The Key Role of Monovalent Indium and Anionic Vacancies in Oxyfluorides Inducing Fluoride Ion Mobility and...

Novel Ba_2-xSr_xInO₃F Ruddlesden-Popper phases were synthesized by solid-state routes at high temperatures under Ar and dry air. The structural features were determined through XRD analysis of well-crystallized powders and isolated single crystals. Two In sites were identified with various occupancies, dependent on the Sr content and on the atmosphere applied during annealing. This indium site distribution leads to the consideration of an anionic disorder on the apical site in the vicinity of In. A maximum in the O2-In-O2 bond angles is identified for the Ba_1.2Sr_0.8InO₃F composition and corresponds to a stronger hybridization between the In (s,p) and O(p) orbitals in the equatorial plane, which should be the signature of monovalent indium. ¹⁹F MAS NMR investigation recorded at various temperatures shows the F^- hopping phenomenon in the mixed (Ba/Sr) environment, which is most present in the Ba_1.2Sr_0.8InO₃F oxyfluoride and linked to the presence of anionic vacancies. Considering the electroneutrality of the composition, monovalent indium should be stabilized in octahedron with a vacant vertex. Excitation (UV range) and emission (visible range) broad bands are clearly detected, but neither excitation nor emission wavelength varies with the Sr content. However, the photoluminescence intensity is strongly correlated with the composition and reaches a maximum for the Ba_1.2Sr_0.8InO₃F compound. Density functional theory calculations allow for the identification of defect states related to anionic vacancies in the band gap, with hybridization mainly between In(s) and O(p) orbitals in the basal plane, thus explaining the self-trapped exciton (STE) mechanism.