The reduction of MoO3 at low temperatures

The reduction of MoO3 in H2 was studied in situ in a high-voltage electron microscope. A new type of defect close to the surface and parallel to the (101) directions was observed, which nucleated abruptly at a temperature below 800C, and which disappeared on exposure to air. It is proposed that a high concentration of anion vacancies is generated at the surface as a result of the chemical reduction, at a temperature at which diffusion is relatively slow. This leads to the nucleation of a collapsed ribbon of vacancies bounded by a partial edge dislocation, Burgers vector type 1/2 (101). The model for the defect is consistent with image-contrast experiments. The conditions for nucleation of the vacancy discs are discussed, and estimates are made of the activation energies of migration of anion vacancies in MoO3. (author)