Doping and Oxidation Effects in Raman Spectra of Manganites
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consider the response of Raman scattering to the composition of manganite single crystals and films. EXPERIMENT Sr- and Ca-doped LMO System Lal.-SrMnO 3 (LSMO) single crystals prepared by a floating-zone technique were examined by x-ray analysis and x-ray diffraction. The Lal-XCaMnO 3 (LCMO) films on LaA103 (LAO) and NdGaO, (NGO) substrates were prepared from ceramic materials by a laser ablation technique [5]. Raman scattering (RS) was excited by the 514.5 nm line of the Ar+ laser and detected in the backscattering geometry. A triple-stage spectrometer with a cooled CCD detector (140 K) was used to obtain Raman spectra with a typical spectral resolution of -4 cm-l. The samples were studied over the temperature range from 4.3 to 350 K. A small distortion of a nearly cubic perovskite crystal lattice results in a complex structure of Raman spectra of La.,RP1MnO 3. Raman spectra of the LSMO/ LCMO system may be presented as a superposition of three components: first-order RS, second-order RS and electronic RS [6]. The first component follows the selection rules for vibrational transitions (phonons). The second component is due to the density of vibrational states and usually contains broad-band features. It was found that for three different dopants R = Ca. Sr and Pb the main Raman features are very similar [6]. In particular, the peaks located at 190 and 435 cm' were found to be the most representative for the spectrum of doped La, -Sr0,MnO 3 single crystal (Fig. 1). These peaks can be 143
Mat. Res. Soc. Symp. Proc. Vol. 574 711999 Materials Research Society
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LaPb
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LaSr
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Raman shift (cm" )
Fig. 2. Raman shift of the Ag mode versus value of doping, x
Fig. 1. Polarized Raman spectra of doped manganites: a, b, c - LSMO, d - Lao. 7Cao.,MnO 3
well separated in the y iv'(AIg) and x iv'(Big) scattering geometries where the prime symbol ' denotes the 45°-rotation of the sample in the xy-plane. According to neutron studies, for x -0.3, La1 .0Sr0Mn0 3 compounds must possess the rhombohedral (D 3 d) symmetry. The observed spectra, however, are more consistent with a tetragonal-distorted structure accounting for the known polarization properties of the corresponding Raman tensor components. The preference for a tetragonal structure over a rhombohedral one is motivated by the presence of the Big mode (435 cm-') and the absence of the E, modes as demonstrated by specific polarization dependence of spectra in different scattering geometries [6, 7]. Raman spectra of LSMO/LCMO samples with different values of doping are presented in Fig. 1. Note that the features related to the D 21,16 orthorhombic structure (the peaks at 493 and 609 cm"), typical for undoped LMO system, are decreasing in intensity with the doping value, x. We attribute this phenomenon to the gradual relative decrease of the orthorhombic lattice distortion due to a reduction of the Jahn-Teller effect in doped materials. These peaks are practically invisible in th
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