EFFECT OF ATMOSPHERE AND DOPANT ON CRYSTAL STRUCTURE AND CONDUCTIVITY OF La_(1-x)Sr_xCuO_(2.5-y) PEROVSKITE

摘要

La_(1-x)Sr_xCuO_(2.5-y) exhibits mixed conducting characteristics with excellent electrical conductivity (La_(0.7)Sr_(0.3)CuO_(2.488) for example, 1610 S/cm at 25°C) and plenty oxygen vacancies. In these mixed conductors, the aliovalent dopant plays an important role on the enhancement of conducting behavior and structural stabilization. With the addition of 10% SrO, the powder mixture of La_2O_3 and CuO shows a monoclinic structure after heated at 800°C. When 20% ～ 30% of Sr was added into La_2O_3 and CuO mixture, single tetragonal phase with perovskite was observed. As more of SrO was added into La_2O_3 and CuO, Sr-rich phases, La_2SrCu_2O_6, Cu_2SrO_3 appeared in addition to the tetragonal phase. The formation of Cu~(3+) is believed to be the main reason for the stabilization of perovskite. When SrO was added into the A-site cation sublattice, Cu~(2+) tend to be oxidized to Cu~(3+). With the addition of SrO, the increasing concentration of Cu~(3+) greatly enhanced the electronic conductivity. When the SrO dopant exceeded the solubility limit, the Sr-rich second phase suppressed the electronic conduction significantly. Under pure oxygen, the concentration of Cu~(3+) and the composition range for stable tetragonal La_(1-x)Sr_xCuO_(2.5-y) were also increased.