Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions
One- and multi-phonon excitations of the single crystalline LaCoO₃ were studied using Raman spectroscopy in the temperature region of 5–300 K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in ter...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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| Cite this: | Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions / V. Gnezdilov, K.-Y. Choi, Yu. Pashkevich, P. Lemmens, S. Shiryaev, G. Bychkov, S. Barilo, V. Fomin, A.V. Yaremenko // Физика низких температур. — 2006. — Т. 32, № 2. — С. 219-226. — Бібліогр.: 38 назв. — англ. |
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Gnezdilov, V. Choi, K.-Y. Pashkevich, Yu. Lemmens, P. Shiryaev, S. Bychkov, G. Barilo, S. Fomin, V. Yeremenko, A.V. 2017-06-11T07:13:11Z 2017-06-11T07:13:11Z 2006 Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions / V. Gnezdilov, K.-Y. Choi, Yu. Pashkevich, P. Lemmens, S. Shiryaev, G. Bychkov, S. Barilo, V. Fomin, A.V. Yaremenko // Физика низких температур. — 2006. — Т. 32, № 2. — С. 219-226. — Бібліогр.: 38 назв. — англ. 0132-6414 PACS: 71.70.Ej, 63.20.Kr, 78.30.Hv https://nasplib.isofts.kiev.ua/handle/123456789/120131 One- and multi-phonon excitations of the single crystalline LaCoO₃ were studied using Raman spectroscopy in the temperature region of 5–300 K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn–Teller (JT) activity of the intermediate-spin (IS) state of Co³⁺ ions. In particular, the 608-cm⁻¹ stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a function of temperature. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck–Condon mechanism that is characteristic for a JT orbital order. This work was supported by INTAS Grant ¹01-0278, NATO Collaborative Linkage Grant PST.CLG.977766 and DFG through SPP1073. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Динамика кристаллической решетки Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions Article published earlier |
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Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions |
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Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions Gnezdilov, V. Choi, K.-Y. Pashkevich, Yu. Lemmens, P. Shiryaev, S. Bychkov, G. Barilo, S. Fomin, V. Yeremenko, A.V. Динамика кристаллической решетки |
| title_short |
Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions |
| title_full |
Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions |
| title_fullStr |
Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions |
| title_full_unstemmed |
Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions |
| title_sort |
low temperature mixed spin state of co³⁺ in lacoo₃ evidenced from jahn–teller lattice distortions |
| author |
Gnezdilov, V. Choi, K.-Y. Pashkevich, Yu. Lemmens, P. Shiryaev, S. Bychkov, G. Barilo, S. Fomin, V. Yeremenko, A.V. |
| author_facet |
Gnezdilov, V. Choi, K.-Y. Pashkevich, Yu. Lemmens, P. Shiryaev, S. Bychkov, G. Barilo, S. Fomin, V. Yeremenko, A.V. |
| topic |
Динамика кристаллической решетки |
| topic_facet |
Динамика кристаллической решетки |
| publishDate |
2006 |
| language |
English |
| container_title |
Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| format |
Article |
| description |
One- and multi-phonon excitations of the single crystalline LaCoO₃ were studied using Raman
spectroscopy in the temperature region of 5–300 K. First-order Raman spectra show a larger
number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes
are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn–Teller
(JT) activity of the intermediate-spin (IS) state of Co³⁺ ions. In particular, the 608-cm⁻¹
stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a
function of temperature. The anomalous temperature dependence of the second-order phonon
excitations spectra is in accordance with the Franck–Condon mechanism that is characteristic for a
JT orbital order.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/120131 |
| citation_txt |
Low temperature mixed spin state of Co³⁺ in LaCoO₃ evidenced from Jahn–Teller lattice distortions / V. Gnezdilov, K.-Y. Choi, Yu. Pashkevich, P. Lemmens, S. Shiryaev, G. Bychkov, S. Barilo, V. Fomin, A.V. Yaremenko // Физика низких температур. — 2006. — Т. 32, № 2. — С. 219-226. — Бібліогр.: 38 назв. — англ. |
| work_keys_str_mv |
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2025-11-25T22:33:24Z |
| last_indexed |
2025-11-25T22:33:24Z |
| _version_ |
1850566841386139648 |
| fulltext |
Fizika Nizkikh Temperatur, 2006, v. 32, No. 2, p. 219–226
Low temperature mixed spin state of Co3 + in LaCoO3
evidenced from Jahn–Teller lattice distortions
V. Gnezdilov1, K.-Y. Choi2, Yu. Pashkevich3, P. Lemmens4, S. Shiryaev5,
G. Bychkov5, S. Barilo5, V. Fomin1, and A.V. Yeremenko1
1B. Verkin Institute for Low Temperature Physics and Engineering
of the National Academy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: gnezdilov@ilt.kharkov.ua
2Institute for Material Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
3A. Galkin Donetsk Physico-Technical Institute of the National Academy of Sciences of Ukraine
72 R. Luksemburg Str., Donetsk 83114, Ukraine
4Institute for Physics of Condensed Matter, TU Braunschweig, Braunschweig D-38106, Germany
5Institute of Physics of Solids & Semiconductors, Academy of Sciences, Minsk 220072, Belarus
Received June 23, 2005, revised August 17, 2005
One- and multi-phonon excitations of the single crystalline LaCoO3 were studied using Raman
spectroscopy in the temperature region of 5–300 K. First-order Raman spectra show a larger
number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes
are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn–Teller
(JT) activity of the intermediate-spin (IS) state of Co3+ ions. In particular, the 608-cm–1
stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a
function of temperature. The anomalous temperature dependence of the second-order phonon
excitations spectra is in accordance with the Franck–Condon mechanism that is characteristic for a
JT orbital order.
PACS: 71.70.Ej, 63.20.Kr, 78.30.Hv
Keywords: lanthanum cobalt oxide, mixed spin state, Jahn–Teller lattice distortions, phonons
1. Introduction
Lanthanum cobalt oxide LaCoO3 is one of a series
of cobaltites with the chemical formula RCoO3, where
R is a rare-earth element or Y. From earlier studies it
is known, that LaCoO3 undergoes successive spin-
state transitions as a function of temperature [1–10].
The first spin transition takes place near 100 K from
a low-spin (LS, t eg g2
6 0, S = 0) state to an intermedi-
ate-spin (IS, t eg g2
5 1,S = 1) state. The second one occurs
near 500 K from an IS state to a high-spin (HS, t eg g2
4 2,
S = 2) state of Co3 + ions. Here note that due to the
partially filled eg level, the IS state is Jahn–Teller
(JT) active creating monoclinic distortions of CoO6
octahedra.
Although the spin-states of cobalt ions and the
underlying nature of the transitions between them
have been investigated for over 50 years, this topic has
recently gained new interest. In midst of the research
activity, the role of the crystal structure of LaCoO3
remains rather controversial. Another issue concerns
a possible orbital ordering in the IS state. Raman
spectroscopy is in general a useful tool for such and
related questions.
As mentioned in our previous work [11], Raman
spectra of LaCoO3 consist of a larger number of
one-phonon modes than what is expected for the
rhombohedral structure. In this paper we report on
polarized Raman spectra of single crystalline LaCoO3
measured in the temperature range of 5–300 K. Ad-
ditional modes observed in whole temperature range
© V. Gnezdilov, K.-Y. Choi, Yu. Pashkevich, P. Lemmens, S. Shiryaev, G. Bychkov, S. Barilo, V. Fomin, and A.V. Yeremenko, 2006
are attributed to phonon scattering that are activated
in the Raman scattering (RS) process by structural
distortions related to a mixed LS/IS state. Detailed
temperature dependent measurements allowed us to
identify a coupling of the 608-cm–1 optic phonon
mode to JT distortions of CoO6 octahedra as well as
the influence of the JT orbital ordering on multi-
phonon scatterings. Moreover, anomalies in peak ener-
gy and relative intensity of the corresponding modes
give evidence for the presence of the IS Co3+ ions
together with the JT distorted CoO6 octahedra even at
lowest temperature of the previously assumed pure LS
state.
2. Experimental details
Single crystals of LaCoO3 were grown using an
anodic electro-deposition technique. In particular, the
approach of McCarrol et al. [12] was modified to use
seeded flux melt growth based on Cs2MoO4–MoO3
mixture in the ratio 2.2:1 as solvent [13]. Appropriate
amount of solvent was added into a 100 cm3 platinum
crucible containing the mixture to grow these single
crystals with a seed served as an anode at
� 950–1000 �C under current density in the range
0.5–0.7 mÀ/ñm2. Simultaneously, the crucible serves
as a cathode of the electrochemical cell.
Raman scattering measurements were carried out in
quasibackscattering geometry using 514.5 nm line of
an argon laser. The incident laser beam of 10 mW
power was focused onto a 0.1 mm spot of the mir-
ror-like chemically etched surface of the as grown
crystal. The sample was mounted on the holder of a
He-gas flow cryostat using silver glue. The scattering
light was analyzed with a DILOR XY triple spectro-
meter combined with a nitrogen-cooled CCD detector.
Provided the naturally grown surfaces of the pe-
rovskite-like crystals are the quasicubic ones, the
measurements were performed in the xx, xz, x’x’, and
x’z’ scattering configurations, where x, z, x’, and z’
are the [100], [001], [101], and [101] quasicubic
directions, respectively.
3. Results and discussion
In certain cases, it is extremely difficult to identify
the exact crystal symmetry. In all previous studies,
based on powder x-ray and neutron diffraction
measurements, the crystal structure of LaCoO3 was
interpreted as rhombohedral without any structural
transitions in the temperature interval of 4.2–1248 K
[14–17]. The rhombohedral R c3 structure can be
obtained from the simple cubic perovskite (Pm3m) by
a rotation of the adjacent CoO6 octahedra in opposite
directions around the cubic [111] direction. For the
rhombohedral structure, the factor group analysis
yields five Raman-active modes (A g1 + 4Eg) out of
the total 20 �-point phonon modes.
In contrast to the common interpretation of the
LaCoO3 crystal symmetry as R c3 , a recent powder and
single crystal x-ray diffraction study [18] as well as
thermal expansion measurements [19] and neutron
pair distribution function analysis [20] provide
evidence for the presence of monoclinic distortions in
LaCoO3. The monoclinic distorted phase is proposed
to be due to the strain caused by a cooperative JT
effect [18]. The averaged structure was found to be
I2/a with three unequal Co–O bond lengths in the
LS/IS state: one short, one long, and one medium
length bond [18]. The long and short Co–O distances
correspond to the bonds in the ab plane while the
medium Co–O distance is the out-of-plane bond all in
I2/a setting. The number of structural items in the
monoclinic phase remains the same as in the rhom-
bohedric one (it contains two formula units of
LaCoO3) and the inversion symmetry also preserves.
The polarized Raman spectra of LaCoO3 measured
on a quasicubic (001) surface at 5 K are shown in
Fig. 1. The inset of Fig. 1 displays the temperature
dependence of the magnetic susceptibility of the studi-
ed sample. The sharpness of the observed phonon
modes in Fig. 1 and an agreement of our susceptibility
data with the previously reported measurements
[15,21,22] indicate a high quality of our single crys-
tals. The total number of the observed modes is at
least three times larger than what is expected for the
R c3 structure. Among them three pronounced peaks
are seen at 562, 657, and 785 cm–1. A closer inspection
reveals that the most intense peak at 657 cm–1 in the
xx and x’x’ polarizations is composed of two addi-
220 Fizika Nizkikh Temperatur, 2006, v. 32, No. 2
V. Gnezdilov et al.
0 200 400 600 800 1000
200
400
600
800
1000
1200
1400
0 100 200 300
0.5
1.0
1.5
2.0
T, K
Raman shift, cm–1
XX
X'X'
X'Z'
XZ
+ 100 un.
In
te
n
si
ty
,a
rb
.u
n
its
S
u
sc
e
p
tib
ili
ty
,1
0
e
m
u
/g
–
5
Fig. 1. Polarized Raman scattering spectra of single
crystalline LaCoO3 at 5 K. The arrows indicate the lines
in the frequency region of stretching-like vibrations. The
inset shows the temperature dependence of the magnetic
susceptibility.
tional peaks at 608 and 701 cm–1, which are well
visible in the x’z’ polarization. In addition, peaks at
48, 82, 122, 138, 167, 196, 247, 286, 340, 371, 405,
and 432 cm–1 with different polarizations as well as a
maximum centered at �485 cm–1 with a three-peak
structure are observed.
The Raman spectra of LaCoO3 are, however, so-
mewhat surprising in the sense that they have no
similarity to those of the isostructural rhombo-
hedral compounds such as LaMnO3+� [23–26],
La1–xAxMnO3 [23,24], LaAlO3 [26]. Even, they diff-
er from the spectra of LaCoO3 reported recently by
Ishikawa et al. [27]. However, the systematic evolu-
tion of the spectra from pure LaMnO3 to pure LaCoO3
in the series of La1–xCoxMnO3 [11] builds confidence
of our data.
The exact assignment of the phonon modes, relying
on their polarization dependence, is complicated due
to the twinning of the crystals which is common for
perovskites. However, compared to lattice-dynamical
calculations and experimental data of Ref. 26 for
rhombohedral LaAlO3, one can safely assign the mode
at 138 cm–1 to a rotation of the oxygen octahedra
around the hexagonal [001] direction, the peak at
167 cm–1 to pure La vibration in the hexagonal (001)
plane, and the peaks at 485 and 657 cm–1 to the
internal (bending- and stretching-like, respectively)
vibrations of the CoO6 octahedra. Note that in com-
parison to our data, the out-of-phase stretching mode
of the isostructural ABO3 compounds is very weak.
In principle, a reduction of the crystal symmetry
from R c3 to I2/a might explain the appearance of
new phonon modes in the Raman spectra. Three A g2
modes of rhombohedric phase become Raman active
3Bg modes of monoclinic phase while every Eg mode
should be split into Ag + Bg . Nonetheless, twelve
(5Ag + 7Bg) Raman active modes expected for I2/a
group are not enough to explain the number of the
peaks observed in our Raman experiments.
Another source of new phonon modes can be
attributed to local lattice distortions associated with
the thermally induced local IS state of Co3 + ions in a
matrix of Co3 + ions of the LS state. Noticeably, an
infrared spectroscopy study of LaCoO3 has shown
anomalies in the phonon spectra, which are ascribed to
local lattice distortions [22]. It is well known that in
such cases the selection rules for an average structure
may be violated. More specific, the short range local
lattice distortions break a rotational symmetry and
can lead to the appearance of new phonon modes
originating from the Brillouin zone boundaries, which
are forbidden in the average, global crystal symmetry.
In this case, the activated phonon modes should be
weak. In our case week phonon peaks are observed in
the frequency regime of 200–450 cm–1 (see Fig. 1).
In the case of phase separation sets of phonons
characteristic for different phases must simultaneously
present in the Raman spectra of the sample. As an
example of such phase coexistence is the observation
of JT distorted and non-JT distorted regions in the
manganite samples [28,29]. Note that Raman intensi-
ties for each phase in this case will be proportional to
the relative phase volume in the sample.
To detect possible structure changes through the
LS–IS state transition, we have examined in detail the
temperature dependence of optical phonon spectra
(Fig. 2). At first glance, there seems to be no appreci-
able changes of the spectra in the temperature interval
of 5–300 K. To extract more detailed information, we
performed a dispersion analysis of the spectra in the
frequency region of 500–850 cm–1, where the most
intense peaks are observed. Lorentzian profiles were
used to fit the spectra as shown in Fig. 3. The
temperature dependence of phonon frequencies is plot-
ted in Fig. 4. With increasing temperature the 657-
and 701-cm–1 modes soften roughly by 8–11 cm–1. In
contrast, the 608-cm–1 mode shows an anomalous
behavior; upon heating up to 130 K, first it hardens by
�10 cm–1 and then it softens by �12 cm–1. The 468-,
483-, and 494-cm–1 modes show a monotonic decrease
of frequency by 7–11 cm–1 with increasing tempera-
ture (not shown here). In order to explain the possible
origins of the exceptional behavior seen in the
608-cm–1 mode we estimate the phonon frequency
shift due to a thermal expansion using a Gruneisen
law, � �� � �/ V/Vi i� � (V is the unit cell volume
and �i is the Gruneisen parameter for the ith phonon
mode). Lattice parameters are taken from Refs. 15 and
Low temperature mixed spin state of Co3 + in LaCoO3 evidenced from Jahn–Teller lattice distortions
Fizika Nizkikh Temperatur, 2006, v. 32, No. 2 221
0 200 400 600 800 1000
1000
2000
3000
Raman shift , cm
–1
5 K, 10 K, 20 K,
30 K, 40 K, 50 K,
60 K, 70 K, 80 K,
90 K, 100 K, 110 K,
120 K, 130 K, 150 K,
170 K, 190 K, 210 K,
230 K, 260 K, 295 K.
In
te
n
si
ty
,a
rb
.u
n
its
Fig. 2. Temperature dependence of the Raman spectra of
single crystalline LaCoO3 in xx scattering configuration.
The spectra from 5 K (bottom) to 295 K (top) are shifted
vertically for clarity.
17 for a calculation of the unit cell volume change.
The results are displayed by the dotted lines together
with the raw data in Fig. 4. There is a good agreement
between the estimated and the observed behavior for
the 657- and 701-cm–1 phonon modes. However, a
strong deviation shows up for the 608-cm–1 phonon
mode in the temperature range of 5–130 K. Notice-
ably, in the corresponding temperature regime the
magnetic susceptibility exhibits a rapid change
[15,21,22]. This can indicate a strong coupling of the
608-cm–1 optic phonon mode to the spin state of the
cobalt ions.
To get more insight, we will examine the tem-
perature dependence of the phonon integrated in-
tensity. In Fig. 5 the relative integrated intensities of
the respective modes, Ii/Itot, are plotted. The in-
tensity of the 657- and 701-cm–1 modes decreases with
increasing temperature while the intensity of the
608-cm–1 mode increases. The behavior of the other
strong peaks in the frequency region of 500–800 cm–1,
the phonon modes at 562 and 785 cm–1, exhibits a
temperature dependence of the integrated intensity
similar to the mode at 608 and 657 cm–1, respectively
(not shown here). The phonon modes related to the
222 Fizika Nizkikh Temperatur, 2006, v. 32, No. 2
V. Gnezdilov et al.
550 600 650 700 750 800
0
1000
2000
3000
0
1000
2000
3000
0
1000
2000
3000
Raman shift , cm –1
290 K
10 K
150 K
LaCoO3
In
te
n
si
ty
,a
rb
.u
n
its
Fig. 3. A fit of the experimental Raman spectra to Loren-
tzian profiles.
0 50 100 150 200 250 300
692
696
700
645
650
655
600
610
620
630
T, K
R
am
an
sh
ift
,c
m
–
1
Fig. 4. Temperature dependence of the Raman shift for
three phonon lines. The dotted lines are the estimated be-
havior of the phonon frequencies according to the Gr�nei-
sen law.
0 50 100 150 200 250 300
0
0.1
0.2
0.8
0.9
1.0
0 100 200 300
35
40
45
50
55
T, K
x
0
T , K
— 608 cm
–1
–1
–1
— 657 cm
— 701 cm
x
1
H
W
H
M
,c
m
–
1
i
to
t
s
/
,x
Fig. 5. Temperature dependence of the integrated in-
tensity, Ii, of one-phonon lines at 608, 657, and 701 cm–1
to their total integrated intensity, I Ii itot � � . The solid
lines present the scaled populations, xs, of the Co3 + ions
in the LS (x0) and IS (x1) states. The inset shows the
temperature dependence of the linewidth for the 657 cm–1
line.
bending-type vibrations are split into three peaks as
the stretching-type modes are. The intensity of these
three peaks as a function of temperature is displayed
in Fig. 6,a. Figure 6,b displays an example of the
spectra fitting in the frequency region of the bend-
ing-type vibrations. Note that the intensity of the
494-cm–1 modes also demonstrates anomalous tem-
perature dependence similar to the 608-cm–1 mode. In
contrast to the 608-cm–1 mode, however, the 494-cm–1
mode shows no substantial softening of its peak energy
upon cooling below 130 K (not shown here).
The contrasting behavior of the observed phonon’s
intensity and frequency (see Figs. 4–6) can be ex-
plained by the following scenario. One can suppose
that oxygen octahedra with Co3 + ions in different spin
states possess a different Raman response (Raman
tensor) because of difference in Co3 + radii and, more
important, due to an increase of covalency of the
oxygen-metal bonds in the IS state [30]. The change
of the covalency has also a strong impact on the lattice
dynamics. However, the effect of all these circum-
stances is very selective to the symmetry of a given
vibration of the oxygen octahedra. For instance, it is
clear that the frequency of the stretching-like mode
must increase upon an increase of covalency while the
frequency of the bending-like mode is not so sensitive
to the p—d hybridization. Applying these consider-
ations to the 494- and 608-cm–1 modes, one can assign
them to the bending and stretching type of oxygen
octahedra vibration, respectively, which both origin-
ate from Eg modes of the rhombohedric phase. The
strongest evidence of this assignment comes from their
intensities as a function of temperature that has to
reflect the increasing number of JT distorted CoO6
octahedra (Co3 + in the IS state) due to the tempe-
rature-induced increasing population of this spin
state. This is indeed observed in our spectra (see
Figs. 5,6). Note that this assignment also is in ac-
cordance with the lattice dynamical calculations of
the Eg stretching- and bending-like modes in LaAlO3
[26].
Then we arrive at the strongest phonon line at 657
cm–1. Assuming that this phonon mode is the Eg
stretching vibration in the rhombohedric phase one
can explain the unusual temperature dependence of its
integrated intensity. Indeed, in this case the contri-
bution to the scattering intensity comes from the
population of undistorted CoO6 octahedra (Co3+ in
the LS state).
Based on above arguments we can finally assign
lines at 468 and 657 cm–1 to Eg bending and stretch-
ing vibrations in the non-JT distorted phase, re-
spectively. Lines at 483, 494 cm–1 can be assigned to
stretching- and lines at 608, 701 cm–1 to bending-type
vibrations of Ag or Bg symmetry in the JT distorted
phase. An assignment of all lines in the spectra of
LaCoO3 need in more detailed study. Herewith, it is
necessary to take into account possible more com-
plicated scenario of crystal structure changing via
temperature starting from single local distortions at
low temperatures to macroscopic phase separation at
higher temperatures.
As was mentioned above, integrated intensities of
phonon modes associated with different phases must
follow the relative volume of the corresponding phase.
Figure 5 presents the scaled populations, xs (S =
= 0, 1), for the Co3 + ions in LS and IS states. We
have used the model of Ref. 15 and the same pa-
rameters for the calculation of xs . The correlation of
the relative integrated intensities between the 608-
and 657-cm–1 modes and the populations x0 and x1,
respectively, are not unreasonable except the tem-
perature regime below 50 K for both modes. Here, the
657-cm–1 mode does not reach its expected maximum.
In contrast, the 608-cm–1 mode has a finite intensity
even at lowest temperature. This implies that a pure
LS state is never achieved even at lowest temperature,
that is, a small amount of IS Co3 + ions are still
present for temperatures below 50 K. A similar con-
jecture has been made in Ref. 21. Furthermore, our
study is consistent with the analysis of thermal ex-
pansion and magnetization measurements [15] which
also show the absence of a pure spin state at finite
temperature. An evidence for a monoclinic distorted
structure of LaCoO3 in the temperature interval
20–300 K was presented in Ref. 18. It was concluded
that the monoclinic distortion state is brought about
by a cooperative JT effect which triggers the long
range orbital ordering of the eg orbitals. In Ref. 27 it
Low temperature mixed spin state of Co3 + in LaCoO3 evidenced from Jahn–Teller lattice distortions
Fizika Nizkikh Temperatur, 2006, v. 32, No. 2 223
0.2
0.3
0.4
0.5
0.2
0.3
0.4
0.5
450 470 490 510
100
200
300
400
– 494 cm–1
–1
–1
a
– 483 cm
Raman shift, cm
–1
b 5 K
XX
0 100 200 300
0.2
0.3
0.4
0.5
– 468 cm
T, K
I
I
i
to
t
/
I
I
i
to
t
/
I
I
i
to
t
/
In
te
n
si
ty
, a
rb
.u
n
its
Fig. 6. Temperature dependence of the integrated intensity,
Ii, of one-phonon lines at 468, 483, and 494 cm–1
normalized to their total integrated intensity (a) and an
example of the spectra fitting in the frequency region of
bending-like vibrations (b).
was concluded that the magnetic state in LaCoO3
changes from the LS to the mixed state with the
thermally excited IS at higher (� 50 K) temperature,
taking no consideration on the fact that the A g2
modes, which are Raman inactive in the rhombohedral
R c3 phase (those modes are active in the monoclinic
I2/a phase), were observed in the spectra in a whole
temperature range from 5 to 300 K. We do not exclude
that the observation of the extra phonon lines at low
temperatures in our experiments might be due the
surface effects: local distortions or presence of Co3 +
ions with IS state near the surface of the sample.
We will turn now to the high-frequency Raman
scattering response. In the LaCoO3 crystal the one-
phonon peak at 785 cm–1 overlaps with a broad
maximum centered at 895 cm–1 (0.11 eV) (Fig. 7).
We will discuss now its possible origin. It might
originate from a photoionization of small polarons,
i.e., an electronic transition from a band of localized
small polaron states to a conduction band. A Ra-
man scattering study of the paramagnetic phase of
A1–xA
’
xMnO3 system [31] shows such a maximum at
�1100 cm–1 (0.14 eV) with nearly the same spectral
shape as well as with a comparable temperature
dependence of the spectrum. However, this origin is
unlikely in our case as undoped LaCoO3 has no holes
which can form polarons. Furthermore, infrared re-
flectivity measurements [32] unveil that electron-
phonon coupling is not large with respect to any
particular phonon and that rather large polaronic
states would be expected. The broad maximum at
895 cm–1 could be considered also as second-order
scattering of the one-phonon modes at 468, 483, and
494 cm–1. Finally, the observed maximum can arise
from an electronic transition through the charge gap.
The energy of the observed excitation is very close to
the calculated (0.2 eV) [33] and the observed values
of the charge gap (� 0.1 eV) [34] by optical con-
ductivity. Most probably, the observed maximum is a
mixture of electronic excitations through the charge
gap and multiphonon scattering.
Finally, we will focus on the higher frequency
region of the spectra where multiphonon scattering is
observed (Fig. 8). Our measurements show three
features at about 1215, 1305, and 1564 cm–1, which
correspond approximately to twice the energy of the
one phonon modes at 608, 657, and 785 cm–1, re-
spectively. However, not all phonon modes show up
as higher-order scattering. For instance, we find no
evidence for the presence of second-order signal of the
224 Fizika Nizkikh Temperatur, 2006, v. 32, No. 2
V. Gnezdilov et al.
800 900 1000 1100
5 K
50 K
100 K
150 K
200 K
295 K
Raman shift , cm –1
In
te
n
si
ty
,a
rb
.u
n
its
XX
Fig. 7. Temperature dependence of the Raman spectra in
the frequency region of 750–1150 cm–1 for the LaCoO3
single crystal in xx scattering configuration. The spectra
at different temperatures are shifted vertically for clarity.
1000 1200 1400 1600
0
50
100
150
200
250
300 5 K
50 K
100 K
150 K
200 K
295 K
Raman shift , cm
In
te
n
si
ty
,a
rb
.u
n
its
–1
XX
Fig. 8. Temperature dependence of the second-order
Raman spectra in the LaCoO3 single crystal. The spectra
at different temperatures are shifted vertically for clarity.
strong polarization-dependent mode at 562 and
701 cm–1. A quantitative characterization of the se-
cond–order Raman response is summarized in Fig. 9.
The ratio of a second- to first-order integrated Raman
intensities, that is, I(2)/I(1) decreases gradually with
increasing temperature for the mode at 608-cm–1.
Such a behavior is rather expected because both the IS
and the LS sites have to contribute to the two–phonon
Raman process. In contrast, the one-phonon intensity
depends just on the population of the IS state. How-
ever, this intensity develops much faster than the ratio
of (x0 + x1)/x1 (this ratio can be easily depicted from
the Ref. 15) does. A nearly temperature-independent
ratio of I(2)/I(1) seen for the 657- and 785-cm–1
modes as well as for their corresponding overtones at
1305 and 1564 cm–1 is observed (see Fig. 9). Such a
difference directs us to emphasize two different me-
chanisms leading to the corresponding two-phonon
scattering intensity.
Recent theoretical [35,36] and experimental
[37,38] studies of multiphonon scattering in man-
ganites reveal that in an orbital ordered state the
conventional two-phonon Raman process is super-
imposed by two-phonon scattering induced by the
Franck–Condon (FC) mechanism. These two processes
depend in different order of perturbation theory on
electron-phonon interaction constant. They are also
selective to the symmetry of vibrations. In the case of
a dominant FC contribution the ratio, I(2)/I(1), is ex-
pected to be temperature independent [36,37]. Thus,
the interrelation between first- and second-order
Raman scattering of the 657- and 785-cm–1 modes,
shown in Fig. 9, strongly indicates the presence of the
FC mechanism, i.e., orbital order [18,27] in LaCoO3
at least on a short range scale and even at low tempe-
ratures. Furthermore, on heating, the ratio I(2)/I(1)
demonstrates a small increase, which is in accordance
with an increasing population of the IS state.
4. Conclusion
In summary, we have reported first- and second-
order Raman scattering on the single crystalline
LaCoO3. The first-order spectra exhibit a larger num-
ber of phonon modes than allowed for rhombohedral
(R c3 ) symmetry. Additional modes are identified as
activated oxygen octahedra modes due to JT di-
stortions associated with Co3 + ions in the inter-
mediate spin state. Moreover, the temperature de-
pendence of the frequencies and intensities of the
bending- and stretching-type phonon modes on cool-
ing suggests the presence of Co3 + ions in the inter-
mediate spin state, even at lowest temperatures. In the
high-frequency region an anomalous temperature
dependence is observed for the two-phonon scattering
at 1305 and 1564 cm–1, which is specific for the
Franck–Condon mechanism. One- and two-phonon
Raman scattering both signal a JT orbital order in
LaCoO3 at least with short range correlations. This
spectroscopic investigation supports and substantiates
earlier thermodynamic experiments with respect to an
intrinsic mixing of low spin and intermediate spin
states at low temperatures in LaCoO3.
This work was supported by INTAS Grant
¹01-0278, NATO Collaborative Linkage Grant
PST.CLG.977766 and DFG through SPP1073.
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