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Roman investigation on structural phase transitions in (C2H5NH3)2CdCl4
CHfhllChL
Volums 93, number 6
RAMAN
INVESTIGATION
PHYSICS
ON STRUCTURAL
LEI”llZRS
24/31 Dcrcmbcr 1982
PHASE TRANSITIONS
IN (C,H,NH3),CdCI,
H. HACEMANN and H. BILL
Rctclvc(l6
Scptmbcr
1982
Wo IKIW mwstydtcd IIW structurJl ph.~sctrdnslttons ~1 I14 and 216 K m (C2HsNH3)zCdCIa (= CACdC) by Rnman spcc~~~casu~c~~~cnts pruwdc more tnformdtlon about the s~mmcrry of tllc low-tcmpuraurc nmnoclm~c ph.w, new rusult~on the phase trdwtlan at II4 K and addltlondl data on the order-duordcr trawlton at 216 K
troscup~. Tlwsc
have equal prob3btltfy
1. Introduction The salr (C,H5NH3),CdC14
the followmg
text)
layer structure
compounds
phase transltwns
(abbrewated
belongs to a family related
which
etiiblt
to reonentatlon
EACdC tn
of perovsklte structural of the otgamc
IOIlS.
In a prevtous paper [I]
spectra of EACdC
and
we have given the Raman
the isostructural
225 K [5]. Below 114
two isotopic analogs below 50
K and have deduced from these data mformation rcgarding the possible factor group of the phase below 114 K. In this work we present the Raman spectra of EACdC
single crystals
at htgher
temperatures.
to
occupy two satesrelated by
a crystallographic mirror plane. At lower temperatures, the organic groups are locked into one of them, thus destroying the nurror plane oT the high-symmetry phase. The phase sequence ISaccording to Dii (OHT) w Diz (OLT). A slmdar transitlon has been observed m (C2HgNH3)zhlnC14
K, rhe structure
(= EAMnC)
of EACdC
at
IS postulated
to be monoclmic (MLT), as proposed previously [I] _
These
data provide ne\c msght hnto the phase lransltion at
3. Experimental details
1I4 K, more tnformation the low-temperature
dence on the transitton
2. Structural
regarding the symmetry of phase and complementary evtat 216 K.
aspects
The &ucture of EACdC can be described as follows. Layers of comer-shanng cadmlum-chlortie octahedra are lsolateJ from each other by Interposed sheets of ethylammonium catlor,: Three phase transitions at 485,216
and 114 K observed in EACdC by calonmetrlc [7_,3], X-ray [4] and Raman [ 11 studies. The phase transition at 216 K is an order-disorder transltion. Above this temperature. the organic groups are disordered and
have been
582
The Raman spectra were obtamed with a laboratoryassembled spectrometer It consists of a Spectra Physics =gon-ion laser, a Spex 1403 double tnonochronutor and a Brookdeal photon counting system. Most of the measurements were perfomled with the 488 nm argon laser line at a nominal output power rangmg from 0.1 to 0.4 W. Typically, the power at the sample was between 0.04 and 0.25 W. An Oxford Instruments hehum cryostat, in conJunction with a temperature controller describedin ref. 111,provided the low-temperature environment of the sample. The temperature of the sample is found to be within 2 K of the reading of the electronically evaluated temperature. The precision of this device is a.1 K. All the temperatures quoted in this work are those read on this temperature controller.
0 009~2614~82/0000-000/$02.75
0
1982 North-Holland
CHChllCAL PHYSICS
Volume 93, number 6
Crystalshave
been grown from aqueous solutions. selected under
The sample crystals were monodomains the
cross-polanzmgmwoscope
Typical
sampIe dlmen-
We have also prepared the crystals (C~~CIi~N~3)~CdCl~
substituted
(EACdC-df
(I)) and (C$f~ND&CdCI,
(II))
(EACdC-d,
to obtain additional information
on the vibrational
spectra. Due to the small amount of CD3CH2NW3CI available to us, the few crystals obtained presented some
defects and some twins. This resulted m a broader Rayleigh wmg and mcompiete evlmction tn the polar-
any change m the Infrared spectra ret tlus temperature The lo~v-synlmetry phase had been identtficd as f I] (factor group CTI,). Among the eight assoctated space groups, the only ones WIIICII enter arc C$, and C& . The plrnc perpendicular to the C2 axis IS a true reflection plane In Cit,, but J ghdc plane in C$,, In the OHT phase, the mirror plane IS due to dynamic disorder. It 1s unli~cly to lmvc 3 mtrror plane in the lowmonoclrmc
temperature
The setting of the crystal axes IS the Same as prevr-
a=7
478,
b = 7.354,~
= 72 1 I a m the
OLT phase.
found
m the parent
I70 K [6 I. The relation of the Raman-acrwc urcduclble represcntutions of D7,, and CzI, is the followmg l?~&,
transition
+, C~h(~~,B~.E~.A~).
A particularly A$b’,c’),
Fig. 1 illustrates the slgnlficant changes of’ the spcctm around II?, K.
compound
n~erhylxnmonwm
MACdCbelow Bi~,B~~,B3~~
4. OLT-MLT
frozen (and ordered) phase. Thus the space
group is very probably C!h. Tlus IS also the space group
Ited spectra. ously used*
Ucccmbrr 1981
This IS the first spectroscopic cvldence of this phase transitlon, as Rao and co-workers [3] did not obscrvc
sionswere2 X 2 X OS mm. isotopic~ly
‘lj31
LCTTEIRS
tntercstrng relatEon IS Ba(b,c)
*+
where the brackets refer to the Raman tensor
componcnrs.
Indeed, rh~s relarmn cuplams
tlon rn the MLT phase of
the obse.rva-
11~s in the a’(c’,b’)c’scatter-
mg geometry whxh are absent in the OLT phase. This IS e~enlpli~ed
hy the line labellcd
3 tn fig.
I wlt~ch IS
not observed at
112 K in the a(c,b)c polarization, but remains intense m the a(b,b)c and b(a,a)c planes. Another remarkable feature IS the hne labellcd 4 in
fig 1 at 259 cm-‘.
It is practicaily absent below the
phasetranstttonand appears clearlyabove.We tcntatively explaintl~s observationin the follo~vln~ m~nncr.
I
100
I
200
There is IIO direct group-subgroup relation between D{i and Cs,,. TIIIS phase transition IS thus of some reconstructive nature. The very important fact IS that the unit cell of the MLT phase (Z = 3) IS half of the one
I
300
cm’
I
1
of the OLT phase (Z = 4) Therefore,
the first Brillouin
zone of the former group doubles tts volume. We assign thts hne labelled 4 to a mode at the zone center III Dii
becoming a zone-boundary
mode in
C~,, Tlus
ac-
counts for the sudden appearance of thushnc wthiu a small temperature
interval around the phase transltlon
r&ion from 40 to I50 cm-l the observed changes are complex. They are not consldered rn thus paper. Frg. 2 shows the varrafion of lmewtdth wth raw In the low-frequency
I 0
,
L
I
100
200
300
I cm”
FIN. 1. Polarued Rsman spectra of EACdC around the lowtcmpcrarure phasetr3nsmon. 1 IO I(- below. 112 Ii above Sin
wtdth 100~~50~~50/100~m. orIent~~ion~(b,~)c.
(A) Orlenl~tion~(c,~)c,
(B)
perature
of the three transitions
labellcd
I.2 and 3 III
fig. I. These transitIons have been chosen because they
allow US to pomt out some additional characteristics of thts phase trans~tron. The cadmium-chorine stretchrng mode (labclled 2) 583
CIIT.XIICAL
Volun:c 93, numbcr 6 cm-’
Involved.
zo-
at different
0
0
co0
X/3
PHYSICS LITTERS
WC are presently temperatures
I Dcccmbcr
1981
lsotoplc
analogs
investrgatrng to obtam
more complete
especially in the low-frequency
formation,
III-
regron [91.
The presentresultssuggestthat this phasetransitron can be related to torsional and hbratlonal matrons of the ethylammomum drstortlon
moteties,
assocrated
with
a small
of the lattice.
5. Order-disorder tnnsition at 216 K the typrcal evolutron of the Raman increasmg temperature m EACdC-d3 (I).
Fig. 3 shows spectra
with
The NH3 remarkable
torsional
mode
the cadmium-chlorme
I and 4 also broaden
3 III fig. 1) etibrts around
stretching
does not seem to be affected Lines
(Ime
shift and broadening
mode
a
116 K, whale at 3-13 cm-l
very much significantly
with
mcrcas-
ing tempernture Addrtronally,
we observe
broad
Raylergh
wmgs In the
at 7-13 cm-l
is not affected by the phase change, and mcreases almost lrnearly from 5 to 160 K. The mode at 329 cln-’ (line3) correspondsto the torsional wbratlon of the NH3 moiety of the cthylIIS lincwrdth
ammonium
group.
This assrgnment
topic substrtutlon and agrees wth s~mlar
compounds
mat~c Increase
[ 7,8]
IS based upon
At 1 I2 K, we observe
of the linewidth
ISO-
Raman spectra of a dra-
as well as a sluft of the
frequency. The hne labelled 1 ctibrts a sundar behavior. These two lmes shift from 15 1 and 329 cm-r at 10 K
to 138 and 318 cm-’ at 155 K. Lme 1 had been assrgned to an external orgamc mode [I ]. Its evolutron with temperature suggests that 11 can be related with a hbrational matron of the whole organic
catIon.
This assignment
that this Iuie compares 154cm-’ These
m (NH3CH,CH2NH3)CdC14 results
outline
dence of the positIon 584
is supported
very well with the strong and hnewdth
by the fact
the hne found [7J.
temperature
depen-
of the translflons
at FIN. 3 Ramsn spectra of EACdC& (I) at different tcmperclturcs from46 to 300 K Ortentatron! b(oa)c + bkblc Slit wdlh. ?00/400/400/200~1m
Volume 93. number 6
CtlChllCAL
PHWCS
uu, bb and ab planes In tile OHT phase. These wings are
Acknowledgement
also observed in the room-temperature
of the analog methylammonnm~
Raman spectra compounds MACdC
24/3 I Dcccmbur I982
LCTTCRS
and MAMnC [IO]. Prelinlmary nleasurements on EAhW reveal a similar behavior as EACdC at the order-dlsorder phase transItIon [9]. These observations show clearly that the observed line broadenmg is mamly caused by the disorder, as It IS found m [sostructural compounds undergomg the same structural phase transltlon. Thus IS confirmed by a FIR study on this phase transition in EACdC [ II]. Wde thus work was m preparation, an Independent Raman mvestigatlon on this structural phase transItton has appeared [ 131. There is good agreement m the experimental observauons. Our forthcommg results 191, based upon the measured and more completely assrgned low-frequency
spectra of EACdC and B set of isotopic
analogs, as well OSan analysis of the internal modes of the erhylammonn~m groups in the dfferent
crysi;ll
phases, wdl prow& additIonal insight Into the structural phase transItIons of the title compound
This work has been supported by tile SWISS NatIonal
Scmce Fouadahon
References
131 C N.R. Rw. S Gankuly. II K.mwAmdr.n S\\JIII) .mJ
J CllNl
I A
77 (1981) 1825. I4 I c CIIIpuIs. I’I1ys SLJI Sol. 432 (1977) 203 151 IV. De~mc~er. J ~clscllc and C W~ldurmu~h..-I Sohd -SM~ Cl~r.r,,:21 (1977)55 R. 6md and II Arcnd. Ph)s Stai Sol 361 1‘31c ChJlWS. (1976) ‘85 I’1 Z IqbA. H Arcnd dnd P Wdchr, J. Ph)s Cl4 (1961) 1497 OXtO%
SOC.
I81 C. Sourlsscauand
l‘JlJdJ)’
-f-rdfls
G LUCJZCJU. J
11
RJmJfl
Spcctrb 8
(1979) 311
191 IlO1
H Hagcmann and II. Ml. lo bc publlhxl hl. COW. A DJOU~ and R Pcrrer, Phys Stat Sol 41~ (1977) 171. hl Pcyrad dnd R Psrrsl, Phys Stat Sol 513 (1979) 521.
IIll II-71 R ~lukl~l~sw. hf. Couzr and C II (1987) I I38
\\‘~a~. 1
CIIPIII
Phyc
585
77
Volums 93, number 6
RAMAN
INVESTIGATION
PHYSICS
ON STRUCTURAL
LEI”llZRS
24/31 Dcrcmbcr 1982
PHASE TRANSITIONS
IN (C,H,NH3),CdCI,
H. HACEMANN and H. BILL
Rctclvc(l6
Scptmbcr
1982
Wo IKIW mwstydtcd IIW structurJl ph.~sctrdnslttons ~1 I14 and 216 K m (C2HsNH3)zCdCIa (= CACdC) by Rnman spcc~~~casu~c~~~cnts pruwdc more tnformdtlon about the s~mmcrry of tllc low-tcmpuraurc nmnoclm~c ph.w, new rusult~on the phase trdwtlan at II4 K and addltlondl data on the order-duordcr trawlton at 216 K
troscup~. Tlwsc
have equal prob3btltfy
1. Introduction The salr (C,H5NH3),CdC14
the followmg
text)
layer structure
compounds
phase transltwns
(abbrewated
belongs to a family related
which
etiiblt
to reonentatlon
EACdC tn
of perovsklte structural of the otgamc
IOIlS.
In a prevtous paper [I]
spectra of EACdC
and
we have given the Raman
the isostructural
225 K [5]. Below 114
two isotopic analogs below 50
K and have deduced from these data mformation rcgarding the possible factor group of the phase below 114 K. In this work we present the Raman spectra of EACdC
single crystals
at htgher
temperatures.
to
occupy two satesrelated by
a crystallographic mirror plane. At lower temperatures, the organic groups are locked into one of them, thus destroying the nurror plane oT the high-symmetry phase. The phase sequence ISaccording to Dii (OHT) w Diz (OLT). A slmdar transitlon has been observed m (C2HgNH3)zhlnC14
K, rhe structure
(= EAMnC)
of EACdC
at
IS postulated
to be monoclmic (MLT), as proposed previously [I] _
These
data provide ne\c msght hnto the phase lransltion at
3. Experimental details
1I4 K, more tnformation the low-temperature
dence on the transitton
2. Structural
regarding the symmetry of phase and complementary evtat 216 K.
aspects
The &ucture of EACdC can be described as follows. Layers of comer-shanng cadmlum-chlortie octahedra are lsolateJ from each other by Interposed sheets of ethylammonium catlor,: Three phase transitions at 485,216
and 114 K observed in EACdC by calonmetrlc [7_,3], X-ray [4] and Raman [ 11 studies. The phase transition at 216 K is an order-disorder transltion. Above this temperature. the organic groups are disordered and
have been
582
The Raman spectra were obtamed with a laboratoryassembled spectrometer It consists of a Spectra Physics =gon-ion laser, a Spex 1403 double tnonochronutor and a Brookdeal photon counting system. Most of the measurements were perfomled with the 488 nm argon laser line at a nominal output power rangmg from 0.1 to 0.4 W. Typically, the power at the sample was between 0.04 and 0.25 W. An Oxford Instruments hehum cryostat, in conJunction with a temperature controller describedin ref. 111,provided the low-temperature environment of the sample. The temperature of the sample is found to be within 2 K of the reading of the electronically evaluated temperature. The precision of this device is a.1 K. All the temperatures quoted in this work are those read on this temperature controller.
0 009~2614~82/0000-000/$02.75
0
1982 North-Holland
CHChllCAL PHYSICS
Volume 93, number 6
Crystalshave
been grown from aqueous solutions. selected under
The sample crystals were monodomains the
cross-polanzmgmwoscope
Typical
sampIe dlmen-
We have also prepared the crystals (C~~CIi~N~3)~CdCl~
substituted
(EACdC-df
(I)) and (C$f~ND&CdCI,
(II))
(EACdC-d,
to obtain additional information
on the vibrational
spectra. Due to the small amount of CD3CH2NW3CI available to us, the few crystals obtained presented some
defects and some twins. This resulted m a broader Rayleigh wmg and mcompiete evlmction tn the polar-
any change m the Infrared spectra ret tlus temperature The lo~v-synlmetry phase had been identtficd as f I] (factor group CTI,). Among the eight assoctated space groups, the only ones WIIICII enter arc C$, and C& . The plrnc perpendicular to the C2 axis IS a true reflection plane In Cit,, but J ghdc plane in C$,, In the OHT phase, the mirror plane IS due to dynamic disorder. It 1s unli~cly to lmvc 3 mtrror plane in the lowmonoclrmc
temperature
The setting of the crystal axes IS the Same as prevr-
a=7
478,
b = 7.354,~
= 72 1 I a m the
OLT phase.
found
m the parent
I70 K [6 I. The relation of the Raman-acrwc urcduclble represcntutions of D7,, and CzI, is the followmg l?~&,
transition
+, C~h(~~,B~.E~.A~).
A particularly A$b’,c’),
Fig. 1 illustrates the slgnlficant changes of’ the spcctm around II?, K.
compound
n~erhylxnmonwm
MACdCbelow Bi~,B~~,B3~~
4. OLT-MLT
frozen (and ordered) phase. Thus the space
group is very probably C!h. Tlus IS also the space group
Ited spectra. ously used*
Ucccmbrr 1981
This IS the first spectroscopic cvldence of this phase transitlon, as Rao and co-workers [3] did not obscrvc
sionswere2 X 2 X OS mm. isotopic~ly
‘lj31
LCTTEIRS
tntercstrng relatEon IS Ba(b,c)
*+
where the brackets refer to the Raman tensor
componcnrs.
Indeed, rh~s relarmn cuplams
tlon rn the MLT phase of
the obse.rva-
11~s in the a’(c’,b’)c’scatter-
mg geometry whxh are absent in the OLT phase. This IS e~enlpli~ed
hy the line labellcd
3 tn fig.
I wlt~ch IS
not observed at
112 K in the a(c,b)c polarization, but remains intense m the a(b,b)c and b(a,a)c planes. Another remarkable feature IS the hne labellcd 4 in
fig 1 at 259 cm-‘.
It is practicaily absent below the
phasetranstttonand appears clearlyabove.We tcntatively explaintl~s observationin the follo~vln~ m~nncr.
I
100
I
200
There is IIO direct group-subgroup relation between D{i and Cs,,. TIIIS phase transition IS thus of some reconstructive nature. The very important fact IS that the unit cell of the MLT phase (Z = 3) IS half of the one
I
300
cm’
I
1
of the OLT phase (Z = 4) Therefore,
the first Brillouin
zone of the former group doubles tts volume. We assign thts hne labelled 4 to a mode at the zone center III Dii
becoming a zone-boundary
mode in
C~,, Tlus
ac-
counts for the sudden appearance of thushnc wthiu a small temperature
interval around the phase transltlon
r&ion from 40 to I50 cm-l the observed changes are complex. They are not consldered rn thus paper. Frg. 2 shows the varrafion of lmewtdth wth raw In the low-frequency
I 0
,
L
I
100
200
300
I cm”
FIN. 1. Polarued Rsman spectra of EACdC around the lowtcmpcrarure phasetr3nsmon. 1 IO I(- below. 112 Ii above Sin
wtdth 100~~50~~50/100~m. orIent~~ion~(b,~)c.
(A) Orlenl~tion~(c,~)c,
(B)
perature
of the three transitions
labellcd
I.2 and 3 III
fig. I. These transitIons have been chosen because they
allow US to pomt out some additional characteristics of thts phase trans~tron. The cadmium-chorine stretchrng mode (labclled 2) 583
CIIT.XIICAL
Volun:c 93, numbcr 6 cm-’
Involved.
zo-
at different
0
0
co0
X/3
PHYSICS LITTERS
WC are presently temperatures
I Dcccmbcr
1981
lsotoplc
analogs
investrgatrng to obtam
more complete
especially in the low-frequency
formation,
III-
regron [91.
The presentresultssuggestthat this phasetransitron can be related to torsional and hbratlonal matrons of the ethylammomum drstortlon
moteties,
assocrated
with
a small
of the lattice.
5. Order-disorder tnnsition at 216 K the typrcal evolutron of the Raman increasmg temperature m EACdC-d3 (I).
Fig. 3 shows spectra
with
The NH3 remarkable
torsional
mode
the cadmium-chlorme
I and 4 also broaden
3 III fig. 1) etibrts around
stretching
does not seem to be affected Lines
(Ime
shift and broadening
mode
a
116 K, whale at 3-13 cm-l
very much significantly
with
mcrcas-
ing tempernture Addrtronally,
we observe
broad
Raylergh
wmgs In the
at 7-13 cm-l
is not affected by the phase change, and mcreases almost lrnearly from 5 to 160 K. The mode at 329 cln-’ (line3) correspondsto the torsional wbratlon of the NH3 moiety of the cthylIIS lincwrdth
ammonium
group.
This assrgnment
topic substrtutlon and agrees wth s~mlar
compounds
mat~c Increase
[ 7,8]
IS based upon
At 1 I2 K, we observe
of the linewidth
ISO-
Raman spectra of a dra-
as well as a sluft of the
frequency. The hne labelled 1 ctibrts a sundar behavior. These two lmes shift from 15 1 and 329 cm-r at 10 K
to 138 and 318 cm-’ at 155 K. Lme 1 had been assrgned to an external orgamc mode [I ]. Its evolutron with temperature suggests that 11 can be related with a hbrational matron of the whole organic
catIon.
This assignment
that this Iuie compares 154cm-’ These
m (NH3CH,CH2NH3)CdC14 results
outline
dence of the positIon 584
is supported
very well with the strong and hnewdth
by the fact
the hne found [7J.
temperature
depen-
of the translflons
at FIN. 3 Ramsn spectra of EACdC& (I) at different tcmperclturcs from46 to 300 K Ortentatron! b(oa)c + bkblc Slit wdlh. ?00/400/400/200~1m
Volume 93. number 6
CtlChllCAL
PHWCS
uu, bb and ab planes In tile OHT phase. These wings are
Acknowledgement
also observed in the room-temperature
of the analog methylammonnm~
Raman spectra compounds MACdC
24/3 I Dcccmbur I982
LCTTCRS
and MAMnC [IO]. Prelinlmary nleasurements on EAhW reveal a similar behavior as EACdC at the order-dlsorder phase transItIon [9]. These observations show clearly that the observed line broadenmg is mamly caused by the disorder, as It IS found m [sostructural compounds undergomg the same structural phase transltlon. Thus IS confirmed by a FIR study on this phase transition in EACdC [ II]. Wde thus work was m preparation, an Independent Raman mvestigatlon on this structural phase transItton has appeared [ 131. There is good agreement m the experimental observauons. Our forthcommg results 191, based upon the measured and more completely assrgned low-frequency
spectra of EACdC and B set of isotopic
analogs, as well OSan analysis of the internal modes of the erhylammonn~m groups in the dfferent
crysi;ll
phases, wdl prow& additIonal insight Into the structural phase transItIons of the title compound
This work has been supported by tile SWISS NatIonal
Scmce Fouadahon
References
131 C N.R. Rw. S Gankuly. II K.mwAmdr.n S\\JIII) .mJ
J CllNl
I A
77 (1981) 1825. I4 I c CIIIpuIs. I’I1ys SLJI Sol. 432 (1977) 203 151 IV. De~mc~er. J ~clscllc and C W~ldurmu~h..-I Sohd -SM~ Cl~r.r,,:21 (1977)55 R. 6md and II Arcnd. Ph)s Stai Sol 361 1‘31c ChJlWS. (1976) ‘85 I’1 Z IqbA. H Arcnd dnd P Wdchr, J. Ph)s Cl4 (1961) 1497 OXtO%
SOC.
I81 C. Sourlsscauand
l‘JlJdJ)’
-f-rdfls
G LUCJZCJU. J
11
RJmJfl
Spcctrb 8
(1979) 311
191 IlO1
H Hagcmann and II. Ml. lo bc publlhxl hl. COW. A DJOU~ and R Pcrrer, Phys Stat Sol 41~ (1977) 171. hl Pcyrad dnd R Psrrsl, Phys Stat Sol 513 (1979) 521.
IIll II-71 R ~lukl~l~sw. hf. Couzr and C II (1987) I I38
\\‘~a~. 1
CIIPIII
Phyc
585
77