Topochemical Oxidation of Perovskite KCoF3 to a K2PtCl6 Structure-Type Oxyfluoride

Analogous Chain Selenite Chlorides Ba2M(SeO3)2Cl2 (M = Cu2+, Ni2+, Co2+, Mn2+) and Pb2Cu(SeO3)2Cl2 with Tunable Spin S: Syntheses and Characterizations

A series of analogous chain selenite chlorides Ba2M(SeO3)2Cl2 (M = Cu 1, Ni 2, Co 3, Mn 4) and Pb2Cu(SeO3)2Cl2 5 with tunable spin S from S = 1/2 to S = 5/2 have been hydrothermally synthesized and characterized. These analogues crystallized in the orthorhombic Pnnm space group (monoclinic P21/n space group for 5) all containing M2+-SeO3-M2+ spin chains, which are further separated by the Ba2+ ions (Pb2+ for 5). The magnetic susceptibility results of 1, 2, and 5 show broad maxima around 80.0, 18.9, and 78.0 K, respectively, indicating good one-dimensional (1D) magnetism. Meanwhile, no long-range order (LRO) is observed down to 2 K for both 1 and 5, while the isostructural compounds 2, 3, and 4 exhibit LRO at 3.4 K, 10.8 K, and 5.7 K, respectively, which are further confirmed by the heat capacity and electron spin resonance results, as well as the observed spin-flop transitions in the M-H curves measured at 2 K below TN. The magnetizations of 1-5 at 7 T are still far from saturation. In addition, thermal stability and FT-IR and UV-vis-NIR spectroscopy of 1-5 are reported.

KMB4O6F3 (M = Co, Fe): two-dimensional magnetic fluorooxoborates with triangular lattices directed by triangular BO3 units

Frustrated magnetic systems are of great interest owing to their spin liquid state for application in quantum computing. However, experimentally, spin liquid has not been realized. Thus, experimental explorations of frustrated magnetic systems including triangular lattices are still urgent, particularly for directed synthesis compared to random exploration. Herein, for the first time, directed by the use of a triangular unit of the BO3 anion group, two novel layered magnetic fluorooxoborates KMB4O6F3 (M = Co 1, Fe 2) with triangular lattices have been hydrothermally synthesized and characterized. Compounds 1 and 2 are isostructural and crystallize in the P21/c space group with layered magnetic triangular lattices, which are further separated by K+ ions. Magnetic susceptibility curves of both 1 and 2 show no λ-anomaly peak down to a low temperature of 2 K in the absence of a magnetic long-range ordering transition, which are further confirmed by the heat capacity results. The magnetic-field dependence of magnetization at 2 K shows saturation of 2.20μB for 1 and 4.24μB for 2, respectively, at 7 T, after roughly subtracting the Van Vleck paramagnetic contribution. Further in-depth investigation of the underlying physics at a lower temperature below 2 K would be subsequently performed. Moreover, thermal stability and FT-IR and UV-vis-NIR spectroscopy with optical bandgap properties are also reported. Most importantly, our work provides a promising method to experimentally realize specific magnetic lattices (e.g. triangular lattices) directed by the use of triangular groups (e.g. BO3) as the functional unit.

Electrochemical Crystal Growth of Titanium Oxyfluorides-A Strategy for Development of Electron-Doped Materials

We report on the growth of single crystals of an electron-doped titanium oxyfluoride, Li₂Ti(O,F)₃, employing high-temperature electrolysis of TiO₂ with a eutectic Li₂MoO₄-LiF melt. Greenish octahedral-shaped crystals (∼30 μm in size) with a cubic rocksalt-type structure were successfully obtained by precisely tuning the applied voltage. The temperature-dependent magnetic susceptibility data revealed a paramagnetic behavior at low temperatures, ensuring the presence of Ti³⁺ ions (mean valence number of +3.78; F/Ti ∼ 0.15). The crystals exhibited clear visible-light absorption and produced H₂ from water in the presence of a sacrificial reagent under UV-light irradiation. Li₂Ti(O,F)₃ more efficiently produced H₂ compared with a nondoped oxyfluoride Li₅Ti₂O₆F, likely due to the doped electrons for the former. This work highlights a promising electrochemical approach toward growing electron-doped oxyfluoride crystals.

Perovskites with d-block metals for solar energy applications

Pb2+ halide organic-inorganic perovskites are excellent semiconductors for use in solar energy applications, but at the expense of robustness and environmental compatibility. Tin (Sn), which sits just above lead in the periodic table, forms pure (or mixed with lead) perovskites when at the 2+ or 4+ oxidation state. It can act as a promising alternative; however, there are still some serious concerns regarding its suitability. This presents a major challenge; viable metal cations have to be identified. A good number of elements, originating from a large range of d-block metal ions, with adequate oxidation states, moderate toxicity, and relative abundance, seem ideal for this purpose. In this review, we present the most characteristic perovskites (conventional perovskites, layered, or double perovskites) that can be formed with the help of these metals. We focus on d-block metal ions with stable oxidation states, such as Ag+ or Ti4+, which have exhibited satisfactory photovoltaic properties until now. Further, we highlight the results involving compounds other than halide perovskites, such as oxides, chalcogenides, and nitrides (as well as oxyhalides, oxysulfides, and oxynitrides); a few of them are ferroelectric (based on Ti4+, Zr4+, Fe3+, and Cr3+) and can yield a photovoltage that exceeds the bandgap of the material. Finally, we present the critical challenges that currently limit the efficiency of these systems and propose prospects for future directions.