Solid State Reaction Synthesis And Characterization Of Double-Perovskited Bi₂FeMnO₆

Multiferroic materials usually possess two or more kinds of ferroic orders like ferroelectricity,ferromagnetism,ferroelasticity and ferrovorticity in single-phase materials,which exhibit the couplings among these ferroic orders such as magneto-electric(ME)coupling.In such ME multiferroic materials,the electrical polarization can be controlled by magnetic field and magnetic polarization can be tuned by electric field.Therefore,ME multiferroic materials have promising applications in the fields of information storage devices,spin-electronic devices,sensors and other fields,which have become as a hot topic of materials science researches.In recent years,studies on perovskite oxides have revealed that BiFeO3 has become as a representative of room-temperature single-phase multiferroic materials due to its excellent ferroelectricity and antiferromagnetism at room temperature.With the development of researches,it is found that BiFeO3 has serious problems such as high leakage currents,weak ME coupling,and difficulty in synthesis,limiting its commercial viability.To solve the above problems of BiFeO3,Bi-based double perovskite oxides such as Bi2FeMnO6,Bi2FeCrO6 and Bi2FeNbO6,have received much attention.Currently,the researches of Bi2FeMnO6 are focused on the form of thin films,only a few of works have reported on the physical properties of Bi2FeMnO6 ceramics.The main reason is attributed to the synthesis difficulty of pure Bi2FeMnO6 ceramic samples by traditional solid-state reaction method due to the easy volatility of bismuth and the instability of phase structure.While some progress has been made in reducing the leakage currents of Bi2FeMnO6 via substituting Bi at A site with La,and in improving its structural stability by using high pressure synthesis method,however,new impurity phases appear in the high pressure produced Bi2FeMnO6.Thus,the synthesis of pure Bi2FeMnO6 is still a problem to be solved.In this thesis,we focus on the synthesis of pure Bi2FeMnO6 ceramic samples by using traditional solid-state reaction method.The effects of reaction temperature and holding time on the microstructures of the synthesized samples were systematically investigated by using X-ray diffraction(XRD),scanning electron microscope(SEM)with an X-ray energy dispersive spectrometer(EDS),and X-ray photoelectron spectroscopy(XPS).XRD patterns revealed that the Bi2FeMnO6 ceramic samples crystallized in a rhombohedral structure with space group and lattice parameters of a=5.55 A and c=16.25 A.The Bi2FeMnO6 ceramic grains exhibited polyhedron morphology with the average grain size of 15 μm.X-ray energy dispersive spectroscopy revealed the ceramic grains with atomic ratio of Bi:Fe:Mn equal to 2.0:1.16:0.90.XPS spectra of Fe 2p and Mn 2p core levels confirmed the mixed chemical valence states of Fe(Fe2+ and Fe3+)and Mn(Mn3+and Mn4+)in the Bi2FeMnO6 ceramics.O 1s XPS spectra revealed the oxygen existing in the forms of lattice oxygen and adsorption oxygen while bismuth was present as Bi3+.Dielectric measurements demonstrated that the Bi2FeMnO6 ceramics had much high dielectric constant in the order of 104 in the frequency below 102 Hz,but decreased fast with increasing the frequency.In the frequency range of 102-106 Hz the dielectric constant approached to 156.Dielectric loss(tnδ)had the same frequency dispersion relation.The data of dielectric loss as a function the temperature measured under different frequencies demonstrated a relaxor-like dielectric behavior with activation energy of 0.34 eV,which was due to the thermal motion of the oxygen vacancies.Magnetic measurements showed that the Bi2FeMnO6 ceramics exhibited antiferromagnetic nature but a weak ferromagnetic behavior at 2 K.Sharp cusp peaks were observed in the zero-field-cooling(ZFC)curves and bifurcations between the field-cooling(FC)and ZFC curves were observed.Both of them shifted to low temperature as increasing the external applied magnetic field,indicating a spin-glass like behavior in the system.At room temperature UV-Vis absorption spectra of Bi2FeMnO6 ceramics demonstrated a broad absorption peak in the wavelength range of 200-800 nm and a direct optical band gap of～1.23 eV was obtained for the Bi2FeMnO6 ceramics.Such narrow optical band gap was attributed to the d-d electronic transition of Mn3+ ions from Mn3+(eg1)to Mn3+(eg2).