Local structure and thermal properties of fluorine-doped perovskite related solid oxides

Abstract

Fluoro-substituted Ba₂CaNbO_{5.5 − 0.5x}F_x double perovskites and Ba₂In₂O_{5 − 0.5x}F_x brownmillerites are synthesized. It is confirmed by X-ray analysis that they are single-phase at 0 ≤ х ≤ 0.40 and 0 ≤ х ≤ 0.25, respectively. It is found that introducing fluoride ions into both double-perovskite and brownmillerite phases reduces the unit cell volume (due to shorter Nb/In–O bond lengths). The preferential positions for fluorine are those in niobium or indium tetrahedra. Energetically inequivalent ОН⁻ groups are the main form in which protons are found in hydrated compositions. Increasing the density of the dopant reduces the fraction of hydroxyl groups involved in strong hydrogen bonds.