SYNTHESIS OF YTTRIUMTRIHYDRIDE FILMS FOR EX-SITU MEASUREMENTS

A new method has been developed to synthesize compact yttriumtrihydrid e by making use of a thin film technique. For electrical measurements yttrium films of typically 500 nm thickness are covered under UHV cond itions by a 5 nm thick palladium overlayer which consists of electrica lly disconnected islands. Loading of these films with hydrogen up to t he trihydride phase can then be done ex-situ in a reasonably short tim e (around 20-40 h) by applying gas pressures of about 60 x 10(5) Pa. F or a thicker Pd layer (above 20 nm) this time can be considerably shor ter (t similar to 125 s). The film morphology stays intact during the loading process although the film thickness increases by approximately 11% and the crystal structure changes from h.c.p. to f.c.c. and back to h.c.p. These samples are: therefore, very well suited for an invest igation of the remarkable electrical and optical properties of trihydr ides, as recently reported by Huiberts et al. (Nature, 380, 1996, 231) . In this article we give evidence for the island structure of the pal ladium overlayer and make a comparison of a number of physical propert ies of yttrium and its related hydrides as thin films with literature values for the same material in bulk form. These properties include la ttice parameters for the different hydride phases, electrical resistiv ity for yttrium and its dihydride and Hall coefficient for yttrium. Th e characteristics of the yttriumhydride thin films are very similar to those of bulk material. Furthermore, we performed concentration measu rements and resistivity measurements during hydrogen loading. It is sh own that the resistivity rises three orders of magnitude when yttrium is loaded up to the trihydride phase at 60 X 10(5) Pa.