Synthesis And Characterization Of Double-Perovskite Sm₂NiMnO₆ And Sr₂FeReO₆ Nanoparticles By Sol-gel Method

As compared with the conventional ABO₃-typed perovskite compounds,double-perovskite oxides A₂BB’O₆ have more diverse crystal structures and more selectivities of the elements at B-site.Theoretical and experimental studies have showed that Sm₂Ni Mn O₆ from the R₂BB’O₆ series（R is rare-earth ion）has excellent ferromagnetic and photovoltaic properties,exhibiting promising applications in the field of solar cells;Re-based double perovskite oxides such as Sr₂FeReO₆ have high ferromagnetic Curie temperature（T_C=445 K）,high saturation magnetization as well as high electron spin polarizability,which have important applications in spintronics.In this thesis,Sm₂Ni Mn O₆ and Sr₂FeReO₆ nanoparticles are selected as the research objects.The samples are synthesized by sol-gel method,and their physical properties and microstructure were characterized and analyzed.The main achievements of this thesis are listed as follows:1.Sm₂Ni Mn O₆ nanocrystalline powders were synthesized by sol-gel method.The results of X-ray diffraction and Rietveld refinements showed that they crystallized in a monoclinic crystal structure（space group P2₁/n）.The cell parameters were determined as a=5.3450（?）,b=5.5087（?）,c=9.2912（?）,β=125.07°,and the cell volume V=223.9（?）³.The structure tolerance factor was calculated to be t=0.923.SEM and TEM images showed that the particles were spherical and the average grain size was 76 nm.EDS spectra confirmed that the molar ratio of metal cations（Sm:Ni:Mn）was equal to1.92:0.94:1.00,which matched well with the nominal ratio of the compound.XPS spectra revealed that the Sm,Ni and Mn elemments existed in the form of Sm³⁺,Ni²⁺and Mn⁴⁺ions in the Sm₂Ni Mn O₆ nanocrystalline powders,respectively,and O element existed as lattice oxygen and hydroxyl oxygen.Sm₂Ni Mn O₆ ceramics have high dielectric constante_r（>2000）in the low frequency region,but it decreases rapidly as the measured frequency is higher than 100 Hz.In the high frequency region（10³-10⁶Hz）,the dielectric constant decreases slowly and tends to a stable value（～62）.The dielectric loss（tand）exhibits a similar frequency dependent behavior.This phenomenon can be attributed to the mechanism of space charge polarization.The modified Curie-Weiss law(¹-¹₈)=^（-8）))was used to fit the dielectric constants as a function of temperature.The results showed that the diffusion phase transition coefficients（g）were in the range of 1.88-1.99,and the dielectric data in the low temperature region were deviated from the Curie-Weiss law obviously.That proved the dielectric behavior of the Sm₂Ni Mn O₆ ceramics had a strong frequency dispersion.Good fitting of the dielectric data by the nonlinear Vogel fulchers formula confirmed the dielectric relaxation behavior in the Sm₂Ni Mn O₆ ceramics.Magnetic hysteresis loop measurements showed that Sm₂Ni Mn O₆nanoparticles exhibited paramagnetism at room temperature,but a weak magnetism at 2K.The remanent magnetic moment（M_r）and coercivity field（H_C）were 2.98 emu/g and 964 Oe,respectively.The magnetizations（M）measured in zero-field cooling（ZFC）and field cooling（FC）mode have a strong dependence of temperature（T）,which shows the magnetic phase transition caused by the coexistence of ferromagnetic（FM）phase（main phase）and antiferromagnetic（AFM）phase（secondary phase）in the sample due to the 3d-4f exchange between Mn-Ni and Sm³⁺magnetic moment.The direct optical band gap（E_g）of the Sm₂NiMnO₆ powders was determined to be 1.42 e V from the UV-Vis spectra and the Tauc relation(h=（h-_g）⁹)).That indicates that Sm₂Ni Mn O₆ nanocrystalline powders can be used as absorbing materials for high efficiency solar cells.2.Sr₂FeReO₆ nanocrystalline powders were synthesized by sol-gel method.XRD and Rietveld refinements showed that they had tetragonal crystal structure with space group I4/m.Cell parameters of the Sr₂FeReO₆ powders were a=b=5.5668（?）,c=7.9015（?）,and the cell volume V=244.9（?）³.The grain size（D=45 nm）was calculated from Scheller formula.SEM images showed that the agglomeration of nanoparticles was obvious;EDS spectra confirmed the loss of Sr and Fe element.The dielectric constant of Sr₂FeReO₆ ceramics was very high in the low frequency region,but it decreased rapidly as the measured frequency exceeded 100 Hz.In the high frequency region of 10⁴-10⁶ Hz,the dielectric constant decreased slowly and tended to a stable value（～134 at 10⁶ Hz）.The dielectric loss（tand）also exhibited a similar frequency dispersion relation.Space charge polarization also existed in the Sr₂FeReO₆ ceramics.Magnetic measurements show that the remanent magnetization（M_r）is 8.23 emu/g and coercive field（H_C）is 3152 Oe at 2 K.The negative magnetoresistance and hysteresis phenomena are also observed at low temperature.The saturated magnetization of 2.00μ_B/f.u.（at 2 K）was obtained by formula fitting of the high field part of the hysteresis loop.According to this data,the antisite defect rate is 16.7%.The M-T curves measured in ZFC and FC modes show a monotonic increasing trend,indicating that the samples had a high ferromagnetic T_C and exhibited apparent ferromagnetism at room temperature.By using the UV-Vis absorption spectra,optical band gap of the Sr₂FeReO₆ powder was determined to be 1.39 e V,which was originated from the electron transition from Re 5d to Fe 3d orbital.