Role of Sr Addition on the Structure Stability and Electrical Conductivity of Sr-Doped Lanthanum Copper Oxide Perovskite
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The structural and electrical properties of lanthanum copper oxide were examined as a function of Sr addition. It was observed that the lanthanum oxide and copper oxide formed La2CuO4 with K2NiF4 structure when the powder mixture was heated at 800 °C in ambient pressure. Interestingly, the samples of Sr-doped (15∼25%) lanthanum copper oxides showed single perovskite-based phase after being heated at 800 °C. Without Sr addition, a single-perovskite phase of lanthanum copper oxide was observed only under the oxygen pressure as high as 65 kbar. The stabilization of perovskite structure in lanthanum copper oxide was effectively achieved by the addition of Sr. Based on the titration analysis and pertinent defect reactions, the enhancement of perovskite stability was due to the presence of trivalent copper ions that were created to balance the electrical charge of doping ion (SrLa⬘). With the increasing concentration of trivalent copper ions (or electron holes equivalently) in Sr-doped samples, lanthanum copper oxide also changed from a semiconductor to metallic conductor. When the Sr dopant exceeded its solubility limit of approximately 25% in the A-site sublattice, the Sr-rich second phases, La2SrCu2O6 and Cu2SrO3, appeared and suppressed the electronic conduction drastically.
I. INTRODUCTION
Lanthanum copper oxides have been considered as potential superconducting materials although the superconductivity has not been obtained. Under the ambient pressure, the stable compound of lanthanum copper oxides is La2CuO4 with K2NiF4 structure. The K2NiF4 structure consists of perovskite layers with alternating rock-salt layers. La2CuO4 exhibits p-type semiconducting behavior.1,2 It has been reported that Sr-doped La2CuO4 or La2-xSrxCuO4 (x ⳱ 0.15∼0.20) showed superconducting behavior based on the measurement of ideal diamagnetism at the temperature as high as 40 K.2,3 The addition of Sr also enhances the stability of the tetragonal K2NiF4 structure although the superconducting behavior only occurred in the distorted K2NiF4 phase. In addition, lanthanum copper oxides may crystallize in a complete perovskite structure with a formula of LaCuO3-␦ when processed at high oxygen pressure. LaCuO3-␦ was first prepared by Demazeau et al.4 at 900 °C and 65 kbar. It was indexed as a rhombohedrally distorted perovskite. The oxygen stoichiometry of LaCuO3-␦ exists with ␦ ranging from 0 to 0.5 under
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 3, Mar 2004
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various oxygen partial pressures. Accordingly, the crystal structure of LaCuO3-␦ also changes from tetragonal, monoclinic to orthorhombic.5,6 The conduction behavior of LaCuO3-␦ oxides changes from an insulator to metallic conductor as ␦ varies from 0.5 to 0. The conductivities of LaCuO3-␦ significantly improve from 10−6 S/cm to 1000 S/cm as ␦ decreased from 0.5 to 0.0, although the superconducting behavior was not observed.7,8 The nonstoichiometry of LaCuO3-␦ perovskite
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