Controlled Synthesis And Characterization Of Mixed Valence Manganese Oxides

Mixed valence manganese oxides are of great interest for physicaland chemical scientists due to its complex and meaningful physical andchemical properties which have been made a lot of research in this field.Traditional synthesis of such compounds is through conventional hightemperature solid state reaction by heating at or above1000°C. Hightemperature can affact the oxygen content of the oxides, therefore thismethod requires extremely precise control of the amount of oxygen inthe reaction atmosphere in order to obtain product with standard oxygencontent.oxygen content can greatly affect the valence of oxide state, tinylittle oxygen defect will cause huge valence change, and changes in the valence states directly determine the physical properties of the oxides.Hydrothermal synthesis method has been proved to overcome the abovedrawbacks, and it is easier to synthesis thermodynamic metastablephase materials according to its lower reaction temperature. Also, bycontrolling solution concentration of precursors, reaction temperature,filling degrees, alkalinity, etc, different products can be prepared withvarious morphologies, which is the cornerstone in the field of materialsapplication.Physical scientists have done a lot of work on discovering theassociation of valence information of mixed valence manganese oxideswith the physical properties. However, the valence state of manganeseinvestigated by core-level Mn2p spectroscopy is difficult to accord withthe inferring results from chemical formula, the intensive study ofmix-valence manganese oxides are still needed.The first part of this paper introduces the substantial progress onmixed valence manganese oxides reported in the pioneer literatures.The second part is experimental part, which divided into threechapters to introduce: the second chapter introduces the composition controlled synthesis of the triple valence manganese oxides viahydrothermal synthesis and characterization of the products. Through asimple method, we can obtain seven La1-x-yCaxKyMnO3samples withdifferent composition. From analysis of the composition maps, A-siteion composition of the seven samples showed a linear distribution.Bycalculation of the Mn average valence, all samples shown the averagevalence of Mn closes to+3.5, and proved that this reaction to bedisproportionation reaction under hydrothermal conditions by the sameMn source. In the FC-ZFC curves, when temperature raises, thedecrease of magnetic susceptibility emerge a knee in the curves, whichindicate that these samples have two ferromagnetic phases, two Curietemperatures. The photoelectron spectroscopy analysis showed that thesamples have three characteristic peaks. In other words, there existsthree Mn valence states in the samples. The valence of Mn of thesamples is apparently different with the change of the A-site ioncontent.The third chapter describes the size and shape control in thesynthesis of the La1-xSrxMnO3samples by hydrothermal method. The pioneer reported results of such compounds by hydrothermal methodwere generally cubes. However, in our work, a series of specialmorphologies have been synthesized, which has been obtained bytemplate-free method. Under the conditions of determined adjustments,the complex morphology of the hollow spheres, nanowires, aggregateshave been obtained. Since these products were mainly determined byalkalinity,we also discuss the impact of alkalinity on the crystal size,form, morphologies, and growing process of crystal.The fourth chapter describes the analysis and control of triplemixed valence manganese oxides by x-ray photoelectron spectroscopy.The surface valence state can be directly observed by Mn2p spectrumphotoelectron spectroscopy by peak positions, which correspond to thedifferent valence states of manganese oxides. Using monochromaticx-ray source and the spherical electron energy analyzer to studyelectronic occupied state of Mn oxides near the Fermi level. Resultsindicate that there apparently exist eg electronic occupied state, whichcan be directly observed using this method. Related study results showed that can be directly observed.There are little work focus on thisaspect. monochromatic light has a similar effect as ultra light source,having a deeper probe depth, and the valence band electrons on thesurface of the sample provibed a better understanding of the electronicstructure.By acid treatment such as argon ion etching, we can controlsurface composition of the mixed-valence manganese oxides andoxygen environment. Mn2p changes under different conditions areanalysized and studied by the system.The results show that the acidtreatment can be a good method to control surface atomic layers in suchoxides.Mn³⁺are obseved under A-site atomic layers, while in thesurface the manganese valence displayed as Mn⁴⁺. At the same timeusing an annealing process to modify the oxygen content of themixed-valence manganese oxides can lead to different oxygen contentin these compounds. The results show that the decreased of oxygencontent lead to increase of the Mn³⁺content in the compound, andfurther decrease would generate the Mn²⁺.Changes in magnetic moment and resistance can also be observed. with the increase of Mn³⁺magnetic and electronicconductivity increases,while the presence of Mn²⁺conductivitydecreases.However, further increase of resistivity, seems to be relatedwith double exchange effect and concentration of electronic chargedcarriers.In this paper, we studied synthisis and following treatments ofmixed-valence mangese oxides and studied the relation between eachother.Comparing with former literatures, we provide more detailed andproper explanation of the mixed valence state. We have a newunderstanding on the mechanism of size and shape control of mixedvalence mangese oxides and mechanism of disproportion reaction underhydrothermal conditions. The impact of surface distribution of mixedvalence Mn oxides, oxygon content,d-d electron strong correlation,d-phybridization on the electronic structure and properties are alsoanalysized.