Magnetism And Cation Vacancies Studies Of Manganite La_0.6Sr_0.1Na_xMnO₃(x≤0.3)

Perovskite manganites RE_1-xT_xMnO₃(RE for rare earth element,T for alkaline-earth element) has attracted much attention,because of its colossal magnetoresistance effect and broad application prospects in the past 10 years.The double exchange mechanism proposed by Zener can be used to explaine qualitatively the electrical and magnetic properties of these kinds of compounds.In this paper,we have studied the influence of heat treated with different temperature and different time,and doped Na ions on the structure and magnetic properies of perovskite manganites with nominal composition La_0.6Sr_0.1MnO₃.(1) As the heat treatment temperature increase and the heat treatment time extension,the unit cell volume of ABO₃ perovskite phase in the samples will increase,because that the vacancies appear at the B sites due to absorbed overmuch oxygen in the heat treatment process.A series of samples with nominal composition La_0.6Sr_0.1MnO₃ were synthesized by sol-gel method and heat treatment at different temperatures for different times in air circumstance.The samples were composite materials with two phases determined by X-ray diffraction spectrum measured by X'pert Pro X-ray diffraction instrument,whose main phase is ABO₃ type rhombohedral perovskite structure,and the second phase is Mn₃O₄ phase.When the heat temperature is higher than 800℃,the(220) diffraction peak of the perovskite phase tends toward the lower 20 with the heat treatment temperature increase and the heat treatment time extension,which means that the distance between crystal planes of(220) crystal face group tends to increase.By theoretical calculating and fitting X-ray powder diffraction data using Rietveld method,it is found that the perovskite unit cell volume will increase with the heat treament temperature increase and the heat treatment time extension.The reason which the unit cell volume increase is that oxygen ion content increase with the heat treatment temperature increase and the heat treatment time extension,resulting in the cation vacancies at B sites in the perovskite phase to increase.Therefore our assumption has been proved that the cation vacancies at B sites in perovskite phase is related to the preparation conditions of the samples.(2) In the preparation process of the samples with the nominal composition La_0.6Sr_0.1Na_xMnO₃,a large part of the Na ions have lost with the volatilization of complexing agent.The X-Ray diffraction spectrum shows that the samples with Na nominal content(x=0, 0.1) possess a perovskite structure main phase and a Mn₃O₄ second phase,and that the samples with Na nominal content(x=0.2,0.3) possess a perovskite structure single phase.By calculating theoretically,fitting X-ray powder diffraction data using Rietveld method,and analyzing with thermogravimetry,it is proved that a large part of Na ions have lost in the heat treatment process.The thermogravimetry shows that there is obvious difference in the volatilizing process of complexing agent between the samples with Na ions and the samples without Na ions.After the heat treatment at 873K for 10 hours,the organic matter in the samples without Na ions has finished to volatilize basically,while there is still organic matter in the samples with Na ions,which is more that 20%of the mass of the final inorganic matter. This part of organic matter will volatilize slowly in the subsequent heat treatment process, while a part of Na ions may volatilize together with the organic matter,and resulting in the loss of Na ions in the end samples.(3) The magnetic properties research results can be acted as the evidences of the above analysis conclusions.The magnetic properties of samples were measured by QuantumDesign Physical Property Measurement System(PPMS),the results show that all the samples are soft magnetic material,and there are the transitions from ferromagnetic to paramagnetic with the temperature increasing.The Curie temperature T_C of the samples was also measured.The bond length d and bond angle(?) of the Mn³⁺-O^2--Mn⁴⁺ bonds in perovskite phase in the above two series of samples were obtained when the XRD powder diffraction data was fitted by Rietveld method,using the above calculation results about the vacancies at B sites and the loss of Na ions.The dependences of Curie temperature T_C on the bond length and bond angle of the Mn³⁺-O^2-Mn⁴⁺ bond,which is caused by the content of the cation vacancies at B sites or Na ions in perovskite phase,can be explained on the basis of the double exchange mechanism,which has proved that the calculation method about the vacancies at B sites and the loss of Na ions are proper.