Effect Of Rare Earth Ion Doping On The Crystallization Behavior Of Bismuth Ferritte

Multiferroics are one important class of functional materials that have two or more ferroic properties simultaneously.Nowadays,most of the researches about multiferroics are focused on those with both electrical and magnetic order parameters,so the ferroelectric and magnetic fields can be coupled and regulated.These materials have potential applications in electromagnetic sensors,polymorphic/high-speed memory,spintronic devices and so on.Among these magnetoelectric multiferroic materials,bismuth ferrite（BiFeO₃）is considered to be the most promising candidate for real application in multifunctional devices.This is owing to the fact that both its Curie temperature T_C（～830℃）and Nèel temperature T_N（～370℃）are much higher than room temperature.In general bulk BiFeO₃ with relatively large size is in form of ceramic or single crystal,and the later usually exhibits better performance due to its higher crystallinity.BiFeO₃ single crystals are commonly grown by melt solidification.However,it is very difficult to obtain high-quality BiFeO₃ single crystals with large size.To explore the reasons for the difficulty of BiFeO₃ crystal growth,this paper conducts a series of studies to investigate the characteristics of its melting and solidification process by using high temperature in-situ observation technique.（1）The effects of precursor states on generation of BiFeO₃ phase were investigagted.Unheated,calcined and sintered precursors of Bi₂O₃-Fe₂O₃ mixture were prepared respectively.Their melting and solidifing processes were visualized by microscopy.It was found that only a small amount of BiFeO₃ phase was generated during these heating processes of unheated and calcined samples.However,much more BiFeO₃ phases were formed in the solidified sample which has been sintered previously.BiFeO₃ grains with regular shape precipitate at about1100℃during heating process.When temperature gets higher,part of BiFeO₃ phase becomes unstable and decomposes into hexagonal Fe₂O₃ phase and irregular Bi₂O₃phase.The results indicate the reason why BiFeO₃ single crystal is difficult to grow.The state of proper precursor is extremely important for the growth of BiFeO₃crystals with high quality.（2）The effects of solid phase synthesis on the crystallization behavior of La³⁺doped Bi_1-xLa_xFeO₃（x=0.05,0.10,0.15,0.20）samples were studied.It is found that the melting and solidifing phenomenon is similar to that of undoped sample with sintered precursor.BiFeO₃ phase can decompose into irregular Bi₂O₃ and hexagonal Fe₂O₃ grains at high temperature during the melting stage.BiFeO₃ grains with regular shape form during the cooling and solidification stage.The size of these BiFeO₃ grains increases firstly and then decrease with the increasing of La³⁺doping content.When the doping ratio is 15%,the size reaches the maximum.The fact that La³⁺doping makes BiFeO₃ crystal grow in a regular shape,hints one potentially effective method to promote the growth of BiFeO₃ single crystals with large size.（3）The effects of solid phase synthesis on the crystallization behavior of La³⁺-Ho³⁺codoped Bi_0.8La_0.2-yHo_yFeO₃（y=0.05,0.10,0.15,0.20）samples were studied.It is found that the doping of Ho³⁺changes the morphology of Fe₂O₃ phase generated from the decomposition of BiFeO₃ phase.Hexagonal Fe₂O₃ grains with some irregular BiFeO₃ phase and Bi₂O₃ phase in their inner part,are found in the La³⁺-Ho³⁺codoped samples.Nevertheless,the shape of Fe₂O₃ grain is quadrilateral in the Ho³⁺single-doped sample when the doping concentration is of 20%.Additionally,the morphology of BiFeO₃ grains formed during cooling process about1000℃is found to be striped for both the La³⁺-Ho³⁺codoped and Ho³⁺single-doped samples,whith is harmful to BiFeO₃ single crystal growth.（4）Bi_1-xLa_xFeO₃（x=0.00,0.05,0.10,0.15,0.20）samples sintered by solid phase synthesis were used as precursors to grow bulk single crystals by vertical Bridgman method.Polycrystalline rods with gray BiFeO₃ as the main phase were successfully grown,and some heterophase Bi₂O₃ and Fe₂O₃ were found to exist in these samples.With the increase of La³⁺doping content,the proportion of heterophase gradually decreases.The electric hysteresis loops of Bi_1-xLa_xFeO₃polycrystalline blocks are elliptic,with large leakage current and high residual polarization intensity.The magnetic hysteresis loops are S-shaped and have large residual magnetization intensity.