Structure Determination Of New Type Oxides And Ionic-doping-induced Structure Evolution

Understanding the structure evolution principle and the correlation between composition,crystal structure and physical properties are the most important content in solid state chemistry.Based on this,design and synthesis new compounds with new type crystal structure,which would supply fundamental study for the development of new materials.The main contents of this thesis are about design and synthesis of new compounds and new structures and solving crystal structure by powder X-ray and neutron diffraction,then further study their structure and symmetry evolution principles.This thesis focus on the “114” family compounds and the CaO-PbO-GaO1.5 ternary phase diagram,the main results are given below.1.The “114” family compounds:(1)A new family of “114” compounds with the general formula MAZn2Ga2O7(M = Ca2+,Sr2+,Ba2+;A = Ca2+,Sr2+)were successfully designed and prepared by substituting of Co2+/3+ and Fe2+/3+ in LnBa(Co/Fe)4O7+δ with no variable chemical valence cations Zn2+ and Ga3+.The influence of non-stoichiometry content of oxygen on the structure could be overcame in MAZn2Ga2O7 effectively.(2)Sr2+ to Ba2+ substitution in(Ca1-xSrx)Ba Zn2Ga2O7 lead to the structure symmetry lowered from P63 mc to P31c(x ≥ 0.4),which was revealed by X-ray and neutron diffraction.The driving force behind this structure symmetry lowering was ascribe to the serious underbonding of Ba2+,and the structure symmetry lowering was helpful for improving the coordination environment of Ba2+.Comparing with the hexagonal phase,the tetrahedra(T)in triangular layer of trigonal phase has a rotation around c-axis,and the rotating angle increase with Sr2+ content.(3)Sr2+-doping-induced structure evolution from P31 c to Pna21(x ≥ 0.6)for Sr(Ba1-xSrx)Zn2Ga2O7,where the rotation model of the tetrahedra in the triangular layer for the orthorhombic phase was changed.Further Sr2+-doping in Sr2-x CaxZn2Ga2O7 had no influence on the structure symmetry,and all the Sr2-x CaxZn2Ga2O7 phases adopt Pna21 symmetry.(4)Al3+-doped MAZn2Ga2O7 induced larger structure distortion,but no symmetry lowering was observed.Layered ordering of Al3+ and Zn2+/Al3+ in the triangular layer and Kagome layer,respectively,was observed in CaBa Zn2Al2O7 and CaBaZn2Al2O7.The T-site cationic ordering distribution was driven by the size difference between T-site cations and local charge balance between triangular layer and Kagome layer.Moreover,the size of M also had an influence on the T-site cation distribution.(5)The structural symmetry lowering of MAZn2(Ga/Al)2O7 is driven by the size mismatch between M and A which can be assessed by the tolerance factor t =(r A+ rO)/√2(rM+ rO).With increasing the size mismatch,the structure symmetry of MAZn2(Ga/Al)2O7 follows the lowering sequence P63mc(t ≥ 0.87)→ P31c(0.87 > t ≥ 0.77)→ Pna21(t < 0.77).(6)Pb2+-doping-induced abnormal cell volume expansion in Ca(Ba1-xPbx)Zn2Ga2O7.Neutron diffraction and DFT calculations suggested the origin of this abnormal phenomenon was attribute to Pb2+-O2-covalency.Local structure refinements based on the neutron PDF data indicated that Ca(Ba1-xPbx)Zn2Ga2O7 exhibit cell volume expansion in both local and long range and no local ordering of Zn2+/ Ga3+ and Pb2+/Ba2+.2.CaO-PbO-GaO1.5 ternary phase diagramThree new compounds Ca2PbGa8O15,CaPbGa4O8,Ca2PbGa6O12 were obtained in the CaO-PbO-GaO1.5 ternary phase diagram.The crystal structures of Ca2PbGa8O15,CaPbGa4O8 were solved by X-ray powder diffraction data.Ca2PbGa8O15 has a new structure type which could be regard as and intergrowth structure with CaGa4O7-and trydimite-type structure.Both CaPbGa4O8 and Ca2PbGa6O12 have super-trydimite type structure but with different ordering of Ca2+ and Pb2+.The former exhibit 1:1 layered ordering of Ca2+ and Pb2+,while 2:1 column ordering of Ca2+ and Pb2+ was observed in Ca2PbGa6O12.