Defect Regulation And Conduction Mechanism Of Melilite(-like) Gallium Germanate Oxide Ion Conductors

The oxide ion conductor electrolyte material is the core of SOFC,which determines commercial application of SOFC.Recently,melilite-type oxide ion conductors with interstitial oxide ions as charge carries have attracted ascending attentions due to their promising oxide ion conductivities,deformation and rotation flexibility of the tetrahedral layers with cations showing variable coordination number.So far,interstitial oxide ion conductions in melilites were observed only in pure gallate or aluminate compositions.There are few reports on the melilite oxide ion conductor with oxygen vacancy conduction,whether oxygen vacancy exists and how to stabilize and migrate remains doubtful.Considering the similar chemical properties of Ga and Ge ions,whether the differences in ionic radii and covalent bonds will affect the incorporation and migration of oxide defects in the mixed tetrahedral layer,and oxide defects are incorporated into melilite（-like）gallium germanate materials,so as to design and screen high-performance oxide ion conductor materials.This thesis mainly focuses on the design and preparation of the gallium germanate Ca₂Ga₂Ge O₇ and Sr Ga₂Ge₂O₈component in the melilite（-like）family.Stabilization of oxide defect and ionic migration mechanism were investigated through a wide variety of experimental and computational methods.The main research contents and results are:1.A new mellite interstitial oxide ion conductor Ca_2-xLa_xGa₂Ge O_7+x/2（0≤x≤0.15）was developed by the substitution of La³⁺for Ca²⁺in Ca₂Ga₂Ge O₇ containing mixed（Ga/Ge）O₄ tetrahedral layers.Neutron powder diffraction data indicated that the introduced interstitial oxide ions were accommodated in the distorted pentagonal rings,close to the coordination environment of（Ga2/Ge1）O₄ containing terminal oxygen.Solid state NMR data show that portion of the 3-linked Ga2 ion changes from a 4-coordination to a 5-coordination.The defect formation energy calculations further reveal that the interstitial oxygen not only favors to approach the Ga O₄ tetrahedra,in agreement with studies of other pure gallate systems,but also is confined to the Ge O₄tetrahedra.Molecular dynamics simulations revealed that the interstitial oxide ions were transported among the pentagonal rings within the tetrahedral layers through a cooperative mechanism involving the synergistic knock-on motion between interstitial and framework oxygens.Ca_1.85La_0.15Ga₂Ge O_7.075 which reached oxide ion conductivities～2.79×10^-5 S/cm-1.68×10^-3 S/cm in 500-1000℃,with oxygen transport numbers of～92.65%,two orders of magnitude higher than the parent material.The lower interstitial oxide ion concentration and conductivity in Ca_2-xLa_xGa₂Ge O_7+x/2reveals that although Ge⁴⁺has similar chemical properties to the adjacent Ga³⁺,the subtle differences in size and the resultant covalent bonding ability make the mixed Ga O₄ and Ge O₄ tetrahedral layers in mellite are unfavorable for accommodation and transportation of interstitial oxygens.2.Melilite-type oxide ion conductors of Ga-excess compositions Ca₂Ga_2+xGe_1-xO_7-0.5x（0≤x≤0.15）demonstrate remarkable oxygen vacancy migration by experimental and computational verifications.Neutron powder diffraction data and defect formation energies of oxygen vacancy revealed that oxygen vacancies favor occurring at the bridging O sites shared by four-linked（Ga1）O₄ tetrahedra,opening the five-fold rings coupled by the local structure relaxation around the oxygen vacancies.Molecular dynamics simulations revealed that the oxygen vacancies migration involves the cooperative rotation and deformation of adjacent tetrahedra,allowing the opening and reforming of five-fold rings in melilite structure.Ca₂Ga_2.15Ge_0.85O_6.925 which reached oxide ion conductivities～7.51×10^-7 S/cm–2.10×10^-4 S/cm in 500-1000℃,one orders of magnitude higher than the parent material.And oxygen transport numbers from63.66%to 85.81%,showing a small amount of p-type electron conduction.3.The La_xSr_1-xGa₂Ge₂O_8+0.5xsolid solution was prepared by Sr site doped La with interstitial oxygen defect by high temperature solid phase method.Neutron powder diffraction data and defect formation energies indicate that interstitial oxygen is accommodated in the four-fold ring and performs coordination with（Ga/Ge）O₄.The molecular dynamics simulation shows that the interstitial oxygen can migrate between the neighboring tetrahedra layers and between the intra-layer four-fold ring and eight-fold ring assisted by synergic rotation and deformation of neighboring tetrahedra.Sr_0.85La_0.15Ga₂Ge₂O_8.075 which reached oxide ion conductivities～3.81×10^-8 S/cm-1.72×10^-4 S/cm in 500-1000℃,with two orders of magnitude higher than the parent material.The lower interstitial oxide ion concentration and conductivity in La_xSr_1-xGa₂Ge₂O_8+0.5x reveals that the interstitial oxygen ions enter the 4-linked（Ga/Ge）O₄tetrahedron and the four-fold ring formed by the ordered occupied Ga/Ge tetrahedra constrains interstitial oxygen ions,which is unfavorable for interstitial oxygen ions migration and accommodation.