Existence and stability of nanometer-thick disordered films on oxide surfaces

Disordered films of 1-2 nm thickness have been observed on the surfaces of binary oxide systems in liquid-solid and solid-state equilibrium The system s Bi2O3-ZnO, Bi2O3-Fe2O3, WO3-TiO2 and MoO3-Al2O3 have been examined. Suffi cient data now exist to interpret the observed films as surface phases of t hermodynamically determined thickness and composition, coexisting at equili brium with one or more bulk phases. Phenomenological comparisons to the for mation of equilibrium-thickness intergranular films, multilayer adsorption, prewetting and surface melting are made. A thermodynamic model that allows the prediction of systems likely to form stable films is proposed, in whic h key variables are the relative surface energies, the sign and strength of the dispersion interaction, the extent of ordering (epitaxy) at the crysta l-film interface, and the free energy of amorphization and mixing. It is su ggested that the relative interfacial energies and volume thermodynamic ter ms are always important contributions, while the dispersion interaction is relatively unimportant well below the solidus temperature, but may be criti cal close to and above the solidus temperature. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All lights reserved.