Hydrogen-induced changes of mechanical stress and optical transmission in thin Pd films

Loading of palladium with hydrogen not only leads to pronounced changes of its optical and electrical properties, but is also accompanied by significant density changes leading to stress formation. Therefore, the loading has been investigated by in-situ mechanical stress and optical transmittance measurements. The exposure of palladium films to hydrogen results in a significant transmittance increase and mechanical stresses of several thousand MPa upon loading with hydrogen. The phase transition from the α-phase to the β-phase was observed in both stress and transmittance data. The influence of a thin oxide layer, which is formed in air and can be removed in a hydrogen gas, was observed. Hence, the stress and transmittance data are governed by a superposition of irreversible oxide removal and reversible loading with hydrogen.