Role of strontium addition on the phase transition of lanthanum copper oxide from K 2NiF 4 to perovskite structure

Abstract

Without Sr addition, the sintered La 2O 3 and CuO powder mixture in a mole ratio of 1:2 formed K 2NiF 4-structured La 2CuO 4 with excess CuO. When 15% of strontium was added, La 2CuO 4 transformed into the single perovskite La 1− x Sr x CuO 2.5− δ phase with orthorhombic structure. As the strontium addition increased to 20%, the perovskite lattice changed from orthorhombic to tetragonal. These phase transitions may be attributed to the enhanced oxidation of the divalent cupper ions (Cu 2+) to trivalent ones (Cu 3+) by the strontium addition. Based on the electroneutrality in an ABO 3 perovskite lattice, a divalent cation is unstable in the B-site cation sub-lattice when the A-site is occupied by a trivalent cation such as La 3+. As strontium was added into the A-site cation sub-lattice, the oxidation of Cu 2+ ion into trivalent Cu 3+ ion was enhanced. The increase of Cu 3+ concentration strengthened the electrostatic bonding (ESB) of copper ions with their neighboring anions. Consequently, the symmetrical tetragonal Sr-doped lanthanum copper oxide was obtained.