YTTRIUM AND LANTHANUM HYDRIDE FILMS WITH SWITCHABLE OPTICAL-PROPERTIES

IN many substances, changes in chemical composition, pressure or tempe rature can induce metal-to-insulator transitions(1). Although dramatic changes in optical and electrical properties accompany such transitio ns, their interpretation is often complicated by attendant changes in crystallographic structure(2). Yttrium, lanthanum and the trivalent ra re-earth elements form hydrides that also exhibit metal-insulator tran sitions(3-5), but the extreme reactivity and fragility of these materi als hinder experimental studies(5,6). To overcome these difficulties, we have coated thin films of yttrium and lanthanum with a layer of pal ladium through which hydrogen can diffuse. Real-time transitions from metallic (YH2 or LaH2) to semiconducting (YH3 or LaH3) behaviour occur in these films during continuous absorption of hydrogen, accompanied by pronounced changes in their optical properties, Although the timesc ale on which this transition occurs is at present rather slow (a few s econds), there appears to be considerable scope for improvement throug h the choice of rare-earth element and by adopting electrochemical mea ns for driving the transition, In view of the spectacular changes in o ptical properties-yttrium hydride, for example, changes from a shiny m irror to a yellow, transparent window-metal hydrides might find import ant technological applications.