Microstructural origin of the optical black state in Mg₂NiH _x thin films

Hydrogen absorption by a Pd capped thin Mg₂Ni film results in the nucleation of the Mg₂NiH₄ phase at the film/substrate interface and thus induces a self-organized two-layer system. This leads to the optical black state in Mg₂Ni thin films upon hydrogenation. This unusual hydrogenation behaviour is completely unexpected since the hydrogen enters through the top film surface. To explain the nucleation of Mg ₂NiH₄ close to the substrate/film interface we performed scanning tunneling microscopy (STM) on as-prepared Mg₂Ni films with various thicknesses (20-150 nm). For films thinner than 50 nm, the film consists of small grains and clusters of small grains whereas on further growth the grain size increases and a columnar microstructure develops. We propose, therefore, that close to the substrate, the relatively porous structure of the film with small Mg₂Ni grains locally reduces the nucleation barrier for Mg₂NiH₄ formation.