Investigation On Cation Distribution In Self-doped Perovskite Phase Of Manganites La_0.7-x1Sr_0.3-x2MnO₃

Colossal magnetoresistance (CMR) materials with ABO₃ perovskite structure have attracted much attention because of their potential application such as the cathodes of the solid oxide fuel cells (SOFCs), multiferroic and microwave dielectric materials. The CMR materials with general formula Ln_1-xM_xMnO₃, where Ln is the lanthanide cation and M is usually an alkaline-earth cation. Zener proposed that the spin structure and the electronic properties of these compounds are correlated via the double exchange (DE) mechanism controlled by the displacement of the eg electrons from Mn³⁺ to Mn⁴⁺ sites. However, there are some basic physical problems, related to the perovskite structure, which should be investigated further. One of these problems is whether or not there are vacancies at the A sites in ABO₃ self-doped perovskite manganites. So far, most studies have focused on vacancy doping in Ln_1-xM_xMnO₃ compounds, and they have all supposed that there are vacancies at the A sites. However, there have been disagreements between these works concerning the dependence of the Curie temperature TC on the content of Mn⁴⁺ ions at B sites.On the basis of the thermal equilibrium theory of crystal defects, Tang et al considered two assumptions regarding the vacancy problem in the ABO₃ self-doped manganite A1?xMnO₃. Firstly, because of the interval of the B site is smaller than that of the A site, the vacancies should be at the B sites, but not the A sites. Secondly, the main source of vacancies in the samples lay with the preparation method, rather than the self-doping level.In this paper, both experimental and theoretical investigations of the self-doped perovskite manganites La0.7-x1Sr0.3-x2MnO₃-δare presented using the above assumptions. Self-doped manganites with nominal composition La0.6-xSr0.4MnO₃-δhave been prepared by sol-gel method. And a serious of nominal composition La_0.70Sr_0.30-xMnO₃ samples have been prepared by the solid state reaction method and sol-gel method, respectively. The experimental results provided the new evidences for the hypothesis of A1?xMnO₃ type self-doped perovskite structure materials'vacancies problem, pointed out by Tang et al. Although the nominal composition is same, because of the different preparation method, the samples phase structure and properties are different. A study of the free energy of the perovskite manganites was performed by fitting experimental results for the temperature dependence of the crystal cell volume using a detailed semiclassical model of the free energy.Perovskite manganites with nominal composition La0.7Sr0.3?xMnO₃?δ(0.00≤x≤0.20) have been prepared by the sol?gel method with the highest heat treatment temperature being 1073 K. The XRD patterns indicate that when the doping level is x≤0.10 the samples have only a single phase, with the R3c perovskite structure, while for x≥0.15, the samples have two phases with the R3c perovskite being the dominant phase and Mn3O4 being the second phase. On the basis of the thermal equilibrium theory of crystal defects, a quantitative analysis and Rietveld fitting of the X-ray powder diffraction data indicate that there are Mn2+ ions at the A sites and Mn³⁺ plus Mn⁴⁺ ions at the B sites in the ABO₃ perovskite phase.The curves of magnetization versus applied magnetic field at 10 K showed that the magnetic moments of the Mn2+ ions at the A sites are antiparallel to those of the Mn³⁺ and Mn⁴⁺ ions at the B sites. The assumption that there are vacancies at the A sites is not supported by the data.Self?doped manganites with nominal composition La_0.6-xSr_0.4MnO_3-δ (0≤x≤0.175) have been prepared by sol?gel method. The XRD patterns and magnetic measurements indicate that the samples possess two phases with the ABO₃ perovskite structure being the dominant phase and Mn3O4 being the minor phase when the doping content x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn2+ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples'X-ray diffraction data. The change tendency of the Curie temperature TC vs. the Mn⁴⁺ ion content ratio RM4 at the B sites of ABO₃ structure is in accordance with the experimental result of the samples La1?xSrxMnO₃. The curves of magnetization versus applied magnetic field at 10 K showed that the magnetic moments of the Mn2+ ions at the A sites are antiparallel to those of the Mn³⁺ and Mn⁴⁺ ions at the B sites. The assumption that there are vacancies at the A sites is not supported by the data.We have prepared manganites with the nominal composition La0.7Sr0.3?xMnO₃?δ(0≤x≤0.10) by the solid state reaction method with the highest heat treatment temperature at 1173 K. Using XRD analysis, the samples were verified as being three phase composities La_0.7-y-zSr_0.3-xMn_1-δ/3O₃-δ/(La2O₃)y/2/(La(OH)₃)z, withδ=3(x+y+z). We therefore have calculated the ion ratios of at A, B and O sites of the perovskite phase, which were proved being reasonable by the Rietveld fitting and the magnetic property analyses. It was found that the dependences of the Curie temperature TC on the Mn⁴⁺ ion content RM4 at B sites, are similar to those of the typical perovskite La1?xSrxMnO₃. At 10 K, the measured specific saturation magnetizationσSE of the samples increases with decreasing Mn⁴⁺ ion content ratio RM4, which change tendency is accordant with the theoretical valueσSC. A study of the free energy of the perovskite manganites was performed by fitting experimental results for the temperature dependence of the crystal cell volume with a detailed semiclassical model of the free energy. The fitted results coincide with the experimental results for all of La_1-xCa_xMnO₃ (x=0.2, 0.25, 0.3, and 0.5).