The Study Of Syntheses,Structures And Physical Properties Of Two Types Of Molybdenum Oxides

Transition metal oxides have attracted much attention over the past years due to their vast components,rich physics,and potential applications.In this dissertation,we have synthesized several molybdenum oxides and investigated the structural and physical properties of(Fe_1-xZn_x)₂Mo₃O₈,Nb_xMo_1-xO₂compounds combined with the method of transmission electron microscopy.The main results and conclusions are presented as follows:1.We have synthesized the compounds(Fe_1-xZn_x)₂Mo₃O₈（x=0,0.25,0.5）by using the solid-state reaction method,and determined the crystal structure of these com-pounds to be hexagonal with space group6₃8)(8.We have analyzed the image data obtained from scanning transmission electron microscopy and quantified the contrast between the tetrahedral and octahedral sites,which can be distinguished in high-angle annular dark-field images.Based on the Z（atomic number）contrast imaging,we find that the Zn ions in Fe Zn Mo₃O₈prefer to occupy the tetrahedron sites.Besides,we have analyzed the physical properties of the series of the compounds,and the results show that(Fe_0.75Zn_0.25)₂Mo₃O₈exhibits ferrimagnetism,and the Fe ions in both Fe₂Mo₃O₈and(Fe_0.75Zn_0.25)₂Mo₃O₈compounds have a+2 valence state.In this work,we have directly observed the preferential occupation site of Zn ions in Fe Zn Mo₃O₈by transmis-sion electron microscopy for the first time,which lays the foundation for further studies on M₂Mo₃O₈compounds.2.We have successfully synthesized polycrystalline Nb_xMo_1-xO₂and determined their structures to be room temperature Mo O₂monoclinic structure（2₁/（8）for x=0.05,0.1,0.2,body-centered tetragonal structure（4₁/（6）for x=0.3,Ti O₂rutile-type structure（4₂/8）9）8）)for x=0.33,0.6,0.65,respectively.The X-ray absorption spectra indicate that the Nb ions in Nb_0.05Mo_0.95O₂possess+5 valence value,and Mo ions pos-sess+4 valence state.The transport property measurements show that Nb_0.05Mo_0.95O₂,Nb_0.1Mo_0.9O₂and Nb_0.2Mo_0.8O₂are metallic.The UV-Vis diffuse reflectance spec-tra support the result of transport measurements.Our work indicates the diversity of crystal structures of Nb_xMo_1-xO₂and displays the related physical properties of the compounds for the first time.3.We have synthesized the polycrystalline compound Nb_0.25Mo_0.75O₂by using the solid-state reaction method and examined the structural and physical properties.We have determined that Nb_0.25Mo_0.75O₂has the body-centered tetragonal crystal structure with the space group4₁/(6,which is the space group of Nb O₂with the room temper-ature phase.The temperature-variability X-ray diffraction results of Nb_0.25Mo_0.75O₂show that the structure is stable within 28^℃-400^℃.The measurement results of trans-port properties show that Nb_0.25Mo_0.75O₂has metal resistivity,and the UV-Vis dif-fuse reflectance spectra experiment results show that it does not have the semiconduc-tor band structure.Considering the grain boundary effect of the synthesized polycrys-talline compound,it is considered that Nb_0.25Mo_0.75O₂has a metallic state.The mag-netic property results show that the paramagnetic behavior of Nb_0.25Mo_0.75O₂includes Pauli paramagnetism and Curie-Weiss paramagnetism.The X-ray absorption spectra reveal that the Nb and Mo ions in Nb_0.25Mo_0.75O₂have+4 valence states.Besides,the electron paramagnetic resonance spectroscopy results indicate no oxygen vacancy in Nb_0.25Mo_0.75O₂,which supports the X-ray absorption spectroscopy results.This work reveals that Nb_0.25Mo_0.75O₂has a space group of4₁/（6,rather than that of the Ti O₂rutile structure（4₂/8）9）8)).We have also systematically analyzed the related physical properties of Nb_0.25Mo_0.75O₂for the first time.