The Application Of X-ray(Electron) Diffraction To Study The Relationships Between The Crystal Structures And Physical Properties Of New Solid Functional Materials

Since Max von Laue discovered X-ray diffraction phenomenon of single crystal in1912,the application of diffraction technologies to study the relationships between material structures and physical properties has been one of the core content in the research of material science.With the developments of hardware and algorithm in the past century,X-ray diffraction and electron diffraction technologies are keeping on progress.At present,the structures of solid functional materials with enough crystal size can be determined by single crystal X-ray diffraction,and the relationships between material structures and physical properties can be studied through structural chemistry.However,it is difficult to grow enough size crystals for most solid functional materials.For this kind of materials,except for the Rietveld refinement of powder X-ray diffraction,the emerging technology,which is the combination of the precession electron diffraction tomography and the structure refinement based on dynamical theory,can be applied to determine the structures.In this thesis,five new functional solid materials were synthesized.They are luminescent materialsα-La_1-xEu_xB₅O₉（0.30≤x≤1.00）andγ-La_1-xEu_xB₅O₉（0.05≤x≤0.30）,relaxor ferroelectric material PbBiNb₅O₁₅ and NLO materials Cs₂TB₄O₉（T=Si,Ge）and Cs SiB₃O₇.Through powder X-ray diffraction,single crystal X-ray diffraction and electron diffraction,combined with fluorescence properties,ferroelectric properties and nonlinear optical properties measurements,the relationships between structures and physical properties of these five new functional solid materials were determined.The results are as follows:1.Forα-La_1-xEu_xB₅O₉（0.30≤x≤1.00）,when the Eu³⁺concentration is beyond 0.60,concentration quenching appears due to the lattice defects introduced by Eu³⁺doping,and the excitation energy is lost through heat.Inγ-La_1-xEu_xB₅O₉（0.00≤x≤0.30）,the average distance of adjacent RE³⁺（<4.5?）is smaller than that of adjacent RE³⁺inα-La_1-xEu_xB₅O₉（0.30≤x≤1.00）（>5.4?）.As a result,inγ-La_1-xEu_xB₅O₉（0.00≤x≤0.30）,when the Eu³⁺concentration is beyond 0.20,concentration quenching appears more easily due to the transformation of the absorbed energy between the luminescent centers.In theα-La_1-xEu_xB₅O₉（0.30≤x≤1.00）,Eu³⁺only occupies the single noncentrosymmetrical RE position.So I（⁵D₀→⁷F₂）is larger than I（⁵D₀→⁷F₁）in the emission spectrums ofα-La_1-xEu_xB₅O₉（0.30≤x≤1.00）both excited by CT and f-f,and the ratio of I（⁵D₀→⁷F₂）to I（⁵D₀→⁷F₁）does not change much with the variations of the Eu³⁺doping amount.Forγ-La_1-xEu_xB₅O₉（0.00≤x≤0.30）,there are three independent positions,and the ratio of Eu³⁺at the noncentrosymmetrical RE1 position reducesfrom greater than 72.4%（x=0.05）to 46.8%（x=0.30）with the Eu³⁺concentration increasing.Correspondingly,the ratio of I（⁵D₀→⁷F₂）to I（⁵D₀→⁷F₁）decreases significantly.2.Different with other incommensurately modulated structure tetragonal tungsten bronze niobates,the incommensurate modulation of PbBiNb₅O₁₅ is not introduced by the distortions of NbO₆ octahedra,and it is from the different distributions and distortions of PbO_x and BiO_x polyhedral.In PbBiNb₅O₁₅,Pb²⁺and Bi³⁺tend to deviate from the centers of PbO_x and BiO_x polyhedral and to produce the ferroelectric polarization in a-b plane due to the 6s² lone pair electrons.To suppress the extremely large LDMs,the long range order incommensurate modulation was appeared along a axis and lead to weak SHG effect and ferroelectric effect of PbBiNb₅O₁₅.3.Generally,the nonlinear coefficients of NLO borate materials depends on the superposition of microscopic nonlinear coefficients of B-O anion groups.However,the microscopic nonlinear coefficients of TO₄ anionic group make large contribution to the nonlinear coefficient of Cs₂GeB₄O₉ and Cs₂SiB₄O₉.In addition,the variations of incommensurately modulated vectors and modulated functions will influence on the nonlinear coefficients of Cs₂GeB₄O₉ and Cs₂SiB₄O₉.Therefore,it is very possible to change the incommensurately modulated vectors and modulated functions of Cs₂TB₄O₉to realize the optimization of NLO properties through element doping.4.The little contribution of s orbit of Si in CsSiB₃O₇ makes the energy of conduction band bottom of CsSiB₃O₇ higher than that of CsGeB₃O₇.Correspondingly,the band gap of CsSiB₃O₇（7.45 eV）is larger than that of CsGeB₃O₇（5.67 eV）.The SHG coefficient of CsSiB₃O₇ is mainly derived from the superposition of microscopic nonlinear coefficients of B₃O₇ and SiO₄ anionic groups,and Cs⁺has almost no contribution to the SHG coefficient of CsSiB₃O₇.For more discussion,the microscopic nonlinear coefficients of B₃O₇ and SiO₄ are mainly derived from the 2p orbit electrons of O.The superposition of parallel 2p orbit electrons of O from the dimeric BO₃ group B₂O₅ plays a decisive role in the microscopic nonlinear coefficients of B₃O₇.For SiO₄,the strong orbit hybridization and electron transfer between Si and O make large contribution to the SHG effect of CsSiB₃O₇.