The changes produced in the electrical resistance of vacuum-condensed films of neodymium and praseodymium when they react with hydrogen and with air

When thin films of vacuum-condensed praseodymium and neodymium are exposed to hydrogen or to air the electrical resistance changes owing to the chemical reactions which occur. The progress of the reactions was studied by means of the resistance changes which occur and by means of electron diffraction.At room temperature, neodymium reacted less readily with hydrogen than did praseodymium and the reaction rate for both metals increased with increasing temperatures. The phases formed in the various experiments ranged from the double h.c.p. structure of the metals to the high resistance cubic structure of the hydrides LnH_x with x approaching 3. Cubic and hexagonal Nd₂O₃ formed when the specimens were strongly heated by the beam of the electron microscope.Exposure to air caused the films to oxidize. Thin films approximately 250 Å thick became insulators but the oxide layer formed on thicker films protected the interior of a given film from rapid oxidation so that the film did not become an insulator.