Physica B 327 (2003) 208–210

Effect of Ni on structure and Raman scatteringof LaMn1�xNixO3+d

Nguyen Van Minha,b,*, Sung-Jin Kimc, In-Sang Yanga

aDepartment of Physics, Ewha Womans University, 120-750 Seoul, South KoreabDepartment of Physics, Hanoi University of Education, Viet Nam

cDepartment of Chemistry, Ewha Womans University, 120-750 Seoul, South Korea

Abstract

In this report we present the structural and Raman scattering properties of the Mn-site-doped perovskite oxides,

LaMn1�xNixO3+d. X-ray diffraction experiments demonstrate that these samples are single perovskite phase below

x=0.5, whereas they are multiphase at high Ni concentration (x > 0:5). Raman spectroscopy has been carried out in thetemperature range from 20 to 300K. The most interesting feature in the spectra is the occurrence of vibrational modes

at frequencies around 530 and 670 cm�1. The frequencies of these phonons depend on the Ni content, and the full-width

at half-maximum (FWHM) depends strongly on the temperature.

r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Manganites; Raman spectroscopy

Recently, the doped LaMnO3 system hasattracted much attention due to its unusualmagnetic and transport properties, especially thecolossal magnetoresistance [1]. This effect can beunderstood in the framework of the doubleexchange (DE) model [2] and the electron–phononcoupling [3]. In particular, ferromagnetic oxideswith Mn in a mixed valence state seem to begood candidates to show CMR. Replacing of Mnby other transition metal ions in LaMnO3 givesrise to considerable changes in their properties. Al-though Mn-site-doped perovskite oxides,LaMn1�xNixO3+d (LMNO) have been widely

studied [4], there still exist controversies, regardingthe structure as a function of Ni doping.The presence of strong electron–phonon inter-

actions, or Jahn–Teller (JT) effects, in the insulat-ing phase of CMR compounds should beassociated with polaron formation [5]. Generally,the perovskite distortions can be divided into twogroups: distortions changing Mn–O–Mn bondangles and changing the Mn–O distances governedby the magnitude and the spatial coherence of theJT distortions of the MnO6 octahedra. Ramanstudies of the vibrational spectrum would beinformative, given that the temperature depen-dence of the vibrational spectrum provides im-portant information on structural changesassociated with the transition to the low-tempera-ture ferromagnetic (FM) phase [6,7]. In this studywe present structure and Raman scattering studiesof LaMn1�xNixO3þd perovskites.

*Corresponding author. Department of Physics, Ewha

Womans University, 120-750 Seoul, South Korea. Tel.: +82-

2-3277-2319; fax: +82-2-3277-2372.

E-mail address: yang@ewha.ac.kr, minhsp@yahoo.com

(N. Van Minh).

0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 2 9 - 5

The samples of LaMn1�xNixO3+d (x ¼ 0:25; 0.3,0.4, 0.5, 0.6 0.75) were prepared by a solid statereaction method [4]. Fig. 1 shows the X-raydiffraction patterns of LMNO samples. Forxp0.5, the phase is the orthorhombic perovskitestructure. In case of x>0.5 the peak becomesbroad and some of them exhibits splitting, imply-ing the coexistence of two or more phases. Solidsolution seems to exist for xp0:5: It is noticed thatthe ion radii of Mn3+, Ni2+ and Ni3+ are 0.0645,0.069 and 0.056 nm, respectively. This clearlyindicates that Ni enters in these compositions asNi3+ [4], giving rise to a decrease of the Mn/Ni–Odistances. Goodenough et al. [4] have shown that,for 0.5pxp1, there is a two-phase region in whichsome La2NiO4 is present. Because of the Nidoping, the Mn–O–Mn, Mn–O–Ni and Ni–O–Ninetwork co-exist, thus enhancing the inhomogene-ity of the system, as indicated in case of Fe doping[8]. In the previous studies on LMNO [9] it wasshown that all compositions up to x ¼ 0:5 in theseries are orthorhombic, and rhombohedral forx > 0:5: Wold et al. [10] observed a slight mono-clinic distortion for x ¼ 0:5 and a monoclinicstructure for x > 0:5: In our case, all samples ofxp0:5 are found to have only the orthorhombicphase, whereas samples of x > 0:5 seem to bemultiphase.The crystallites in the present samples are thus

assumed to have orthorhombic structure withdescribed by the Pnma space group [6]. Ni is

substituted for Mn as Ni3+. There are 24 Ramanactive modes (7Ag+5B1g+7B2g+5B3g). Ramanspectra of LMNO samples (x ¼ 0:25–0.75) weretaken from 20K to room temperature. Typicalspectra at 20K are shown in Fig. 2. As thetemperature is lowered a shoulder peak around610 cm�1 appears (Fig. 2). These modes have beenidentified as the activated oxygen modes due to acooperative JT distortion [11].In these spectra we focus on the two modes

located around 530 and 670 cm�1. These modesare hard modes with a frequency slightly depen-dent on the temperature and on the JT distortionof the MnO6 octahedra [12]. Both Raman modesare observed to shift to high frequencies as the Nicontent increases (Fig. 3). The FWHM of the twomodes of various samples are plotted in Fig. 4. Asthe temperature increases, a dramatic broadeningof the Raman modes is observed. Taking intoaccount the sign of the change in frequencies andthe difference in the atomic weights of Mn (55) andNi (59), this behavior is unusual in terms of what isknown for mixed crystals where the frequencychange is driven mainly by the mass effect. Weconsider therefore that the above anomalouschange is due to the nature of the interactionbetween Mn/Ni ions and MnO6 octahedra thatcan overcompensate the mass effect. The broad-ening of the stretching peak may be induced by thefollowing two vibrations: (i) the Mn(Ni)–Ostretching vibration in the Mn–O–Ni structureFig. 1. X-ray diffraction patterns of LMNO samples.

200 300 400 500 600 700 8000

500

1000

1500

2000LaMn

(1-x)Ni

xO

3+δ at 20K

0.75

0.6

0.50.40.3

0.25

Inte

nsity

(a.u

.)

Raman shift (cm-1)

Fig. 2. Raman spectra of LMNO samples at 20K.

N. Van Minh et al. / Physica B 327 (2003) 208–210 209

and (ii) the Ni–O stretching vibration in the NiO6octahedra. The anomalous broadening of the hardmodes can be understood assuming the followingscenario for the JT transition in LMNO. At 20Kthe oxygen octahedra show a cooperative JTdistortion associated with the ordering of theMn3+ eg orbitals. The long phonon mean lifetimeobserved at this temperature indicates low defectconcentration, and high ordering in the sample. Asthe temperature increases, due to a thermally

activated disorder of the orientation of the egorbitals in some of the Mn3+ ions, local deviationsfrom the mean crystal structure and a consequentdisorder in the oxygen network is expected. Suchstructural fluctuations may decrease the phononmean lifetime and account for the anomalousphonon broadenings observed in LMNO.In summary, structural and Raman scattering

properties of LaMn1�xNixO3þd compound arestudied. Below x=0.5, the structure is orthorhom-bic, whereas samples of x>0.5 are in multiphase.Two Raman modes, located around 530 and670 cm�1, are found to shift toward higherfrequencies as Ni content increases, implying thatthere are changes of angle and distance of bonds inthe MnO6 octahedra. No abrupt change in theFWHM across the metal–insulator transitiontemperatures, indicates that it may be associatedwith structural distortions that remain approxi-mately constant as the temperature is lowered,consistent with the absence of structural phasetransition in the range of measured temperature.

Acknowledgements

This work was supported by BK 21 project. SJKwishes to thank Korea Research FoundationGrant KRF-1997-011-D5519, ISY, KRF-2000-015-DS0014. We would like to thank Yim Sunahfor her help in X-ray experiments.

References

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0 80 160 240 32020

40

60

(*)- 530 cm-1

(**)- 670 cm-1

**(.6)*(.6)

**(.5)

*(.5)

**(.4)

*(.4)

FWH

M (c

m-1)

Temperature (K)

Fig. 4. Temperature dependence of the FWHM of the 530 and

670 cm�1 modes of LMNO. For clarity, only the sets of x ¼0.4,0.5, 0.6 are shown.

0.2 0.3 0.4 0.5 0.6 0.7 0.8516

520

524

528

532 20 K

Ni content

530

cm-1 m

ode

656

660

664

668

672

676

670

cm-1 m

ode

Fig. 3. Peak positions of the 530 and 670 cm�1 modes as a

function of Ni doping.

N. Van Minh et al. / Physica B 327 (2003) 208–210210