Synthesis,Structure And Properties Of The A₈B₇O₂₄Twinned-shifted Hexagonal Perovskites

In this thesis,we prepared some new A₈B₇O₂₄ hexagonal perovskite oxides by the high temperature solid state reaction,including the first 14-layer twinned hexagonal perovskite with long-period and more 8-layer shifted or twined hexagonal perovskites.The structure determination and phase analysis were performed by multiple efficient and complementary techniques,which including?synchrotron or XRD?X-ray powder diffraction and neutron powder diffraction and the selected electron diffraction and high-resolution transmission electron microscopy and scanning transmission electron microscopy et al.The valence of element was examined by the magnetic susceptibility and X-ray photoelectron spectroscopy?XPS?data.Elements were characterized by energy dispersive spectrometer?EDS?.We have also focused on the conduction properties and microwave dielectric properties and UV-Vis spectrum,and payed more attention to structure,microwave dielectric properties and the relationship between structure and properties of the 8-layer hexagonal perovskite oxides compositions.The research work can be composed of six parts:In the first part,Ba₈MnTa₆O₂₄ materials were synthesized and founded unlike an 8-layer twinned hexagonal perovskite Ba₈ZnTa₆O₂₄,to display the unexpected formation of a 14-layer twinned hexagonal perovskite with a staking sequence?cccccch?₂ for the BaO₃ layers on the Ba₁₄Mn_1.75Ta_10.5O₄₂(Ba₈MnTa₆O₂₄)composition.Atomic-resolution imaging and high-spin d⁵Mn²⁺cation ordering in the d⁰ Ta⁵⁺host are characterized by multiple efficient and complementary techniques including neutron and synchrotron powder diffraction,scanning transmission electron microscopy-high angle annular dark field?STEM-HAADF?imaging,and electron energy loss spectroscopy?EELS?and X-ray energy dispersive spectroscopy?EDS?elemental mapping.Atomic-resolution STEM-HAADF imaging and EELS/EDS elemental mapping demonstrating the great interest of this technique for probing cation ordering and performing structure determination.The large high-spin Mn²⁺cation and Ta-vacancy pair formation in face-sharing octahedral sites play key roles on both the stabilization of this 14-layer twinned hexagonal perovskite structure and the Mn²⁺ordering in the central corner-sharing octahedral?CSO?positions within the five-consecutive CSO layers.Compared with the 8-layer twinned Ba₈ZnTa₆O₂₄4 material,14-layer hexagonal perovskite displayed the low quality factor in microwave frequency and enhanced ultra-violet and visible light absorption of Ba₁₄Mn_1.75Ta_10.5O₄₂2 as well as weakly the photocatalytic activity on water splitting.In the second part,a new 8-layer shifted hexagonal perovskite Ba₈MnNb₆O₂₄ was synthesized and the crystal structure was characterized using a combination of neutron powder diffraction?NPD?,X-ray powder diffraction?XRD?,selected area electron diffraction and high-resolution transmission electron microscopy.The sample,which features long-range B-cation ordering with nanometer-scale separation by¹.9 nm of octahedral d⁵ cation Mn²⁺layers within the purely corner-sharing octahedral d⁰ catioic Nb⁵⁺host.The long-range ordering of the B-site vacancy and out-of-center distortion of the highly-charged d⁰ Nb⁵⁺that is assisted by the second-order Jahn-Teller effect contribute to the unusual B-cation ordering in Ba₈MnNb₆O₂₄.XPS data confirmed the high-spin Mn²⁺in Ba₈MnNb₆O₂₄ structure.The microwave dielectric properties and AC conductivities were investigated.Ba₈MnNb₆O₂₄pellet exhibit high dielectric permittivity??³8,much lower Qf⁹87 and??_f²0 ppm/?.Compared with the analogous 8-layer shifted structure Ba₈ZnNb₆O₂₄,Ba₈MnNb₆O₂₄4 ceramic exhibited much lower quality factor at microwave frequency and stronger ultra-violet and visible light absorption of Ba₈MnNb₆O₂₄.The calculation of First Principle show Ba₈MnNb₆O₂₄ was an indirect semiconductor and the valence band is mainly Mn-3d orbital and conduction band is mainly made up of O-2p and Nb-3d orbital.In the third part:a new 8-layer twinned hexagonal perovskite Ba₈FeTa₆O₂₄ was synthesized in air atmosphere.The density of Ba₈FeTa₆O₂₄ pellet?90%?exhibited?²1,Qf²6 000-29000 GHz and??_f⁶4 ppm/?.Ba₈Fe_4-x-x Ta_4+0.6x+0.6x O₂₄?x=3.0³.4?were also synthesized by the same method.The density of Ba₈Fe_4-xTa_4+0.6xO₂₄?x=3.2³.4?pellet?65%?exhibited dielectric permittivity??¹6-18,lower Qf⁵ 600�7 900 GHz and??_f²3 ppm/?.The B-cation order/disorder and the density of ceramic may led to the different microwave dielectric properties for Ba₈FeTa₆O₂₄4 and Ba₈Fe_4-xTa_4+0.6xO₂₄?x=3.2³.4?compositions.In the fourth part:a new 8-layer shifted hexagonal perovskite Ba₈FeNb₆O₂₄ was synthesized in different atmosphere via the high-temperature solid state reaction,and eventually a single phase was obtained in 5%H₂+95%N₂ atmosphere.The valence of Fe element was examined by the magnetic susceptibility and X-ray photoelectron spectroscopy?XPS?measurement.AC data confirm that Ba₈FeNb₆O₂₄4 is semiconductor.Ultra-violet and visible light absorption of Ba₈FeNb₆O₂₄4 displays a strong absorption in visible light.In the fifth part:a new 8-layer twinned hexagonal perovskite Ba₈CuNb₆O₂₄ was synthesized.The structure was characterized by XRD data and further confirmed Ba₈CuNb₆O₂₄ is an 8-layer twinned hexagonal perovskite and different from 8-layer shifted structure.Ba₈CuNb₆O₂₄4 ceramics possess exceptionally high microwave dielectric loss like Ba₈CuTa₆O₂₄.Impedance spectroscopy measurement demonstrates that Ba₈CuNb₆O₂₄ show the electrical heterogeneous microstructure,consisting of leaky insulating grains and more resistive grain boundary regions,which induced internal barrier layer capacitance effects on Ba₈CuNb₆O₂₄ ceramics.The electrical heterogeneous microstructure is could be associated with partial reduction of Cu²⁺to Cu⁺and oxygen loss during the sintering procedure and limited reoxidization along grain boundary regions on cooling.X-ray photoelectron spectroscopy measurement confirm the existence of Cu⁺in Ba₈CuNb₆O₂₄ ceramic.The leaky insulting bulk property for the Ba₈CuNb₆O₂₄ ceramic is compared with the highly insulating bulk behavior of other low dielectric loss analogs,which indicates that the significant defects of Cu⁺and oxygen vacancies are responsible for the high microwave dielectric loss of the Ba₈CuNb₆O₂₄ ceramic.In the sixth part:A new 8-layer hexagonal perovskite Ba₈NiNb₆O₂₄4 was synthesized and its structure was characterized using selected area electron diffraction,high-resolution transmission electron microscopy and synchrotron X-ray diffraction.Unlike the 8-layer ordered shifted Ba₈CoNb₆O₂₄ and Ba₈ZnNb₆O₂₄,Ba₈NiNb₆O₂₄ adopts a twinned structure with stacking sequence?ccch?₂ for the BaO₃ layers and displays more disordered cation and vacancies over the face-sharing octahedral?FSO?sites than the twinned tantalates Ba₈MTa₆O₂₄?M=Zn,Ni,Co?.The stabilization of twinned structure and cation/vacancy ordering in Ba₈NiNb₆O₂₄ composition is correlated with the smaller size difference between Ni²⁺and Nb⁵⁺in comparison with those between?Zn/Co?²⁺and Nb⁵⁺in the shifted Ba₈CoNb₆O₂₄ and Ba₈ZnNb₆O₂₄.The Ba₈NiNb₆O₂₄4 pellet exhibits high dielectric permittivity ?⁴0,modest Qf⁴1319 GHz and large temperature coefficient of resonant frequency??_f⁶0 ppm/?.The lower Qf value compared with the high-Q Ba₈MTa₆O₂₄ is ascribed to the reduced short-range B-cationic ordering inside the FSO dimers in Ba₈NiNb₆O₂₄.In summary:the B-cation order/disorder,the valence of B-cation,Jehn-Teller,the cationic size and electronic replusive play an important role on the twin-shift option of hexagonal perovskites.Further investigations are necessary for elucidate the detailed mechanisms of stabilization of 8-layer twinned and shifted hexagonal perovskites.