Topochemical fluorination of n = 2 Ruddlesden–Popper type Sr₃Ti₂O₇ to Sr₃Ti₂O₅F₄ and its reductive defluorination

Within this study, we show that a sequence of substitutive topochemical fluorination of the n = 2 Ruddlesden–Popper type compounds Sr₃Ti₂O₇ to Sr₃Ti₂O₅F₄ followed by reductive topochemical defluorination reactions between the oxyfluoride and the reducing agent sodium hydride allows for a substantial reduction of the oxidation state of Ti due to selective extraction and hydride substitution of fluoride ions. The oxyfluoride Sr₃Ti₂O₅F₄ has been synthesized and characterized structurally for the first time. The defluorination experiments have been conducted at temperatures as low as 300 °C, enabling also the reduction of this metastable compound. The evolution of phase fractions and unit cell volumes of various reduced phases as well as of side products has been monitored by an X-ray diffraction study as a function of the amount of sodium hydride used. Strong structural changes within the reduced phases, involving considerable decreases in the c lattice parameters partly accompanied by symmetry, lowering have been observed. To gain a deeper understanding of the structural changes, selected reduction reaction products have been further investigated by coupled analysis of X-ray and neutron powder diffraction data. Moreover, changes in the oxidation state of Ti have been studied using magnetic measurements and X-ray photoelectron spectroscopy examining differences between the bulk and the surface properties. Additionally, similarities and differences between previously published results on the topochemical defluorination of the n = 1 Ruddlesden–Popper type compound Sr₂TiO₃F₂ are discussed.