Rare earths in general absorb hydrogen readily forming solid solutions and/or dyhydrides and trihydrides. In this work the change in optical transmittance of hydrogenated-Pd coated-Gd and Dy films (250-300 nm thick), as well as thinner (12 -28 mm thick) uncovered Gd films is studied. Dramatic changes are observed in the transmittance as a function of hydrogen pressure indicating that a transition to a semiconductor state, and possibly to an insulator state, occurs. The optical transmission of the hydrogenated films measured at λ=632.S nm, increases by a factor of the order of 100.
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