A method for forming and protecting high quality bismuth oxide films comprises depositing a transparent thin film on a substrate comprising one of Si, alkali metals, or alkaline earth metals. The transparent thin film is stable at room temperature and at higher temperatures and serves as a diffusion barrier for the diffusion of impurities from the substrate into the bismuth oxide. Reactive sputtering, sputtering from a compound target, or reactive evaporation are used to deposit a bismuth oxide film above the diffusion barrier.