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Raman and infrared spectra of some tetrahalide crystals
95
Journal of Molecular Structure, 143 (1986) 95-100 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
RAMAN AND INFRARED SPECTRA OF SOME TETRAHALIDE CRYSTALS
Anthony Anderson and Bruce Torrie Guelph-WaterlooProgram for Graduate Work in Physics, Waterloo Campus, University of Waterloo, Waterloo, Ontario, Canada, NZL 3Gl
ABSTRACT Recent Raman and infrared spectra of a number of tetrahalide crystals are reported. While some examples of isotopic and crystal field splittings of the internal molecular modes are included, the emphasis is on the external lattice vibrations which are important for investigations of intermolecular forces and lattice dynamics calculations. Because of the weak signals from these non-polar near-sphericalmolecules and other experimental difficulties, these modes have not been investigated in detail in earlier work. Examples to be discussed include CCl4, CBr4 and CF4, all of which exhibit solid state phase transitions; the tetrachloridesof Ge, Ti, Si and Sn, all of which are thought to have similar crystal structures; and SnBr4, the structure of which is accurately known and is used as a basis for lattice dynamics calculations.
INTRODUCTION As part of a continuing program on the vibrational spectra and lattice dynamics of molecular crystals (refs. 1,2), we have recently investigated a number of tetrahalide compounds, both organic and inorganic, using Raman and infrared spectroscopy,with particular emphasis on the low frequency lattice vibrations. In these crystals, the important intermolecular forces will be those involving halogen-halogeninteractions, and the lattice spectra will provide fundamental data with which to test simple dynamical models of these systems and investigate the nature of these interactions. In this presentation,we review the progress made on a number of tetrahalide crystals, including CCl4, CBr4 and CF4, all of which have more than one solid modification, four group IV tetrachlorides (GeC14, TiCl4, SIC14 and SnC14), which are thought to have similar structures, and SnBr4, for which accurate structural information is available, and which is therefore suitable for lattice dynamics calculations. Tetrabalide molecules in the free state are tetrahedrally shaped and their spherical top symmetry is described by point group Td.
Since they contain 5
atoms, there are 3 x 5 - 6 = 9 internal degrees of freedom. A group theoretical analysis shows that there are 4 vibrational modes: non-degenerate
1
(Al), 1 doubly degenerate (E) and 2 triply degenerate (Fg).
pure rotational modes (FI) are optically inactive. In the spectra of the
0022-2860/86/$03.50 01986 Elsevier SciencePublishers B.V.
96
crystal, several interesting effects may be observed. For site symmetries which are lower than tetrahedral, the degeneracies of the E and F modes are lifted and uultiplets may be seen.
If the crystal field at the site is
sufficiently strong and anisotropic, the molecule may be distorted from its tetrahedral shape, as has been suggested for SnBrq (ref. 3), in which case these splittings will be large. If there are several molecules in the unit cell, coupling between their vibrations also leads to multiplet structures, corresponding to differently phased modes. The effects also change the optical activity. For example, some components of the A and E modes will be weakly infrared active. New features will be observed at low frequencies, corresponding to lattice vibrations,which may be classified as librations (hindered rotations about the principal molecular axes) or translations (linear motions of the molecular centres of mass). Mixing between modes of the same symmetry species may occur, but this effect is usually only significant if these frequencies are similar. For the external modes of these non-polar, near-spherical molecules, oscillatory changes in the polarizabilitiesand dipole moments result only from small intermolecularperturbations. Hence their Raman and infrared activity is expected to be weak.
For this reason, and because of experimental
difficulties in the low frequency region, very little has been published on the lattice modes of the tetrahalide crystals.
EXPERIMENTAL TECHNIQUES Raman spectra are excited by the 514.5 and 488.0 nm lines of an argon ion laser operating at powers up to 1.5 W.
Light scattered at 90" is analysed by
a double monochromatorand detected with a photomultiplierand photon counting electronics. Signals are fed to a microcomputer for processing, storage and display. For some samples, a third monochromator is used to improve sensitivity at low frequencies. Far infrared spectra are recorded on a Fourier transform spectrometerwith a mercury/quartzsource, Mylar beam divider and germanium bolometric detector, operating at 4.2 K.
Interferogram
signals are digitized and sent to a microcomputer for Fourier transforming ratioing against background spectra, storage and display. Samples, most of which are liquid or solid at room temperature, are sealed in glass tubes for the Raman and polyethylene cells for the infrared experimentsand cooled to temperatures down to 20 K in conventional optical cryostats. For some samples with solid state phase changes, annealing at temperatures just below the transition point is necessary. For CF+, which is a gas at room temperature, distillation directly into the cooled cryostat sample tube and vapour deposition on to a cold substrate have been used.
97
RESULTS AND
DISCUSSION
1. Carbon Tetrachloride (Refs. 4,s) CC14 has at least 5 solid modifcations, 3 of which are stable at normal pressures. At higher temperatures, the lattice spectra show only weak broad features, characteristicof disordered phases. In contrast, an ordered monoclinic phase, stable between 225 and 20 K, gives Raman and far infrared spectra with many sharp peaks, as shave in figure 1. The complexity of these spectra (22 Raman and 13 infrared peaks are resolved) are compatible with the large unit cell (16 molecules) proposed for this phase (ref. 6). 2.
Carbon Tetrabromide CBr4 has two solid phases, the one stable below 320 K being ordered and
isomorphous to that of CC14 (ref. 7). Preliminary Raman and far infrared spectra (figure 2) are indeed similar to the corresponding ones for Ccl,, with peaks shifted to lower frequencies because of the larger mass and moment of inertia.
I
I
I
I
1
,
30
40
50
60
70
81
FREQUENCY
cm-’
Fig. 1. Raman and Far Infrared Spectra of Solid CCll,at 20 K.
3.
0
20
30
40
FREQUENCY
50
60
cm-’
Fig. 2. Raman and Far Infrared Spectra of Solid CBrb at 20 K.
Carbon Tetrafluoride CFI,also has two solid phases, the transition tsmperature being 76 K
(ref. 8,9). Because of the low polarizability of this molecule, attempts to obtain a Raman lattice spectrum have so far been unsuccessful. Far infrared spectra have been recorded and are similar to those already published (ref.10).
98 4.
Carbon Tetraiodide CI, also has a monoclinic structure, but is unstable under normal
atmospheric and light conditions. Attempts to obtain a sample pure enough for spectrscopic investigationshave so far failed. 5.
Group IV Tetrahalides The Raman spectra in the lattice region of solid GeC14, TiCl4, SIC14 and
SnC14 are shcwn in figure 3.
Similarities in the spectra would suggest that
the crystal structures are identical or closely related. The structures of TIC14 and SnC14 have been investigated (ref. 11) and are thought to isomorphous with SnBr4 (describedbelow), CF4 and a high pressure form of cc14. Most, but not all, of the predicted 12 Raman peaks have been observed. If it is assumed that the packing and dominant intermolecular interactions in these crystals are similar, one would expect frequency correlations to exist for the translations (inversely proportional to the square root of the total molecular mass) and for the librations (inversely proportionalto the square root of the moment of inertia or to the cation anion distance). Relative intensities of correspondingpeaks should also be similar. However, the observed variations in frequency and intensity patterns are indicative of either structural differences (especially the case for SiC14) or coupling between translationaland librational modes of the same symmetry, resulting in frequency shifts and intensity redistributions.
25
50
75 FREQUENCY
25
50
75
cm-l
Fig. 3. Raman Spectra of the Lattice Modes of A) C) SIC14 and D) SnC14 at 20 K.
GeC14, B)
TiC14,
99
6.
Tin Tetrabromide SnBr4 is one of the few tetrahalide crystals whose structure is accurately
known, but even here data is available only at room tenperature (ref. 3).
It
has a centrosymmetricmonoclinic u;it cell containing 4 molecules on general sites, with space group P21/c or C2h.
There is evidence that the
tetrahedral shape of the molecule is distorted in the crystal. Baman spectra of the v1 and
~3
intramolecularregions are shown in figure 4 A and B.
The
multiplet structure results from the presence of 5 isotopically different molecules (VI) distortion of the molecules from their Td symmetry (us), and coupling of molecules in the unit cell (~1 and US). The 12 Kaman active lattice modes predicted for this structure have all been observed, as shown in figure 4C.
Preliminary far infrared spectra have also been recorded, but in
these the strongly absorbing internal mode v4 overlaps and obscures some of the much weaker lattice peaks. I
I
220
225-
270
280
290
FREQUENCY
,
20
L
40
6l
cm-’
Fig. 4. l&man Spectra of Solid SnBrr,at 20 K. A) v1 region; B) ~3 region; C) lattice region. LATTICE DYNAMICS Because the structural data for SnBrq is precisely known, it is the most suitable candidate for an attempt to model its vibrational spectra through lattice dynamics calculations. These are based on simple force constants representing springs between neighbouring atoms and follow procedures successfully used for the halogen crystals (ref. 12). The advantage of this approach over the conventional one, using atom-atom interactions represented by potential functions of the Lennard-Jones or Buckingham type, is that the former includes the effects of the crystal field on the intramolecularmodes, while the latter usually adopts the rigid molecule approximation. Although
100 these calculations,which involve the diagonalizationof a 60 x 60 secular matrix, are in their preliminary stages, they have shown that isotopic splitting is quite small (e.g. -0.6 cm-' between components of the u1 mode), that the effects of distortion of the molecules can be modelled by small changes in the intramolecularforce constants, and that the lattice mode frequencies may be reproduced reasonably well by considering only Br-Br interactions between nearest neighbour molecules.
SDMMARY Considerable progress has been made in obtaining Raman and far infrared spectra of the lattice modes of tetrahalide crystals, to supplement the existing data on the internal modes. However, interpretationof the spectra has proved to be difficult. The usual aids to assignment (such as isotopic substitution, intensity arguments, differences in moments of inertia, structrual inferences, etc.) are not applicable or helpful in these cases. The most promising approached to distinguishing the various peaks appears to be lattice dynamics calculations, but these require precise crystallographic data (including atomic positions) and these are not available at present for many of the crystals investigated in this work.
ACKNOWLEDGEMENTS This research has been supported by grants from the Natural Sciences and Engineering Research Council of Canada. We are grateful to B. Andrews, H. Basista, K.M. Hird, W.S. Tse and W.Y. Zeng for their assistance in these experiments.
REFERENCES 1. A. Anderson, J.W. Leech and B.H. Torrie, Indian J. Pure Appl. Phys. -16 (1978), 243-253. 2. A. Anderson, B. Andrews and B.H. Torrie, J. de Chim. Phys. -82 (19851, 99-109. 3. P. Brand and H. Sackmann, Acta Cryst. -16 (19631, 446-451. 4. A. Anderson, B.H. Torrie and W.S. Tse, Chem. Phys. Lett. -61 (1979), 119-123. 5. A. Anderson, H. Basista, B.H. Torrie and W.Y. Zeng, Chem. Phys. Lett. (accepted for publication). 6. S. Cohen, R. Powers and R. Rudman, Acta. Cryst. B35 (1979), 1670-1674. 7. M. More, F. Baert and J. Lefebvre, Acta. Cryst. B33 (19771, 3681-3684. a. S.C. Greer and L. Meyer, J. Chem. Phys. 51 (1969r4583-4585. 9. D.N. Bol'Shutkin, V.M. Gasan, A.I. Prokhzatilov and A.I. Erenburg, Acta Cryst. B28 (1972), 3542-3547. 10. Y.A. SaGy, A. Ron and F.H. Herbstein, J. Chem. Phys. -62 (1975), 1094-1097. 11. P. Brand and H. Sackmann, Z. Anorg. Chem. 321 (19631, 262-269. (accepted for publication). 12. J.F. Higgs and A. Anderson, Phys. Stat. Sorb
Journal of Molecular Structure, 143 (1986) 95-100 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
RAMAN AND INFRARED SPECTRA OF SOME TETRAHALIDE CRYSTALS
Anthony Anderson and Bruce Torrie Guelph-WaterlooProgram for Graduate Work in Physics, Waterloo Campus, University of Waterloo, Waterloo, Ontario, Canada, NZL 3Gl
ABSTRACT Recent Raman and infrared spectra of a number of tetrahalide crystals are reported. While some examples of isotopic and crystal field splittings of the internal molecular modes are included, the emphasis is on the external lattice vibrations which are important for investigations of intermolecular forces and lattice dynamics calculations. Because of the weak signals from these non-polar near-sphericalmolecules and other experimental difficulties, these modes have not been investigated in detail in earlier work. Examples to be discussed include CCl4, CBr4 and CF4, all of which exhibit solid state phase transitions; the tetrachloridesof Ge, Ti, Si and Sn, all of which are thought to have similar crystal structures; and SnBr4, the structure of which is accurately known and is used as a basis for lattice dynamics calculations.
INTRODUCTION As part of a continuing program on the vibrational spectra and lattice dynamics of molecular crystals (refs. 1,2), we have recently investigated a number of tetrahalide compounds, both organic and inorganic, using Raman and infrared spectroscopy,with particular emphasis on the low frequency lattice vibrations. In these crystals, the important intermolecular forces will be those involving halogen-halogeninteractions, and the lattice spectra will provide fundamental data with which to test simple dynamical models of these systems and investigate the nature of these interactions. In this presentation,we review the progress made on a number of tetrahalide crystals, including CCl4, CBr4 and CF4, all of which have more than one solid modification, four group IV tetrachlorides (GeC14, TiCl4, SIC14 and SnC14), which are thought to have similar structures, and SnBr4, for which accurate structural information is available, and which is therefore suitable for lattice dynamics calculations. Tetrabalide molecules in the free state are tetrahedrally shaped and their spherical top symmetry is described by point group Td.
Since they contain 5
atoms, there are 3 x 5 - 6 = 9 internal degrees of freedom. A group theoretical analysis shows that there are 4 vibrational modes: non-degenerate
1
(Al), 1 doubly degenerate (E) and 2 triply degenerate (Fg).
pure rotational modes (FI) are optically inactive. In the spectra of the
0022-2860/86/$03.50 01986 Elsevier SciencePublishers B.V.
96
crystal, several interesting effects may be observed. For site symmetries which are lower than tetrahedral, the degeneracies of the E and F modes are lifted and uultiplets may be seen.
If the crystal field at the site is
sufficiently strong and anisotropic, the molecule may be distorted from its tetrahedral shape, as has been suggested for SnBrq (ref. 3), in which case these splittings will be large. If there are several molecules in the unit cell, coupling between their vibrations also leads to multiplet structures, corresponding to differently phased modes. The effects also change the optical activity. For example, some components of the A and E modes will be weakly infrared active. New features will be observed at low frequencies, corresponding to lattice vibrations,which may be classified as librations (hindered rotations about the principal molecular axes) or translations (linear motions of the molecular centres of mass). Mixing between modes of the same symmetry species may occur, but this effect is usually only significant if these frequencies are similar. For the external modes of these non-polar, near-spherical molecules, oscillatory changes in the polarizabilitiesand dipole moments result only from small intermolecularperturbations. Hence their Raman and infrared activity is expected to be weak.
For this reason, and because of experimental
difficulties in the low frequency region, very little has been published on the lattice modes of the tetrahalide crystals.
EXPERIMENTAL TECHNIQUES Raman spectra are excited by the 514.5 and 488.0 nm lines of an argon ion laser operating at powers up to 1.5 W.
Light scattered at 90" is analysed by
a double monochromatorand detected with a photomultiplierand photon counting electronics. Signals are fed to a microcomputer for processing, storage and display. For some samples, a third monochromator is used to improve sensitivity at low frequencies. Far infrared spectra are recorded on a Fourier transform spectrometerwith a mercury/quartzsource, Mylar beam divider and germanium bolometric detector, operating at 4.2 K.
Interferogram
signals are digitized and sent to a microcomputer for Fourier transforming ratioing against background spectra, storage and display. Samples, most of which are liquid or solid at room temperature, are sealed in glass tubes for the Raman and polyethylene cells for the infrared experimentsand cooled to temperatures down to 20 K in conventional optical cryostats. For some samples with solid state phase changes, annealing at temperatures just below the transition point is necessary. For CF+, which is a gas at room temperature, distillation directly into the cooled cryostat sample tube and vapour deposition on to a cold substrate have been used.
97
RESULTS AND
DISCUSSION
1. Carbon Tetrachloride (Refs. 4,s) CC14 has at least 5 solid modifcations, 3 of which are stable at normal pressures. At higher temperatures, the lattice spectra show only weak broad features, characteristicof disordered phases. In contrast, an ordered monoclinic phase, stable between 225 and 20 K, gives Raman and far infrared spectra with many sharp peaks, as shave in figure 1. The complexity of these spectra (22 Raman and 13 infrared peaks are resolved) are compatible with the large unit cell (16 molecules) proposed for this phase (ref. 6). 2.
Carbon Tetrabromide CBr4 has two solid phases, the one stable below 320 K being ordered and
isomorphous to that of CC14 (ref. 7). Preliminary Raman and far infrared spectra (figure 2) are indeed similar to the corresponding ones for Ccl,, with peaks shifted to lower frequencies because of the larger mass and moment of inertia.
I
I
I
I
1
,
30
40
50
60
70
81
FREQUENCY
cm-’
Fig. 1. Raman and Far Infrared Spectra of Solid CCll,at 20 K.
3.
0
20
30
40
FREQUENCY
50
60
cm-’
Fig. 2. Raman and Far Infrared Spectra of Solid CBrb at 20 K.
Carbon Tetrafluoride CFI,also has two solid phases, the transition tsmperature being 76 K
(ref. 8,9). Because of the low polarizability of this molecule, attempts to obtain a Raman lattice spectrum have so far been unsuccessful. Far infrared spectra have been recorded and are similar to those already published (ref.10).
98 4.
Carbon Tetraiodide CI, also has a monoclinic structure, but is unstable under normal
atmospheric and light conditions. Attempts to obtain a sample pure enough for spectrscopic investigationshave so far failed. 5.
Group IV Tetrahalides The Raman spectra in the lattice region of solid GeC14, TiCl4, SIC14 and
SnC14 are shcwn in figure 3.
Similarities in the spectra would suggest that
the crystal structures are identical or closely related. The structures of TIC14 and SnC14 have been investigated (ref. 11) and are thought to isomorphous with SnBr4 (describedbelow), CF4 and a high pressure form of cc14. Most, but not all, of the predicted 12 Raman peaks have been observed. If it is assumed that the packing and dominant intermolecular interactions in these crystals are similar, one would expect frequency correlations to exist for the translations (inversely proportional to the square root of the total molecular mass) and for the librations (inversely proportionalto the square root of the moment of inertia or to the cation anion distance). Relative intensities of correspondingpeaks should also be similar. However, the observed variations in frequency and intensity patterns are indicative of either structural differences (especially the case for SiC14) or coupling between translationaland librational modes of the same symmetry, resulting in frequency shifts and intensity redistributions.
25
50
75 FREQUENCY
25
50
75
cm-l
Fig. 3. Raman Spectra of the Lattice Modes of A) C) SIC14 and D) SnC14 at 20 K.
GeC14, B)
TiC14,
99
6.
Tin Tetrabromide SnBr4 is one of the few tetrahalide crystals whose structure is accurately
known, but even here data is available only at room tenperature (ref. 3).
It
has a centrosymmetricmonoclinic u;it cell containing 4 molecules on general sites, with space group P21/c or C2h.
There is evidence that the
tetrahedral shape of the molecule is distorted in the crystal. Baman spectra of the v1 and
~3
intramolecularregions are shown in figure 4 A and B.
The
multiplet structure results from the presence of 5 isotopically different molecules (VI) distortion of the molecules from their Td symmetry (us), and coupling of molecules in the unit cell (~1 and US). The 12 Kaman active lattice modes predicted for this structure have all been observed, as shown in figure 4C.
Preliminary far infrared spectra have also been recorded, but in
these the strongly absorbing internal mode v4 overlaps and obscures some of the much weaker lattice peaks. I
I
220
225-
270
280
290
FREQUENCY
,
20
L
40
6l
cm-’
Fig. 4. l&man Spectra of Solid SnBrr,at 20 K. A) v1 region; B) ~3 region; C) lattice region. LATTICE DYNAMICS Because the structural data for SnBrq is precisely known, it is the most suitable candidate for an attempt to model its vibrational spectra through lattice dynamics calculations. These are based on simple force constants representing springs between neighbouring atoms and follow procedures successfully used for the halogen crystals (ref. 12). The advantage of this approach over the conventional one, using atom-atom interactions represented by potential functions of the Lennard-Jones or Buckingham type, is that the former includes the effects of the crystal field on the intramolecularmodes, while the latter usually adopts the rigid molecule approximation. Although
100 these calculations,which involve the diagonalizationof a 60 x 60 secular matrix, are in their preliminary stages, they have shown that isotopic splitting is quite small (e.g. -0.6 cm-' between components of the u1 mode), that the effects of distortion of the molecules can be modelled by small changes in the intramolecularforce constants, and that the lattice mode frequencies may be reproduced reasonably well by considering only Br-Br interactions between nearest neighbour molecules.
SDMMARY Considerable progress has been made in obtaining Raman and far infrared spectra of the lattice modes of tetrahalide crystals, to supplement the existing data on the internal modes. However, interpretationof the spectra has proved to be difficult. The usual aids to assignment (such as isotopic substitution, intensity arguments, differences in moments of inertia, structrual inferences, etc.) are not applicable or helpful in these cases. The most promising approached to distinguishing the various peaks appears to be lattice dynamics calculations, but these require precise crystallographic data (including atomic positions) and these are not available at present for many of the crystals investigated in this work.
ACKNOWLEDGEMENTS This research has been supported by grants from the Natural Sciences and Engineering Research Council of Canada. We are grateful to B. Andrews, H. Basista, K.M. Hird, W.S. Tse and W.Y. Zeng for their assistance in these experiments.
REFERENCES 1. A. Anderson, J.W. Leech and B.H. Torrie, Indian J. Pure Appl. Phys. -16 (1978), 243-253. 2. A. Anderson, B. Andrews and B.H. Torrie, J. de Chim. Phys. -82 (19851, 99-109. 3. P. Brand and H. Sackmann, Acta Cryst. -16 (19631, 446-451. 4. A. Anderson, B.H. Torrie and W.S. Tse, Chem. Phys. Lett. -61 (1979), 119-123. 5. A. Anderson, H. Basista, B.H. Torrie and W.Y. Zeng, Chem. Phys. Lett. (accepted for publication). 6. S. Cohen, R. Powers and R. Rudman, Acta. Cryst. B35 (1979), 1670-1674. 7. M. More, F. Baert and J. Lefebvre, Acta. Cryst. B33 (19771, 3681-3684. a. S.C. Greer and L. Meyer, J. Chem. Phys. 51 (1969r4583-4585. 9. D.N. Bol'Shutkin, V.M. Gasan, A.I. Prokhzatilov and A.I. Erenburg, Acta Cryst. B28 (1972), 3542-3547. 10. Y.A. SaGy, A. Ron and F.H. Herbstein, J. Chem. Phys. -62 (1975), 1094-1097. 11. P. Brand and H. Sackmann, Z. Anorg. Chem. 321 (19631, 262-269. (accepted for publication). 12. J.F. Higgs and A. Anderson, Phys. Stat. Sorb