The burgeoning field of anion engineering in oxide‐based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent‐bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide‐only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion‐engineering principles in heteroanionic materials are reviewed.

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The slow kinetics of the oxygen evolution reaction (OER) is the main cause of energy loss in many low-temperature energy storage techniques, such as metal-air batteries and water splitting. A better understanding of both the OER mechanism and the degradation mechanism on different transition metal oxides is critical for the development of the next generation of oxides as OER catalysts. In this paper, we systematically investigated the catalytic mechanism and lifetime of ABO 3-δ perovskite catalysts for OER, where A = Sr or Ca and B = Fe or Co. During the OER process, the Fe-based AFeO 3-δ oxides with δ ≈ 0.5 demonstrate no activation of lattice oxygen or pH dependence of OER activity, which is different from the SrCoO 2.5 with similar oxygen 2p-band position relative to the Fermi level. The difference was attributed to the larger changes in the electronic structure during the transition from the oxygen-deficient brownmillerite structure to the fully-oxidized perovskite structure and the poor conductivity in Fe-based oxides, which hinders the uptake of oxygen from the electrolyte to the lattice under oxidative potentials. The low stability of Fe-based perovskites under OER conditions in basic electrolyte also contribute to the different OER mechanism compared with the Co-based perovskites. This work reveals the influence of transition metal composition and electronic structure on the catalytic mechanism and operational stability of perovskite OER catalysts.

Anion redox in lithium transition metal oxides such as Li 2 RuO 3 and Li 2 MnO 3, has catalyzed intensive research efforts to find transition metal oxides with anion redox that may boost the energy density of lithium-ion batteries. The physical origin of observed anion redox remains debated, and more direct experimental evidence is needed. In this work, we have shown electronic signatures of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O 2− / (O 2 ) n− ), in charged Li 2-x RuO 3 after Ru oxidation (Ru 4+ /Ru 5+ ) upon first-electron removal with lithium de-intercalation. Experimental Ru L 3 -edge high-energy-resolution fluorescence detected X-ray absorption spectra (HERFD-XAS), supported by ab-initio simulations, revealed that the increased intensity in the high-energy shoulder upon lithium de-intercalation resulted from increased O-O coupling, inducing (O-O) σ*-like states with π overlap with Ru d-manifolds, in agreement with O K-edge XAS spectra. Experimental and simulated O K-edge X-ray emission spectra (XES) further supported this observation with the broadening of the oxygen non-bonding feature upon charging, also originated from (O-O) σ* states. This lattice oxygen redox of Li 2-x RuO 3 was accompanied by a small amount of O 2 evolution in the first charge from *

The double perovskite CaMnTiO, is a rare A-site ordered perovskite oxide that exhibits a sizable ferroelectric polarization and relatively high Curie temperature. Using first-principles calculations combined with detailed symmetry analyses, we identify the origin of the ferroelectricity in CaMnTiO. We further explore the material properties of CaMnTiO, including its ferroelectric polarization, dielectric and piezoelectric responses, magnetic order, electronic structure, and optical absorption coefficient. It is found that CaMnTiO exhibits room-temperature-stable ferroelectricity and moderate piezoelectric responses. Moreover, CaMnTiO is predicted to have a semiconducting energy band gap similar to that of BiFeO, and its band gap can further be tuned via distortions of the planar Mn-O bond lengths. CaMnTiO exemplifies a new class of single-phase semiconducting ferroelectric perovskites for potential applications in ferroelectric photovoltaic solar cells.