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Raman spectra of a single crystal of BiOCl
Spectrochimica Acta, Vol. 30A, pp. 311 to 313. Pergamon Press 1974. Printed in l~orthern Ireland
Raman spectra of a single crystal of Bi001 A . RULMONT I)6partement de Chimie G~n6rale et de Chimie Physique, Institut de Chlmie, Saxt Tilman Pax B 4000, Liege I, Belgium
(Received 9 April 1973) Abstract--The Raman spectrum of BiOC1 single crystal has been recorded and the symmetry of the different peaks assigned on the basis of polarization data. However, these data do not allow the discrimination between Az~ and Bzg modes. Intensity and frequency considerations strongly suggest that the two observed non-degenerate modes belong to the Az~ class, and that the predicted Big mode is missing in the actual spectra. INTRODUCTION THE EXISTING data on the vibrational spectrum of bismuth oxyhalides BiOX (X ---- C1, Br, I) have been obtained with powder [1] and the assigments are incomplete. One Raman-active mode only was mentioned in this paper. We have grown single crystals of BiOC1 by the transport method recently reported by SHTILVEHA and CHEPUR [2] and we present in this paper the results of a R a m a n investigation of these crystals. EXPERIMENTAL
A. Spectroscopic technique The spectra have been recorded with the Coderg p H 0 double monochromator spectrometer equipped with a Spectra Physics He-Ne 50 m W laser. The spectral slitwidth was 2 cm -1. The crystal was oriented by means of a goniometric device. The polarization planes of the incident and scattered light was controlled by a half-wave plate. The light was collected at 90 ° .
B. Synthesis of the compound The synthesis of the BLOC1powder is reported elsewhere [3]. This powder is put into an evacuated silica tube of 1 cm diameter; water vapour is then introduced under 10 mm Hg pressure, and the tube is sealed off. I t is put into a gradient temperature oven as described in Ref. [2]. The crystals obtained were square plates 3 or 4 mm on an edge. The crystal habit is very simple, with predominant 0 0 1 face; 1 0 0 and 1 1 1 faces are also present but their development is very small. Unfortunately, there is a more or less important intergrowth of the crystals and it is not always easy to select a transparent part whose orientation is known. STRUCTURE AND FACTOR GROUP ANALYSIS We shall mention only the main features of the structure which have been completely studied in an other paper [3]. [1] A. RULMONT,Spectrochim. Acta 28A, 1287 (1972). [2] M. V. SHTILZKHAand D. V. CH~PU~, KristaUografiya 16, 840 (1971). [3] F. A. BA~rZSTER,Mineral. Mag. 149, 49 (1935). 311
312
A. RELMONT Table 1. l~aman frequencies of BLOC1
Powder Crystal Atoms in movement
Eg
Azg
Eg
Azg
Bza
58.5 59.5 Bi, O, C1
144.5 Bi, C1
144 150 Bi, O, C1
200.5 201.5 Bi, C1
--0
The space group is D4h7 and the number of molecule per unit cell is two. The crystallographic cell is primitive. The crystal is made up of two-dimentional layers (Bi--O)~ + linked together by two layers of chlorine. The factor group analysis has already been given [1, 4, 5] but a re-examination of this analysis showed t h a t the Bg mode should be a Bzg and not a B2, mode as previously deduced by BON~AIRV,. T = 2Azg(R ) + Blg(R ) + 3Eg(R) + 3E~ + 3A~ We have indicated in Table 1 the atoms which are allowed to move during the different normal vibrations. I:~ESULTS AND DISCUSSION The spectra are shown in Fig. 1 for several orientations of the crystal and of the polarization planes. The extinction observed is good and allow the direct discrimination between the doubly degenerate modes of E~ symmetry and the symmetrical ones of Axg and Bzg type. Table 1 gives the proposed assignments for the observed frequencies. Two points should now be discussed, namely the number of observed fundamentals and the assignment of the non-degenerate modes. Three peaks only were observed in the R a m a n spectrum of the powder, but the single crystal data show t h a t the 144 cm -z peak correspond to the overlapping of
Z(YX)Y
K(ZZ)Y ~,,(,,,, )
B~g
Z(YZ)Y E~
!
(~ 2 0
i I IO0 5 0
i 200
I I tO0 50
I
200
I
I 0 0 5O
z(xx)Y N )
z(xz)Y E~
A 2(~0
J
I00 = 50
200
i I00
i 50
Fig. 1. Raman spectra of the BiOC1 single crystal. [4] R. BO~-~AI~., Compt. Rend. 266, 1415 (1968). [5] L. J. BASILE,J. R. FERRAROsnd D. GRO~rERT,J. Inorg. 1Vucl. Chem. 33, 1047 (1971).
Raman spectra of a single crystal of BiOC1
313
two modes, one A1g and one Eg. Nevertheless, we find four fundamentals only, out of a t o t a l of six predicted by the factor group analysis. Finally, there remains some doubt about the assignment of the non-degenerate fundamentals to either Azg or Bzg modes. These assignments m a y be attempted on the following bases: (1) The most reliable assignment should be deduced from the correlation between the peak intensities and a suitable notation of the crystal. This is unfortunately impossible in the actual case because of the intergrowth and the platy habit of the crystals. (2) I f we compare the spectra corresponding to the two orientations Z ( X X ) Y and X ( Z Z ) Y , we notice t h a t the intensity of the 200 cm -z peak is considerably lower for the first orientation; now for a Bza mode it m a y be predicted t h a t ~ = - - ~ and a~ ---- 0. We m a y thus conclude t h a t this peak is not of Bzg symmetry. (3) Finally we may consider the fact t h a t the oxygen atoms only are allowed to move during the Big vibration because of the s y m m e t r y conditions related to this type of vibrations [1]. As a consequence, the corresponding frequency should remain nearly constant along the hole serie BiOX (X ---- F, C1, Br, I). Experiments which are actually carried out on this family of compounds show t h a t neither the 144 nor the 200 em -z peak exhibit this type of behaviour. I t appears from this discussion t h a t both 200 and 145 cm -z peaks belong to the Azg class, and t h a t the Bzg fundamental is missing in our spectra. This conclusion does not agree with the previous assignment of the 59 cm -z peak to the Big class but it should be pointed out t h a t this assignment was proposed on the basis of powder spectra alone [1]. Further work is actually carried out on this family of compounds, and the results will be reported in a forth-coming paper.
Acknowledgements--We wish to thank Professor P. Tarte for fruitful discussions and encouragements during this work. Note added in proof. Recently, J. E. D. Davies [6] has reported that a small peak appears at nearly 400 cm-z in the spectrum of the BLOC1powder. A further crystal reexamination has indicated that this peak is necessary of Bzg type. [6] J . E . D . DAvI~.S:J. Inorg. 1Vucl. Chem., 35, 1531 (1973).
Raman spectra of a single crystal of Bi001 A . RULMONT I)6partement de Chimie G~n6rale et de Chimie Physique, Institut de Chlmie, Saxt Tilman Pax B 4000, Liege I, Belgium
(Received 9 April 1973) Abstract--The Raman spectrum of BiOC1 single crystal has been recorded and the symmetry of the different peaks assigned on the basis of polarization data. However, these data do not allow the discrimination between Az~ and Bzg modes. Intensity and frequency considerations strongly suggest that the two observed non-degenerate modes belong to the Az~ class, and that the predicted Big mode is missing in the actual spectra. INTRODUCTION THE EXISTING data on the vibrational spectrum of bismuth oxyhalides BiOX (X ---- C1, Br, I) have been obtained with powder [1] and the assigments are incomplete. One Raman-active mode only was mentioned in this paper. We have grown single crystals of BiOC1 by the transport method recently reported by SHTILVEHA and CHEPUR [2] and we present in this paper the results of a R a m a n investigation of these crystals. EXPERIMENTAL
A. Spectroscopic technique The spectra have been recorded with the Coderg p H 0 double monochromator spectrometer equipped with a Spectra Physics He-Ne 50 m W laser. The spectral slitwidth was 2 cm -1. The crystal was oriented by means of a goniometric device. The polarization planes of the incident and scattered light was controlled by a half-wave plate. The light was collected at 90 ° .
B. Synthesis of the compound The synthesis of the BLOC1powder is reported elsewhere [3]. This powder is put into an evacuated silica tube of 1 cm diameter; water vapour is then introduced under 10 mm Hg pressure, and the tube is sealed off. I t is put into a gradient temperature oven as described in Ref. [2]. The crystals obtained were square plates 3 or 4 mm on an edge. The crystal habit is very simple, with predominant 0 0 1 face; 1 0 0 and 1 1 1 faces are also present but their development is very small. Unfortunately, there is a more or less important intergrowth of the crystals and it is not always easy to select a transparent part whose orientation is known. STRUCTURE AND FACTOR GROUP ANALYSIS We shall mention only the main features of the structure which have been completely studied in an other paper [3]. [1] A. RULMONT,Spectrochim. Acta 28A, 1287 (1972). [2] M. V. SHTILZKHAand D. V. CH~PU~, KristaUografiya 16, 840 (1971). [3] F. A. BA~rZSTER,Mineral. Mag. 149, 49 (1935). 311
312
A. RELMONT Table 1. l~aman frequencies of BLOC1
Powder Crystal Atoms in movement
Eg
Azg
Eg
Azg
Bza
58.5 59.5 Bi, O, C1
144.5 Bi, C1
144 150 Bi, O, C1
200.5 201.5 Bi, C1
--0
The space group is D4h7 and the number of molecule per unit cell is two. The crystallographic cell is primitive. The crystal is made up of two-dimentional layers (Bi--O)~ + linked together by two layers of chlorine. The factor group analysis has already been given [1, 4, 5] but a re-examination of this analysis showed t h a t the Bg mode should be a Bzg and not a B2, mode as previously deduced by BON~AIRV,. T = 2Azg(R ) + Blg(R ) + 3Eg(R) + 3E~ + 3A~ We have indicated in Table 1 the atoms which are allowed to move during the different normal vibrations. I:~ESULTS AND DISCUSSION The spectra are shown in Fig. 1 for several orientations of the crystal and of the polarization planes. The extinction observed is good and allow the direct discrimination between the doubly degenerate modes of E~ symmetry and the symmetrical ones of Axg and Bzg type. Table 1 gives the proposed assignments for the observed frequencies. Two points should now be discussed, namely the number of observed fundamentals and the assignment of the non-degenerate modes. Three peaks only were observed in the R a m a n spectrum of the powder, but the single crystal data show t h a t the 144 cm -z peak correspond to the overlapping of
Z(YX)Y
K(ZZ)Y ~,,(,,,, )
B~g
Z(YZ)Y E~
!
(~ 2 0
i I IO0 5 0
i 200
I I tO0 50
I
200
I
I 0 0 5O
z(xx)Y N )
z(xz)Y E~
A 2(~0
J
I00 = 50
200
i I00
i 50
Fig. 1. Raman spectra of the BiOC1 single crystal. [4] R. BO~-~AI~., Compt. Rend. 266, 1415 (1968). [5] L. J. BASILE,J. R. FERRAROsnd D. GRO~rERT,J. Inorg. 1Vucl. Chem. 33, 1047 (1971).
Raman spectra of a single crystal of BiOC1
313
two modes, one A1g and one Eg. Nevertheless, we find four fundamentals only, out of a t o t a l of six predicted by the factor group analysis. Finally, there remains some doubt about the assignment of the non-degenerate fundamentals to either Azg or Bzg modes. These assignments m a y be attempted on the following bases: (1) The most reliable assignment should be deduced from the correlation between the peak intensities and a suitable notation of the crystal. This is unfortunately impossible in the actual case because of the intergrowth and the platy habit of the crystals. (2) I f we compare the spectra corresponding to the two orientations Z ( X X ) Y and X ( Z Z ) Y , we notice t h a t the intensity of the 200 cm -z peak is considerably lower for the first orientation; now for a Bza mode it m a y be predicted t h a t ~ = - - ~ and a~ ---- 0. We m a y thus conclude t h a t this peak is not of Bzg symmetry. (3) Finally we may consider the fact t h a t the oxygen atoms only are allowed to move during the Big vibration because of the s y m m e t r y conditions related to this type of vibrations [1]. As a consequence, the corresponding frequency should remain nearly constant along the hole serie BiOX (X ---- F, C1, Br, I). Experiments which are actually carried out on this family of compounds show t h a t neither the 144 nor the 200 em -z peak exhibit this type of behaviour. I t appears from this discussion t h a t both 200 and 145 cm -z peaks belong to the Azg class, and t h a t the Bzg fundamental is missing in our spectra. This conclusion does not agree with the previous assignment of the 59 cm -z peak to the Big class but it should be pointed out t h a t this assignment was proposed on the basis of powder spectra alone [1]. Further work is actually carried out on this family of compounds, and the results will be reported in a forth-coming paper.
Acknowledgements--We wish to thank Professor P. Tarte for fruitful discussions and encouragements during this work. Note added in proof. Recently, J. E. D. Davies [6] has reported that a small peak appears at nearly 400 cm-z in the spectrum of the BLOC1powder. A further crystal reexamination has indicated that this peak is necessary of Bzg type. [6] J . E . D . DAvI~.S:J. Inorg. 1Vucl. Chem., 35, 1531 (1973).