Electrical resistance and electron diffraction studies of thin films vacuum condensed from cerium vapour and subsequently exposed to hydrogen or oxidized

Abstract

Cerium vapour was evaporated in a conventional vacuum coating unit and was condensed onto glass substrates or onto thin carbon films supported by standard electron microscope grids. When a given deposit was exposed to hydrogen there was an initial increase in the electrical resistance corresponding to the formation of hydrogen-rich cerium. Further exposure led to a decrease in the resistance because of the development of the low resistivity phase CeH x with x ≈ 2; finally the resistance increased as more hydrogen was absorbed because the resistivity of CeH x increases markedly as x increases from 2 to 3. When the films were heated in air in a furnace to 600 °C or when they were heated in an intense electron beam in an electron microscope, f.c.c. CeO 2 was observed. Less intense heating by the electron beam caused cubic C-Ce 2O 3 to develop. Electron diffraction studies were made of the asdeposited films and of the films after exposure to hydrogen. The results were variable but usually the diffraction pattern was that of an f.c.c. material with a 0 in the range 5.39–5.59 Å. Studies of the weight increase accompanying oxidation to CeO 2 showed that the initial films were largely metallic cerium or cerium hydride.