Synthesis And Characterization Of La-Doped Bismuth Ferrite Multiferroics

In this dissertation,BiFeO₃(BFO) and Bi_1-xLa_xFeO₃(BLFO,x≤0.3) powders have been successfully synthesized by chemical coprecipitation method.The obtained samples were characterized by X-ray diffraction(XRD), Thermal analysis(TG-DTA),Transmission electron Microscopy(TEM),and X-ray photoelectron Spectroscopy(XPS).BLFO ceramics have been odtained by two-phase sintered progess,the delectric and ferromagnetic properties of which were studied.The effects of precipitator concentrations,reacting temperature,Bi/Fe, pH value,aging process,and calcining process on the phase formation of the powders were studied.The experimental results show that exorbitant precipitator concentrations and Bi/Fe,reacting temperature more than 50℃, pH value less than 14,and no considering of aging process,and calcining process will lead to the appearance of a second minor phase,when precipitator concentrations 2.5-4 M,reacting temperature 25-35℃,Ba/Ti 1-1.03,pH value≥14,aging temperature 50℃,and aging time 24h,finer powders can be obtained.The calcining process make the crystal structure of bismuth ferrite transit from amorphous to perovskite structure.The feasible calcining temperature is 750-800℃.A series of single-phase BLFO powders were successfully synthesized by optimal chemical coprecipitation method.When the content of La is below 0.15,the samples are.single-phase BLFO crystallizing in the same structure as the parent BFO compound,and La substitution does not affect the crystal structure of the parent compound.It also found that tha La content has a benefical effect on restriction to the appearance of a second phase.The unit-cell symmetry of BLFO can be seen to shift from R3c to C222 with increasing La content.The results of XPS show that the element of Fe in the powders only existed in the form of Fe³⁺.BLFO ceramics were obtained by two-phase sintered progess,which can make the ceramics samples get 8-15%higher relative density compared with direct sintered progess.The room dielectric constant of BLFO ceramics at the test frequency of 10²-10⁶ Hz is 40-110,and which is evidently enhanced with the increase of La content.Furthermore,the dielectric loss of BLFO ceramics is apparently lower than undoped BFO at the frequency of above 10,000 Hz.M-H curves at room temperature show that La substitution make the magnetic properties of BFO ceramics shift from antiferromagnetism to weak ferromagnetism,and it enhances with the increase of La content.