A Raman scattering study of structural changes

in LaMn1�xCoxO3+d system

Nguyen Van Minh a,b, In-Sang Yang a,*a Division of Nanosciences and Department of Physics, Ewha Womans University, 120-750 Seoul, Republic of Korea

b Department of Physics, Hanoi University of Education, 136 Xuan-Thuy Road, Hanoi, Vietnam

www.elsevier.com/locate/vibspec

Vibrational Spectroscopy 42 (2006) 353–356

Available online 21 June 2006

Abstract

We present results of Raman scattering studies on LaMn1�xCoxO3+d over a wide range of doping content (x = 0.1–0.75) and temperature range

of 20–300 K. Powder X-ray diffraction patterns show that there is a structural change from orthorhombic to rhombohedral at x = 0.5 as x increases.

Raman spectra of all LaMn1�xCoxO3+d samples show peaks near 260, 500, and 650 cm�1. However, the Raman spectra are not drastically different

from each other across the structural phase transition at x = 0.5. On the other hand, the peak frequencies of the modes near 260 and 500 cm�1 as

functions of Co content (x) show slope changes at x = 0.5. The full-width at the half-maximum (FWHM) of the mode near 650 cm�1 as a function

of Co content (x) shows minimum at x = 0.5. Normally, larger values of FWHM are expected at near x = 0.5, if the mode were affected by the

structural disorder at the phase boundary. Therefore, it is likely due to lowest charge concentration at x = 0.5, which results in lowest screening

effect. This is consistent with the fact that the intensity of the phonons is strongest at x = 0.5. As the temperature decreases, the two peaks near 500

and 650 cm�1 of different Co contents, related with octahedral distortions, are found to shift to lower frequencies unlike the usual temperature

behavior. However, no abrupt change in the peak frequencies and the FWHM is observed across measured temperature range, regardless of the Co

content.

# 2006 Elsevier B.V. All rights reserved.

Keywords: LaMn1�xCoxO3+d; Manganite; Perovskite; Raman scattering; Electron–phonon interaction; Structural phase transition

1. Introduction

Recently, interests in the lanthanum manganites belonging

to the perovskite family have strongly increased because of

their unique properties. These materials find wide range of

application areas of current technologies since doping of the

compounds with bivalent Ca or Sr cations induces colossal

magnetoresistance (CMR). There are many Raman studies on

La-site doped manganites, but only a few study on Mn-site

doped [1]. Raman spectroscopy has the advantage of being very

sensitive not only to the structural phase transition but also to

the subtle changes in the local structure or the electronic states.

It is a useful tool to simultaneously measure the structural

distortion and the changes in the charge carrier density induced

by the doping for the Mn-site in the LaMnO3+d structure.

* Corresponding author. Tel.: +82 2 3277 2332; fax: +82 2 3277 2372.

E-mail address: yang@ewha.ac.kr (I.-S. Yang).

0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.vibspec.2006.05.027

Co doping into the Mn-site in LaMn1�xCoxO3+d is expected

to affect the MnO6 octahedra. Raman modes of LaMnO3+d are

already well known to show three major peaks at 280, 490, and

610 cm�1, which are identified as rotational-, bending-, and

stretching-like vibrations of the MnO6 octahedra, respectively

[2]. Therefore, the effect of Co doping in LaMn1�xCoxO3+d is

expected to appear as changes in the Raman spectra of the

compounds as the Co content changes. In the present work, the

changes in the local structure and the charge carrier density

induced by the Co doping in the LaMn1�xCoxO3+d compound

are investigated by Raman spectroscopy.

2. Experimental details

Polycrystalline ceramic samples were prepared by conven-

tional solid-state reaction methods, heating stoichiometric

mixtures of La2O3, MnO2, and CoO at 1000 8C for 36 h with

intermediate grindings. The powders were then pelletized and

sintered at 1400 8C for 36 h. The structure and phase purity

were checked by powder X-ray diffractometer (Rigaku) with

N.V. Minh, I.-S. Yang / Vibrational Spectroscopy 42 (2006) 353–356354

Fig. 2. Raman spectra of LaMn1�xCoxO3+d (x = 0.1–0.75) samples at room

temperature. The inset shows details of the Raman mode near the 260 cm�1 for

some selected values (not all) of x.

Cu Ka radiation (1.5415 A). Raman scattering measurements

were performed in a backscattering geometry using a Jobin

Yvon T64000 triple spectrometer equipped with a charge-

coupled device (CCD) camera. The spectra were excited with

the 514.5 nm line of an argon ion laser with low power

density to avoid the laser heating. For Raman measurements

at low temperatures, the samples were mounted on the cold

finger of a closed-cycle He refrigerator. Temperature-

dependent Raman measurements were performed in the

warming cycle.

3. Results and discussion

As shown in Fig. 1, powder X-ray diffraction patterns of

LaMn1�xCoxO3+d indicate a structural transition from orthor-

hombic to rhombohedral structure when the Co content exceeds

0.5. The crystal structures of the end members, LaMnO3 and

LaCoO3 have been known for decades to be orthorhombic

(space group Pnma, Z = 4) [3] and rhombohedral (space group

R3c, Z = 2) [4], respectively. The crystal structures of

LaMn1�xCoxO3+d compounds have been studied and found

to be orthorhombic x < 0.5, and rhombohedral x > 0.5. At

x = 0.5, the compounds are known to have the mixture of the

two structural phases [5].

Raman spectra of LaMn1�xCoxO3+d (x = 0.1–0.75) com-

pounds are measured in the temperature range of 20–300 K.

The Raman spectra for LaMn1�xCoxO3+d are not drastically

different from each other across the structural phase transition

at x = 0.5, showing peaks near 260, 500, and 650 cm�1.

According to lattice dynamic calculations, the most intense

modes, near 260, 500, and 650 cm�1, are associated with

rotational-, bending-, and stretching-like vibrations of the

Mn(Co)O6 octahedra, respectively. Room temperature mea-

surements, lattice dynamical calculations, and assignment of

the Raman modes of undoped LaMnO3 were done previously

by Iliev et al. [2]. Two bands near 500 and 650 cm�1 were

related to the Jahn-Teller octahedral distortions. We will

concentrate on the effect of the Co substitution for Mn on the

rotation (�260 mode), bending (�500 mode), and stretching

(�650 mode) vibrations of the MnO6 octahedra.

Fig. 1. Powder X-ray diffraction patterns of LaMn1�xCoxO3+d (x = 0.1–0.75)

samples.

Fig. 2 displays the Co-content dependence of Raman spectra

of LaMn1�xCoxO3+d measured at room temperature. The inset

shows details of the Raman mode near the 260 cm�1 for some

selected values (not all) of x only, for clarity. It is noted that the

intensity of the Raman spectra is strongest at x = 0.5. With

increasing Co content x, the Raman scattering intensity of the

phonon modes is increasing, and when the Co content exceeds

0.5, the intensity of the Raman modes decrease again. Direct

comparison of intensities of Raman spectra measured in

different runs of experiment requires careful normalization of

all the conditions. Intensity of a known Raman peak such as Si

peak at 521 cm�1 may serve as a reference in calibrating the

intensities of Raman spectra measured in different runs.

At x = 0.5, phase transition from orthorhombic to rhombo-

hedral structure occurs [5]. The Mn(Co)O6 octahedra are

rotated around the [0 1 0] and then [1 0 1] directions in the

orthorhombic phase, while the same octahedral are rotated

around [1 1 1] direction in the rhombohedral phase. In other

words, there is a change only in the direction of the rotation of

the Mn(Co)O6 octahedra at the structural phase transition

boundary x = 0.5. However, the observed Raman modes are

from the internal vibrations of the Mn(Co)O6 octahedra. Thus,

there is no apparent reason for the changes in the intensity of the

Raman modes from the structural point of view. Rather, it is

reasonable to assume that the screening effect of the charge

carriers is minimum at x = 0.5 as Co is doped into the

LaMnO3+d system.

This assumption is clearly consistent with the behavior of

the FWHM of the 650 mode as function of Co content as seen in

Fig. 3. The charge carriers affect the phonons through electron–

phonon interaction, giving rise to the broadening of the phonon

modes due to dissipation of the phonon energy. The FWHM of

the 650 mode as a function of Co content shows minimum at

x = 0.5. This is unusual for that we might expect maximal

disorder due to substitution, if there is any, at x = 0.5 in

LaMn1�xCoxO3+d. Minimum FWHM of the 650 mode means

that the dissipation of the phonon mode is lowest at x = 0.5. It

could be due to lowest charge concentration x = 0.5, which

results in lowest screening effect of the phonons at x = 0.5. This

is correlated with the strongest intensity of the phonon modes at

x = 0.5 as seen in Fig. 2.

N.V. Minh, I.-S. Yang / Vibrational Spectroscopy 42 (2006) 353–356 355

Fig. 3. The full-width at the half-maximum (FWHM) of the mode near

650 cm�1 at room temperature as a function of the Co content x. The solid

line is guide for the eye.

Fig. 5. Temperature dependences of the peak frequencies of the mode near

650 cm�1 for several Co contents as indicated. The lines are guide for the eye.

Fig. 6. Temperature dependences of the FWHM of the mode near 650 cm�1 for

several Co contents as indicated.

The peak frequencies of the 260, 500 and 650 modes at room

temperature as functions of the Co content are shown in Fig. 4.

The two modes near 260 and 500, involving rotation and

bending of the Mn(Co)O6 octrahedra, show changes in the

frequency at x = 0.5. On the other hand, the 650 mode, which

involves stretching vibration of the Mn(Co)O6 octahedra, does

not show any change in the frequency throughout the Co

content. This is consistent with the fact that there is a change

only in the direction of the rotation of the Mn(Co)O6 octahedra

at the structural phase transition boundary x = 0.5. Internal

stretching vibrations would not be affected by the orientational

change of the Mn(Co)O6 octahedra, but the rotational and

bending vibrations would be affected by the relative orienta-

tions of the neighboring Mn(Co)O6 octahedra.

As the temperature decreases, intensifying and sharpening

of all the Raman modes are observed. Unlike the usual

temperature behavior, the two phonon modes near 500 and

650 cm�1, related with octahedral distortions, are found to shift

to lower frequencies for all x values of LaMn1�xCoxO3+d. Such

Fig. 4. The peak frequencies of the modes near 260, 500, and 650 cm�1 at room

temperature as functions of the Co content x. The lines are guide for the eye.

peak shifts of the two Raman modes involving the octahedral

distortions indicate softening of the bonds in the Mn(Co)O6

octahedra at low temperatures. The peak frequencies and the

FWHM for the 650 mode as functions of the temperature for

several values of x in LaMn1�xCoxO3+d are shown in Figs. 5 and

6, respectively. No abrupt change in the peak frequencies and

the FWHM is observed across measured temperature range.

This shows that there is no dramatic change in the octahedral

distortions below 300 K. The softening of the two phonon

modes persists down to the lowest temperature, therefore, it

may not be associated with the JT transition.

4. Conclusion

Results of Raman scattering studies on LaMn1�xCoxO3+d

over a wide range of doping content (x = 0.1–0.75) and

temperature range of 20–300 K are presented. Powder X-ray

diffraction patterns confirm that there is a structural change

from orthorhombic to rhombohedral at x = 0.5 as x increases.

N.V. Minh, I.-S. Yang / Vibrational Spectroscopy 42 (2006) 353–356356

However, the Raman spectra for LaMn1�xCoxO3+d are not

drastically different from each other across the structural phase

transition at x = 0.5. On the other hand, the intensity of the

phonon modes is strongest at x = 0.5. The peak frequencies of

the modes near 260 and 500 cm�1 as functions of Co content (x)

show slope changes at x = 0.5. The full-width at the half-

maximum (FWHM) of the mode near 650 cm�1 as a function of

Co content (x) shows minimum at x = 0.5. This means that the

dissipation of the phonon mode is lowest at x = 0.5. It could be

due to lowest charge concentration at x = 0.5, which results in

lowest screening effect, thus strongest intensity of the phonons

at x = 0.5. As the temperature decreases, the two peaks near 500

and 650 cm�1 of different Co contents, are found to shift to

lower frequencies, unlike the usual temperature behavior.

However, no abrupt change in the peak frequencies and the

FWHM is observed across measured temperature range,

regardless of the Co content.

Acknowledgments

This work was supported by the Korea Research Foundation

Grant (KRF-2004-005-C00057). We would like to thank Prof.

Sung-Jin Kim for her help in X-ray experiments. N.V.M.

acknowledges the support from the National Basic Research

Program of Vietnam.

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