Hysteresis in YH x films observed with in situ measurements

Giant hysteretic effects in the YHx hydrogen switchable mirror system are observed between x51.9 and x53 in pressure composition isotherms, optical and electrical properties, and mechanical stress. Polycrystalline Y films are studied by simultaneous in situ measurements of electrical resistivity, optical transmittance and x-ray diffractometry. These experiments are linked to optical microscopy of the samples. During hydrogen loading above x51.9 the films stay in the metallic fcc phase until the optical transmittance reaches its minimum and the electrical resistance curve exhibits a characteristic feature at x52.1. Upon further loading the system crosses the miscibility gap in which the fcc phase coexists with the hcp phase before hydrogen saturation is reached in the pure hcp phase. While the fcc phase stays at a concentration of x52.1 in the coexistence region during loading, it remains at a concentration of x51.9 during unloading. The hysteretic effects observed in optical transmission and electrical resistivity result from the different properties of the low concentration fcc phase YH1.9 and the high concentration fcc phase YH2.1. They can be explained on the basis of the bulk phase diagram if the different stress states during loading and unloading are taken into account. These results contradict earlier interpretations of the hysteresis in thin film YHx , based on nonsimultaneous measurements of the optical and structural properties on different films.