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Oxynitrides such as LaTa(O,N)3 are attractive materials as photoelectrodes for photoelectrocatalytic solar water splitting. The potential anionic ordering in their perovskite-type structure has been shown to impact the materials' properties. Given the importance attributed to it, the present study reports a detailed experimental analysis supported by simulations of the anionic ordering of La1−xYxTa(O,N)3. The influence of O/N and yttrium content on the anionic order was assessed. Neutron diffraction analysis was performed on four different nominal compositions—LaTaON2, LaTaO2N, La0.9Y0.1TaON2, and La0.9Y0.1TaO2N—at 10 K and 300 K to study potential long-range ordering. Neutron pair distribution function (PDF) analysis was performed on all samples at 10 K and on non-Y-substituted samples at 300 K to evaluate short-range ordering. There was no evidence of long-range O/N order in any of the compounds. In contrast, at a short range (1.5 Å ≤ r < 6 Å), a Pnma (a−b+a−) tilting pattern and local cis-ordering of the anions were seen. The latter faded rapidly, leaving the Pnma tilting pattern in a 6 Å ≤ r ≤ 11 Å range. At higher distances, the PDF analysis agreed with the Imma (a−b0a−) O/N disordered long-range structure. As the O/N content changed, not much difference in behavior was observed. Yttrium substitution introduced some disorder in the structure; nonetheless, it showed marginal influence on octahedral tilting and anionic ordering. [ABSTRACT FROM AUTHOR]

Perovskite-type oxynitrides hold great potential for optical applications due to their excellent visible light absorption properties. However, only a limited number of such oxynitrides with modulated physical properties are available to date and therefore alternative fabrication strategies are needed to be developed. Here, we introduce such an alternative strategy involving a precursor microstructure controlled ammonolysis. This leads to the perovskite family member LaTa(IV)O2N containing unusual Ta4+ cations. The adjusted precursor microstructures as well as the ammonia concentration are the key parameters to precisely control the oxidation state and O:N ratio in LaTa(O,N)3. LaTa(IV)O2N has a bright red colour, an optical bandgap of 1.9 eV and a low (optically active) defect concentration. These unique characteristics make this material suitable for visible light-driven applications and the identified key parameters will set the terms for the targeted development of further promising perovskite family members. Perovskite-type oxynitrides hold great potential for optical applications due to their excellent visible light absorption properties. Here, a defined precursor microstructure is used to modulate the oxidation state of tantalum during ammonolysis leading to a bright red perovskite containing tantalum in a rare 4+ -state. [ABSTRACT FROM AUTHOR]

For the synthesis of perovskite-type oxynitrides AB (O,N) 3 in most cases cationic substitution is applied for charge compensation, while anionic substitution, e.g., using fluoride is by far less investigated. Herein, we have investigated the possibility to use sequential double anionic substitution for charge compensation while forming perovskite-type oxynitride fluorides AB (O,N,F) 3 with A = Ca, Sr, Ba and B Ti, Zr. As expected from Goldschmidt's tolerance factor, the cubic SrTi(O,N,F) 3 and BaZr(O,N,F) 3 were found to be easily formed. The synthesis required a lower number of rigorous thermal treatment cycles to obtain single-phase materials. The non-cubic crystal structures favored a factor six to ten higher nitrogen content of e.g., up to 0.06 per formula unit in SrZr(O,N,F) 3 , while the fluoride content did not change within measurement uncertainty. Overall, the amount of anionic substitution remained low and a presence of oxygen vacancies in the range of 0.1 per formula unit was indicated in all cases. In comparison to the corresponding oxides the double anionic substituted oxynitride fluorides displayed a band gap reduction by about 0.7 eV on average, however, the obtained band gap values of around 3 eV were still too large for a potential application in solar water splitting (SWS). Nonetheless, a clear correlation of the nitrogen content and the band gap reduction was obtained, e.g., SrZr(O,N,F) 3 revealing the highest nitrogen content also showing the highest band gap reduction. While no band gap change was observed in CaZr(O,N,F) 3 showing a negligible nitrogen amount. The main drawback of a sequential double anionic substitution strategy using consecutive heterovalent substitution by F− and N3− is found to be the need of intermediate charge compensation. In parallel the Fermi level is altered. Both allowed only for a very limited substitution of the anionic sublattice. Instead, alternative synthesis pathways enabling a simultaneous anionic double substitution will be needed to realize increased partial substitution of nitride and fluoride in perovskite-type AB (O,N,F) 3 reducing the band gaps to values suitable for SWS. [Display omitted] • Cubic SrTi(O,N,F) 3 & BaZr(O,N,F) 3 are easier to synthesize, but have lower N content. • The higher the N content, the higher the band gap reduction. • Non-cubic structures helped to stabilize higher anionic substitution degree. • Intermediate charge compensation during sequential synthesis limits anionic substitution. [ABSTRACT FROM AUTHOR]

Most of the currently used perovskite-based oxygen-transporting membranes have insufficient resistance towards CO 2 and high material costs that potentially limit their commercial applications. In the present work, a highly CO 2 -tolerant oxygen permeation membrane based on La 0.6 Ca 0.4 Co 1– x Fe x O 3− δ (x = 0, 0.3, 0.5, 0.7, 1) was designed and prepared by a scalable reverse co-precipitation method. The oxygen permeation flux through the dense membranes was evaluated and found to be highly dependent on the Co/Fe ratio. La 0.6 Ca 0.4 Co 0.3 Fe 0.7 O 3− δ possessed the highest permeation flux among the investigated samples, achieving 0.76 ml min−1 cm−2 under an Air/He gradient and 0.5 ml min−1 cm−2 under an Air/CO 2 gradient at 1173 K for a 1 mm thick membrane. A combination study of first principles calculations and experimental measurements was conducted to advance the understanding of Co/Fe ratio effects on the oxygen migration behavior in La 0.6 Ca 0.4 Co 1– x Fe x O 3− δ. The observed oxygen permeability is three times higher than that reported in literature under similar conditions. The presented results demonstrate that this highly CO 2 -tolerant membrane is a promising candidate for high temperature oxygen separation applications. Density functional theory is combined with experiment to advance the understanding of substitutional doping on oxygen transport behavior and CO 2 stability. Image 1 • La 0.6 Ca 0.4 Co 1– x Fe x O 3− δ (x = 0, 0.3, 0.5, 0.7, 1) was designed and prepared by a scalable reverse co-precipitation method. • Highest O 2 permeability under CO 2 atmosphere compared to reported values. • DFT calculation was used to advance the understanding of the oxygen ion migration behavior. • In-depth study of CO 2 tolerance and O 2 permeability for LCCF membrane. [ABSTRACT FROM AUTHOR]