Abnormal lattice expansion and double periodicity in La 0.7 Sr 0.3 MnO 3 thin films under electron irradiation
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H.B. Lu, Z.H. Chen, and G.Z. Yang Laboratory of Optical Physics, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China (Received 6 November 2004; accepted 24 February 2005)
Perovskite-based SrNb0.2Ti0.8O3/La0.7Sr0.3MnO3 bilayer films were grown on (001) SrTiO3 substrate. By means of in situ transmission electron microscopy, lattice expansion and double periodicity were identified in both plan-view and cross-section La0.7Sr0.3MnO3 films under electron irradiation for 25 s. After the electron beam was removed from specimens, the original perovskite structure recovered within 10 min. However, when irradiation time was more than 1 min, the original perovskite structure could not recover and became an amorphous phase or a cavity created by irradiation. According to first-principle calculation and electron diffraction pattern simulation, formation mechanism of the lattice expansion and double periodicity is proposed based on oxygen deficiency during electron irradiation.
I. INTRODUCTION
The family of perovskite-based oxides has attracted much attention because their electrical, magnetic, and catalytic properties are of great research value and technological applications.1,2 Among them, La1−xSrxMnO3 not only has a colossal magnetoresistance (CMR) effect, which usually occurs near the magnetism transition temperature (Tc) with a metal-to-insulator transition,3,4 but also is one of the most commonly used cathode and interconnection materials for zirconia based solid oxide fuel cells (SOFCs) due to its high catalytic potential for oxygen reduction, large electrical conductivity, and relatively high chemical stability.5,6 In common La1−xSrxMnO3 cathode material, partial substitution of Sr2+ for La3+ in LaMnO3 oxide leads to charge compensation by the formation of ionic and electronic defects. The former ionic defects are related to the concentration of oxygen vacancies and oxygen pressure. At low oxygen pressure, the predominant defects in La1−xSrxMnO3 are oxygen ion vacancies, which have an important influence on magnetic, electronic, and magnetotransport properties of perovskite-based oxide thin films. Molecular dynamics calculation of oxygen ion diffusion in perovskite-based
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0221 1778
J. Mater. Res., Vol. 20, No. 7, Jul 2005
oxides confirms the very high oxygen diffusion and catalytic activity in La1−xSrxMnO3.7,8 In the past several years, a few transition metal oxides have been investigated under electron irradiation.9–13 Oxygen diffusion and desorption/adsorption are found to be an important characteristic. Su et al. studied the effect of electron irradiation on V2O5 and MoO3 by electron energy loss spectroscopy and found reduction of V5+ and Mo6+, respectively, due to oxygen desorption.10 For example, the valence state of vanadium is reduced from V5+ to V2+, and the structure changes from orthorhombic V2O5 to cubic VO. MoO3 is reduced to a phase wit
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