- Email: [email protected]
High pressure phase transition studies of CsSnCl3
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 326 (1994) 73-80
High pressure phase transition studies of CsSnC13 Z.X. Shen*, W.L. Loo, M.H. Kuok, S.H. Tang Department
of Physics, National University of Singapore, Lower Kent Ridge Road, 0511 Singapore
Received 7 March 1994
Abstract The phase transitions of CsSnCls have been investigated by Raman scattering and optical observations under high pressure up to 60kbar in a diamond anvil cell. Two new high pressure phases have been discovered. The first phase
transition II/III is tirst order and occurs at 3.3 kbar. It is marked by the softening of the v1 mode of the S&l; ion and dramatic spectral changes for all the Raman bands. The second transition III/IV is also first order and occurs at 25.3 kbar. This phase transition is manifested by the disappearance of all the Raman peaks in phase IV. In this phase, the molecules are more closely packed than in the monoclinic phase II. The q mode which is at 296cm-’ for the free SnCl; ion appears as a strong Raman peak at 167 cm-’ for CsSnCls in phase II.
1. Introduction
Two solid phases (I and II) have been reported for CsSnCls in the literature and both of them can be obtained by varying temperature alone. Under ambient conditions, CsSnCls (phase II) is monoclinic, space group P2i/n, Cih, with a = 16.1OA, b = 7.425& c = 5.748A and p = 93.2”. There are four CsSnC13 molecules per unit cell (Z = 4) [l-3]. The structure may be described as being constructed from Cs+ ions and tetrahedral SnCli ions. Both the Csf and SnCl; ions occupy sites of Ci symmetry. It transforms to the high temperature phase I at about 385 K, which has a cubic structure of space group Prn3m, Ok, Z = 1, and a = 5.56 A [l, 3, 4, 51. Single crystals of CsSnCls are colourless in phase II and light yellow in phase I. The II/I phase transition is found to be first order and irreversible [l], but traces of water make the reverse transformation I to II possible [2].
Spectrally, the II/I phase transition is marked by the complete disappearance of all the Raman active bands in the cubic phase I [4]. Raman scattering studies carried out by our group [4, 6, 71 show that phase II is stable in the low temperature region at least down to 10K. There is some confusion about the assignment of the u1 symmetric stretching mode for the SnClT ion. The frequency of this vibration in diethyl ether extracts is found at 297cm-’ and it is by far the strongest Raman active band with As symmetry just as expected [8, 91. In CsSnCls, the frequencies of all the other internal modes (y, y and v.J of the SnCl; ion do not differ much from those in extracts, the strongest Raman band which also has As symmetry is however at 169 cm-‘. No high pressure studies have been reported in the literature.
2. Experimental
* Corresponding author. 0022-2860/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-2860(94)08341-E
CsSnC13 crystals
are prepared
by dissolving
14
Z.X. Shen et al./J. Mol. Struct. 326 (1994) 73-80
Table 1 The four vibrational modes of the S&l;
free ion
Mode
Frequency (cm-‘)
Optical activity
297 (single) 128 (single) 256 (double) 103 (double)
IR IR IR IR
and and and and
Raman Raman Raman Raman
Raman intensity active active active active
stoichiometric amounts of CsCl and SnC12 in a hot solution of glycerol. Crystals obtained by slow cooling of the solution are colourless and needle shaped. The needle direction coincides with the c-axis of the crystals [l]. Raman spectra were recorded using a Spex 1403 double monochromator in conjunction with a photon-counting system. A Spectra-Physics argon ion laser operating at 514.5 or 488 nm was used as the excitation source. Its power was kept below 0.1 W since CsSnCls showed signs of decomposition at about 0.3 W of laser power in the atmosphere [lo]. Backscattering geometry was used. The spectral slit width was set at about 2cm-’ and sharp peaks were determined to within f0.5 cm-‘. High pressure was generated using a diamond anvil cell (DAC). A total of four loadings were made with either stainless steel (s/s) or inconel gaskets. In each loading, the gasket was placed on the lower diamond of the DAC and the centre of the gasket hole coincided with that of the diamond. A few pieces of single crystals together with one or two ruby chips were then loaded into the gasket hole. A 4:l methanol-ethanol mixture was used as a pressure transmitting medium. The sample occupied about one-third of the total volume. The size of the hole was 0.3mm in diameter and 0.2mm in thickness in the case of s/s gaskets and 0.2 and 0.28mm respectively for inconel gaskets. All the loadings were done under a polarized microscope which was also used to monitor colour changes of the samples. The pressure was calibrated using the ruby R, fluorescence line.
Strong, sharp Weak, sharp Medium, broad Medium, broad
CsVsymmetry with the three CI atoms forming an equilateral triangle and the Sn atoms lying on the Cs axis. Its six degrees of internal vibrational freedom form four vibrational modes. Table 1 lists these modes together with their optical activities and vibrational frequencies found in diethyl ether extracts [8, 91. 3.2. Factor group analysis of the CsSnC13 (II) crystal
From group theory, 60 modes are expected, of which 24 are internal modes of the SnCl; ions and 33 are optical external modes with the other three being acoustic modes. Factor group analysis gives the following results. The 24 internal modes: v~,vz:A~+A,+B~+B, Y,
v4
:2(A,+A,+B,+B,)
The three acoustic modes: A, + 2B, The 33 optical external modes: I’(translationa1) = 6As + 5A, + 6Bs + 4Bu I’(librationa1) = 3As + 3A, + 3Bs + 3B, where the gerade modes are Raman active and the ungerade modes are IR active. Hence we expect 12 Raman active internal bands (6As + 6B,): two bands (As + Bs) for each of the vl and y modes and four (2As + 2Bs) for each of the v3 and u4 modes. There are 18 Raman active external modes (9As + 9Bs).
3. Results and discussion
3.3. Raman spectra at ambient conditions
3.1. The SnCl; free ion
The Raman spectra of the CsSnCls crystal are shown in Fig. 1. These spectra were obtained by holding the crystal at different orientations. In the
The SnCl; free ion has a pyramidal structure of
Z.X. Shen et al./J. Mol. Struct. 326 (1994) 73-80
0
100
200
300
400
Wavenumber Fig. 1. Raman spectra of the C&Cl3
(II) single crystal at different orientations at 0.0 kbar.
first spectrum, the crystal was held vertically, while in the second it was held horizontally. In both orientations, the laser beam was incident normal to the crystal axis. As for the third spectrum, the laser was directed along the crystal axis. The three spectra obtained are significantly different due to their different orientations. The plasma lines at 66, 77 and 116 cm-’ are marked by “P”. The v1 (As) mode, which is the totally symmetric stretching of all SnCl; ions in the unit cell, is expected to give rise to the strongest Raman band. Thus the 165cn-’ band is assigned to this mode and its shoulder at 145cm-’ to the other vI mode of B, symmetry. The polarized Raman study by Kuok [4] confirms the symmetries of these bands. Woodward and Taylor [8, 91 have reported the frequency of the v1 mode for SnCl; free ion at 297 cm-‘, so it is rather surprising that the frequency has changed so much in the crystal. It has been reported that subjecting the free ion to different liquid surroundings would result in totally different Raman spectra [8]. Aqueous solution of stannous chloride in hydrochloric acid gives a Raman spectrum consisting of very broad and diffuse bands, but ether extracts show distinct line
spectra attributable with confidence to the single complex SnCl; ion. We have also recorded Raman spectra for aqueous solution of CsSnCls in the 15-35Ocm-’ region. No welldefined Raman peaks could be observed in agreement with Ref. [8]. This demonstrates that the interaction between the SnCli ion and its environment greatly affects its optical activities. In CsSnCls crystal, this interaction is stronger and more directional. The internal modes of SnCl; are therefore expected to differ even more from those of the free ion. The symmetric stretching vl mode is the most sensitive among the internal modes to interaction with neighbouring molecules, so it is reasonable that this mode shows the greatest shifts in the crystal. But it is very unusual for a vi mode to reduce its frequency by as much as 44%. Four Raman active bands are predicted for the u3 mode in phase II and they are found at 227,249, 263 and 278cm-’ respectively. The splitting of 51 cm-’ for this mode is much bigger than that of any other internal modes. This is expected as this asymmetric stretching mode has the largest dipole moment which in turn gives rise to the greatest interaction with the crystal field.
16
Z.X. Shen et al&L Mol. Struct. 326 (1994) 73-80
Table 2 Raman bands and their assignments in phases II and III Phase
II
P (kbar) Lattice (cm-‘)
0.0 ~ 23.8 ~ 33.3 ~ 37.7 ~ 51.7 ~ 15.5 ~ 88.4 ~ 117
2.6 24.9 35.0 II 40.6 II 54.1 II 81.0 II 96.1 II
145 __ 165 ~
134.0 II 159.1 II 182.5 III 209.5 III 225.9 II 234.1 III 248.2 II 265.3 II 272.7 III
v4
VI
Y
227 -
VI Y
11/111a
II/III”
249 263 ~
6.8 28.2 ~ 36.0 43.6 57.5 ~
~ ~ -
III
III 11.3 29.4 -
III
19.5 31.2 -
24.0 32.7 -
III 25.5 32.9 -
IIIb 28.5 35.5
59.2
101.3 128.7 -
138
~
143.4 ~
138.0 ___
144.3
193.0 220.0 -
194.8 ~ 220.4 ~
200.0 220.0 /
211.8 ~
217.7
235.0 -
235.8 -
237.0 -
237.0 -
237.2
~ ~ ~ -
152.6 185.4 ~ 214.2 223.0 235.5 -
-
264.3 274.9 -
276.0 -
279.5 -
283.0 -
281.0 -
283.5
289.8 III ~
290.0 ~
291.0 ~
294.4 ~
296.9 ~
297.0 ~
298.6
278
' Pressure region in which phases II and III co-exist. b Remnant of phase III in the phase IV stable region.
9.7 kbar 4.4 kbar
2.6 kbar
0.0 kbar
0
100
200
300
400
Wavenumber Fig. 2. High pressure Raman spectra showing the II/III phase transition between 2.6 and 4.4kbar
Z.X. Shen et al./J. Mol. Struct. 326 (1994) 73-80
II
0.0 kbar after 18.0 kbar
200 Wavenumber Fig. 3. High pressure Raman spectra showing the III/IV phase transition at about 25.3 kbar. The spectrum at 0.0 kbar after pressing to 18.0 kbar (phase II) resembles that of phase II.
The broad band at 117 cm-i is assigned to the v4 mode. The optical activities of the v4 mode are the same as those of y, so that four Raman bands are expected. The crystal field splitting of the v4 mode should be smaller than that of y since the corresponding dipole moment is smaller. Hence the frequencies of the u4 bands spread over a smaller region and band overlapping is expected to be more severe. As a result, only an overall contour is observed. Since the Raman bands of the y mode are weaker than that of the v4 mode, no peaks are detected. All the remaining Raman bands are found below 100 cm-’ and are attributed to lattice vibrations. Eighteen Raman active lattice modes are predicted by group theory analysis. A total of seven bands were observed in the frequency range 20-lOOcm-’ (Table 2). In the same frequency region, 15 bands (7As + 8Bs) are observed by polarized Raman work [4] at 10 K. Below 2Ocm-‘, Lee [7] has observed two more bands at 12.0cm-’ and 18.7cm-‘, respectively, at 21°C so that at least 17 of the 18 expected bands can be observed. At room temperature,
some of the bands are very strong and fairly broad thus the other predicted bands may overlap with them and become unresolvable. Detailed polarization work should detect more bands. Three plasma lines are observed at 66, 77 and 116cm-’ with the band at 77cm-’ being the strongest. The plasma line at 66cm-’ overlaps with a Raman peak and thus appears broader and stronger than that at 77 cm-’ (see spectra 1 and 3 in Fig. 1). 3.4. High pressure Raman spectra Figs. 2 and 3 show the representative Raman spectra of C&Cl3 under high pressure. The spectra have a higher background scattering than those in Fig. 1 due to the smaller sample volume used in the DAC. The two spectra at 0.0 and 2.6 kbar look similar. The spectrum changes dramatically at 4.4 kbar. The strongest v1 peak at 165cm-’ disappears completely at 4.4 kbar and a new strong band appears at 220 cm-‘. The spectral changes agree with our optical observations using a microscope which shows a first order phase transition
78
Z.X. Shen et al./J. Mol. Siruct. 326 (1994) 73-80
24.0 kbar
6.8 kbar
2.6 kbar
0.0 kbar
I
0
100
200
300
4:o
Wavenumber Fig. 4. High pressure Raman spectra with fast pressure increase. The spectrum at 0.0 kbar shows bands of pure phase II and that at 24.0 kbar shows bands of pure phase III. The spectra at 2.6 and 6.8 kbar show mixtures of both phases.
from transparent single crystals (phase II) to a polycrystalline form, which makes the samples look opaque (phase III) over the same pressure range. The intensities of all the bands drop signilicantly in phase III. This may also be linked to the fact that the samples are polycrystalline in phase III whereas they are single crystals in phase II before the transition. We take 3.3 kbar, which is the average of 2.6 and 4.4 kbar, as the II/III transition pressure. In the lattice region below lOOcm_‘, only a sharp band at 30.4cm-’ and a very broad band centred at 61 cm-’ are clearly seen at 9.7 kbar (Fig. 2). The intensities of both bands decrease with pressure. The broad band disappears at about 20.8 kbar (Fig. 3). The phase III/IV phase transition occurs at 25.3 kbar. It is marked by the disappearance of all the Raman peaks in phase IV (see the spectrum at 30.2 kbar in Fig. 3). The spectrum recorded at ambient pressure after the sample has been pressed to 18.0 kbar (phase III) (Fig. 3) clearly resembles that of phase III rather than phase II. Another ambient pressure spectrum (not shown) recorded after the pressure had been released from the
phase IV region does not show any peaks which is a characteristic feature of phase IV. We therefore conclude that both phase transitions II/III and III/IV are irreversible. Spectra shown in Figs. 2 and 3 are obtained with pressure held constant for periods of about 70 h near phase transition and about 6 h at other pressures before each pressure increment. Fig. 4 shows the Raman bands mainly near the II/III transition. These spectra are obtained by a quick pressure increment of about 2 h interval, so that the sample is in a less equilibrium state and a large mixing of the bands of phases II/III is anticipated. It clearly show that the spectra at 2.6 and 6.8 kbar contain bands of both phases, while the spectrum at 0.0 kbar is that of pure phase II and the spectrum at 24.0 kbar is that of pure phase III. We do not take 2.6 kbar as the II/III transition pressure since the sample is in a non-equilibrium state. The vl mode frequency decreases with pressure in phase II which is characteristic of a soft mode, so it is not surprising that it disappears in phase III. Since the II/III transition is sluggish, bands from phase II are expected to exist in the phase III region
19
2X. Shen et al.lJ. Mol. Strut. 326 (1994) 73-80
240
210 280
v1’::/ Y,
120
-
+--II 240
II
t
VI II
220
I
1
I
I
0
5
10
15
Pressure
I
/
20
25
@-+--El
n
I
0
30
I
I
I
I
I
I
I
0
5
10
15
20
25
30
kbar Pressure
/
kbar
Fig. 5. Pressure dependence of the vl and u4 modes.
Fig. 6. Pressure dependence. of the y modes.
near the transition pressure. We therefore take the spectrum near, but below, the III/IV transition at 24.0 kbar, as representative of phase III bands. At this pressure, we assign the strongest band which consists of three components centred at 200, 220 and 237 cm-’ to the vi modes and the two bands at 283 and 297cm-’ to the v3 modes. The broad band centred at 143cm-’ is assigned to the v4 mode. The Raman frequencies of all the bands are presented in Table 2 for some representative pressures, together with their assignments in their respective phases. More bands are observed in the 2.6-6.8 kbar range owing to the co-existence of phases II and III. All the bands in this region can be correlated to either phase II or III. At 11.3 kbar (spectrum not shown), all the bands except one are those of phase III. The ~4 (phase III) band is not observable until 11.3 kbar. This is because being weak and broad, it is overshadowed by the strong vl (phase II) band in the same region. The low frequency v1 band has bigger pressure dependence so that it overlaps with another vi band at about 25.5 kbar and it becomes very difficult to determine the frequencies of the two bands. The average frequency of the two is taken instead. This accounts for the decrease of the 220cm-i band at 24.0 kbar to 211.8 cn-’ at 25.5 kbar (Table 2 and Fig. 5). The presence of phase III
bands in the phase IV stable region at 28.5 kbar is also due to the sluggishness of the III/IV transition. The frequency shifts of the Raman bands are plotted in Figs. 5-7. The vI bands of phase II show clear softening. Two of the v3 bands (phase II) also show a slight frequency decrease with pressure. All the internal modes show clear discontinuities between phases II and III indicating that the transition is first order. They move to higher frequencies in phase III implying a shortening of the bonds for the SnClF ion and this in turn suggests that phase III is a more close packed structure. In general, molecules are more closely packed under pressure. The fact that there are fewer bands in phase III than in phase II implies a structure with a symmetry higher than monoclinic for phase III. The softening of the internal modes vl and y at the II/III transition indicates major changes of the Sn-Cl bond lengths. In phase II, the average distance between the Sn atom and its three nearest Cl atoms is 2.52A, and that between Sn and it next nearest three Cl atoms is 3.46A [I]. The coordination of Sn may have changed so that it has more than three nearest Cl atoms in phase III. This will necessarily involve significant changes in the internal vibrational modes. This may also account for the three v1 Raman bands observed in phase III. No Raman bands are observed in
2.X. Shen et al./J. Mol. Struct. 326 (1994) 73-80
80
obtained by varying the temperature at ambient pressure suggests that the II/III phase transition line is ahnost parallel to the temperature axis in the P-T phase diagram, which in turn indicates a large volume drop for this transition [l 11. Phase IV has a structure of even higher symmetry. Both phase transitions are irreversible.
Acknowledgement
I
I
I
I
I
I
0
5
10
15
20
25
Pressure
/
1 30
This work is supported by the National University of Singapore by a research grant no. RP910682. We thank Drs. I-I. Gong and T.J. Lu for helpful discussion.
kbor
Fig. 7. Pressure dependence of the lattice modes.
phase IV. As Raman active modes are also absent for CsSnQ in phase I, it is possible that phase IV also possess cubic symmetry.
4. Conclusion Two new phases are found for pressure up to 60 kbar at room temperature. Phase III is stable between 3.3 and 25.3 kbar and has a structure with s~etry higher than monoclinic (phase II). The Sn atom might have more than three nearest Cl atoms so that the SnCl; ion loses its identity. The Sn-Cl bond lengths are shorter in phase III than in phase II. This together with the possible increase in coordination number mentioned above makes the molecules much more closely packed in phase III and a large volume drop at the II/III phase transition. The fact that phase III cannot be
References [l] F.R. Poulsen and SE. Rasmussen, Acta Chem. &and., 24 (1970) 150. [2] D.E. Scaife, P.F. Weller and W.G. Fisher, J. Solid State Chem., 9 (1974) 308. [3] S. Sharma, N. Weiden and A. Weiss, Z. Nat~o~h., Teil A, 46 (1991) 329. [4] M.H. Kuok, J. Raman Spectrosc., 23 (1992) 225. [5] J. Barrett, S.R.A. Bird, J.D. Donaldson and J. Silver, J. Chem. Sot. A, (1971) 3105. [6] M.H. Kuok, Phys. Status Solidi B, 141 (1987) k167. [7] PC Lee, Hons. Report, Department of Phys., National ~~v~ity of Singapore, 1988/1989. A. Tan, Hons. Report, Department of Phys., National University of Singapore, 1991/1992. [8] L.A. Woodward and M.J. Taylor, J. Chem. Sot., (1962) 407. [9] M.J. Taylor, J. Raman Spectrosc., 20, (1989) 663. [lo] Z.X. Shen, unpublished work, 1992. [1 l] W.F. Sherman and G.R. Wilkinson, in R.J. Clark and R.E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, vol. 6, Heyden, London 1980, chapter 4.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 326 (1994) 73-80
High pressure phase transition studies of CsSnC13 Z.X. Shen*, W.L. Loo, M.H. Kuok, S.H. Tang Department
of Physics, National University of Singapore, Lower Kent Ridge Road, 0511 Singapore
Received 7 March 1994
Abstract The phase transitions of CsSnCls have been investigated by Raman scattering and optical observations under high pressure up to 60kbar in a diamond anvil cell. Two new high pressure phases have been discovered. The first phase
transition II/III is tirst order and occurs at 3.3 kbar. It is marked by the softening of the v1 mode of the S&l; ion and dramatic spectral changes for all the Raman bands. The second transition III/IV is also first order and occurs at 25.3 kbar. This phase transition is manifested by the disappearance of all the Raman peaks in phase IV. In this phase, the molecules are more closely packed than in the monoclinic phase II. The q mode which is at 296cm-’ for the free SnCl; ion appears as a strong Raman peak at 167 cm-’ for CsSnCls in phase II.
1. Introduction
Two solid phases (I and II) have been reported for CsSnCls in the literature and both of them can be obtained by varying temperature alone. Under ambient conditions, CsSnCls (phase II) is monoclinic, space group P2i/n, Cih, with a = 16.1OA, b = 7.425& c = 5.748A and p = 93.2”. There are four CsSnC13 molecules per unit cell (Z = 4) [l-3]. The structure may be described as being constructed from Cs+ ions and tetrahedral SnCli ions. Both the Csf and SnCl; ions occupy sites of Ci symmetry. It transforms to the high temperature phase I at about 385 K, which has a cubic structure of space group Prn3m, Ok, Z = 1, and a = 5.56 A [l, 3, 4, 51. Single crystals of CsSnCls are colourless in phase II and light yellow in phase I. The II/I phase transition is found to be first order and irreversible [l], but traces of water make the reverse transformation I to II possible [2].
Spectrally, the II/I phase transition is marked by the complete disappearance of all the Raman active bands in the cubic phase I [4]. Raman scattering studies carried out by our group [4, 6, 71 show that phase II is stable in the low temperature region at least down to 10K. There is some confusion about the assignment of the u1 symmetric stretching mode for the SnClT ion. The frequency of this vibration in diethyl ether extracts is found at 297cm-’ and it is by far the strongest Raman active band with As symmetry just as expected [8, 91. In CsSnCls, the frequencies of all the other internal modes (y, y and v.J of the SnCl; ion do not differ much from those in extracts, the strongest Raman band which also has As symmetry is however at 169 cm-‘. No high pressure studies have been reported in the literature.
2. Experimental
* Corresponding author. 0022-2860/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-2860(94)08341-E
CsSnC13 crystals
are prepared
by dissolving
14
Z.X. Shen et al./J. Mol. Struct. 326 (1994) 73-80
Table 1 The four vibrational modes of the S&l;
free ion
Mode
Frequency (cm-‘)
Optical activity
297 (single) 128 (single) 256 (double) 103 (double)
IR IR IR IR
and and and and
Raman Raman Raman Raman
Raman intensity active active active active
stoichiometric amounts of CsCl and SnC12 in a hot solution of glycerol. Crystals obtained by slow cooling of the solution are colourless and needle shaped. The needle direction coincides with the c-axis of the crystals [l]. Raman spectra were recorded using a Spex 1403 double monochromator in conjunction with a photon-counting system. A Spectra-Physics argon ion laser operating at 514.5 or 488 nm was used as the excitation source. Its power was kept below 0.1 W since CsSnCls showed signs of decomposition at about 0.3 W of laser power in the atmosphere [lo]. Backscattering geometry was used. The spectral slit width was set at about 2cm-’ and sharp peaks were determined to within f0.5 cm-‘. High pressure was generated using a diamond anvil cell (DAC). A total of four loadings were made with either stainless steel (s/s) or inconel gaskets. In each loading, the gasket was placed on the lower diamond of the DAC and the centre of the gasket hole coincided with that of the diamond. A few pieces of single crystals together with one or two ruby chips were then loaded into the gasket hole. A 4:l methanol-ethanol mixture was used as a pressure transmitting medium. The sample occupied about one-third of the total volume. The size of the hole was 0.3mm in diameter and 0.2mm in thickness in the case of s/s gaskets and 0.2 and 0.28mm respectively for inconel gaskets. All the loadings were done under a polarized microscope which was also used to monitor colour changes of the samples. The pressure was calibrated using the ruby R, fluorescence line.
Strong, sharp Weak, sharp Medium, broad Medium, broad
CsVsymmetry with the three CI atoms forming an equilateral triangle and the Sn atoms lying on the Cs axis. Its six degrees of internal vibrational freedom form four vibrational modes. Table 1 lists these modes together with their optical activities and vibrational frequencies found in diethyl ether extracts [8, 91. 3.2. Factor group analysis of the CsSnC13 (II) crystal
From group theory, 60 modes are expected, of which 24 are internal modes of the SnCl; ions and 33 are optical external modes with the other three being acoustic modes. Factor group analysis gives the following results. The 24 internal modes: v~,vz:A~+A,+B~+B, Y,
v4
:2(A,+A,+B,+B,)
The three acoustic modes: A, + 2B, The 33 optical external modes: I’(translationa1) = 6As + 5A, + 6Bs + 4Bu I’(librationa1) = 3As + 3A, + 3Bs + 3B, where the gerade modes are Raman active and the ungerade modes are IR active. Hence we expect 12 Raman active internal bands (6As + 6B,): two bands (As + Bs) for each of the vl and y modes and four (2As + 2Bs) for each of the v3 and u4 modes. There are 18 Raman active external modes (9As + 9Bs).
3. Results and discussion
3.3. Raman spectra at ambient conditions
3.1. The SnCl; free ion
The Raman spectra of the CsSnCls crystal are shown in Fig. 1. These spectra were obtained by holding the crystal at different orientations. In the
The SnCl; free ion has a pyramidal structure of
Z.X. Shen et al./J. Mol. Struct. 326 (1994) 73-80
0
100
200
300
400
Wavenumber Fig. 1. Raman spectra of the C&Cl3
(II) single crystal at different orientations at 0.0 kbar.
first spectrum, the crystal was held vertically, while in the second it was held horizontally. In both orientations, the laser beam was incident normal to the crystal axis. As for the third spectrum, the laser was directed along the crystal axis. The three spectra obtained are significantly different due to their different orientations. The plasma lines at 66, 77 and 116 cm-’ are marked by “P”. The v1 (As) mode, which is the totally symmetric stretching of all SnCl; ions in the unit cell, is expected to give rise to the strongest Raman band. Thus the 165cn-’ band is assigned to this mode and its shoulder at 145cm-’ to the other vI mode of B, symmetry. The polarized Raman study by Kuok [4] confirms the symmetries of these bands. Woodward and Taylor [8, 91 have reported the frequency of the v1 mode for SnCl; free ion at 297 cm-‘, so it is rather surprising that the frequency has changed so much in the crystal. It has been reported that subjecting the free ion to different liquid surroundings would result in totally different Raman spectra [8]. Aqueous solution of stannous chloride in hydrochloric acid gives a Raman spectrum consisting of very broad and diffuse bands, but ether extracts show distinct line
spectra attributable with confidence to the single complex SnCl; ion. We have also recorded Raman spectra for aqueous solution of CsSnCls in the 15-35Ocm-’ region. No welldefined Raman peaks could be observed in agreement with Ref. [8]. This demonstrates that the interaction between the SnCli ion and its environment greatly affects its optical activities. In CsSnCls crystal, this interaction is stronger and more directional. The internal modes of SnCl; are therefore expected to differ even more from those of the free ion. The symmetric stretching vl mode is the most sensitive among the internal modes to interaction with neighbouring molecules, so it is reasonable that this mode shows the greatest shifts in the crystal. But it is very unusual for a vi mode to reduce its frequency by as much as 44%. Four Raman active bands are predicted for the u3 mode in phase II and they are found at 227,249, 263 and 278cm-’ respectively. The splitting of 51 cm-’ for this mode is much bigger than that of any other internal modes. This is expected as this asymmetric stretching mode has the largest dipole moment which in turn gives rise to the greatest interaction with the crystal field.
16
Z.X. Shen et al&L Mol. Struct. 326 (1994) 73-80
Table 2 Raman bands and their assignments in phases II and III Phase
II
P (kbar) Lattice (cm-‘)
0.0 ~ 23.8 ~ 33.3 ~ 37.7 ~ 51.7 ~ 15.5 ~ 88.4 ~ 117
2.6 24.9 35.0 II 40.6 II 54.1 II 81.0 II 96.1 II
145 __ 165 ~
134.0 II 159.1 II 182.5 III 209.5 III 225.9 II 234.1 III 248.2 II 265.3 II 272.7 III
v4
VI
Y
227 -
VI Y
11/111a
II/III”
249 263 ~
6.8 28.2 ~ 36.0 43.6 57.5 ~
~ ~ -
III
III 11.3 29.4 -
III
19.5 31.2 -
24.0 32.7 -
III 25.5 32.9 -
IIIb 28.5 35.5
59.2
101.3 128.7 -
138
~
143.4 ~
138.0 ___
144.3
193.0 220.0 -
194.8 ~ 220.4 ~
200.0 220.0 /
211.8 ~
217.7
235.0 -
235.8 -
237.0 -
237.0 -
237.2
~ ~ ~ -
152.6 185.4 ~ 214.2 223.0 235.5 -
-
264.3 274.9 -
276.0 -
279.5 -
283.0 -
281.0 -
283.5
289.8 III ~
290.0 ~
291.0 ~
294.4 ~
296.9 ~
297.0 ~
298.6
278
' Pressure region in which phases II and III co-exist. b Remnant of phase III in the phase IV stable region.
9.7 kbar 4.4 kbar
2.6 kbar
0.0 kbar
0
100
200
300
400
Wavenumber Fig. 2. High pressure Raman spectra showing the II/III phase transition between 2.6 and 4.4kbar
Z.X. Shen et al./J. Mol. Struct. 326 (1994) 73-80
II
0.0 kbar after 18.0 kbar
200 Wavenumber Fig. 3. High pressure Raman spectra showing the III/IV phase transition at about 25.3 kbar. The spectrum at 0.0 kbar after pressing to 18.0 kbar (phase II) resembles that of phase II.
The broad band at 117 cm-i is assigned to the v4 mode. The optical activities of the v4 mode are the same as those of y, so that four Raman bands are expected. The crystal field splitting of the v4 mode should be smaller than that of y since the corresponding dipole moment is smaller. Hence the frequencies of the u4 bands spread over a smaller region and band overlapping is expected to be more severe. As a result, only an overall contour is observed. Since the Raman bands of the y mode are weaker than that of the v4 mode, no peaks are detected. All the remaining Raman bands are found below 100 cm-’ and are attributed to lattice vibrations. Eighteen Raman active lattice modes are predicted by group theory analysis. A total of seven bands were observed in the frequency range 20-lOOcm-’ (Table 2). In the same frequency region, 15 bands (7As + 8Bs) are observed by polarized Raman work [4] at 10 K. Below 2Ocm-‘, Lee [7] has observed two more bands at 12.0cm-’ and 18.7cm-‘, respectively, at 21°C so that at least 17 of the 18 expected bands can be observed. At room temperature,
some of the bands are very strong and fairly broad thus the other predicted bands may overlap with them and become unresolvable. Detailed polarization work should detect more bands. Three plasma lines are observed at 66, 77 and 116cm-’ with the band at 77cm-’ being the strongest. The plasma line at 66cm-’ overlaps with a Raman peak and thus appears broader and stronger than that at 77 cm-’ (see spectra 1 and 3 in Fig. 1). 3.4. High pressure Raman spectra Figs. 2 and 3 show the representative Raman spectra of C&Cl3 under high pressure. The spectra have a higher background scattering than those in Fig. 1 due to the smaller sample volume used in the DAC. The two spectra at 0.0 and 2.6 kbar look similar. The spectrum changes dramatically at 4.4 kbar. The strongest v1 peak at 165cm-’ disappears completely at 4.4 kbar and a new strong band appears at 220 cm-‘. The spectral changes agree with our optical observations using a microscope which shows a first order phase transition
78
Z.X. Shen et al./J. Mol. Siruct. 326 (1994) 73-80
24.0 kbar
6.8 kbar
2.6 kbar
0.0 kbar
I
0
100
200
300
4:o
Wavenumber Fig. 4. High pressure Raman spectra with fast pressure increase. The spectrum at 0.0 kbar shows bands of pure phase II and that at 24.0 kbar shows bands of pure phase III. The spectra at 2.6 and 6.8 kbar show mixtures of both phases.
from transparent single crystals (phase II) to a polycrystalline form, which makes the samples look opaque (phase III) over the same pressure range. The intensities of all the bands drop signilicantly in phase III. This may also be linked to the fact that the samples are polycrystalline in phase III whereas they are single crystals in phase II before the transition. We take 3.3 kbar, which is the average of 2.6 and 4.4 kbar, as the II/III transition pressure. In the lattice region below lOOcm_‘, only a sharp band at 30.4cm-’ and a very broad band centred at 61 cm-’ are clearly seen at 9.7 kbar (Fig. 2). The intensities of both bands decrease with pressure. The broad band disappears at about 20.8 kbar (Fig. 3). The phase III/IV phase transition occurs at 25.3 kbar. It is marked by the disappearance of all the Raman peaks in phase IV (see the spectrum at 30.2 kbar in Fig. 3). The spectrum recorded at ambient pressure after the sample has been pressed to 18.0 kbar (phase III) (Fig. 3) clearly resembles that of phase III rather than phase II. Another ambient pressure spectrum (not shown) recorded after the pressure had been released from the
phase IV region does not show any peaks which is a characteristic feature of phase IV. We therefore conclude that both phase transitions II/III and III/IV are irreversible. Spectra shown in Figs. 2 and 3 are obtained with pressure held constant for periods of about 70 h near phase transition and about 6 h at other pressures before each pressure increment. Fig. 4 shows the Raman bands mainly near the II/III transition. These spectra are obtained by a quick pressure increment of about 2 h interval, so that the sample is in a less equilibrium state and a large mixing of the bands of phases II/III is anticipated. It clearly show that the spectra at 2.6 and 6.8 kbar contain bands of both phases, while the spectrum at 0.0 kbar is that of pure phase II and the spectrum at 24.0 kbar is that of pure phase III. We do not take 2.6 kbar as the II/III transition pressure since the sample is in a non-equilibrium state. The vl mode frequency decreases with pressure in phase II which is characteristic of a soft mode, so it is not surprising that it disappears in phase III. Since the II/III transition is sluggish, bands from phase II are expected to exist in the phase III region
19
2X. Shen et al.lJ. Mol. Strut. 326 (1994) 73-80
240
210 280
v1’::/ Y,
120
-
+--II 240
II
t
VI II
220
I
1
I
I
0
5
10
15
Pressure
I
/
20
25
@-+--El
n
I
0
30
I
I
I
I
I
I
I
0
5
10
15
20
25
30
kbar Pressure
/
kbar
Fig. 5. Pressure dependence of the vl and u4 modes.
Fig. 6. Pressure dependence. of the y modes.
near the transition pressure. We therefore take the spectrum near, but below, the III/IV transition at 24.0 kbar, as representative of phase III bands. At this pressure, we assign the strongest band which consists of three components centred at 200, 220 and 237 cm-’ to the vi modes and the two bands at 283 and 297cm-’ to the v3 modes. The broad band centred at 143cm-’ is assigned to the v4 mode. The Raman frequencies of all the bands are presented in Table 2 for some representative pressures, together with their assignments in their respective phases. More bands are observed in the 2.6-6.8 kbar range owing to the co-existence of phases II and III. All the bands in this region can be correlated to either phase II or III. At 11.3 kbar (spectrum not shown), all the bands except one are those of phase III. The ~4 (phase III) band is not observable until 11.3 kbar. This is because being weak and broad, it is overshadowed by the strong vl (phase II) band in the same region. The low frequency v1 band has bigger pressure dependence so that it overlaps with another vi band at about 25.5 kbar and it becomes very difficult to determine the frequencies of the two bands. The average frequency of the two is taken instead. This accounts for the decrease of the 220cm-i band at 24.0 kbar to 211.8 cn-’ at 25.5 kbar (Table 2 and Fig. 5). The presence of phase III
bands in the phase IV stable region at 28.5 kbar is also due to the sluggishness of the III/IV transition. The frequency shifts of the Raman bands are plotted in Figs. 5-7. The vI bands of phase II show clear softening. Two of the v3 bands (phase II) also show a slight frequency decrease with pressure. All the internal modes show clear discontinuities between phases II and III indicating that the transition is first order. They move to higher frequencies in phase III implying a shortening of the bonds for the SnClF ion and this in turn suggests that phase III is a more close packed structure. In general, molecules are more closely packed under pressure. The fact that there are fewer bands in phase III than in phase II implies a structure with a symmetry higher than monoclinic for phase III. The softening of the internal modes vl and y at the II/III transition indicates major changes of the Sn-Cl bond lengths. In phase II, the average distance between the Sn atom and its three nearest Cl atoms is 2.52A, and that between Sn and it next nearest three Cl atoms is 3.46A [I]. The coordination of Sn may have changed so that it has more than three nearest Cl atoms in phase III. This will necessarily involve significant changes in the internal vibrational modes. This may also account for the three v1 Raman bands observed in phase III. No Raman bands are observed in
2.X. Shen et al./J. Mol. Struct. 326 (1994) 73-80
80
obtained by varying the temperature at ambient pressure suggests that the II/III phase transition line is ahnost parallel to the temperature axis in the P-T phase diagram, which in turn indicates a large volume drop for this transition [l 11. Phase IV has a structure of even higher symmetry. Both phase transitions are irreversible.
Acknowledgement
I
I
I
I
I
I
0
5
10
15
20
25
Pressure
/
1 30
This work is supported by the National University of Singapore by a research grant no. RP910682. We thank Drs. I-I. Gong and T.J. Lu for helpful discussion.
kbor
Fig. 7. Pressure dependence of the lattice modes.
phase IV. As Raman active modes are also absent for CsSnQ in phase I, it is possible that phase IV also possess cubic symmetry.
4. Conclusion Two new phases are found for pressure up to 60 kbar at room temperature. Phase III is stable between 3.3 and 25.3 kbar and has a structure with s~etry higher than monoclinic (phase II). The Sn atom might have more than three nearest Cl atoms so that the SnCl; ion loses its identity. The Sn-Cl bond lengths are shorter in phase III than in phase II. This together with the possible increase in coordination number mentioned above makes the molecules much more closely packed in phase III and a large volume drop at the II/III phase transition. The fact that phase III cannot be
References [l] F.R. Poulsen and SE. Rasmussen, Acta Chem. &and., 24 (1970) 150. [2] D.E. Scaife, P.F. Weller and W.G. Fisher, J. Solid State Chem., 9 (1974) 308. [3] S. Sharma, N. Weiden and A. Weiss, Z. Nat~o~h., Teil A, 46 (1991) 329. [4] M.H. Kuok, J. Raman Spectrosc., 23 (1992) 225. [5] J. Barrett, S.R.A. Bird, J.D. Donaldson and J. Silver, J. Chem. Sot. A, (1971) 3105. [6] M.H. Kuok, Phys. Status Solidi B, 141 (1987) k167. [7] PC Lee, Hons. Report, Department of Phys., National ~~v~ity of Singapore, 1988/1989. A. Tan, Hons. Report, Department of Phys., National University of Singapore, 1991/1992. [8] L.A. Woodward and M.J. Taylor, J. Chem. Sot., (1962) 407. [9] M.J. Taylor, J. Raman Spectrosc., 20, (1989) 663. [lo] Z.X. Shen, unpublished work, 1992. [1 l] W.F. Sherman and G.R. Wilkinson, in R.J. Clark and R.E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, vol. 6, Heyden, London 1980, chapter 4.