The Raman-active phonons in perovskite-like LaMn₁–xCoxO₃ (x = 0, 0.2, 0.3, 0.4, and 1.0) were studied by measuring Raman spectra at temperatures of 295 and 5 K. The changes in the spectra with Co doping are correlated with the decrease of orthorhombic distortions. Surprisingly more phonon lines than allowed for the rhombohedral LaCoO₃ structure were observed

Barilo, S.N.

Bychkov, G.L.

Gnezdilov, V.P.

Guntherodt, G.

Lemmens, P.

Pashkevich, Yu.G.

Shiryaev, S.V.

Yeremenko, A.V.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

Fizika Nizkikh Temperatur, 2003, v. 29, No. 11, p. 1269–1272Phonon Raman scattering in LaMn1–xCoxO3(x = 0, 0.2, 0.3, 0.4, and 1.0)V.P. Gnezdilov1, Yu.G. Pashkevich2, A.V. Yeremenko1, P. Lemmens3,4,G. Guntherodt3, S.V. Shiryaev5, G.L. Bychkov5, and S.N. Barilo51B. Verkin Institute for Low Temperature Physics and Engineering of the National Academyof Sciences of Ukraine, 47 Lenin Ave., Kharkov 61164, UkraineE-mail: gnezdilov@ilt.kharkov.ua2A.A. Galkin Donetsk Phystech of the National Academy of Scienses of Ukraine72 R. Luksemburg str., Donetsk 83114, Ukraine32.Physikalisches Institut, RWTH Aachen, D-52056 Aachen, Germany4Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany5Institute of Physics of Solids & Semiconductors, Academy of Sciences, Minsk 220072, BelarusReceived April 24, 2003The Raman-active phonons in perovskite-like LaMn1–xCoxO3 (x = 0, 0.2, 0.3, 0.4, and 1.0)were studied by measuring Raman spectra at temperatures of 295 and 5 K. The changes in the spec-tra with Co doping are correlated with the decrease of orthorhombic distortions. Surprisingly morephonon lines than allowed for the rhombohedral LaCoO3 structure were observed.PACS: 78.30.Hv, 63.20.–eThe manganese perovskites of the typeR1–xAxMnO3 (R = rare earth, A = Ca, Sr, Ba, or Pb)have been subject of scientific investigations for manydecades. Recently, this interest has been renewed dueto the observation of a colossal magnetoresistance[1,2], charge, spin, and orbital ordering effects as afunction of Mn3+/Mn4+ ratio [3–5]. Another system,Mn-site-doped, with the composition LaMn1–xDxO3(D = Cr, Fe, Co, or Ni) was intensively studied in the60’s, but colossal magnetoresistance was not men-tioned in them until the 90’s [6]. While Raman spec-tra of La-site-doped compounds have been reported innumerous of publications [7–13], surprisingly nothingwas done on Mn-site-doped compounds. In this workwe report the results of an optical phonon study in theperovskite oxides LaMn1–xCoxO3 (x = 0, 0.2, 0.3,0.4, and 1.0). The end member of this system, namelyLaCoO3 has been the subject of continuing interestsince the 50’s due to unusual magnetic properties andtwo spin-state transitions [14,15].© V.P. Gnezdilov, Yu.G. Pashkevich, A.V. Yeremenko, P. Lemmens, G. Guntherodt, S.V. Shiryaev,G.L. Bychkov, and S.N. Barilo, 2003Mn(Co)LaxyzOFig. 1. Unit cell of the simple perovskite structure.Raman scattering measurements were carried out inquasi-backscattering geometry using 514.5 nm argonlaser line. The incident laser beam of 10 mW power wasfocused onto a 0.1 mm spot of the mirror-like chemi-cally etched as grown crystal surface. The sample wasmounted on the holder of a He-gas-flow cryostat usingsilver glue. The scattering light was analyzed with aDILOR XY triple spectrometer combined with a nitro-gen-cooled CCD detector. The measurements weredone in the xx(zz) scattering configuration, where xand z are the [100] and [001] quasicubic directions, re-spectively.The structural properties of LaMn1–xCoxO3 werecharacterized in several publications [15–19]. Thecrystal structures of the end members, LaMnO3 andLaCoO3 were found to be orthorhombic (space groupPnma, Z = 4) [16] and rhombohedral (space groupR c3 , Z = 2) [17], respectively. The compounds with0.15 < x < 0.50 have been found to be orthorhombic[18,19]. When x > 0.50 and near to 1.0, the compoundshave a rhombohedral structure [19]. Around 0.5 dopinglevel, the compounds show a mixture of two structuralphases, the orthorhombic and rhombohedral [19]. Theidealized cubic perovskite structure of the LaMn(Co)O3crystal is shown in Fig. 1. The orthorhombic Pnmastructure can be obtained be two consequent rotations ofthe Mn(Co)O6 octahedra around the [010] and [101] di-rections of cubic perovskite. The rhombohedral R c3structure is generated by the rotation of the same octahe-dral about the cubic [111] direction.Results of group-theoretical analysis for zone-centervibrations are presented in Table for orthorhombicLaMnO3 and rhombohedral LaCoO3. Of the total 30-point phonon modes, only 5 (A1g + 4Eg) are Ramanactive for rhombohedral LaCoO3 structure and of 60-point phonon modes, 24 (7Ag + 5B1g + 7B2g + 5B3g)are Raman-active for orthorhombic LaMnO3 structure.The increase in the phonon modes from 5 to 24 on goingfrom rhombohedral to orthorhombic structure is due to(i) lowering of crystal symmetry which splits the doublydegenerate Eg modes into nondegenerate B2g + B3g,(ii) displacement of oxygen atoms into the lowersymmetry site of the La-plane which introduces newRaman-active vibrations, (iii) doubling of the unit cellwhich folds the zone-boundary modes of the rhom-V.P. Gnezdilov et alFizika Nizkikh Temperatur, 2003, v. 29, No. 11 1270TableFactor group analysis and selection rules for the zone-center vibrational modes of orthorhombic LaMnO3 andrhombohedral LaCoO3Sample andspace groupAtomNumber ofequivalentpositions(Wyckoffnotation)Site symmetryIrreducible representation ofmodesActivity and selection rulesLaMnO3PnmaLa 4(c) Cs 2 21 2 3A B B Bg g g g      A B B Bu u u u2 21 2 3Raman   7 5 1A Bg g 7 52 3B Bg g( )D h216 Mn 4(b) Ci 3 3 3 31 2 3A B B Bu u u u   IR   9 7 91 2 3B B Bu u uZ = 4 O1 4(c) Cs 2 21 2 3A B B Bg g g g      A B B Bu u u u2 21 2 3acoustic   B B Bu u u1 2 3silent  8AuO28(d) C1 3 3 31 2A B Bg g g  + 3 3 33 1B A Bg u u   3 32 3B Bu uA a a axx yy zzg: , ,B a B a B axy g xz g yz1g: ; : ; :2 3LaCoO3La 2(b) C i3 A A B Eu u u u1 2 22 2   Raman  A Eg g1 4R c3 Co 2(a) D3 A A E Eg u g u2 2   IR  3 52A Eu u( )D d36Z = 2O 6(e) C2 A A B Eg g g g1 22 3 3     A A Eu u u1 22 3acoustic  A Eu u2silent  3 22 1A Ag uA a a axx yy zz1g: ,E a a a a axx yy xy xz yzg: ( , ), ( , )bohedral structure into zone-center modes of theorthorhombic structure.Raman spectra of LaMn1–xCoxO3 compounds at295 K are shown in Fig. 2. Room temperaturemeasurements, lattice dynamical calculations, and anassignment of the Raman modes of undoped LaMnO3were done previously by Iliev et al. [20]. Our spectraof LaMnO3 are consistent with spectra at 300 K re-ported earlier [9,20]. Three broad bands centered near280, 490, and 610 cm–1 are observed. The line near280 cm–1 was assigned to a rotation-like mode [20].The other two bands near 490 and 610 cm–1 are relatedto Jahn-Teller octahedral distortions [20]. Given thatthe Jahn-Teller distortions are static and ordered inorthorhombic LaMnO3, these bands are Raman-al-lowed modes of bending- and stretching-type charac-ter, respectively. The fourths peak in the spectrum at 310 cm–1 is associated with vibrations of the apexoxygen (O1) atoms along the x direction. The spectraof the x = 0.2, 0.3, and 0.4 samples look quite similarto the x = 0 sample, except for the width and positionof the bands in the region of 500 and 600 cm–1. Atroom temperature the spectra of the LaCoO3 sampleexhibit peaks centered near 130, 160, 480, 555, 610,and 780 cm–1, and four broad bands at 70, 270, 340,and 400 cm–1.Lowering the temperature, more phonon peaks be-come visible in the Raman spectra of LaMn1–xCoxO3.In Fig. 3 we present spectra measured at 5 K.LaMnO3: The spectra on the LaMnO3 sample show10 resolved peaks at 80, 110, 130, 154, 184, 257, 280,314, 496, and 610 cm–1. The strong high frequencylines in the studied sample are broader than the corre-sponding lines in the spectra of measured earlier[9,20]. The reason for this broadening is the presenceof a small amount of excess oxygen in our sample.Using x-ray diffraction, magnetic susceptibility, andchemical analysis, the oxygen content was estimatedto be 3.071.LaMn1–xCoxO3 (x = 0.2, 0.3, 0.4): It is well knownthat Raman spectroscopy is a sensitive tool for the studyof both local and spatially coherent structural changes.Spectra of the samples with x > 0 differ from the one ofpure LaMnO3 and in the following we will concentrateon the effect of Mn substitution by Co on the rotation-,bending-, and stretching-like vibrations of the MnO6octahedra. The rotational mode at 280 cm–1 in LaMnO3shifts to lower energy ( 270 cm–1) in the samples withx = 0.2, 0.3, and 0.4. The frequency of this mode is ameasure of the degree of the rotational distortions (theaveraged angle of octahedral tilts). For example, com-paring the spectra of more distorted YMnO3 and lessdistorted LaMnO3 a large shift from 396 to 284 cm–1was observed [20]. Therefore, the softening of the rota-tion-like mode with Co doping is the indication of an de-creasing orthorhombic distortion.Phonon Raman scattering in LaMn1-xCoxO3 (x = 0, 0.2, 0.3, 0.4, and 1.0)Fizika Nizkikh Temperatur, 2003, v. 29, No. 11 12710 100 200 300 400 500 600 700 800 900Raman shift,  cm 1Intensity, arb. units400300200100x = 1.0x = 0.4x = 0.3x = 0.2x = 00.25LaMn       Co  O1 x x 3xx295 KFig. 2. Raman spectra of single crystals of LaMn1–xCoxO3at 295 K. Spectra are shifted for clarity. The right part ofthe spectra are multiplied by a factor indicated there.0 100 200 300 400 500 600 700 800 900100200300400500600700Raman shift,  cm 1Intensity, arb. unitsx = 0x = 0.2x = 0.3x = 0.4x = 1.00.2LaMn        Co  O1 x x 3xx5 KFig. 3. Raman spectra of single crystals of LaMn1–xCoxO3at 5 K. Spectra are shifted for clarity. The right part ofthe spectra are multiplied by a factor indicated there.The scattering intensity of the phonon modes at 496and 610 cm–1 decreases in the Co-doped samples. Thedecreasing of intensity of these modes reasonably be re-lated with the reduction of the Jahn-Teller distortionsin the averaged structure, introduced by the presence ofCo. Moreover, each of these modes splits into two com-ponents. This splitting can be attributed to the coexis-tence of two types of octahedra, namely MnO6 andCoO6 in the composition. The frequency positions ofsome high-frequency lines are plotted in Fig. 4.LaCoO3: The spectra of LaCoO3 are, however,somewhat surprising since (i) they are not similar tothe spectra of isostructural rhombohedral LaMnO3+[9,10,21,22], La1–xAxMnO3 [9,10], and LaAlO3 [22],(ii) due to the neutron-diffraction data the sampleshould have a rhombohedral symmetry D d36 and only 5Raman-active phonon modes are expected. The mainpeaks in the spectra are at 75, 135, 169, 486, 560, 656,and 785 cm–1. Note that high-frequency lines, withlarge intensity in LaCoO3 even at room temperature,are not observed in the spectra of isostructural com-pounds. Moreover, wide bands centered approximatelyat 198, 281, 366, 407 cm–1 and a shoulder at 702 cm–1are present in the spectrum. Probably a part of the fea-tures observed in these spectra are a contribution of sec-ond-order Raman processes, but it is more reasonable toassume that the LaCoO3 crystal has a lower symmetrythan a rhombohedral one. Precise measurements of po-larized Raman spectra could provide valuable informa-tion about the structure of this compound.In conclusion, we have performed a phonon Ramanstudy on the LaMn1–xCoxO3 (x = 0, 0.2, 0.3, 0.4, and 1.0)system at temperatures of 295 and 5 K. Characteristicchanges have been observed in the phonon spectra with Coconcentration and are assigned to the reduction of octahe-dral distortions. The spectra of pure LaCoO3 exhibit alarger number of peaks than allowed for the rhombohedral(D d36 ) symmetry and a more accurate definition of thecrystal structure of this compound is necessary.This work was supported by INTAS Grant No.01-0278 and NATO Collaborative Linkage GrantPST.CLG.977766.1. R. Von Helmolt, J. Wecker, B. Holzapfel, L. Schultz,and K. Samwer, Phys. Rev. Lett. 71, 2331 (1993).2. S. Jim, T.H. Tiefel, M. McCormack, R.A. Fastnacht,R. Manesh, and L.H. Chen, Science 264, 413 (1994).3. H. Yoshizawa, H. Kawano, T. Tomioka, and Y. Tokura,Phys. Rev. B52, 13145 (1995).4. J.B. Googenough, Phys. Rev. 164, 785 (1967).5. C.N. Rao, J. Phys. Chem. B104, 5877 (2000).6. J.H. Park, S.W. Cheong, and C.T. Chen, Phys. Rev.B55, 11072 (1997).7. V.B. Podobedov, A. Weber, D.B. Roberto, J.P. Rice,and H.D. Drew, Solid State Commun. 105, 589 (1998).8. V.B. Podobedov, A. Weber, D.B. Roberto, J.P. Rice,and H.D. Drew, Phys. Rev. B58, 43 (1998).9. E. Granado, N.O. Moreno, A. Garcia, J.A. Sanjurjo,C. Rettori, I. Torriani, S.B. Oseroff, J.J. Neumeier,K.J. McClellan, S.-W. Cheong, and Y. Tokura, Phys.Rev. B58, 11435 (1998).10. Chaitali Roya and R.S. Budhari, J. Appl. Phys. 85,3124 (1999).11. E. Granado, A. Garcia, J.A. Sanjurjo, C. Rettori, I.Torriani, F. Prado, R.D. Sanchez, A. Caneiro, andS.B. Oseroff, Phys. Rev. B60, 11879 (1999).12. E. Liarokapis, Th. Leventouri, D. Lampakis, D. Palles,J.J. Neumeier, and D.H. Goodwin, Phys. Rev. B60,12758 (1999).13. M.V. Abrashev, J. Bäckström, L. Börjesson, M. Pissas,N. Kolev, and M.N. Iliev, Phys. Rev. B64, 144429(2001).14. Kichizo Asai, Atsuro Yoneda, Osamu Yokokura, J.M.Tranquada, G. Shirane, and Key Kohn, J. Phys. Soc.Jpn. 67, 290 (1998) (and references therein).15. P.G. Radaelli and S.-W. Cheong, Phys. Rev. B66,094408 (2002).16. J.B.A.A. Elemans, B. Van Laar, K.R. Van der Veen,and B.O. Loopstra, J. Solid State Chem. 3, 238 (1971).17. G. Thornton, B.C. ToField, and A.W. Hewat, J.Solid State Chem. 61, 301 (1986).18. G.H. Jonker, J. Appl. Phys. 37, 1424 (1966).19. J.B. Goodenough, A. Wold, R.J. Arnott, and N. Menyuk,Phys. Rev. 124, 372 (1961).20. M.N. Iliev, M.V. Abrashev, H.-G. Lee, V.N. Popov,Y.Y. Sun, C. Thomsen, R.L. Meng, and C.W. Chu,Phys. Rev. B57, 2872 (1998).21. E. Granado, J.A. Sanjurjo, C. Rettori, F. Prado, R.D.Sanches, A. Caneiro, and S.B. Oseroff, Phys. StatusSolidi 220, 609 (2000).22. M.V. Abrashev, A.P. Litvinchuk, M.N. Iliev, R.L.Meng, V.N. Popov, V.G. Ivanov, R.A. Chakalov, andC. Thomsen, Phys. Rev. B59, 4146 (1999).1272 Fizika Nizkikh Temperatur, 2003, v. 29, No. 11V.P. Gnezdilov et al660640620600580560540520500480Raman shift,  cm10 0.2 0.4 0.6 0.8 1.0Co content, xFig. 4. Raman shift of some high-frequency phonon modesversus x in LaMn1–xCoxO3 single crystals. The lines areguides to the eyes.

Phonon Raman scattering in LaMn₁₋xCoxO₃ (x = 0, 0.2, 0.3, 0.4, and 1.0)

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Phonon Raman scattering in LaMn₁₋xCoxO₃ (x = 0, 0.2, 0.3, 0.4, and 1.0)

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