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.