Long-range propagation of protons in single-crystal VO₂ involving structural transformation to HVO₂

Vanadium dioxide (VO₂) is a strongly correlated electronic material with a metal-insulator transition (MIT) near room temperature. Ion-doping to VO₂ dramatically alters its transport properties and the MIT temperature. Recently, insulating hydrogenated VO₂ (HVO₂) accompanied by a crystal structure transformation from VO₂ was experimentally observed. Despite the important steps taken towards realizing novel applications, essential physics such as the diffusion constant of intercalated protons and the crystal transformation energy between VO₂ and HVO₂ are still lacking. In this work, we investigated the physical parameters of proton diffusion constants accompanied by VO₂ to HVO₂ crystal transformation with temperature variation and their transformation energies. It was found that protons could propagate several micrometers with a crystal transformation between VO₂ and HVO₂. The proton diffusion speed from HVO₂ to VO₂ was approximately two orders higher than that from VO₂ to HVO_2. The long-range propagation of protons leads to the possibility of realizing novel iontronic applications and energy devices.