Synthesis, Structure And Magnetic Properties Of Doped Perovskite Manganese Oxides

The perovskite manganites are attracted much interest in recent years because of their rich physics as well as application potential.In this thesis,we take the optimum doped La_0.67Ca_0.33MnO₃ as a parent compound,two A site doping methods are adopted:(1) Na⁺(r_A=1.39(?)),whose ionic radius is approximately equal to Ca²⁺(r_A=1.34(?)) ionic radius,while the Mn³⁺/Mn⁴⁺ ratio can be changed by Na⁺ doping.(2) Mg²⁺(rA=0.72(?)) and Zn²⁺(r_A=0.74(?)),whose ionic radii are greatly less than Ca²⁺(r_A=1.34(?)) ionic radius,which can fix the value of the Mn³⁺/Mn⁴⁺.The three series of polycrystalline samples were prepared by conventional solid state reaction method,and then the structural analysis was carried out by Rietveld refinement using the FullProf_Suite package.The main results are as follows:1.For La_0.67Ca_0.33-xNa_xMnO₃ polycrystal,in all the compounds sintered at 1573K,the samples with x≥0.16 are materials with two phases,whose main phase is ABO₃ type orthorhombic perovskite structure,and the second phase is Mn₃O₄ phase with the tetragonal structure;and the samples with x=0.12 are single phase without a detectable secondary phase. The diffraction peaks are indexed with respect to the orthorhombic perovskite type structure with a space group of Pbnm.With increasing Na⁺ concentration,approximate increasing tendency of the lattice parameters of the ABO₃ phase is revealed.The average ionic radius of A site A> and the tolerance factor t increase,this shows that the stability of the samples is enhanced.However,the approximate narrowing tendency of the single electronic bandwidth W is found and the magnetism of the samples tends to be enhanced.This may be attributed to the dominant role of the double-exchange interaction competing with electron-phonon coupling since the Jahn-Teller effect is weakened by the increase of Mn⁴⁺. For the polycrystalline samples sintered at 1673K,the compound with x=0.12 shows paramagnetism,which is the orthorhombic perovskite type structure with a space group of Pbnm.Meanwhile ferromagnetism is observed in the samples with x=0.16,0.18,0.20 in which structural aberration probably occurred.On the whole,ferromagnetism of the samples becomes stronger with Na⁺ dopant content increasing.In conclusion,the structural transitions may occur and magnetism can be stronger in proper sintered condition.When Na⁺ is introduced,the magnetism of the samples becomes stronger,further more the magnetism is stronger when the samples are sintered at lower temperature2.For the two series of polycrystalline samples La_0.67Ca_0.33-xMg_xMnO₃ and La_0.67Ca_0.33-xZn_xMnO₃,the samples with x=0.10,0.15,0.20 are materials with two phases, whose main phase is ABO₃ type orthorhombic perovskite structure,and the second phase is Mn₃O₄ phase with the tetragonal structure;and the rest samples doped Mg with x=0.00,0.05,0.075 and the samples doped Zn with x=0.025,0.05 are all the orthorhombic perovskite type structure with a space group of Pbnm.By introducing of Mg²⁺(r_A=0.72(?)) and Zn²⁺(r_A=0.74(?)) whose ionic radiuses are greatly less than Ca²⁺(r_A=1.34(?)) ionic radius,the lattice parameters and lattice volume of the ABO₃ phase increase abnormally.The average ionic radius of A site A> and the tolerance factor t decrease monotonously,which shows the stability of the polycrystalline samples is weakened.The magnetic permeability is reduced.In this work,two-layered perovskite samples of La_1.4-xR_xSr_1.6Mn₂O₇ and R_2-2xSr_1+2xMn₂O₇(R=Nd,Er) are also synthesized by conventional solid state reaction method, the experimental results show:1.The formation temperature of Sr₃Ti₂O₇ type structure is above 1723K;2.The more R content,the more difficult of the formation of Sr₃Ti₂O₇ type structure;3.For Er-doped samples,the formation of Sr₃Ti₂O₇ type structure is more difficult than that of Nd-doped samples.The sintering temperature of Er-doped samples is higher than that of Nd-doped samples.