With an aim of decreasing the temperature at which metallic Y reacts with H-2 to form the semiconductor phase YH3 (gamma phase), we employed Pd and Ni co-capping layers as catalysts. For this purpose, various types of Pd/Ni co-capping layers having different Ni thicknesses varied from 5 to 80 nm while having a Pd thickness of 80 nm were prepared as a catalytic capping layer on metal Y. We achieved an onset temperature (50 degrees C) of the. phase formation, which is approximately independent of Ni thickness except for 40 nm. A directing principle for decreasing the onset temperature of the. phase formation was discussed in terms of crystal texture and crystallite size of dihydride phase (beta phase).