Growth and characterizations of Ba2Ti2Fe2As4O single crystals

Layered Anion-Mixed Oxycompounds: Synthesis, Properties, and Applications

Layered anion-mixed oxycompounds have emerged as pivotal materials across diverse technological domains encompassing electronics, optics, sensing, catalysis, and energy applications. Capitalizing on the unique properties imparted by the additional anion, these compounds exhibit exceptional characteristics including ultra-large charge carrier mobility, giant second-harmonic generation, visible-light-driven photocatalysis, and outstanding thermoelectricity. This article aims to provide a comprehensive summary of layered anion-mixed oxychalcogenides, oxyhalides, oxynitrides, and oxypnictides. Organized by chemical composition and crystal structures, the classification of these oxycompounds precedes an in-depth exploration of various synthesis methodologies. Subsequently, their properties are elucidated in electronics, optics, magnetics, and ferroelectrics, contextualizing their utility in electronic, optical, and catalytic applications. The review culminates in a critical assessment of extant challenges and opportunities within this realm. Furthermore, insights are proffered into the future trajectory of research, underpinning the significance of advancing novel 2D multi-anion oxygenated compounds and their attendant applications.

Correlation-induced self-doping in the iron-pnictide superconductor Ba2Ti2Fe2As4O

The electronic structure of the iron-based superconductor Ba2Ti2Fe2As4O (Tc(onset)=23.5  K) has been investigated by using angle-resolved photoemission spectroscopy and combined local density approximation and dynamical mean field theory calculations. The electronic states near the Fermi level are dominated by both the Fe 3d and Ti 3d orbitals, indicating that the spacer layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect; i.e., 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, in which the self-doping effect is attributed to the electronic correlations in the 3d shells. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.