The Structures And Physical Properties Of Multi-elements-doped Pr-containing Perovskite Oxides

Both the high-Tc cuprate superconductor (HTCS) REBa₂Cu₃O_7-δ (RE is rare-earth elements) and the colossal magnetoresistance (CMR) manganites A_1-xA'_xMnO₃ (A is trivalent rare-earth ions, A'is divalent ions) are the strongly correlated electronic systems which have the perovskite or perovskite-like structures. The physical properties affected by doping to the RE or A sites with the valence-variable rare-earth elements in the two materials are interesting. In the Pr doping REBa₂Cu₃O_7-δ materials, there always have been some debates about the the valence of Pr and the physical principles of the suppression of superconductivity by Pr doping. In the A_1-xA'_xMnO₃ materials, there also have been debates about the valence of Ce and its different effects to the crystal structures, the electronic transport properties, and the magnetic properties. In this dissertation, we studied the doping-effects of the valence-variable rare-earth elements both in the two materials. Some important results about the regularitys of the variations of valence, and the variations of crystal structure, electronic transport property, and magnetic property caused by doping were obtained. The dissertation contains four parts:In Chapter 1, firstly, the crystal sturctures, physical properties and doping effects of the HTCS REBa₂Cu₃O_7-δ were simply reviewed, and the physical properties of Pr doping REBa₂Cu₃O_7-δ, debates of the physical principles of the suppression of superconductivity by Pr doping,debates of the valence of Pr were especially reviewed. Secondly, the crystal sturctures, physical principles, physical properties, and the doping effects of the CMR manganites A_1-xA'_xMnO₃ were simply reviewed, and the different physcial properties caused by Ce substituting for A, the debates of the valence of Ce were also reviewed,. At last,the similarity of and the difference between those two doping effects of valence-variable elements in the two materials were also discussed.In Chapter 2, by investigating the variation of resistivity with temperature of the Pr and Ca double doping Y_1-2xPr_xCa_xBa₂Cu₃O_7-δ with 0≤x≤0.14, a deviation of the linearly decreasing of critical temperature of superconductivity Tc with increasing x which caused by the equal amount double doping of Pr⁴⁺ and Ca²⁺ was observed when x<0.10. when x>0.10, the relation between Tc decreasing and x increasing are linear. The residual resistivity, 300K resistivity and the temperature coefficient of resistivity at 300K also exhibit abnormal behaviors around x=0.10. We suggested that it is the transition of Pr valence from +3 to +4 and the variation of the principles of the depression of TC caused by the valence variation which result in those physical phenomenons. When x<0.10, the valence of Pr is close to +3 and the Tc depression is mainly caused by hole location. When x>0.10, the valence of Pr is close to +4 and the Tc depression is mainly caused by hole filling. The X-ray photoelectron spectroscopy (XPS) measurements also support our conclusion.In Chapter 3, by reviewing the the doping effects of Ce in A_1-xA'_xMnO₃, we found that there are so many physical properties and different physical principles appearing from the wide-band system to narrow-band system in the Ce doping A_1-xA'_xMnO₃ (the band means the one electron conduction band of eg of Mn). In narrow-band systems, the appearance of abnormal variations of crystal structure and the investigation of magnetic properties support the Ce⁴⁺. In wide-band systems, the appearance of Kondo-like interaction and the abnormal magnetic order of Ce moments support the Ce³⁺. By Ca²⁺ substituting for Sr²⁺, which decreasing the average ion radii of A site and the width of one electron conduction band of Pr_0.3Ce_0.2Ca_xSr_0.5-xMnO₃ (x≤0.125), we found that, the abnormal variation of crystal structure appearing in Ce⁴⁺ doping narrow-band manganites and the Kondo-like interaction and abnormal magnetic order of Ce³⁺ appearing in Ce³⁺ doping wide-band manganties coexist in our samples. The Kondo-like interaction caused by Ce³⁺ weakens and the difference between the temperature of the onset of magnetic ordering of Ce³⁺ moments Tmax and the Curie Temperature TC decreases with the width of band decreasing(or the x increaing). All the phenomenons mentioned above support the transition from Ce³⁺ in wide-band system to Ce⁴⁺ when the width of band decreases. The saturation magnetic moment of our samples increases with the average ion radii decreasing or x increasing, which supports the Ce³⁺ decreasing and Mn³⁺ increasing. We suggested that in manganites A0.5A'0.5MnO3, there are transition of the valence of Ce ion from +3 to +4 and the variation of physical principles of the materials when the width of one electron conduction band decreases. In addition, although the temperatures of the onset of magnetic ordering of Ce³⁺ moments disappear in the measurements of magnetization,they appear in the curves of the intensity of electron spin resonance (ESR) vs Temperture.In Chapter 4, we found that annealing Pr_0.3Ce_0.2Sr_0.5MnO₃ at different atmosphere, which causes different content of oxygen in the meterials, affects the TC hardly, but causes lots of difference of the physical properties. The datas of X-ray diffraction (XRD) and the Raman spectrum suggest that the tilting and distortion of the octahedron increase with the content of oxygen either increasing or decreasing, which affect the double exchange interaction. By investigating the results of our experiments, we suggested that, the Pr_0.3Ce_0.2Sr_0.5MnO₃ annealed in nitrogen change the valence of Mn ions only, and affect the competition between ferromagnetic (FM) and antiferromagnetic (AFM) phase of Mn only; the Pr_0.3Ce_0.2Sr_0.5MnO₃ annealed in oxygen can change both the valence of Mn ions and the one of Ce ions, and affect not only the competition between FM and AFM phase of Mn, but also the Kondo-like interaction and magnetic order of Ce, which cause more complex electronic transport properties and magnetic properties. Although the Ce doping induces a new insulator-metal(IM) transtion in manganites, because of the magnetic order of Ce inducing by the one of Mn, the ultimate effects to the electronic transport properties and magnetic properties come from the competition between FM and AFM phase of Mn ions.