Mihiro Hoshino | Searcharxiv

arXiv2025

Diffusion-Controlled Anion Conversion into Dense Polycrystalline and Single-Crystalline Oxyhydrides

Oxyhydrides represent a new class of functional materials, yet the synthesis of dense polycrystals or single-crystals suitable for transport studies remains a significant challenge due to hydrogen desorption at elevated temperatures. The co-diffusion of oxygen and hydrogen in densely sintered BaTiO3 enables the topochemical formation of millimeter-scale bulk BaTiO3-xHx via high-pressure diffusion control (HPDC). Hydride ions selectively occupy oxygen-deficient sites, as confirmed by neutron diffraction, TPD, TG, and NMR. Systematic tuning of the hydrogen content and precise control of the electronic conductivity were achieved via HPDC. Hydrogen desorption analysis reveals distinct bonding states between near-surface and interior-bulk regions, which significantly affect the oxynitride conversion under N2 flow. Importantly, the diffusion-based nature of HPDC allows direct anion conversion even in single-crystalline oxides, as demonstrated by the synthesis of SrTiO3-xHx single crystals. These results establish HPDC as a general platform for accessing dense, metastable oxyhydrides with tunable anionic composition and transport properties.

cond-mat.mtrl-sci↗

arXiv2025

Compositional Tuning in NaxAlB14 via Diffusion Control

A uniform Na distribution in NaxAlB14 was achieved using high-pressure diffusion control (HPDC), which promotes Na deintercalation through enhanced diffusion under high pressure, combined with post-annealing. NaxAlB14 with a non-stoichiometric Na composition is thermodynamically metastable, and conventional solid-state reactions with adjusted starting compositions typically result in the formation of stoichiometric NaAlB14 and side products. While HPDC alone typically leads to concentration gradients, intentionally halting the Na removal process before complete extraction, followed by annealing, enabled a uniform composition across the bulk. This allowed structural and electronic properties to be examined over a wide range of Na concentrations. As Na content decreased, electrical conductivity increased, and the optical band gap narrowed. NMR measurements showed an increase in the density of states at the Fermi level, consistent with DFT calculations predicting boron-related in-gap states. Boron vacancies at specific sites were found to generate deep levels near the band gap center, which can explain experimentally observed optical gap reduction. These results demonstrate that diffusion-controlling methods can be effectively applied to synthesize metastable compounds with tunable compositions in covalent frameworks. Furthermore, they provide a foundation for designing functional boride-based materials with adjustable electronic properties by controlling Na extraction and inducing defect formation.

cond-mat.mtrl-sci↗