Molten Salt Synthesis And Characterization Of Double-Perovskited Ba₂FeNbO₆ Nanoparticles

Double perovskite oxides are generally represented by a chemical formula of A2B'B"O6.The ideal structure of A2B'B"O6 crystal can be viewed as an alternating arrangement of B'O6 and B"O6 octahedrons,while B' and B"cations are separated by O2-ions,forming the arrangements of B'-O-B".A-site cations are located at the interstitial positions surrounding by oxygen octahedrons.As compared with the perovskite ABO3 oxides,double perovskite oxides have a more complex crystal structure and richer physical properties.By changing the combination forms of the different elements at B sites,the stoichiometry and doping of elements,electrical and magnetic properties and optical band gaps of double perovskite oxides can be modulated effectively,making the double perovskite oxides exhibit multifunctional behaviors of metal,semi-metal,semiconductor,insulator,and spin glass state.Therefore,double perovskite oxides have important applications in the fields of spin electronics,electrically controlled magnetic memory,ferroelectric photovoltaic,and so on.As one kind of double perovskite oxide,Ba2FeNbO6(BFNO)has good dielectric properties and appropriate optical band gap,which can be used as lead-free relaxor ferroelectric and visible light catalyst.In this thesis,BFNO nanoparticles with cubic phase structure were synthesized by molten salt method.The effects of the processing parameters(e.g.,annealing temperature,dwell time,and molten salt concentration)on the physical properties and microstructures of BFNO nanoparticles were investigated.Micro structures of BFNO nanoparticles were examined by X-ray diffraction(XRD)patterns and scanning electron microscopy(SEM),XRD data and Rievtveld refinements reveal that the BFNO nanoparticles synthesized at 1000? for 5 h under the molar ratio of BaCO3:Fe2O3:Nb2O5:NaCl:KCl= 4:1:1:100:100,have good crystallinity and pure phase structure with cubic morphology.The lattice parameter a was determined to be 8.12 A,and the average particle size was 500 nm.With increasing the annealing temperature from 700? to 1100?,the average particle size was increased from 200 nm to 600 nm.Quantitative X-ray energy-dispersive spectroscopy(EDS)analysis demonstrates that the atomic cation ratio of Ba:Fe:Nb in the BFNO nanoparticles is close to the nominal value.The UV-Vis absorption spectra of the BFNO nanoparticles were measured with the wavelength from 300 nm to 800 nm,from which the band gaps of the BFNO nanoparticles synthesized at different conditions were determined in the range of 2.12 eV-2.29 eV.Dielectric measurements demonstrate that the dielectric constants of the BFNO nanoparticles synthesized at 1000? for 5 h under the molar ratio of BaCO3:Fe2O3:Nb2O5:NaCl:KCl = 4:1:1:100:100,decrease quickly in the low frequency region whereas tend to be constant value of 55 in the high frequency region.That is ascribed to the dielectric polarizations from space charges and dipole orientations in the BFNO nanoparticles.At room temperature the dielectric constant of the BFNO nanoparticles was 450 measured at 1 kHz,and corresponding dielectric loss was 0.65.The temperature dependence of the dielectric constants of the BFNO nanoparticles shows that in the low temperature region(from-150 to 200?),the dielectric constants measured at different frequencies increase slowly with increasing the temperature whereas decrease with the frequency,and the temperature corresponding to the peak value of the dielectric loss shifts to higher temperatures as increasing the frequency.That indicates the BFNO nanoparticles exhibit a typical dielectric relaxor behavior.The relaxor temperature(T)and the corresponding frequency(f)follows the Arrhenius law f= f0 exp(-Ea/kBT),where f0 is frequency factor,kB is Boltzmann's constant,and Ea is the activation energy of the frozen local dipoles,which is measured to be 0.188 eV.As the temperature is over 200?,the dielectric constants of the BFNO nanoparticles increase quickly and reach a maximum value,which is attributed to the frozen local dipoles activated by the thermal energy.Similar phenomenon is also observed in the temperature dependence of the dielectric losses of the BFNO nanoparticles measured at different frequencies.