Sol-Gel Preparation Of La_1-xSr_xMnO₃ And Its Microstructure Studied By Confocal Micro-Raman Spectroscopy

Perovskite materials represented by La_1-xSr_xMnO₃ have attracted more and more attention and interests due to their special crystal and electronic structure, transport properties and applications in many fields such as magnetic recording, magnetic sensor, magnetocaloric, multiferroics and thermal control etc. It was found that the crystal structure and microstructure of La_1-xSr_xMnO₃ can greatly influence the electronic and transport properties. In this paper, La_1-xSr_xMnO₃ with different doping degree were prepared by sol-gel method and the influences of Jahn-Teller effect and tolerance factor on the crystal structure of La_1-xSr_xMnO₃ were studied using confocal micro-Raman spectroscopy. The results were as follow:1. Polycrystalline La_1-xSr_xMnO₃ bulks of single phase were successfully prepared by simple sol-gel method at a relative low temperature (900℃) and a relative short time (10 hours), and La_1-xSr_xMnO₃ films were also prepared by sol-gel and spin casting method on crystalline silicon substrates.2. The influences of the Sr doping level on the microstructure of La_1-xSr_xMnO₃ bulks and La_1-xSr_xMnO₃/Si films were systematically studied. Results showed that the crystal structures of La_1-xSr_xMnO₃ bulks were rhombohedral in the doping range of 0≤x≤0.3. Lattice axes of films were varied when the doping level increased, it was because the tolerance factor increased and lattice distortion reduced with the increase of doping level, and the radius of Sr²⁺ is bigger than that of La³⁺. Interface stresses between the films and the substrates were formed attributed to the lattice mismatch between the La_1-xSr_xMnO₃ films and the substrates, which leaded to the formation of orthorhombic films. In the doping range of 0≤x≤0.3, the most part of films structure was orthorhombic and a little part was rhombohedral. As the Sr²⁺ doping level increased, rhombohedral structure of films increased because the stresses from lattice mismatch in the films decreased.3. The microstructure of La_1-xSr_xMnO₃ films with different thicknesses were studied by confocal micro-Raman spectroscopy and it was revealed that La_0.9Sr_0.1MnO₃/Si film has double layered structure: the structures between films and substrates were orthorhombic, while that of surface were rhombohedral. Through similar structure was found in La_0.83Sr_0.17MnO₃ prepared by MOCVD, there was no literature which reported this structure of La_1-xSr_xMnO₃ films prepared by sol-gel method. The peculiar structures were induced by lattice mismatch and the tensile stress and compression stress from the crystal axis of films, which made the static Jahn-Teller distortion enhanced.