Comparative study of optical phonons in the rhombohedrally
distorted perovskites LaAlO
3
and LaMnO
3
M. V. Abrashev,* A. P. Litvinchuk, M. N. Iliev, and R. L. Meng
Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5932
V. N. Popov and V. G. Ivanov
Faculty of Physics, Sofia University, BG-1164 Sofia, Bulgaria
R. A. Chakalov
Institute of Electronics, Bulgarian Academy of Sciences, Tzarigradsko Chausse 72, 1784 Sofia, Bulgaria
C. Thomsen
Institut fu ¨r Festko ¨rperphysik, Technische Universita ¨t Berlin, Hardenbergstrasse 36, D-10623 Berlin, Federal Republic of Germany
Received 10 August 1998
The Raman and infrared phonons of isostructural rhombohedral LaMnO
3
and LaAlO
3
are studied at room
temperature. The experimental spectra are compared with the prediction of lattice-dynamical calculations and
the lines observed are assigned to definite atomic vibrations. It is shown that the Raman mode of A
1g
symmetry
in LaAlO
3
and LaMnO
3
at 123 cm
-1
and 236 cm
-1
, respectively involves atomic motions that cause the
rhombohedral distortion, i.e., it is a ‘‘soft’’ mode, and its position could be used as a measure of the degree of
the distortion. It is also argued that the broad Raman bands in the high-frequency range of LaMnO
3
are not
proper modes of the rhombohedral R 3
¯
c structure, but are rather induced by the dynamic Jahn-Teller effect.
S0163-18299902706-X
I. INTRODUCTION
The manganese perovskites with general formula
R
1 -x
A
x
MnO
3
( R =rare earth, A =Ca, Sr, Ba, Pb) are char-
acterized by a strong interplay of the structural, electronic,
and magnetic properties. Recently, they attracted a renewed
interest due to the effect of ‘‘colossal’’ negative magnetore-
sistance CMR exhibited near the temperature where metal-
insulator and paramagnetic-ferromagnetic transitions occur.
1
Shortly after the discovery of these materials, the concept of
the ‘‘double-exchange’’ DE has been developed to describe
both the ferromagnetism FM and the metallic conductivity
in the low-temperature phase. Within the DE model the
transport in the FM metallic phase involves hopping of spin-
polarized charge between Mn
3 +
and Mn
4 +
sites.
2
However,
recently Millis et al.
3
have shown that the DE alone is insuf-
ficient to account for either the low transition temperature or
the large change in resistivity near T
c
. It has been proposed
that in addition to the DE mechanism, the strong electron-
phonon coupling in the form of dynamical Jahn-Teller dis-
tortions tends to localize the conduction electrons into po-
laronic states. While the Jahn-Teller distortions are large in
the insulating state, they are at least partly removed below
T
c
. The large isotope shift of T
c
( 20 K) upon
18
O for
16
O
substitution does indicate a strong spin-lattice coupling.
4
Therefore it is expected that the Raman and infrared spec-
troscopies could be powerful tools to clarify the role of the
phonons in CMR and related phenomena.
The crystal structure of the undoped parent compound
LaMnO
3
depends on the synthesis conditions.
5,6
At least two
well-determined phases exist at room temperature: the
orthorhombically distorted space group Pnma ) and the
rhombohedrally distorted space group R 3
¯
c ), both are para-
magnetic insulators.
6
The rhombohedral phase is of special
interest as it is typical for some R
1 -x
A
x
MnO
3
materials ex-
hibiting CMR. It is plausible to expect the phonon spectra of
this phase to be sensitive to some specific features associated
with the CMR. Such are the variations of Jahn-Teller distor-
tions with the Mn
4+
/Mn
3 +
ratio as well as with the tempera-
ture near T
c
. The phonon spectra will also be sensitive to the
degree of rhombohedral distortions. This concerns, in par-
ticular, the first-order Raman spectra, which become allowed
exclusively due to lattice distortions as no Raman-active
modes are anticipated in the ideal perovskite structure
( Pm 3 m ). At present there is, however, scanty data on the
Raman spectra of rhombohedral LaMnO
3
and related
La
1 -x
A
x
MnO
3
compounds and the assignment of the ob-
served spectral structures is still ambiguous. The main diffi-
culties arise because of the very low Raman intensities,
which has given rise to speculations that some of the broad
bands in the experimental spectra may correspond to second-
order Raman processes.
7
To our knowledge there are no re-
ports on either polarized Raman spectra of rhombohedral
LaMnO
3
that could unambiguously determine the symme-
tries of experimentally observed lines, or on lattice dynami-
cal calculations LDC that could predict the shapes of Ra-
man modes of given symmetry and the expected range of
their wave numbers.
In this work we report the results of our study of optical
phonons in isostructural rhombohedral LaMnO
3
and
PHYSICAL REVIEW B 1 FEBRUARY 1999-II VOLUME 59, NUMBER 6
PRB 59 0163-1829/99/596/41468/$15.00 4146 ©1999 The American Physical Society