Mechanochemical Synthesis of Perovskite Oxyhydrides: Insights from Shear Modulus.

Perovskite oxyhydrides have attracted recent attention due to their intriguing properties such as ionic conductivity and catalysis, but their repertoire is still restricted compared to perovskite oxynitrides and oxyfluorides. Historically, perovskite oxyhydrides have been prepared mostly by topochemical reactions and high-pressure (HP) reactions, while in this study, we employed a mechanochemical (MC) approach, which enables the synthesis of a series of ABO2H-type oxyhydrides, including those with the tolerance factor (t) much smaller than 1 (e.g., SrScO2H with t = 0.936) which cannot be obtained by HP synthesis. The octahedral tilting, often present in perovskite oxides, does not occur, suggesting that the lack of π-symmetry of the H 1s orbital and the large polarization destabilize tilted low-symmetry structures. Interestingly, SrCrO2H (t = 0.997), previously reported with the HP method, was not achieved with the MC method. A comparative analysis revealed a correlation between the feasibility of MC reactions and the (calculated) shear modulus of the starting reagents (binary oxides and hydrides). Notably, this indicator is not exclusive to oxyhydride perovskites but extends to oxide perovskites (SrMO3). This study demonstrates that MC synthesis offers unique opportunities not only to expand the compositional space in oxyhydrides in various structural types but also to provide a guide for the choice of starting materials for the synthesis of other compounds.

Impact of Surface Roughness on Recrystallization of an α-Al2O3(001) Single Crystal to α-AlO(OH) Diaspore Microcrystals.

We demonstrate that the surface of an α-Al2O3(001) single crystal recrystallizes to α-AlO(OH) under ultrahigh pressure (8 GPa) at 600 °C. The recrystallization depends on the degree of surface roughness. A polished surface topotaxially recrystallizes to (100)-oriented α-AlO(OH) microcrystals, while unpolished surface recrystallizes to polycrystalline α-AlO(OH). This study demonstrates a new synthetic route to obtain oriented crystals of ultrahigh-pressure-phase materials and paves the way for the investigations of the physical and chemical properties of such materials.

Preparation of a Si(111) Atomically Flat Substrate via Wet Etching and Evaluation as an AFM Substrate for Observations of Isolated Chains, Crystals, and Crystallization of Isotactic Poly(methyl methacrylate) at the Molecular Level.

To observe a polymer chain deposited on a substrate by atomic force microscopy (AFM) at the molecular level, the substrate should be atomically flat and stable under laboratory conditions and adsorb polymer chains firmly. Therefore, substrates used under laboratory conditions are practically limited to mica, highly ordered pyrolytic graphite, and atomically stepped sapphire, and polymers observed by AFM at the molecular level are also limited. A silicon wafer is frequently used as a substrate for AFM observation for somewhat macroscopic observations, but the surface of the silicon wafer is too rough to observe polymer chains deposited on it at the molecular level. In this study, we prepared an atomically stepped Si(111) substrate via wet etching in NH4F and evaluated it as an AFM substrate. The Si(111) substrate was stable as an AFM substrate, and isolated poly(methyl methacrylate) (it-PMMA) chains and a crystalline monolayer deposited on the substrate were observed by AFM at the molecular level. An it-PMMA amorphous monolayer deposited on mica crystallized under high humidity, but that on the Si(111) substrate did not because of the difference in the surface nature and the crystal structure of the substrates. The Si(111) substrate was hydrophobic, and the it-PMMA monolayers could be deposited as a multilayer, which could not be formed on hydrophilic mica. The crystallization behavior of an it-PMMA amorphous multilayer and an amorphous/crystalline mixed multilayer on the Si(111) substrate was also evaluated.