SwissFEL

Layered perovskites of general formula AA'CuFeO₅ are characterized by the presence of spiral magnetic phases whose ordering temperatures 𝑇_spiral can be tuned far beyond room temperature by introducing modest amounts of Cu/Fe chemical disorder in the crystal structure. This rare property makes these materials prominent candidates to host multiferroicity and magnetoelectric coupling at temperatures suitable for applications. Moreover, it has been proposed that the highest 𝑇_spiral value that can be reached in this structural family ( ∼400 K) corresponds to a paramagnetic-collinear-spiral triple point with potential to show exotic physics. Since generating high amounts of Cu/Fe disorder is experimentally difficult, the phase diagram region beyond the triple point has been barely explored. To fill this gap we investigate here eleven YBa_1−𝑥⁢Sr_𝑥⁢CuFeO₅ solid solutions (0≤𝑥≤1 ), where we replace Ba with Sr with the aim of enhancing the impact of the experimentally available Cu/Fe disorder. Using a combination of bulk magnetization measurements, synchrotron x-ray and neutron powder diffraction we show that the spiral state with 𝐤𝑠=(1/2,1/2,1/2±𝑞) is destabilized beyond a critical Sr content, being replaced by a fully antiferromagnetic state with ordering temperature 𝑇_coll⁢2≥𝑇_spiral and propagation vector 𝐤_𝑐⁢2=(1/2,1/2,0). Interestingly, both 𝑇_spiral and 𝑇_coll⁢2 increase with 𝑥 with comparable rates. This suggests a common, disorder-driven origin for both magnetic phases, consistent with theoretical predictions.

Specific signatures of fractionalization have been observed in a three-dimensional system known as quantum spin ice.

The conversion of solar energy into chemical energy, stored in the form of hydrogen, bears enormous potential as a sustainable fuel for powering emerging technologies. Photoactive oxynitrides are promising materials for splitting water into molecular oxygen and hydrogen. However, one of the issues limiting widespread commercial use of oxynitrides is degradation during operation. While recent studies have shown the loss of nitrogen, its relation to reduced efficiency has not been directly and systematically addressed with experiments. In this study, we demonstrate the impact of the anionic stoichiometry of BaTaO_xN_y on its electronic structure and functional properties. Through experimental ion scattering, electron microscopy, and photoelectron spectroscopy investigations, we determine the anionic composition ranging from the bulk toward the surface of BaTaO_xN_y thin films. This further serves as input for band structure computations modeling the substitutional disorder of the anion sites. Combining our experimental and computational approaches, we reveal the depth-dependent elemental composition of oxynitride films, resulting in downward band bending and the loss of semiconducting character toward the surface. Extending beyond idealized systems, we demonstrate the relation between the electronic properties of real oxynitride photoanodes and their performance, providing guidelines for engineering highly efficient photoelectrodes and photocatalysts for clean hydrogen production.