point defects, imperfections, and higher dimensional extended defects are known to severely impact the overall functional properties. [3,4] As a matter of example, oxygen vacancies were shown to enhance oxygen conductivity in oxide-ion electrolytes [5] or to boost oxygen incorporation and catalytic activity in mixed ionic electronic conductors (MIECs) [6,7] while weakening electronic and magnetic order in ferromagnetic oxides. [8] Besides, the presence of heterogeneous and homogenous interfaces in thin films was shown to drastically impact defect concentrations in such layers, which gives rise to deviations from bulk defect chemistry and, eventually, to new and unexpected properties. [9-11] Therefore, the knowledge and quantification of the chemical reactions that dominate the defect concentration in oxide thin films (i.e., defect chemistry) is essential for understanding the material behavior and for engineering their properties. This is especially relevant at intermediate-to-low temperatures (below 500 °C), where a high electrochemical activity and the nanometric dimensions of the thin films allow the point defect equilibrium with the environment, [12] hampering the use of the high temperature defect chemistry model from the bulk counterpart. A paradigmatic example of the large effect of point defects on the functional properties of transitional metal oxides can be found in the La 1−x Sr x FeO 3−δ (LSF) model family. LSF compounds crystalize in a perovskite ABO 3 structure and find application in many renewable energy technologies, such as solid oxide fuel cells (SOFC), [13] or electrochemical and photoelectrochemical water splitting. [14,15] In LSF, the substitution of trivalent La by divalent Sr gives rise to the generation of electronic holes and/or oxygen vacancies for electronic compensation, depending on the electrochemical equilibrium with the oxygen partial pressure of the environment. Interestingly, both point defects were found to be strongly correlated to many functional properties of LSF. For instance, the increase of holes concentration was found responsible for a large modification of the electronic structure, [16] affecting not just the electronic and magnetic transport properties [17] but also the oxygen evolution properties in aqueous media. [15] Meanwhile, oxygen vacancies were shown to take part into the rate-limiting step of oxygen incorporation at high temperature. [18] For these reasons, the development of a reliable and flexible in situ method for tracking the point defects of LSF thin films on any substrate and environment is fundamental for tailoring their properties. Unveiling point defects concentration in transition metal oxide thin films is essential to understand and eventually control their functional properties, employed in an increasing number of applications and devices. Despite this unquestionable interest, there is a lack of available experimental techniques able to estimate the defect chemistry and equilibrium constants in such oxides at intermediate-to-low temperatures. In ...

In this work, a mechanochemical process using high-energy milling conditions was employed to synthesize La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3-δ (LSGM) powders from the corresponding stoichiometric amounts of La 2 O 3 , SrO, Ga 2 O 3 , and MgO in a short time. After 60 min of milling, the desired final product was obtained without the need for any subsequent annealing treatment. A half solid oxide fuel cell (SOFC) was then developed using LSGM as an electrolyte and La 0.8 Sr 0.2 MnO 3 (LSM) as an electrode, both obtained by mechanochemistry. The characterization by X-ray diffraction of as-prepared powders showed that LSGM and LSM present a perovskite structure and pseudo-cubic symmetry. The thermal and chemical stability between the electrolyte (LSGM) and the electrode (LSM) were analyzed by dynamic X-ray diffraction as a function of temperature. The electrolyte (LSGM) is thermally stable up to 800 and from 900 • C, where the secondary phases of LaSrGa 3 O 7 and LaSrGaO 4 appear. The best sintering temperature for the electrolyte is 1400 • C, since at this temperature, LaSrGaO 4 disappears and the percentage of LaSrGa 3 O 7 is minimized . The electrolyte is chemically compatible with the electrode up to 800 • C. The powder sample of the electrolyte (LSGM) at 1400 • C observed by HRTEM indicates that the cubic symmetry Pm-3m is preserved. The SOFC was constructed using the brush-painting technique; the electrode-electrolyte interface characterized by SEM presented good adhesion at 800 • C. The electrical properties of the electrolyte and the half-cell were analyzed by complex impedance spectroscopy. It was found that LSGM is a good candidate to be used as an electrolyte in SOFC, with an Ea value of 0.9 eV, and the LSM sample is a good candidate to be used as cathode.

Ion intercalation of perovskite oxides in liquid electrolytes is a very promising method for controlling their functional properties while storing charge, which opens up its potential application in different energy and information technologies. Although the role of defect chemistry in oxygen intercalation in a gaseous environment is well established, the mechanism of ion intercalation in liquid electrolytes at room temperature is poorly understood. In this study, the defect chemistry during ion intercalation of La0.5Sr0.5FeO3−δ thin films in alkaline electrolytes is studied. Oxygen and proton intercalation into the La1–x Sr x FeO3−δ perovskite structure is observed at moderate electrochemical potentials (0.5 to –0.4 V), giving rise to a change in the oxidation state of Fe (as a charge compensation mechanism). The variation of the concentration of holes as a function of the intercalation potential is characterized by in situ ellipsometry, and the concentration of electron holes is indirectly quantified for different electrochemical potentials. Finally, a dilute defect chemistry model that describes the variation of defect species during ionic intercalation is developed.

High energy planetary ball milling has been used to synthesize pseudo-cubic highly-pure LaGaO3 in one hour from its oxide components in an air atmosphere.

The defect chemistry of La1−xSrxFeO3−δ (LSF) thin films is unveiled for intermediate‐to‐low temperature range using a novel in situ ellipsometry approach. The evolution of the concentration of holes in the LSF thin films with oxygen partial pressure and temperature is obtained. This technique pushes the limits for tracking the defect chemistry in LSF thin films to lower temperature. More details can be found in article number 2001881 by Francesco Chiabrera, Albert Tarancón, and co‐workers.