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.

BaCoO2+δ: a new highly oxygen deficient perovskite-related phase with unusual Co coordination obtained by high temperature reaction with short reaction times.

A new highly oxygen deficient metastable modification of perovskite-related BaCoO2+δ (δ ∼ 0.01-0.02) has been prepared using high temperature reactions with short heating times. This defect rich compound has at least partially square planar coordination of the Co2+ ions, a highly unusual coordination environment for Co. Low temperature neutron powder diffraction showed a G-type antiferromagnetic ordering, confirmed by SQUID magnetic measurements, which indicate a high Néel temperature of 220 K. This work shows how novel defective phases can be synthesized by exploiting short reaction times in solid state synthesis, thus offering an alternative route for new materials synthesis.

Synthesis and characterisation of fluorinated epitaxial films of BaFeO2F: tailoring magnetic anisotropy via a lowering of tetragonal distortion.

In this article, we report on the synthesis and characterisation of fluorinated epitaxial films of BaFeO2F via low-temperature fluorination of thin films of BaFeO2.5+d grown by pulsed laser deposition. Diffraction measurements show that fluoride incorporation only results in a contraction of the film perpendicular to the film surface, where clamping by the substrate is prohibitive for strong in-plane changes. The fluorinated films were found to be homogenous regarding the fluorine content over the whole film thickness, and can be considered as single crystal equivalents to the bulk phase BaFeO2F. Surprisingly, fluorination resulted in the change of the tetragonal distortion to a nearly cubic symmetry, which results in a lowering of anisotropic orientation of the magnetic moments of the antiferromagnetically ordered compound, confirmed by Mössbauer spectroscopy and magnetic studies.

Topochemical Fluorination of La2NiO4+d: Unprecedented Ordering of Oxide and Fluoride Ions in La2NiO3F2.

The Ruddlesden-Popper (K2NiF4) type phase La2NiO3F2 was prepared via a polymer-based fluorination of La2NiO4+ d. The compound was found to crystallize in the orthorhombic space group Cccm ( a = 12.8350(4) Å, b = 5.7935(2) Å, c = 5.4864(2) Å). This structural distortion results from an ordered half occupation of the interstitial anion layers and has not been observed previously for K2NiF4-type oxyfluoride compounds. From a combination of neutron and X-ray powder diffraction and 19F magic-angle spinning NMR spectroscopy, it was found that the fluoride ions are only located on the apical anion sites, whereas the oxide ions are located on the interstitial sites. This ordering results in a weakening of the magnetic Ni-F-F-Ni superexchange interactions between the perovskite layers and a reduction of the antiferromagnetic ordering temperature to 49 K. Below 30 K, a small ferromagnetic component was found, which may be the result of a magnetic canting within the antiferromagnetic arrangement and will be the subject of a future low-temperature neutron diffraction study. Additionally, density functional theory-based calculations were performed to further investigate different anion ordering scenarios.