Enhanced Medium Range Order in Vapor Deposited Germania Glasses at Elevated Temperatures

Glasses are nonequilibrium solids with properties highly dependent on their method of preparation. It is established that in vapor-deposited molecular glasses structural organization and properties could be readily tuned with deposition rate and substrate temperature. In contrast, it has not yet been demonstrated whether the atomic packing of strong network forming glasses such as GeO2 could be modified at short and medium range (< 2 nm) through vapor deposition at elevated temperatures. In this work, we show that the structure of vapor-deposited amorphous GeO2 (a-GeO2), characterized by GeO4 tetrahedra connected in rings of various sizes, evolves into a more ordered configuration containing an increased population of 6-membered rings at elevated temperatures. It is also demonstrated that deposition near the glass transition temperature (Tg) is more efficient than post-growth annealing in modifying the atomic organization at medium range. The improvement in medium range order correlates with the reduction of the room temperature internal friction which decreases by as much as 44% when a-GeO2 is deposited at 0.83 Tg. In combination, these results show a strong correlation between medium range order and internal friction as predicted by theory.