Role of Sr addition on the structure stability and electrical conductivity of Sr-doped lanthanum copper oxide perovskites

摘要

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 La_2CuO_4 with K_2NiF_4 structure when the powder mixture was heated at 800 deg C in ambient pressure. Interestingly, the samples of Sr-doped (15-25 percent) lanthanum copper oxides showed single perovskite-based phase after being heated at 800 deg 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 (Sr_(La)'). 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 percent in the A-site sublattice, the Sr-rich second phases, La_2SrCu_2O_6 and Cu_2SrO_3, appeared and suppressed the electronic conduction drastically.