Defect chemistry, oxygen ion conduction, and proton conduction of oxides with brownmillerite and related structures

This dissertation presents a study on defect structure, oxygen ion conductivity, proton conductivity, electronic conductivity, and high temperature equilibrium redox properties of brownmillerite related oxides with general formula A{dollar}sb{lcub}rm n{rcub}{dollar}B{dollar}sb{lcub}rm n{rcub}{dollar}O{dollar}sb{lcub}rm 3n-1{rcub}{dollar} (n = 2 to {dollar}infty{dollar}).; A defect chemistry model is proposed for the brownmillerite oxides with high oxygen ion conductivity. Ba{dollar}sb2{dollar}In{dollar}sb2{dollar}O{dollar}sb5{dollar} was chosen as the model material and its electrical conductivity and transport properties have been studied in detail. The oxygen ion conduction above the order-disorder temperature, T{dollar}sb{lcub}rm d{rcub}{dollar} {dollar}approx{dollar} 925{dollar}spcirc{dollar}C, and a mixed ionic-electronic conduction below T{dollar}sb{lcub}rm d{rcub}{dollar}, was studied by conductivity and EMF measurements as a function of temperature and oxygen activity. The main defects are intrinsic anion Frenkel defects below T{dollar}sb{lcub}rm d{rcub}{dollar}, and above T{dollar}sb{lcub}rm d{rcub}{dollar} the oxide can be treated as acceptor doped perovskite with extrinsic oxygen vacancies. Charge compensation involves only ionic defects over the whole P(O{dollar}sb2{dollar}) range used in this study. The formation and mobility enthalpies of the Frenkel defects, the redox enthalpies, and the band gap have been obtained for this oxide. A similar study has been done for other compositions in Ba{dollar}sb{lcub}rm n{rcub}{dollar}In{dollar}sb2{dollar}Zr{dollar}sb{lcub}rm n-2{rcub}{dollar}O{dollar}sb{lcub}rm 3n-1{rcub}{dollar} system. The proposed model is in good agreement with the experimental results.; Evidence for protonic conduction was also found in these materials, especially at low temperatures. Three regions of protonic conduction in the Arrhenius plot have been observed and analyzed. The proton formation and mobility enthalpies have been obtained. The observed proton conductivity transition at the oxygen order-disorder transition temperature directly confirms the proton formation mechanism by incorporation of H{dollar}sb2{dollar}O molecules into oxygen vacancies.; A defect chemistry study was also conducted for the brownmillerite oxides with high electronic conductivity with Ca{dollar}sb2{dollar}(Al{dollar}sb{lcub}rm x{rcub}{dollar}Fe{dollar}sb{lcub}rm 2-x{rcub}{dollar})O{dollar}sb5{dollar} chosen as the model system. The main defects are intrinsic electronic defects and the charge compensation mechanism is by electrons and holes. The band gap of this system was obtained and the value agrees with other similar Fe{dollar}sp{lcub}+3{rcub}{dollar} oxides. The redox behavior of Ca{dollar}sb2{dollar}(Al{dollar}sb{lcub}rm x{rcub}{dollar}Fe{dollar}sb{lcub}rm 2-x{rcub}{dollar})O{dollar}sb5{dollar} and (Sr,Ca)MnO{dollar}sb{lcub}3-delta{rcub}{dollar} system also support this defect model.; A low activation enthalpy of ionic conductivity in (BaLa)(YCd)O{dollar}sb5{dollar} and La{dollar}sb2{dollar}Sr{dollar}sb2{dollar}O{dollar}sb5{dollar} systems was observed. The preliminary results encourage the further study of these systems.; In conclusion, defect structure, defect transport, and redox behavior of oxides with brownmillerite-related structures have been studied. Complete and self-consistent defect models are proposed for compounds with cations in different oxidation states. A clear picture of proton conduction is also obtained for this type of oxide. The investigation of several new systems suggests the possibility to find new ionic conductors in the brownmillerite structures.