- Email: [email protected]
Correlation between k = 0 optical phonons in NbS3 and phonons in ZrS3
Physica 105B (1981) 361-365 North-Holland Publishing Company
C O R R E L A T I O N B E T W E E N k = 0 OPTICAL P H O N O N S IN NbS3 A N D P H O N O N S IN ZrS3
A. ZWICK, M.A. RENUCCI, R. CARLES, N. SAINT-CRICQ and J.B. RENUCCI Laboratoire de Physique des Solides-Universite Paul Sabatier, 31062 Toulouse Cedex, France
We have investigated by Raman spectroscopy the long wavelength optical phonons of N-bS3 at 300 and 80 K. We have detected 20 phonons among the 24 k = 0 Raman active modes predicted by group theory. The Raman spectra exhibit a remarkable similarity with the analogous spectra of ZrS3. This may be understood from the close relationship between the structures of the two compounds. We could relate most of the k = 0 phonons in NbS3 to zone center and zone edge phonons of ZrS3 in the b* direction. The occurrence of doublets may be explained by the flatness of the optical branches of the dispersion curves along the b*-reciprocal axis of ZrS3, deduced from second order Raman measurements.
1. Introduction
The transition-metal trichaicogenides MX3 (M = IVb or Vb transition metal, X = S, Se, Te) are layer compounds of special interest, exhibiting both chain structure and cleavage properties. Their structural anisotropy results in highly anisotropic physical properties and induces a quasi-one-dimensional behavior. Recently, considerable attention has been devoted to the lattice properties of the IVb trichalcogenides [1-5], in contrast to the Vb compounds [6]. We report in this paper a Raman study of the long wavelength optical phonons in the semiconducting compound NbS3.
The symmetries of the k = 0 modes of the crystal are determined by the decomposition of the representation generated by the 3x 16 atomic displacements in the unit cell into the irreducible representations of the i point group. This gives r = 24 Ag(~24 Au. Only the 24 even parity modes will be Raman active. In contrast to what occurs in the other IVb and Vb trichalcogenides, the normal modes of NbSa cannot be classified according to atomic displacements along the chain or perpendicular • Nb a t o m s
2. Structure and group analysis
The structure of NbS3 is triclinic, and belongs to the P1 non-symmorphic space group [7]. As for the other trichalcogenides, the crystal may be viewed as built up of chains arranged in infinite layers. The chains are made of distorted trigonal MX6 prisms stacked one on top of the other. This structure is closely related to the "ZrSe3" structure [8]. A pairing of the Nb atoms along the chains lowers the symmetry to triclinic and produces a doubling of the unit cell along the b-axis relative to the ZrSe3 monoelinic structure. Therefore, the unit cell contains four molecular units. Fig. 1 shows the 16-atom, one layer thick, crystal unit cell.
0 S atoms
aX LLL~ZC Fig. 1. Four molecular unit cell of NbS3, with a chain viewed edge on. abc and XYZ are the sets of crystallographic and principal axes, respectively.
0378-4363/81/0000-0000/$2.50 © North-Holland Publishing Company and Yamada Science Foundation
362
A . Zwick et al./Raman spectra of Nb$3
to it, since the chain axis is no longer a two-fold screw axis of the crystal. NbS 3
3. Experiments
A:51/~5 A T :80 K
The NbS3 samples were thin, ribbon-shaped crystals typically 5 mm long, and about S0/~m wide and less than 40/~m thick. The chain axis, which is the b-crystal axis, is parallel to the long dimension of the ribbons. Although the ribbon surfaces are cleavage planes parallel to the ab plane, it was impossible in practice to establish their orientation. Therefore, a backscattering geometry was used perpendicular to the crystal b axis, with incident and scattered light polarized either parallel or perpendicular to this axis. The Raman spectra were excited with the 4880 and 5145~ lines of an argon ion laser and analyzed in a T800 Coderg triple spectrometer in conjunction with photon counting electronics. The low temperature measurements were performed in a CF204 Oxford Instruments cryostat. Even at room temperature, the sample was kept in helium atmosphere to prevent overheating and avoid inelastic scattering from air. The spectral range covered was 15-600cm -~ and the resolution about 2.5 cm -1. 4. Results and discussion We present in fig. 2 the Raman spectra of NbS3 at 80 K for the two scattering configurations Eil[Esl[b and Ei[lEs±b. We do not reproduce the structureless spectrum for the configuration Eill b &Es. Twenty phonons among the 24 predicted by group theory are detected, some as very weak structures. The wavenumbers and associated polarizabilities of all the structures observed at 300 and 80 K are given in table I. The Raman spectra of NbS3 are characterized by two features: first of all, most of the lines appear in doublets; secondly, the spectra show remarkable similarity, up to the frequency scale, with their counterparts in ZrSa. Fig. 3(a,b), which displays unanalyzed spectra of NbS3 and ZrS3 on the same frequency scale, emphasizes the close relationship between the two compounds. We observe seven dominant peaks in
!! RAMAN INTENSITY (arb. u)
E.i HE_sHb
0
100
200 300 400 WAVENUMBER (crn-I )
500
600
Fig. 2. Two polarized Raman spectra of NbS3 at 80 K excited with the 5145 ~ line of the A r + laser in a backscattering geometry perpendicular to the b-axis. (a) The incident and scattered light polarizations are parallel to the b-axis; Co) the incident light polarization, perpendicular to the b-axis is parallel to the scattered light polarization.
NbS3 at 153, 162, 197, 266, 343, 395 and 575 instead of the five in ZrS3. Most of them are coupled with other lines, generally weaker, to form doublets: 153-162, 197-202, 343-355, 385395 and 563-575 cm -1. Several weak structures are detectable in the range 230-340 cm -1 at 240, 305, 313 and 326 cm -1. Four low-lying frequency peaks occur only at low temperature at 69, 84, 107.5 and 133cm -1, as the two low frequency lines in ZrS3. Such a relationship between the vibrational properties of NbS3 and ZrS3 may be understood from their closely related structures. ZrS3 crystallizes in the ZrSe3 structure. The main difference in NbS3, if one excludes the electron screening, is that the Nb atoms are slightly shifted from the mirror planes of the trigonal prisms to form Nb-Nb pairs. This small distortion results in a doubling of the NbSa pseudo unit cell along the b-axis, which produces the folding in half of the pseudo Brillouin zone along the b*-reciprocal axis. This makes zone edge phonons in the pseudo BZ equivalent to zone center phonons in the real BZ and allows the optical activity of the corresponding phonons. As we expect from the
363
A. Zwick et al./Raman spectra of NbS3 Table I Wavenumbers (era -]) of long wavelength optical phonons of NbS3 with associated polarizabilities 300K 70 K
EdlEsllb gdlgs±b EdlEsllb
68.5
EdlEs±b
68
107
152 160.5 195
262 303
(84) 107.5 133 153 162.5 197.5 202 240 266 305
266
T : 7'7 K
INTENSITY . ~
i
]!i.2~ ,
~(arb. u
326 343 355 (365)
313
385 395 563 395 575
have appeared recently in the literature [1,2,5,6]. At this time, not all the k = 0 phonons have been identified and there are still some discrepancies in the assignment of symmetries. Therefore, our task was to measure second order Raman spectra with the aim of confirming our k = 0 phonons assignment and obtaining information on the zone edge phonons. Fig. 3b displays the unanalyzed spectrum of ZrS3 excited with the 6764 A line of a krypton ion laser far away from resonance with any electronic transition. The scan covers the range 15-
molecular approach of the vibrational properties of ZrS3 [2, 5, 6], the dispersion curves of the N-bS3 pseudolattice should present rather fiat optic branches covering a narrow range in energy. Therefore, the k = 0 phonons in NbS3 which originate from optical k=O and k=b*/2 phonons belonging to the same branch of the dispersion curves of the pseudolattice should occur as doublets in Raman spectra. This approach needs a good knowledge of the lattice dynamics of ZrS3. Many studies concerning mainly long wavelength phonons in ZrS3
RAMAN
391 558 391 572
323 340 350.5
NbS~ x= 51,~5 i
ii ,'
I; i
A
~ 2.5
. •;~: 5145~,
+
+10
200
II
i
3;0
1.00 ZrS3 z(YY)z ~: 6764~,
_
. . . . Ii
100
i
200
3;0
'
~o
zrs3
~: ~6s8
z(YYI2
'
, li A 200
~ ~'120
, 400
,
, , , 600 800 WAVENUMBER (crn-~ )
, 10'00
Fig. 3. The spectra are not analyzed. The incident fight is polarized parallel to the b-axis. (a) Raman spectrum of NbS3 at 80 K excited with the 5145/~ line of an Ar + laser. (b) Raman spectrum of ZrS3 at 77 K excited with the 6764 ~ line of a Kr + laser, far from resonance. The insert displays the low-lying modes of which the 109 cm -1 line grows up with increasing incident energy. (c) Raman spectrum of ZrS3 at 77 K excited with the 4658/~ line of an Ar + laser, showing multiple order scattering up to the third.
364
A. Zwick et al./Raman spectra of NbS3
Table II Symmetry assignments of long wavelength optical phonons of ZrS3 21
84
110
J.Y. Harbec et al. [9] A. Grisel et al. [1] A. Zwick [10]
and present work
A~
123
152
9
E" A2 + B2
Bm
(RCM) Ag
Bz
(Libr.)
As
?
246
279
285
E' Al + BI
324
335
532
A' Al
E' B1
A2
AI
AI
AI
Ag Bg
Ag
Ag
As
(RCM)
Ag A1 (RCM)
237
Bl (RCM) Ae
6 0 0 c m -1. Fig. 3c shows the same spectrum measured near resonance using, as exciting source, the 4658 A, line of an argon ion laser. The frequency scale has been doubled in order to display the whole range covered, 1 5 - 1 2 0 0 c m -1, and to compare directly the k = O phonons with their overtones. Previous k = 8 p h o n o n s assignments are summarized in table II. Actually, the discrepancies concerning the symmetry assignment of the 152cm peak seem to be due to the degeneracy of two m o d e s that can be lifted under hydrostatic pressure [11]. N o t e that one more phonon is clearly observed at 335 cm -~, as shown in fig. 3b. It could be the missing k = 8 Ag mode, for it disappears in the crossed polarization configuration Z(YX)Z. It can be seen from fig. 3c that the most prominent features in the second order spectrum near resonance is exactly at twice its wavenumber, while other structures may unambiguously be attributed to its additive combinations with the other major k = 8 Ag phonons. Otherwise, most of the second order structures can be assigned to overtones and additive combinations of k = 8 R a m a n active phonons. These assignments are given in table III. A s was expected, the optic branches of the dispersion curves are so flat that new peaks originating from overtones and combinations of z o n e edge p h o n o n s do not appear in the second order spectrum of Z r 8 3. In the spirit of the preceding remarks, we can relate n o w k = 8 p h o n o n s in NbS3 to p h o n o n s in
Bi A2 (Libr.)
B2
B2
B1
A1
A1
BI
AI
A e Bg
Be
Be
Ae
Ag
Ag
Ag
Ag
ZrS3. The three peaks at 265.5, 343 and 395 cm -~ corresponds to the m o d e s at 246, 285 and 3 2 4 c m -1 in ZrS3. The weaker lines at 355 and 385 cm -~ should be the z o n e edge p h o n o n mates of the last ones. The very weak structures in the range 300-330 cm -~ may be related to the weaker peaks at 280 and 335 cm -1 in ZrS3 and to their zone edge counterparts. The structure at 5 7 5 c m -1, which grows up in the Eil[Esllb configuration, compares well with the upper frequency Ag m o d e at 5 3 2 c m -~ in ZrS3. It is assigned to the S-S bond length stretching m o d e
Table III Assignment of second order scattering structures in terms of overtones and additive combinations of Raman active zone center phonons for ZrS3 v (cm ~) Overtones 302 427 435 468 473 490 564 596 607 618 652 662 672 688
Combinations
2 × 152 152 + 279 152 + 285 2 × 237 2 × 245 2×279
2×285
152 + 324 152+ 335 279+285 285 + 324 285 + 335
2 × 324 324 + 335 2 × 335
279 + 324
A . Zwick et al./Raman
at k = 0. Its mate at 562 cm -1 corresponds to the zone edge phonon counterpart in ZrS3. In the case of the lower energy modes in NbS3, the assignment to "old" or "new" k = 0 phonons is more difficult, since the "new" ones come from zone edge phonons belonging to more dispersed acoustic branches along the b*-reciprocal axis. The eight low-lying modes in NbS3 correspond to the rigid chain modes of ZxS3 at 84, 123 and the double degenerated mode at 152 cm -1, with the "new" k = 0 modes mostly originating from the zone edge acoustical phonons in the b* direction. Nevertheless, we can confidently attribute the lowest energy mode at 69.5 cm -~ to a rigid chain mode.
Acknowledgments The samples used in these experiments were obtained through the courtesy of Professor A. Kjekshus (ZrS3) and Dr. P. Monceau (NbS3).
spectra of NbS 3
365
References [1] A. Cn'isel, F. Levy and T.J. Wieting, Physica 99B (1980) 365. [2] S. Jandl, C. Deville Cavellin and J.Y. Harbec, Solid State Commun. 31 (1979) 351. [3] C. Deville Cavellin and S. Jandl, Solid State Commun. 33 (1980) 813. [4] D.W. Galliardt, W.R. Nieveen and R.D. Kirby, Solid State Commun. 34 (1980) 37. [5] A. Zwick and M.A. Renucci, Phys. Stat. Sol. Co) 96 (1979) 757. [6] T.J. Wieting, A. Grisel, F. Levy and Ph. Schmid, in: Proc. of the Intern. Conf. on Quasi-One-Dimensional Conductors, Dubrovnik 1978, Lecture Notes in Physics 95 (Springer-Verlag, Berlin, 1979). [7] J. Rijnsdorf and F. Jellinek, J. Solid State Chem. 25 (1978) 325. [8] S. Furuseth, L. Brattas and A. Kjekshus, Acta Chem. Scand. A29 (1975) 623. [9] J.Y. Harbec, C. Deville Caveliin and S. Jandl, Phys. Stat. SOl. Co) 96 (1979) Kl17. [10] A. Zwick and M.A. Renucci, J. Phys. C: Solid State Phys., in press. [11] C. Deville Cavellin, G. Martinez and A. Zwick, private communication.
C O R R E L A T I O N B E T W E E N k = 0 OPTICAL P H O N O N S IN NbS3 A N D P H O N O N S IN ZrS3
A. ZWICK, M.A. RENUCCI, R. CARLES, N. SAINT-CRICQ and J.B. RENUCCI Laboratoire de Physique des Solides-Universite Paul Sabatier, 31062 Toulouse Cedex, France
We have investigated by Raman spectroscopy the long wavelength optical phonons of N-bS3 at 300 and 80 K. We have detected 20 phonons among the 24 k = 0 Raman active modes predicted by group theory. The Raman spectra exhibit a remarkable similarity with the analogous spectra of ZrS3. This may be understood from the close relationship between the structures of the two compounds. We could relate most of the k = 0 phonons in NbS3 to zone center and zone edge phonons of ZrS3 in the b* direction. The occurrence of doublets may be explained by the flatness of the optical branches of the dispersion curves along the b*-reciprocal axis of ZrS3, deduced from second order Raman measurements.
1. Introduction
The transition-metal trichaicogenides MX3 (M = IVb or Vb transition metal, X = S, Se, Te) are layer compounds of special interest, exhibiting both chain structure and cleavage properties. Their structural anisotropy results in highly anisotropic physical properties and induces a quasi-one-dimensional behavior. Recently, considerable attention has been devoted to the lattice properties of the IVb trichalcogenides [1-5], in contrast to the Vb compounds [6]. We report in this paper a Raman study of the long wavelength optical phonons in the semiconducting compound NbS3.
The symmetries of the k = 0 modes of the crystal are determined by the decomposition of the representation generated by the 3x 16 atomic displacements in the unit cell into the irreducible representations of the i point group. This gives r = 24 Ag(~24 Au. Only the 24 even parity modes will be Raman active. In contrast to what occurs in the other IVb and Vb trichalcogenides, the normal modes of NbSa cannot be classified according to atomic displacements along the chain or perpendicular • Nb a t o m s
2. Structure and group analysis
The structure of NbS3 is triclinic, and belongs to the P1 non-symmorphic space group [7]. As for the other trichalcogenides, the crystal may be viewed as built up of chains arranged in infinite layers. The chains are made of distorted trigonal MX6 prisms stacked one on top of the other. This structure is closely related to the "ZrSe3" structure [8]. A pairing of the Nb atoms along the chains lowers the symmetry to triclinic and produces a doubling of the unit cell along the b-axis relative to the ZrSe3 monoelinic structure. Therefore, the unit cell contains four molecular units. Fig. 1 shows the 16-atom, one layer thick, crystal unit cell.
0 S atoms
aX LLL~ZC Fig. 1. Four molecular unit cell of NbS3, with a chain viewed edge on. abc and XYZ are the sets of crystallographic and principal axes, respectively.
0378-4363/81/0000-0000/$2.50 © North-Holland Publishing Company and Yamada Science Foundation
362
A . Zwick et al./Raman spectra of Nb$3
to it, since the chain axis is no longer a two-fold screw axis of the crystal. NbS 3
3. Experiments
A:51/~5 A T :80 K
The NbS3 samples were thin, ribbon-shaped crystals typically 5 mm long, and about S0/~m wide and less than 40/~m thick. The chain axis, which is the b-crystal axis, is parallel to the long dimension of the ribbons. Although the ribbon surfaces are cleavage planes parallel to the ab plane, it was impossible in practice to establish their orientation. Therefore, a backscattering geometry was used perpendicular to the crystal b axis, with incident and scattered light polarized either parallel or perpendicular to this axis. The Raman spectra were excited with the 4880 and 5145~ lines of an argon ion laser and analyzed in a T800 Coderg triple spectrometer in conjunction with photon counting electronics. The low temperature measurements were performed in a CF204 Oxford Instruments cryostat. Even at room temperature, the sample was kept in helium atmosphere to prevent overheating and avoid inelastic scattering from air. The spectral range covered was 15-600cm -~ and the resolution about 2.5 cm -1. 4. Results and discussion We present in fig. 2 the Raman spectra of NbS3 at 80 K for the two scattering configurations Eil[Esl[b and Ei[lEs±b. We do not reproduce the structureless spectrum for the configuration Eill b &Es. Twenty phonons among the 24 predicted by group theory are detected, some as very weak structures. The wavenumbers and associated polarizabilities of all the structures observed at 300 and 80 K are given in table I. The Raman spectra of NbS3 are characterized by two features: first of all, most of the lines appear in doublets; secondly, the spectra show remarkable similarity, up to the frequency scale, with their counterparts in ZrSa. Fig. 3(a,b), which displays unanalyzed spectra of NbS3 and ZrS3 on the same frequency scale, emphasizes the close relationship between the two compounds. We observe seven dominant peaks in
!! RAMAN INTENSITY (arb. u)
E.i HE_sHb
0
100
200 300 400 WAVENUMBER (crn-I )
500
600
Fig. 2. Two polarized Raman spectra of NbS3 at 80 K excited with the 5145 ~ line of the A r + laser in a backscattering geometry perpendicular to the b-axis. (a) The incident and scattered light polarizations are parallel to the b-axis; Co) the incident light polarization, perpendicular to the b-axis is parallel to the scattered light polarization.
NbS3 at 153, 162, 197, 266, 343, 395 and 575 instead of the five in ZrS3. Most of them are coupled with other lines, generally weaker, to form doublets: 153-162, 197-202, 343-355, 385395 and 563-575 cm -1. Several weak structures are detectable in the range 230-340 cm -1 at 240, 305, 313 and 326 cm -1. Four low-lying frequency peaks occur only at low temperature at 69, 84, 107.5 and 133cm -1, as the two low frequency lines in ZrS3. Such a relationship between the vibrational properties of NbS3 and ZrS3 may be understood from their closely related structures. ZrS3 crystallizes in the ZrSe3 structure. The main difference in NbS3, if one excludes the electron screening, is that the Nb atoms are slightly shifted from the mirror planes of the trigonal prisms to form Nb-Nb pairs. This small distortion results in a doubling of the NbSa pseudo unit cell along the b-axis, which produces the folding in half of the pseudo Brillouin zone along the b*-reciprocal axis. This makes zone edge phonons in the pseudo BZ equivalent to zone center phonons in the real BZ and allows the optical activity of the corresponding phonons. As we expect from the
363
A. Zwick et al./Raman spectra of NbS3 Table I Wavenumbers (era -]) of long wavelength optical phonons of NbS3 with associated polarizabilities 300K 70 K
EdlEsllb gdlgs±b EdlEsllb
68.5
EdlEs±b
68
107
152 160.5 195
262 303
(84) 107.5 133 153 162.5 197.5 202 240 266 305
266
T : 7'7 K
INTENSITY . ~
i
]!i.2~ ,
~(arb. u
326 343 355 (365)
313
385 395 563 395 575
have appeared recently in the literature [1,2,5,6]. At this time, not all the k = 0 phonons have been identified and there are still some discrepancies in the assignment of symmetries. Therefore, our task was to measure second order Raman spectra with the aim of confirming our k = 0 phonons assignment and obtaining information on the zone edge phonons. Fig. 3b displays the unanalyzed spectrum of ZrS3 excited with the 6764 A line of a krypton ion laser far away from resonance with any electronic transition. The scan covers the range 15-
molecular approach of the vibrational properties of ZrS3 [2, 5, 6], the dispersion curves of the N-bS3 pseudolattice should present rather fiat optic branches covering a narrow range in energy. Therefore, the k = 0 phonons in NbS3 which originate from optical k=O and k=b*/2 phonons belonging to the same branch of the dispersion curves of the pseudolattice should occur as doublets in Raman spectra. This approach needs a good knowledge of the lattice dynamics of ZrS3. Many studies concerning mainly long wavelength phonons in ZrS3
RAMAN
391 558 391 572
323 340 350.5
NbS~ x= 51,~5 i
ii ,'
I; i
A
~ 2.5
. •;~: 5145~,
+
+10
200
II
i
3;0
1.00 ZrS3 z(YY)z ~: 6764~,
_
. . . . Ii
100
i
200
3;0
'
~o
zrs3
~: ~6s8
z(YYI2
'
, li A 200
~ ~'120
, 400
,
, , , 600 800 WAVENUMBER (crn-~ )
, 10'00
Fig. 3. The spectra are not analyzed. The incident fight is polarized parallel to the b-axis. (a) Raman spectrum of NbS3 at 80 K excited with the 5145/~ line of an Ar + laser. (b) Raman spectrum of ZrS3 at 77 K excited with the 6764 ~ line of a Kr + laser, far from resonance. The insert displays the low-lying modes of which the 109 cm -1 line grows up with increasing incident energy. (c) Raman spectrum of ZrS3 at 77 K excited with the 4658/~ line of an Ar + laser, showing multiple order scattering up to the third.
364
A. Zwick et al./Raman spectra of NbS3
Table II Symmetry assignments of long wavelength optical phonons of ZrS3 21
84
110
J.Y. Harbec et al. [9] A. Grisel et al. [1] A. Zwick [10]
and present work
A~
123
152
9
E" A2 + B2
Bm
(RCM) Ag
Bz
(Libr.)
As
?
246
279
285
E' Al + BI
324
335
532
A' Al
E' B1
A2
AI
AI
AI
Ag Bg
Ag
Ag
As
(RCM)
Ag A1 (RCM)
237
Bl (RCM) Ae
6 0 0 c m -1. Fig. 3c shows the same spectrum measured near resonance using, as exciting source, the 4658 A, line of an argon ion laser. The frequency scale has been doubled in order to display the whole range covered, 1 5 - 1 2 0 0 c m -1, and to compare directly the k = O phonons with their overtones. Previous k = 8 p h o n o n s assignments are summarized in table II. Actually, the discrepancies concerning the symmetry assignment of the 152cm peak seem to be due to the degeneracy of two m o d e s that can be lifted under hydrostatic pressure [11]. N o t e that one more phonon is clearly observed at 335 cm -~, as shown in fig. 3b. It could be the missing k = 8 Ag mode, for it disappears in the crossed polarization configuration Z(YX)Z. It can be seen from fig. 3c that the most prominent features in the second order spectrum near resonance is exactly at twice its wavenumber, while other structures may unambiguously be attributed to its additive combinations with the other major k = 8 Ag phonons. Otherwise, most of the second order structures can be assigned to overtones and additive combinations of k = 8 R a m a n active phonons. These assignments are given in table III. A s was expected, the optic branches of the dispersion curves are so flat that new peaks originating from overtones and combinations of z o n e edge p h o n o n s do not appear in the second order spectrum of Z r 8 3. In the spirit of the preceding remarks, we can relate n o w k = 8 p h o n o n s in NbS3 to p h o n o n s in
Bi A2 (Libr.)
B2
B2
B1
A1
A1
BI
AI
A e Bg
Be
Be
Ae
Ag
Ag
Ag
Ag
ZrS3. The three peaks at 265.5, 343 and 395 cm -~ corresponds to the m o d e s at 246, 285 and 3 2 4 c m -1 in ZrS3. The weaker lines at 355 and 385 cm -~ should be the z o n e edge p h o n o n mates of the last ones. The very weak structures in the range 300-330 cm -~ may be related to the weaker peaks at 280 and 335 cm -1 in ZrS3 and to their zone edge counterparts. The structure at 5 7 5 c m -1, which grows up in the Eil[Esllb configuration, compares well with the upper frequency Ag m o d e at 5 3 2 c m -~ in ZrS3. It is assigned to the S-S bond length stretching m o d e
Table III Assignment of second order scattering structures in terms of overtones and additive combinations of Raman active zone center phonons for ZrS3 v (cm ~) Overtones 302 427 435 468 473 490 564 596 607 618 652 662 672 688
Combinations
2 × 152 152 + 279 152 + 285 2 × 237 2 × 245 2×279
2×285
152 + 324 152+ 335 279+285 285 + 324 285 + 335
2 × 324 324 + 335 2 × 335
279 + 324
A . Zwick et al./Raman
at k = 0. Its mate at 562 cm -1 corresponds to the zone edge phonon counterpart in ZrS3. In the case of the lower energy modes in NbS3, the assignment to "old" or "new" k = 0 phonons is more difficult, since the "new" ones come from zone edge phonons belonging to more dispersed acoustic branches along the b*-reciprocal axis. The eight low-lying modes in NbS3 correspond to the rigid chain modes of ZxS3 at 84, 123 and the double degenerated mode at 152 cm -1, with the "new" k = 0 modes mostly originating from the zone edge acoustical phonons in the b* direction. Nevertheless, we can confidently attribute the lowest energy mode at 69.5 cm -~ to a rigid chain mode.
Acknowledgments The samples used in these experiments were obtained through the courtesy of Professor A. Kjekshus (ZrS3) and Dr. P. Monceau (NbS3).
spectra of NbS 3
365
References [1] A. Cn'isel, F. Levy and T.J. Wieting, Physica 99B (1980) 365. [2] S. Jandl, C. Deville Cavellin and J.Y. Harbec, Solid State Commun. 31 (1979) 351. [3] C. Deville Cavellin and S. Jandl, Solid State Commun. 33 (1980) 813. [4] D.W. Galliardt, W.R. Nieveen and R.D. Kirby, Solid State Commun. 34 (1980) 37. [5] A. Zwick and M.A. Renucci, Phys. Stat. Sol. Co) 96 (1979) 757. [6] T.J. Wieting, A. Grisel, F. Levy and Ph. Schmid, in: Proc. of the Intern. Conf. on Quasi-One-Dimensional Conductors, Dubrovnik 1978, Lecture Notes in Physics 95 (Springer-Verlag, Berlin, 1979). [7] J. Rijnsdorf and F. Jellinek, J. Solid State Chem. 25 (1978) 325. [8] S. Furuseth, L. Brattas and A. Kjekshus, Acta Chem. Scand. A29 (1975) 623. [9] J.Y. Harbec, C. Deville Caveliin and S. Jandl, Phys. Stat. SOl. Co) 96 (1979) Kl17. [10] A. Zwick and M.A. Renucci, J. Phys. C: Solid State Phys., in press. [11] C. Deville Cavellin, G. Martinez and A. Zwick, private communication.