In complex oxides, oxygen defects play a crucial role for physical functionalities. For instance, the presence of vacant anion sites can induce a large ionic conductivity, with relevant applications including solid oxide fuel cells, electrolyte membranes, catalysts, and oxygen sensors. [1][2][3][4] In addition, oxygen vacancy migration driven by low voltages possesses excellent resistance switching characteristics, promoting the emergence of novel oxide devices for resistive switching random access memory. [5][6][7][8] Beyond these, especially among transition metal oxides with mixed valence, oxygen off-stoichiometry alters oxidation states of the transition metal, the metal-oxygenmetal bond angles and the overall charge carrier concentration, which are essential for a multitude of unique phenomena, such as various forms of magnetism, superconductivity, magnetoresistance, and multiferroics. [9,10] Due to the profound influence of oxygen defects, particular attention has been devoted to functional oxides accommodating oxygen content variations. A significant development in recent studies is the realization of a layered vacancy ordered brownmillerite (BM) structure that can be topotactically transformed from a perovskite (PV) structure framework, in which one-sixth of the oxygen anions have been removed. [11][12][13] The special feature of this structure type is demonstrated to provide additional mechanisms for proton and oxide-ion conduction. [14][15][16] However, very few studies have been conducted regarding the dynamic behavior of oxygen vacancies and the transformation mechanisms between distinctive topotactic phases. Such mechanistic details will not only aid the fundamental understanding of oxygen vacancy physics, but also open comprehensive prospects for novel ionic conductor devices. In this view, a real-time investigation of the structural evolution is crucial for addressing the full details of the phase transition process.Strontium-doped lanthanum manganites (La 1−x Sr x MnO 3 ) exhibit a variety of unique tunable functionalities such as colossal magnetoresistance, half-metallicity, and ferromagnetism above room temperature, which can be considered as a novel platform to study the topotactic transition mechanism andThe vacancy distribution of oxygen and its dynamics directly affect the functional response of complex oxides and their potential applications. Dynamic control of the oxygen composition may provide the possibility to deterministically tune the physical properties and establish a comprehensive understanding of the structure-property relationship in such systems. Here, an oxygen-vacancy-induced topotactic transition from perovskite to brownmillerite and vice versa in epitaxial La 0.7 Sr 0.3 MnO 3−δ thin films is identified by real-time X-ray diffraction. A novel intermediate phase with a noncentered crystal structure is observed for the first time during the topotactic phase conversion which indicates a distinctive transition route. Polarized neutron reflectometry confirms an oxygen-deficient in...

Depth-resolved structural analysis reveals strong size selectivity of self-assembled iron oxide nanoparticles.

The High-Brilliance Neutron Source (HBS) project aims to design a scalable compact accelerator driven neutron source (CANS) which is competitive and cost-efficient. The concept allows one to optimize the whole facility including accelerator, target, moderators and neutron optics to the demands of individual neutron instruments. Particle type, energy, timing, and pulse structure of the accelerator are fully defined by the requirements of a given neutron instrument. In the following, we present the current status of the HBS project.

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Strontium cobaltite (SrCoO2.5+δ , SCO) is a fascinating material because of its topotactic structural phase transition caused by a change in oxygen stoichiometry. In the brownmillerite phase (δ = 0) it is an insulating antiferromagnet whereas in the perovskite phase (δ = 0.5) it is a conducting ferromagnet. In contrast, the impact of the varying Co/Sr stoichiometry on the structure has not yet been studied in SCO thin films. Using molecular beam epitaxy we have fabricated SCO thin films of varying Co/Sr stoichiometry. Films with Co excess exhibit a brownmillerite crystal structure with CoO precipitates within the thin film and on the surface. Co deficient films are amorphous. Only for 1:1 stoichiometry a pure brownmillerite structure is present. We find a clear dependence of the Reflection High Energy Electron Diffraction (RHEED) pattern of these thin films on the stoichiometry. Interestingly, RHEED is very sensitive to a Co excess of less than 12% while x-ray diffraction fails to reveal that difference. Hence, using RHEED, the stoichiometry of SCO can be evaluated and tuned in-situ to a high degree of precision, which allows for a quick adjustment of the growth parameters during a sample series.