Perovskite Transition Metal Oxides Nanomaterials Preparation And Physical Properties,

Nanosized materials, due to their novel chemical, physical properties and their potential applications in electronics, have become the focus in the fields of chemistry, materical science and condensed matter physics and so on. In this dissertation, we have synthesisized the nanosized pervoskite manganites, cobaltites and cuprates by a sol-gel method and investigated their structural, magnetic and transport properties. The dissertation is composed of five chapters which were, respectively, abstracted as following:In Chap.1, the structural, magnetic and transport properties of pervoskite oxides, especially, manganites and cobaltites exhibiting colossal magnetroresistivity effect and cuprates showing high-temperature superconductivity, were firstly introduced. Then, the recent research developments in their materials in nanoscale were reviewed.In Chap.2, The structural, transport and electron spin resonance properties of bulk and nanosized La_0.875Sr_0.125MnO₃ prepared by a sol-gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T_C In addition to a sharp peak in the vicinity of T_C, another sharp peak close to Too is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 75 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.In Chap.3, The structural, transport and magnetic properties of La_0.875Sr_0.125MnO_3+δ samples synthesized by a sol-gel method at different annealing temperatures were investigated. As the annealing temperature is lowered, a structural transition from orthorhombic to rhombohedral and a crossover from ferromagnetic insulating to ferromagnetic metallic state are observed. Raman spectra show two sets of phonons for the low temperature annealing samples, which correspond to the orthorhombic and rhombohedral structures, respectively. Electronic spin resonance signals reveal the coexistence of ferromagnetic insulating and ferromagnetic metallic phases at low temperatures for all samples.These results suggest that an annealing temperature dependent phase segregation exists in this system.In Chap.4, we have investigated the structural and magnetic properties of LaCoO₃ nanoparticles prepared by a sol-gel method. A ferromagnetic order with T_C～85 K has been observed in the nanoparticles. The infrared spectra give evidence for a stabilizing of higher spin state and a suppression of the Jahn-Teller distortions in the nanoparticles with respect to the bulk LaCoO₃, which is well consistent with the recent reports in the strained LaCoO₃ films and proposed to be the possible origin of the observed ferromagnetic order in LaCoO₃.In Chap.5, Lightly doped La_2-xSr_xCuO₄ (x = 0.04) nanoparticles with different particle sizes have been successfully prepared by a sol-gel method and characterized by X-ray diffraction, scanning electron microscopy, infrared transmission spectra and superconducting quantum interference device magnetometer. All samples are single phase and have an orthorhombic unit cell. As the particle size reduces, it is found that the IR band at around 685 cm^-1 corresponding to the in-plane Cu-O asymmetrical stretching mode shifts to higher frequency and the magnetization exhibits a large enhancement at low temperature. The magnetic susceptibility of all samples follows a modulated Curie law between～20 K and～100 K and the Curie constant displays a strong dependence on the particle size. It is suggested that as the particle size decreases surface effects should play an important role in the magnetic properties of the nanoparticles.