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 ([TiO_m] 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 temperature, 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.