Search for: All records

Perovskite oxides containing cations with unusually high-valent states such as Fe4+, Ni3+, and Cu3+ have attracted significant attention, often requiring a strong oxidizing atmosphere. This study addresses the challenges associated with the conventional precursor preparation for the high-pressure synthesis of BiNi1–xFexO3, a material exhibiting negative thermal expansion (NTE), specifically the need for mixed oxidants and the emission of NOx. We successfully established a modified method for preparing a highly oxidized, amorphous precursor containing Bi5+ and Ni3+ ions, ensuring high elemental dispersion by employing a reverse coprecipitation method with simultaneous oxidation using hypochlorite ions, enabling the synthesis of the target phase without the addition of external oxidizing agents. When subjected to high-pressure and high-temperature treatment, the BiNi1–xFexO3 phase crystallized directly from the amorphous precursor at a relatively low temperature (750 °C) and in a short time of less than 1 min, unlike the crystalline precursor prepared by conventional methods. Furthermore, leveraging the advantage of direct crystallization from the amorphous phase, we demonstrated that reducing the heating duration allows for the fabrication of fine particles, decreasing the size from 15 to 5 μm. These fine particles exhibited NTE over a wider temperature range without any degradation of the volume shrinkage magnitude. This synthetic method provides a safe, pollution-free, and potentially scalable route for BiNi1–xFexO3 fine particles and is expected to be applicable to the synthesis of other oxides containing anomalously high-valent ions. 

The design of both molecular and non-molecular solid materials with specific properties fundamentally relies on the controlled synthesis of crystals with desired functional groups, bonding motifs, polarity, chirality, and more. To this end, fluoride and oxide-fluoride anions have been utilized as basic building units (BBUs) in the synthesis of noncentrosymmetric racemic materials for their ability to create polar axes that facilitate the breaking of an inversion center as demonstrated in a series of compounds with [MF6]2- anions (M = Ti, Zr, Hf). Targeting an analog with a [TaOF5]2- anion, the phase space of (CuO, Ta2O5)/bpy/HF(aq)/H2O (bpy = 2,2′- bipyridine) was investigated and three new compounds with Cu-bpy cations and Ta-fluoride or Ta-oxyfluoride anions were synthesized: [Cu(bpy)2][TaF6], [Cu (bpy)2][Ta2OF10], and [Cu(bpy)F(H2O)2]2[TaF7]•H2O with the anions [TaF6]-, [Ta2OF10]2-, and [TaF7]2-, respectively. The formation of these anions was found to be a product of both the concentration of hydrofluoric acid in solution and the ratio of metal-oxide starting materials to ligand. This work contributes to the understanding of mixed anion formation in the solid state. 

Crystalline solids are governed by universal structure-property relationships derived from their crystal symmetry, leading to paradigmatic rules on what properties they can and cannot exhibit. A long-held structure-property relationship is that centrosymmetric crystals cannot differentially absorb circularly polarized light. In this study, we demonstrate the design, synthesis, and characterization of the centrosymmetric material Li2Co3(SeO3)4, which violates this relationship not by defying symmetry-imposed selection rules but by invoking a photophysical process not previously characterized for crystalline solids. This process originates from an interference between linear dichroism and linear birefringence, referred to as LD-LB, and involves strong chiroptical signals that invert upon sample flipping. In addition to enabling a chiroptical response under centrosymmetry, this process opens up photonic engineering opportunities based on crystalline solids. 

We present electronic structure calculations on layered nickelate oxyfluorides derived from the Ruddlesden- Popper aristotype structure in search of unidentified materials that may host nickelate superconductivity. By performing anion exchange of oxygen with fluorine, we create two heteroanionic La2NiO3F polymorphs and stabilize Ni1+ in four-coordinate and five-coordinate square planar and square pyramidal geometries, respectively. We further predict chemical reactions with high thermodynamic driving forces to guide their synthesis. These oxyfluorides are weakly correlated antiferromagnetic insulators, and their nonmagnetic phases exhibit quasi-two-dimensional Fermi surfaces dominated by Ni dx2−y2 states, which strikingly resemble undoped cuprate superconductors. We discuss how the oxyfluoride anion chemistry promotes Ni-O covalency and single-band character that is more similar to the cuprates than homoanionic infinite-layer nickelates. We use our understanding to propose doping strategies and layered LaSrNiO2F2 and La3Ni2O4F3 nickelate oxyfluorides with tunable electronic and magnetic structures for experimentation.