Hydrothermal Synthesis And Spectra Analysis Of The Perovskite Manganites

In the past few decades, the research development of ABO₃ perovskites manganites has always been an interesting subject due to this family has diverse structures and rich optic, electric and magnetic properties. So as a functional material, manganite has much application value. The manganese perovskite oxide with complicated mixed valence states is a strong electron correlation system, the interaction among spin, charge, orbit and lattice of this system is very strong, such as spin-spin interaction, spin-lattice interaction, charge-lattice interaction and so on. These interactions coexist and compete, which resulted in complicated physical effect on electrics, magnetism, thermodynamics, thus this subject is widely investigated.It is known that understanding the electronic/lattice structure often plays a crucial role in studying the properties and characteristics. In idea state, perovskite oxides with a general formula ABO₃ (A = 12-coordinated ions and B = 6-coordinated ions) provide cubic structural model, but in fact, due to Jahn-Teller effect and the mismatch effect of the average A-cation size, strong lattice distortions are often found in perovskite manganites. The super exchange and double exchange theory has been used to describe the spin interaction in perovskite manganites. The interesting magnetic properties are related to the mixed-valence of the manganese ions in the perovskite structure. LaMnO₃ and CaMnO₃, the prototype of these perovskite oxides, is an antiferromagnetic insulator while the partial substitution La_1-xCa_xMnO₃ compounds induce ferromagnetism.Spectroscopy is the study of the interaction of electromagnetic radiation with matter. There are three aspects to spectroscopic measurements: irradiation of a sample with electromagnetic radiation; measurement of the absorption, emission, reflection and scattering from the sample; analysis and interpretation of these measurements. The main subject of spectroscopy is to study the interactions by different types of spectra technique and theoretical analysis, and then supply their electronic structure, composition, physical and chemical natures. It is known that spectroscopy supplies us credible theoretical and experimental basis, which enables us to investigate valence state, coordination, character of crystal field and so on. So being an important part of chemical basic research, spectroscopy is also a useful approach to promote the development of material science.In this dissertation, we synthesized the triplet valence states (three oxidation states) of manganese (Mn³⁺, Mn⁴⁺, Mn⁵⁺) in the perovskite oxides. Their conformations and properties are complicated and confused. In order to enrich the knowledge of this family of perovskite manganites, we try to determine their crystalline structure, electronic characteristics and magnetism mechanisms by various spectra.1 We synthesized a serious of perovskite manganites using mild hydrothermal conditions. The use of hydrothermal conditions, where an aqueous reaction mixture is heated in a sealed reaction container, permits a wider range of reaction conditions to be accessed for the synthesis of oxides. Under conditions of elevated temperature and autogenous pressure, the solubilities of starting materials and reagents can be very different than in ambient condition. A family of triplet valence states (three oxidation states) of manganeses (Mn³⁺, Mn⁴⁺, Mn⁵⁺) in the perovskite oxides was synthesized in the condition of strong alkali media. These perovskite oxides, which were substituted the A site with La³⁺,Ca²⁺ and K⁺, create complicated three oxidation states of Mn in B site. The hydrothermal method would permit rapid mixing of several chemical elements, leading to homogeneous product and also offer the potential for control of crystal growth leading to perfect crystal. It provides us advantages for the further character study.2 The study on the molecular spectra of this family of perovskite manganites is divided into two parts, firstly, electronic spectra, including UV-visible absorption spectra and emission spectra; secondly, vibrational spectra, including Raman and infrared spectra. In electronic spectra, according to the idea cubic structural of perovskite oxides, we simulated the absorption spectra of manganites, in which, manganese ions hold +3, +4, +5 valence state, respectively. We detected the characteristic absorptions of charge- transfer from O to Mn and d-d transition of Mn, which correspond to UV ray energy and visible ray energy, respectively. The typical emissions in NIR band for Mn⁵⁺ion were also found in our La_0.60Ca_0.32K_0.08MnO₃ sample. It is an additional evidence showing the existence of Mn5+ in this family of perovskite manganites. The vibrational characteristics of the specimens have been analyzed by Raman and infrared spectra. A part of characteristic frequencies of the infrared and Raman spectra of octahedral and tetrahedral structure in La_0.60Ca_0.32K_0.08MnO₃,SrMnO₃ and Li3MnO4 have been determined and assigned.3 Electron paramagnetic resonance (EPR) has been used to study resonance signals in La_1-x-yCa_yK_xMnO₃ system and interaction mechanisms, Jahn–Teller distortions in perovskite manganites. At room temperature, we investigated Mn⁵⁺ resonance signal in Li₃MnO₄ compound and analyzed its zero-field-splitting and hyperfine structure theoretically. Based on understanding the interaction and lattice distortion, we observed linewidth decreased while La concentration increased and considered it is caused by Jahn-Teller effect and double exchange interaction. To further understand the considering system, EPR signals for La_0.614Ca_0.197K_0.166MnO₃ and La_0.661Ca_0.108K_0.213MnO₃ samples at some representative temperatures are discussed. Linewidth reaches a minimum value at about Curie temperature, above this temperature, linewidth and Lande factor g are nearly constant, which is caused by spin–spin and/or spin–lattice interactions;below this temperature, ferromagnetic interaction is dominate, which interprets why linewidth and Lande factor g change greatly.