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

Extraordinary large hysteresis effects in optical, electrical, and structur al properties are observed in switchable mirrors based on thin yttrium hydr ide (YHx) films, deposited on quartz, glass or sapphire. The pressure-compo sition isotherms of the YHx system between x=2 and 3 for absorption and des orption, determined electrochemically, differ by approximately three orders of magnitude. The optical transmittance exhibits a distinct minimum when l oading 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 bull; yttrium, but the absorption results show lar ge deviations. Most important for the metal-insulator transition is that du ring hydrogen loading YHx remains in a single hcp phase for x>2.1. The hyst eresis is discussed in terms of strains (and consequently stress) at the in terface between fee dihydride and hcp trihydride.