This thesis gives a detailed study of heavily cation-substituted, disordered Brownmillerite based compounds. The focus is on their proton conduction behaviour when being hydrated as well as their structural properties in the dry state. The study is predominantly conducted by DFT calculations (both static and Born-Oppenheimer molecular dynamics) combined with XANES experiments which confirmed certain theoretical results. Amongst others, by means of these techniques, the coordination preference of different substituents, the protonic mobility and vibrational spectra can be obtained and compared to experimental evidence. The focus lies on the Ba2 In2(1 − x)Ti2xO5+x (BITx) family, proven to be a well suited electrolyte material for proton conducting solid-oxide fuel cells (PC-SOFC). Moreover, neighbouring compounds such as Sr2In2(1 − x)Ti 2xO5+x (SITx), Ba2In2(1 − x)Zr2xO5+x (BIZx) and Ba2In2(1 − x)Y2xO5 (BIYx), are being considered as they prove useful to systematically clarify the influence of different chemical environments on proton diffusivity (e.g. different "proton affinities" of the Ti- or Zr-substituents, more or less pronounced hydrogen bonding, distinction between extra- and intra-octahedrally bonded protons, etc.). All those aspects are obtained in the molecular dynamics framework, naturally integrating temperature or entropic effects.
