On the other hand, the octahedral lithium accommodation was encountered in the wavenumbers range 320-350 cm-’ [21]. These wavenumbers compare favourably with those at 328 cm-’ in Li,SnSe, intercalates [23]. Considering a simple local-symmetry model, the tetrahedral LiS, entities have a Td symmetry. Fig. 3 shows the displacement of atoms in such a symmetry. The four normal modes of vibration of a tetrahedral XY, molecule are all Raman active, whereas only vj and vq (with vq
C. Julien, C. Pe’rez-Vicente I Solid State tonics 89 (1996) 337-343
v1
(Al) R
Fig. 3. Representation
of the vibrational
v2
~4 (F2)
03 R
modes considering
~3 W2) ir,R
ir, R a simple local model in the Td symmetry
for the tetrahedral
attribution of the peaks in the region 400-450 cm-‘. Spectra recorded between 5 and 300 K are displayed in Fig. 4. These results show that the v3(F2) mode is split in two components located at 420 and 444 cm-’ which correspond to a site effect in LiSnS,. The high-frequency peak at 444 cm-’ is well resolved and dominates the IR spectrum at low temperature. A simplified linear-chain model has been applied to SnS, intercalated with lithium as shown by the one-dimensional representation in Fig. 5. Considering one displacement direction, either longitudinal or transverse, in this figure the chain direction corresponds to the direction perpendicular to the layered plane [9]. We assumed that there is a diatomic molecule in the host unit cell which interacts with the nearest-neighbour molecules by a weak interlayer coupling. Moreover, we inserted a tetrahedrally accommodated intercalant between the layers and a repulsive interaction with a force constant, t, exists between two nearest-neighbouring intercalants. The host atoms are positioned equidistantly and the intercalants are located on the tetrahedral holes. Using the dynamical Eq. (l), we obtain the three frequencies given by w,, = 0,
(3a)
WOP= {(K + k)l/_&}“*,
(3b)
win = {(K + k + 2t)lm}“2.
(3c)
Of course, the frequency wac corresponds to that of the /acoustic mode. The frequency of the optical (intermolecular) mode woP shifts to a slightly higher
LiS, entities.
200 K\
100
2@@
300
400
500
6
Frequency (cm-l) Fig. 4. The temperature dependence of the m-IR spectrum of LiSnS,. These results show that the v,(F,) mode is well resolved at low temperature with a site effect for the LiSnS, sample.
C. Julien, C. P&ez-Vicente
342
km
K
km
X3"-1
'h-3
X3ll
! Solid State tonics 89 (1996) 337-343
K
'3n+2 '3n+3
Fig. 5. The linear-chain model of lattice dynamics in the intercalated layered materials where M and m are the masses of the SnS2 molecule and of the lithium atom, respectively.
frequency. Using this simple model of lattice dynamics, the coupling force constants are determined (Table 2) and can be compared with those of the pristine material. The new mode win appears in the intercalated materials. Therefore, we can easily guess that the frequency of the intercalation mode is almost independent of the concentration of intercalant. This result is consistent with the experimental data in many intercalated layered compounds [2-41. Using the frequency 4, =444 cm-’ measured by infrared absorption, one obtains the interlayer force constant K + k=68.2 N/m and the Li-Li repulsive force constant t= -7.5 N/m. Furthermore, it can be remarked that: (a) the frequency of the intercalation mode is always higher than woP even if t is close to zero and (b) if the tetrahedral sites have quite different occupation, the linear chain model does not give good agreement with experimental data. These results are in good agreement with the recent data obtained from the solid state NMR studies of 6Li and ‘Li in Li,SnS, [24]. An analysis of the NMR spectra revealed the existence of at least two different environments of the lithium which is consistent with an occupation of the vacant sites in the SnS, structure, i.e., both tetrahedral and octahedTable 2 The coupling
force constants
(in N/m) in SnS, and LiSnS,
Force
SnS,
LiSnS,
constant
Ilc
Ic
Ilc
Ic
K k
148.7 20.1
52.6 14.2
144.5 24.0
54.1 14.2
ral sites can be occupied simultaneously. It has been also reported that the two tetrahedral sites are not equivalent for intercalants but one of them is preferently occupied. Our spectroscopic results show that bands due to vibration of LiS, entities are observed. By other side, the IR-active modes of LiSnS, at ca. 420 and 444 cm-’ can be ascribed to the two different LiS, sites being dominant. Thus, from the Raman and IR experiments, we can conclude that there are two tetrahedral sites occupied by Li ions in SnS, and that the dominant occupation corresponds to the tetrahedral site with an active mode located at 444 cm-‘.
4. Conclusion We have studied lattice dynamics in lithium intercalated SnS, crystal. The effect of lithium intercalation on the vibrational spectra of these compounds is twofold. First, the normal modes of the host material remain almost unchanged; only a frequency shift (few wavenumbers) of the lattice modes of the SnS, occurs. Secondly, new broad bands are observed which are located in the high frequency region at around 400 cm-‘. Using lattice dynamical models such as a simple local-symmetry approach and a simplified linear-chain model we found that the lithium intercaled atoms have a tetrahedral environment in the van der Waals gap of the layered host structure. We conclude that the two types of tetrahedral sites are occupied with one of them which is a preferential position giving rise to an infrared vibrational mode located at high frequency.
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