Hysteresis and the single-phase metal-insulator transition in switchable YHx films

Extraordinary large hysteresis effects in optical, electrical, and structural properties are observed in switchable mirrors based on thin yttrium hydride (YHx) films, deposited on quartz glass or sapphire. The pressure-composition isotherms of the YHx system between x=2 and 3 for absorption and desorption, determined electrochemically, differ by approximately three orders of magnitude. The optical transmittance exhibits a distinct minimum when loading the films from the dihydride to the trihydride state; however, upon unloading this minimum is absent. The desorption data are in good agreement with literature data on bulk yttrium, but the absorption results show large deviations. Most important for the metal-insulator transition is that during hydrogen loading YHx remains in a single hcp phase for x>2.1. The hysteresis is discussed in terms of strains (and consequently stress) at the interface between fcc dihydride and hcp trihydride.