- Email: [email protected]
Cr3+−Cr3+ excited state Raman scattering from molecular vibrations in binuclear Cs3Cr2Cl9 induced by exchange striction
(~‘)So1idState Communicaitons, Vol.35, pp.689—692. ‘~“PergamonPress Ltd. 1980. Printed in Great Briitan.
34 EXCITED STATE RANAN SCATTERING FROM MOLECULAR VIBRATIONS IN BIN1JCLEAR Cs
Cr~~Cr
3Cr2C19 INDUCED BY EXCHANGE STRICTION I.W. Johnstone, B. BrIat* and D.J. Lockwood Physics Division, National Research Council, Ottawa, Canada K1A 0R6
*Laboratoire d’Optique Physique, ERS du CNRS, E.P.C.f., 10 rue Vauquelin, 75231 Paris Cedex 05, France (ReceIved 6 May 1980 by M.F. Collins) On increasing the temperature, several new bands appear at higher energy from their parent vibrational modes In the Raman spectrum of the molecular complex Cr2C1~. The parent and new bands have 1’A intensities that follow the thermal population factors of various ~A22 2gCr~ pair states. This behaviour is attributed to exc}cange striction which produces a change in vibrational energy with excited state for those normal modes with large net axial Cr~—Cr~ displacements.
1.
Introduction
dependence of E.P.R. measurementsb and the 4A 2Tjgspectrum transitions7,8. optical absorption associated with the Thus 2g~A2g the + ~A2g ~A~g A 2g manifold comprises four levels at 0, 11, 35 and 80 cm’ having total spin S = 0, 1, 2 and 3, respectively, with ~ab —14 cm~ and j = +0.5 cm 8, 2. Experiment
observation In this of paper spin—dependent we report the excited first state Raman scattering from crystal vibrations. In Cs3Cr2Cl9 new phonon scattering is observed whose intensity 34~Cr34pairs. follows the population of the excited ~A2g~’A2g The bandsassociated appear because exchange striction spin new States with the Cr within these states induces a shift in the vibrational frequency of particular internal
Large single crystals of
Cs 7 3Cr2C19 and Raman were samples cleaving — 5x5x3 mm3 grown bywere the obtained Bridgeman bytechnique sections from a boule. Specimens were mounted in
date, observation of spin dependent modes the of the Cr2Cl~’molecular complex. Raman To scattering has been confined to magnetically ordered materials (e.g. CdCr 2Se~,FeCl2.2H20, CsCoBr3 and VI2 in ref. 1—4) where the Reman intensity of certain bands varies considerably near T~. This behaviour has been attributed to modulation of the superexchange path by the particular phonorms involved. The difference here is that the temperature dependent scattering arises from modification of the phonons by the exchange interaction. 5 and contains Cs3Cr2C19 is hexagonal with the essentially D~h isolated, (P63/mmnc) space exchange—coupled group Cr 2C19 units having atoms ~ h symmetry. These units are composed34of lying on sharing the Z axis of the with pair which two face octahedra the Crcoincides with the crystal c—axis. Because of their relatively simple structure, compared to Ruby for example, the 11 3Cr2X9 (N Cs, Rb, NEt~, and X = Cl, Br) series are ideal for studying exchange coupled CrJ+ pairs and have attracted 1’A2g~Ainterest. The splittings of the considerable using Hamiltonlan: ground the exchange 2gmanifold may be represented
K
a ~
+
+
+
a Thor S500 cryostat, where the sample is cooled by both thermal conduction and a helium exchange gas. A gold—iron/chromel thermocouple, placed near the sample, was used to monitor the crystal temperature. RaLnan spectra were excited using 50— 75 mW of 476.5 nm argon laser radiation where the incident polarisation was determined using a Glan— Thomson prism coupled to a half—wave plate. The 90° scattered light was passed through a polaroid aanalyser Spex 14018 monochromator anddispersed detected with and double scrambler before being with a RCA 31034A photomultiplier. The spectra counting techniques elsewhere9. were recorded under discussed computer control using photon 3.
Results
Polarised Raman spectra of Cs3Cr2C19 recorded at 7K are given in figure 1. Our symmetry assignments agree with those given by 1 region. Black et-al.~based on their results at 77K In contrast the bands lattice except forto the in modes the 375andcmmost internal modes of the Cr 2Cl~complex, the band t change markedly intensities temperature. of Wethree now consider Aig with modes increasing inat detail 135, 200 the and 370 cm temperature dependence of the internal mode
+ .
The magnitudes of the exchange parameters
spectrum.
have been determined from the temperature
The band at 135 cm 689
(4K) is assigned to the
690
RANAI~ SCATTERING FROM MOLECULAR VIBRATIONS IN BINUCLEAR Cs
3Cr2C19
Vol. 35, No. 9 19, whose normal coordinate is depicted (Cr—C13—Cr) A~gbreathing mode in figure 2. With increasing temperature this band develops a complex lineshape (see figure 2). This was computer resolved into three components at 134.7, 137 and 139 cm1, whose widths are comparable with their frequency separation. The temperature dependence of the band envelope can then be understood as follows: on warming, the Intensity of the lower component decreases rapidly while the central component increases strongly. At even high temperatures the much weaker band at 139 cm~ appears. Thus the envelope peak frequency shifts to higher energy as each component becomes significant. In contrast, the band assigned to the E~g (Cr—Cl 3) wag at 147 cm~ decreases only weakly in intensity and shows no anomalous shift in frequency with increasing temperature (see figure 2). The 161 1 bands behave (E2g), 172 (E1 ), 194at(E2g), 196 (E1g), simIlar~y. The cm~ is the A 237 (E2 ) and ~83band (A1g) 200.3 cm 1g
X(ZZ)Y AIg
>.. I-
f(ZX)YEI~~
C 5,
4-
E C 0
E C
~XE2g
_________________________________ 00 200 300 400 I
I
I
—
Frequency, Fig. I
cm
I
terminal bending mode whose normal coordinate is given in figure 3 along with the band’s temperature dependence. On warming, this band rapidly loses intensity and a new band appears at 204.5 cm’~, being clearly visible at 25K. An extremely weak band at 315 cm~ shown in detail
Polarised Reman spectra (7K) showing scattering from molecular vibrations in Cs3Cr2Cl9.
T(K)
t1.. S.
I••
7
——
S.
•.. 23 —34
I’
.
65 I. I’ I. >‘
~
C a 4C 0
E
ZZ ~
~ ~••_
“_~‘
C
It
I’ ‘S
~ ~
7
——
‘‘.23 —
55
zx 35
47
I
I —l
Frequency,
Fig. 2
cm
Temperature dependence of the A1g (Cr—Cl3—Cr) breathing mode and E1 (Cr—Cl3) wag phonon scattering. ~he relevant normal coordinates for the Cr2C1~complexare given for each mode.
Vol. 35, No. 9
R.ANAN SCATTERING FROM MOLECULAR VIBRATIONS IN BINIJCLEAR Cs
3Cr2Cl9
691
1\ TIK) ‘I I I
4 ... 24
—4
——
>‘
39 --—-92
t ~
~ C
I fl
4-
~
C
a,
f..j
4-
~_i
U
~
C
‘I
‘
I
E
A19
,,
L/
I, / ~I
zz~
72.5S. 3755 378 I
-
200.3
204.5
I
I
Frequency, Fig. 3
Frequency,
•S—~~S~.
- -
~~_~95
I
0
0 ItE
S /I~
/ ~I \.
0
C
T (K) ...14 —— 27
‘
~
Fig. 5
cm’
I
cm~
Temperature dependence of the A18 terminal stretch phonon scattering near 370 cm_i.
Temperature dependence of the 200.3 and 204.5 cm~’1 components of the A1g terminal bend phonon scattering,
1) scattering combination mode. ~ in figure 4 is assigned to from an A18 increasing temperature (135 cm’1) the X A18 (200 cm” from 4K this band follows the behaviour of its parent first—order modes showing a rapid decrease in intensity while developing an asymmetry to higher energy. In comparison, the nearby 337 (Eig) and 346 (E 1 bands show no observable variation. 2g) Finally, the structure near 375 cm1 j~ cm associated with the A 1 terminal stretch whose normal coordinate is g~venin figure 5. In contrast to the 95K spectrum, the 4K 1, spectrum while the shows a dominant feature at 370 cm’ intensity (see figure In fact, underinhigh 378 cm’1 component has 5). dropped markedly resolution at least four bands are detected at 370, 372.5, 375.5 and 378 cm1 respectively,
Their presence complicates the extraction of the temperature dependence for all but the 378 cm_I band. In this region, scattering from many two— phonon combination modes is expected (e.g. principle,ElgI
Discussion
An examination of the displacements in each of the that shows normal onlymodes the 135, of the200 Cr2Cl~’complex and 370 cm1 A 3~—Cr3~’ net axial1g displacements. It is these vibrations involve large Cr same modes that show the most marked temperature dependent behaviour and give rise to sidebands to higher energy with increasing temperature. The variation in intensity of the structure within these three A~g bands closely parallels the population factors for the spin states IS>exp(—E whicha /kT) comprise the a ~ (2S+1)exp(—E/kT)
K) —
a
suggests that we are observing excited state ~A Reman 2g~A2g scattering pair ground involving manifold these (figure Crl+ pair 6). This
states. —
-
,,_
However, in the absence of electron—phonon
I
Frequency, Fig. 4
In principle, excited state Reman scattering
from phonons is always possible through population of low lying electronic states.
4~
cm~
1 (E Temperature dependence of the bands at 315 cm~ (A1g~A1g), 337 cm” 18) and 346 cm” (Em).
coupling each contribution to the scattering is superimposed on that involving the ground state because in each instance the phonon scattering frequency is identical. However, it is possible that the total integrated band intensity may vary from the expected Bose behaviour if the Reman
692
RAMAI’) SCATTERING FROM MOLECULAR VIBRATIONS IN BINUCLEAR Cs
3Cr2Cl9
scattering cross—section varies according to the initial state. This possibility would explain 1 the behaviour of the E8 modes at 147 and 161 ca decreases in intensity with increasing temperature but no these accompanying shift in for example since bands show small position. However, the temperature dependence of the three Ajg modes can only be explained by the coupling is to be expected since earlier work68 presence of electron-’phonon coupling. Such has shown that in Cs 3Cr2Cl9 exchange striction effects cause significant departure of the ~A2~A2g energy level scheme from the Lancm~ interval rule. This effect is known to be large when the exchange interaction depends strongly on the configurational coordinates and when these coordinates correspond to easily deformuable bonds with small elastic Constants. Both these 3+ criterion in our case. the Hence, pair is apply thermally excited, Cr3+_ Cr3+ when axial the Crseparation changes significantly. It immediately follows that only those internal modes modulating this same separation will show sidebands, which is consistent with experiment. The exchange striction induced frequency shifts are determined by the sensitivity of the particular mode to each of the excited state configurations so that for some Aig modes sidebands are clearly resolved whereas for others they are not. Hence the 134.7, 137, and 139 cm’1
a 0,8
I
I
I
I
Vol. 35, No. 9 I
0.6 C a am 4-
02
_________________________________ 0
0
20
30
I
40
50
60
I
(b)
1.0 S
08 5,
~
E 0 Z
\~ -
0.2
0
~ P0+P1
\
0.6 0.4
-
~——--~ ~
/
~•--~
‘N
~ 10
4
-——_
____
000
20
30
40
50
Temperature, Fig. 6
70
60
70
K
Temperature dependence of the normnalised scattering intensities associated with
band components of the Ajg Cr—C1 of the (a) 1components the 134.7 (0), 137 A(o) and 139 (A) cm~ 1g (Cr—Cl —Cr)
3-’Cr breathing mode follow P0, P1 and P2 population factors while the 200.3 and 204.5 cm’1 components of the A18 terminal bending mode exhibit (P0~1) and P2 behaviour (figure 6). The 378 cm~ sideband to the Aig terminal stretch follows ~2 although for this Dand and its neighbours the temperature dependence is complicated by the Fermi resonances with second order features noted earlier. Acknowledgements — Preliminary measurements were conducted by I.W.J. and B.B. whilst at the Research School of Chemistry, Australian National
breathing mode and (b) the 200.3 ~0) and 204.5 (a) cm~ comuponermis of the A1g terminal bending mode. The solid curves are the population factors P8 for the A2g~’A2gspin states defined in Section 4.
University. One of us (BB) wishes to thank J. Ferguson and the Australian National University for a visiting research followship.
References 1. 2.
3. 4. 5.
E.F. Steigmeier and G. Harbeke, Phys. Kondense Materie 12, 1 (1970). L. Graf and C. Schaack, in Light Scattering in Solids, eds. M. Balkanski, R.C.C. Leite and S.P.S. Porto (Flamarion, Paris 1976) p. 264. I.W. Johnstone and L. Dubicki, .1. Phys. C: Solid St. Phys. to be published (1980). G. Gilntherodt, W. Bauhofer and G. Benedek, Phys. Rev. Lett. 43, 1427 (1979). .J. Weasel and D.J.W. Ijdo, Acts Cryst. 10, ~66 (1957).
6. 7.
8. 9. 10.
J.R. Beswick and D.E. Dugdale, J. Phys. C: Solid St. Phys. 6, 3326 (1973). B. Briat, M.F. Russel, J.C. Rivoal, J.P. Chapelle and 0. Kahn, Molec. Phys. 34, 1357 (1977). — 1.14. Johnstone and K.J. Maxwell, J. Phys. C: Solid St. Phys. to be published. N.L. Rowell, D.J. Lockwood and P. Grant, J. Raman Spectrosc. to be published. J.D. Black, J.T. Dunamuir, 1.14. Forrest and A.P. Lane, Inorg. Chain. 14, 1257 (1975).
34 EXCITED STATE RANAN SCATTERING FROM MOLECULAR VIBRATIONS IN BIN1JCLEAR Cs
Cr~~Cr
3Cr2C19 INDUCED BY EXCHANGE STRICTION I.W. Johnstone, B. BrIat* and D.J. Lockwood Physics Division, National Research Council, Ottawa, Canada K1A 0R6
*Laboratoire d’Optique Physique, ERS du CNRS, E.P.C.f., 10 rue Vauquelin, 75231 Paris Cedex 05, France (ReceIved 6 May 1980 by M.F. Collins) On increasing the temperature, several new bands appear at higher energy from their parent vibrational modes In the Raman spectrum of the molecular complex Cr2C1~. The parent and new bands have 1’A intensities that follow the thermal population factors of various ~A22 2gCr~ pair states. This behaviour is attributed to exc}cange striction which produces a change in vibrational energy with excited state for those normal modes with large net axial Cr~—Cr~ displacements.
1.
Introduction
dependence of E.P.R. measurementsb and the 4A 2Tjgspectrum transitions7,8. optical absorption associated with the Thus 2g~A2g the + ~A2g ~A~g A 2g manifold comprises four levels at 0, 11, 35 and 80 cm’ having total spin S = 0, 1, 2 and 3, respectively, with ~ab —14 cm~ and j = +0.5 cm 8, 2. Experiment
observation In this of paper spin—dependent we report the excited first state Raman scattering from crystal vibrations. In Cs3Cr2Cl9 new phonon scattering is observed whose intensity 34~Cr34pairs. follows the population of the excited ~A2g~’A2g The bandsassociated appear because exchange striction spin new States with the Cr within these states induces a shift in the vibrational frequency of particular internal
Large single crystals of
Cs 7 3Cr2C19 and Raman were samples cleaving — 5x5x3 mm3 grown bywere the obtained Bridgeman bytechnique sections from a boule. Specimens were mounted in
date, observation of spin dependent modes the of the Cr2Cl~’molecular complex. Raman To scattering has been confined to magnetically ordered materials (e.g. CdCr 2Se~,FeCl2.2H20, CsCoBr3 and VI2 in ref. 1—4) where the Reman intensity of certain bands varies considerably near T~. This behaviour has been attributed to modulation of the superexchange path by the particular phonorms involved. The difference here is that the temperature dependent scattering arises from modification of the phonons by the exchange interaction. 5 and contains Cs3Cr2C19 is hexagonal with the essentially D~h isolated, (P63/mmnc) space exchange—coupled group Cr 2C19 units having atoms ~ h symmetry. These units are composed34of lying on sharing the Z axis of the with pair which two face octahedra the Crcoincides with the crystal c—axis. Because of their relatively simple structure, compared to Ruby for example, the 11 3Cr2X9 (N Cs, Rb, NEt~, and X = Cl, Br) series are ideal for studying exchange coupled CrJ+ pairs and have attracted 1’A2g~Ainterest. The splittings of the considerable using Hamiltonlan: ground the exchange 2gmanifold may be represented
K
a ~
+
+
+
a Thor S500 cryostat, where the sample is cooled by both thermal conduction and a helium exchange gas. A gold—iron/chromel thermocouple, placed near the sample, was used to monitor the crystal temperature. RaLnan spectra were excited using 50— 75 mW of 476.5 nm argon laser radiation where the incident polarisation was determined using a Glan— Thomson prism coupled to a half—wave plate. The 90° scattered light was passed through a polaroid aanalyser Spex 14018 monochromator anddispersed detected with and double scrambler before being with a RCA 31034A photomultiplier. The spectra counting techniques elsewhere9. were recorded under discussed computer control using photon 3.
Results
Polarised Raman spectra of Cs3Cr2C19 recorded at 7K are given in figure 1. Our symmetry assignments agree with those given by 1 region. Black et-al.~based on their results at 77K In contrast the bands lattice except forto the in modes the 375andcmmost internal modes of the Cr 2Cl~complex, the band t change markedly intensities temperature. of Wethree now consider Aig with modes increasing inat detail 135, 200 the and 370 cm temperature dependence of the internal mode
+ .
The magnitudes of the exchange parameters
spectrum.
have been determined from the temperature
The band at 135 cm 689
(4K) is assigned to the
690
RANAI~ SCATTERING FROM MOLECULAR VIBRATIONS IN BINUCLEAR Cs
3Cr2C19
Vol. 35, No. 9 19, whose normal coordinate is depicted (Cr—C13—Cr) A~gbreathing mode in figure 2. With increasing temperature this band develops a complex lineshape (see figure 2). This was computer resolved into three components at 134.7, 137 and 139 cm1, whose widths are comparable with their frequency separation. The temperature dependence of the band envelope can then be understood as follows: on warming, the Intensity of the lower component decreases rapidly while the central component increases strongly. At even high temperatures the much weaker band at 139 cm~ appears. Thus the envelope peak frequency shifts to higher energy as each component becomes significant. In contrast, the band assigned to the E~g (Cr—Cl 3) wag at 147 cm~ decreases only weakly in intensity and shows no anomalous shift in frequency with increasing temperature (see figure 2). The 161 1 bands behave (E2g), 172 (E1 ), 194at(E2g), 196 (E1g), simIlar~y. The cm~ is the A 237 (E2 ) and ~83band (A1g) 200.3 cm 1g
X(ZZ)Y AIg
>.. I-
f(ZX)YEI~~
C 5,
4-
E C 0
E C
~XE2g
_________________________________ 00 200 300 400 I
I
I
—
Frequency, Fig. I
cm
I
terminal bending mode whose normal coordinate is given in figure 3 along with the band’s temperature dependence. On warming, this band rapidly loses intensity and a new band appears at 204.5 cm’~, being clearly visible at 25K. An extremely weak band at 315 cm~ shown in detail
Polarised Reman spectra (7K) showing scattering from molecular vibrations in Cs3Cr2Cl9.
T(K)
t1.. S.
I••
7
——
S.
•.. 23 —34
I’
.
65 I. I’ I. >‘
~
C a 4C 0
E
ZZ ~
~ ~••_
“_~‘
C
It
I’ ‘S
~ ~
7
——
‘‘.23 —
55
zx 35
47
I
I —l
Frequency,
Fig. 2
cm
Temperature dependence of the A1g (Cr—Cl3—Cr) breathing mode and E1 (Cr—Cl3) wag phonon scattering. ~he relevant normal coordinates for the Cr2C1~complexare given for each mode.
Vol. 35, No. 9
R.ANAN SCATTERING FROM MOLECULAR VIBRATIONS IN BINIJCLEAR Cs
3Cr2Cl9
691
1\ TIK) ‘I I I
4 ... 24
—4
——
>‘
39 --—-92
t ~
~ C
I fl
4-
~
C
a,
f..j
4-
~_i
U
~
C
‘I
‘
I
E
A19
,,
L/
I, / ~I
zz~
72.5S. 3755 378 I
-
200.3
204.5
I
I
Frequency, Fig. 3
Frequency,
•S—~~S~.
- -
~~_~95
I
0
0 ItE
S /I~
/ ~I \.
0
C
T (K) ...14 —— 27
‘
~
Fig. 5
cm’
I
cm~
Temperature dependence of the A18 terminal stretch phonon scattering near 370 cm_i.
Temperature dependence of the 200.3 and 204.5 cm~’1 components of the A1g terminal bend phonon scattering,
1) scattering combination mode. ~ in figure 4 is assigned to from an A18 increasing temperature (135 cm’1) the X A18 (200 cm” from 4K this band follows the behaviour of its parent first—order modes showing a rapid decrease in intensity while developing an asymmetry to higher energy. In comparison, the nearby 337 (Eig) and 346 (E 1 bands show no observable variation. 2g) Finally, the structure near 375 cm1 j~ cm associated with the A 1 terminal stretch whose normal coordinate is g~venin figure 5. In contrast to the 95K spectrum, the 4K 1, spectrum while the shows a dominant feature at 370 cm’ intensity (see figure In fact, underinhigh 378 cm’1 component has 5). dropped markedly resolution at least four bands are detected at 370, 372.5, 375.5 and 378 cm1 respectively,
Their presence complicates the extraction of the temperature dependence for all but the 378 cm_I band. In this region, scattering from many two— phonon combination modes is expected (e.g. principle,ElgI
Discussion
An examination of the displacements in each of the that shows normal onlymodes the 135, of the200 Cr2Cl~’complex and 370 cm1 A 3~—Cr3~’ net axial1g displacements. It is these vibrations involve large Cr same modes that show the most marked temperature dependent behaviour and give rise to sidebands to higher energy with increasing temperature. The variation in intensity of the structure within these three A~g bands closely parallels the population factors for the spin states IS>exp(—E whicha /kT) comprise the a ~ (2S+1)exp(—E/kT)
K) —
a
suggests that we are observing excited state ~A Reman 2g~A2g scattering pair ground involving manifold these (figure Crl+ pair 6). This
states. —
-
,,_
However, in the absence of electron—phonon
I
Frequency, Fig. 4
In principle, excited state Reman scattering
from phonons is always possible through population of low lying electronic states.
4~
cm~
1 (E Temperature dependence of the bands at 315 cm~ (A1g~A1g), 337 cm” 18) and 346 cm” (Em).
coupling each contribution to the scattering is superimposed on that involving the ground state because in each instance the phonon scattering frequency is identical. However, it is possible that the total integrated band intensity may vary from the expected Bose behaviour if the Reman
692
RAMAI’) SCATTERING FROM MOLECULAR VIBRATIONS IN BINUCLEAR Cs
3Cr2Cl9
scattering cross—section varies according to the initial state. This possibility would explain 1 the behaviour of the E8 modes at 147 and 161 ca decreases in intensity with increasing temperature but no these accompanying shift in for example since bands show small position. However, the temperature dependence of the three Ajg modes can only be explained by the coupling is to be expected since earlier work68 presence of electron-’phonon coupling. Such has shown that in Cs 3Cr2Cl9 exchange striction effects cause significant departure of the ~A2~A2g energy level scheme from the Lancm~ interval rule. This effect is known to be large when the exchange interaction depends strongly on the configurational coordinates and when these coordinates correspond to easily deformuable bonds with small elastic Constants. Both these 3+ criterion in our case. the Hence, pair is apply thermally excited, Cr3+_ Cr3+ when axial the Crseparation changes significantly. It immediately follows that only those internal modes modulating this same separation will show sidebands, which is consistent with experiment. The exchange striction induced frequency shifts are determined by the sensitivity of the particular mode to each of the excited state configurations so that for some Aig modes sidebands are clearly resolved whereas for others they are not. Hence the 134.7, 137, and 139 cm’1
a 0,8
I
I
I
I
Vol. 35, No. 9 I
0.6 C a am 4-
02
_________________________________ 0
0
20
30
I
40
50
60
I
(b)
1.0 S
08 5,
~
E 0 Z
\~ -
0.2
0
~ P0+P1
\
0.6 0.4
-
~——--~ ~
/
~•--~
‘N
~ 10
4
-——_
____
000
20
30
40
50
Temperature, Fig. 6
70
60
70
K
Temperature dependence of the normnalised scattering intensities associated with
band components of the Ajg Cr—C1 of the (a) 1components the 134.7 (0), 137 A(o) and 139 (A) cm~ 1g (Cr—Cl —Cr)
3-’Cr breathing mode follow P0, P1 and P2 population factors while the 200.3 and 204.5 cm’1 components of the A18 terminal bending mode exhibit (P0~1) and P2 behaviour (figure 6). The 378 cm~ sideband to the Aig terminal stretch follows ~2 although for this Dand and its neighbours the temperature dependence is complicated by the Fermi resonances with second order features noted earlier. Acknowledgements — Preliminary measurements were conducted by I.W.J. and B.B. whilst at the Research School of Chemistry, Australian National
breathing mode and (b) the 200.3 ~0) and 204.5 (a) cm~ comuponermis of the A1g terminal bending mode. The solid curves are the population factors P8 for the A2g~’A2gspin states defined in Section 4.
University. One of us (BB) wishes to thank J. Ferguson and the Australian National University for a visiting research followship.
References 1. 2.
3. 4. 5.
E.F. Steigmeier and G. Harbeke, Phys. Kondense Materie 12, 1 (1970). L. Graf and C. Schaack, in Light Scattering in Solids, eds. M. Balkanski, R.C.C. Leite and S.P.S. Porto (Flamarion, Paris 1976) p. 264. I.W. Johnstone and L. Dubicki, .1. Phys. C: Solid St. Phys. to be published (1980). G. Gilntherodt, W. Bauhofer and G. Benedek, Phys. Rev. Lett. 43, 1427 (1979). .J. Weasel and D.J.W. Ijdo, Acts Cryst. 10, ~66 (1957).
6. 7.
8. 9. 10.
J.R. Beswick and D.E. Dugdale, J. Phys. C: Solid St. Phys. 6, 3326 (1973). B. Briat, M.F. Russel, J.C. Rivoal, J.P. Chapelle and 0. Kahn, Molec. Phys. 34, 1357 (1977). — 1.14. Johnstone and K.J. Maxwell, J. Phys. C: Solid St. Phys. to be published. N.L. Rowell, D.J. Lockwood and P. Grant, J. Raman Spectrosc. to be published. J.D. Black, J.T. Dunamuir, 1.14. Forrest and A.P. Lane, Inorg. Chain. 14, 1257 (1975).