Rare-earth oxyhydride thin films show a color-neutral photochromic effect, but the origins of both this photochromism and the memory effect (bleaching slows progressively upon photodarkening-bleaching cycling) remain unclear. Using in situ illumination Doppler broadening positron annihilation spectroscopy (DB-PAS) and positron annihilation lifetime spectroscopy (PALS) on magnetron-sputtered oxyhydride films, we show that the photochromic origin is linked to the reversible formation of metallic-like nanodomains. PALS reveals that as-deposited films contain abundant cation monovacancies (10^-5-10^-4), vacancy clusters, and nanopores. Both the S-parameter from DB-PAS and positron lifetimes from PALS increase with the bleaching time constant upon cycling (fig. 1) [1-2], demonstrating that slower bleaching (the memory effect) involves vacancy formation and agglomeration. These findings establish a nanoscale structure-optical property relationship in photochromic oxyhydride films.

photochromic effect,positron annihilation spectroscopy,thin films,oxyhydrides