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CuFeO2 delafossite materials have been researched for their promising photoactivity for CO2 reduction (CO2R) due to their intrinsic p-type conductivity. However, its practical application is limited by its poor stability and low photocurrent densities. In this work, we investigated the mechanistic origin of CuFeO2 degradation under CO2R conditions. Through photoelectrochemical measurements combined with ex situ X-ray photoelectron spectroscopy and in situ surface-enhanced Raman spectroscopy, we show that CO2-saturated sodium bicarbonate electrolytes enhance photoelectrochemical corrosion by facilitating iron leaching from the catalyst. Systematic control experiments reveal that this instability is not governed solely by thermodynamic surface stability but arises from a nonequilibrium interfacial speciation of CO2, bicarbonate, and carbonate. The presence of carbonate species at the catalyst interface facilitates iron(II) complexation and degrades the CuFeO2 surface. These findings establish carbonate-driven photoelectrochemical corrosion as a key degradation pathway for CuFeO2 and underscore the importance of speciation at the interface-electrolyte in dictating the long-term performance of a catalyst for CO2R.   

To investigate the scope of ferroelectric behavior in La-substituted BiFeO3 films, LaxBi1−xFeO3 epitaxial films were synthesized using off-axis co-sputtering on SrTiO3(001) and DyScO3(110) substrates with a SrRuO3 bottom electrode layer. A digital-doping deposition method was used to enable precise control and continuous tuning of La concentration in high-quality LaxBi1−xFeO3 films across a wide range of x = 0.05–0.60, which was systematically investigated using piezoresponse force microscopy. Robust and reversible out-of-plane ferroelectric switching has been observed up to x = 0.35, while films with x ≥ 0.37 exhibit no measurable ferroelectric behavior, indicating a sharp ferroelectric-to-paraelectric phase transition between x = 0.35 and 0.37. This represents the highest reported La concentration in LaxBi1−xFeO3 films that retains ferroelectric ordering, highlighting opportunities to engineer ferroelectric and multiferroic properties in complex oxide heterostructures. 

Abstract Structural domains and domain walls, inherent in single crystalline perovskite oxides, can significantly influence the properties of the material and therefore must be considered as a vital part of the design of the epitaxial oxide thin films. We employ 4D-STEM combined with machine learning (ML) to comprehensively characterize domain structures at both high spatial resolution and over a significant spatial extent. Using orthorhombic LaFeO₃as a model system, we explore the application of unsupervised and supervised ML in domain mapping, which demonstrates robustness against experiment uncertainties. The results reveal the consequential formation of multiple domains due to the structural degeneracy when LaFeO₃film is grown on cubic SrTiO₃. In situ annealing of the film shows the mechanism of domain coarsening that potentially links to phase transition of LaFeO₃at high temperatures. Moreover, synthesis of LaFeO₃on DyScO₃illustrates that a less symmetric orthorhombic substrate inhibits the formation of domain walls, thereby contributing to the mitigation of structural degeneracy. High fidelity of our approach also highlights the potential for the domain mapping of other complicated materials and thin films. 

Hybrid n = 1 Ruddlesden-Popper perovskites with aromatic ammonium cations like benzylammonium (BzA) and phenethylammonium (PEA) have been shown to adopt polar structures and exhibit ferroelectricity, but many of the examples discovered thus far contain either Pb or Cd. Here, we describe the synthesis and structural characteriza-tion of four layered halide double perovskites: (BzA)4AgBiBr8, (PEA)4AgBiBr8, (BzA)4AgInCl8, and (PEA)4AgInCl8. In all four compounds the inorganic layers exhibit a chessboard ordering of Ag+ and Bi3+/In3+ and the layers stack in a coherent pattern that maintains the ordering over three-dimensional space. The octahedra sur-rounding Ag+ show a large axial compression, which results in much shorter bonds to the terminal halide ions than to the bridging halide ions, whereas the bismuth- and indium-centered octahedra show only small distortions. There appears to be a competition between polar distortions of the octahedra and octahedral tilting, both of which can optimize hydrogen bonding interactions between the ammonium cations and the inorganic layers. Unlike the Pb- or Cd-containing analogs, the double perovskites seem to favor patterns of octahedral tilting that suppress po-lar ordering of the organic cations. The packing of the organic cations depends on both their conformational flexi-bility and the lateral dimensions of the inorganic layer. These forces favor intra-layer edge-to-face interaction be-tween aromatic rings in the three of the four compounds. The lone exception is (PEA)4AgBiBr8, which forms weak inter-layer edge-to-face interactions between aromatic rings and slip-stacked packing within each organic layer. 

The synthesis, crystal structures, and optical properties of four ternary and six quaternary halides containing the Rh3+ ion are reported here. Rb3RhCl6 adopts a monoclinic structure with isolated [RhCl6]3− octahedra. Rb3Rh2Cl9, Cs3Rh2Cl9, and Cs3Rh2Br9 crystallize in a vacancy ordered variant of the 6H hexagonal perovskite structure, which contains isolated Rh2X93− (X = Cl, Br) dimers of face-sharing octahedra. Cs2AgRhCl6 and Cs2NaRhCl6 adopt the 12R rhombohedral perovskite structure, featuring [M2RhCl12]7− face-sharing octahedral trimers, connected to one another through rhodium-centered octahedra. A4AgRhCl8 and A4AgRhBr8 (A = CH3CH2CH2CH2NH3+, (CH3)2CHCH2CH2NH3)+) crystallize in a cation-ordered variant of the n = 1 Ruddlesden Popper structure, which features layers of corner-connected octahedra with a chessboard ordering of Ag+ and Rh3+ ions separated by double layers of organic cations. The diffuse reflectance spectra of all compositions studied feature peaks in the visible that can be attributed to spin-allowed d-to-d transitions and peaks in the UV that arise from charge transfer transitions. Electronic structure calculations reveal moderate Rh–X–Ag hybridization when rhodium- and silver-centered octahedra share corners, but minimal hybridization when they share faces. Many of the compositions studied have an electronic structure that is effectively zero-dimensional, but Cs2AgRhCl6 is found to possess a two-dimensional electronic structure. The results are instructive for controlling the electronic dimensionality of compositionally complex halide perovskite derivatives.