Structure of amorphous silica–hafnia and silica–zirconia thin-film materials: The role of a metastable equilibrium state in non-glass-forming oxide systems

• Amorphous thin films of SiO2–HfO2 and SiO2–ZrO2 were made by ion beam sputtering.
• Short-range structure was quantified by Si-29 and O-17 NMR.
• Structures of thin films and comparable amorphous sol–gel materials are very similar.
• These materials may pass through a metastable, liquid-like state during formation.
• A simple thermodynamic model of oxygen speciation describes the structures accurately.

Amorphous heavy metal oxide thin films have important technological applications, including multi-layer mirror coatings in instruments such as the Laser Interferometric Gravitational Wave Observatory (LIGO). However, the structures of these materials are much less well-known than those of conventional glass-forming oxides. For example, it has not been clear whether films deposited by methods such as ion beam sputtering (IBS) ever pass through a state of local (metastable) thermodynamic equilibrium, and hence could be amenable to modeling by thermodynamic approaches long explored in glass-forming liquid systems. We present new 17O and 29Si NMR data on IBS films and sol–gel equivalents in the silica–hafnia and silica–zirconia binary systems. Similar distributions of local structural groups are obtained by different synthesis routes, suggesting that indeed a transient, possibly liquid-like metastable state is reached during film formation. All oxygen species are well-resolved in the spectra and can be readily quantified. Their concentrations are indeed well-predicted by a simple thermodynamic treatment, similar to that applied for decades to binary metal oxide-silica liquids, based on the reaction of bridging oxygens with added “free” metal oxide ions to form non-bridging oxygens.