Role of strontium addition on the phase transition of lanthanum copper oxide from K₂NiF₄ to perovskite structure

Without Sr addition, the sintered La₂O₃ and CuO powder mixture in a mole ratio of 1:2 formed K₂NiF₄-structured La₂CuO₄ with excess CuO. When 15% of strontium was added, La₂CuO₄ transformed into the single perovskite La_1-xSr_xCuO_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 (Cu²⁺) to trivalent ones (Cu³⁺) by the strontium addition. Based on the electroneutrality in an ABO₃ 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 La³⁺. As strontium was added into the A-site cation sub-lattice, the oxidation of Cu²⁺ ion into trivalent Cu³⁺ ion was enhanced. The increase of Cu³⁺ concentration strengthened the electrostatic bonding (ESB) of copper ions with their neighboring anions. Consequently, the symmetrical tetragonal Sr-doped lanthanum copper oxide was obtained.