Unconventional floppy network structures in titanate glasses

Annealed titanate glasses are important candidate materials for high-performance electronic components, but their structural response to temperature-induced properties upon thermal annealing remains elusive. Here, using high-energy synchrotron X-ray scattering, high-temperature Raman spectroscopy, and empirical potential structure refinement simulation, we track the structural evolution in situ of a barium dititanate (BaTi2O5) glass upon annealing around and below the glass transition temperature. We find that the network structure is intrinsically floppy, as the network units ([TiOm] polyhedra) themselves and their topological packing can be easily modulated by annealing temperature. The floppy nature of the glassy titanate network challenges the currently well-cognized Rigid-Unit Mode model explaining temperature-driven structural reorganization of prototypical network glasses (e.g., silicate/borate glasses). As temperature increases, the floppy network of BaTi2O5 glass undergoes a non-monotonic, two-stage network rearrangement. After cooling back to room tem-perature, the distortion of network units is found recoverable whereas the change of network connectivity in the intermedium-range order is irreversible. Simulation ensembles further show the irreversible connectivity change can be ascribed to the local development of crystal-like cationic motifs, which in turn induce excess enthalpy relaxation. Our findings provide an atomic-scale perspective on temperature-dependent structural and enthalpy relaxation of a model titanate glass upon annealing, which are crucial for understanding thermal-related changes in the properties of titanate glasses.

Automated summary

Annealed titanate glasses have a flexible network structure that can be modulated by annealing temperature, which is different from the structural changes in traditional network glasses. The rearrangement of the network in barium dititanate glass occurs in a non-monotonic, two-stage process. The irreversible change in network connectivity is attributed to the development of crystal-like cationic motifs.