Atomic scale structure and dynamical properties of (TeO₂)_1-x-(Na₂O)_x glasses through first-principles modeling and...

We resort to first-principles molecular dynamics, in synergy with experiments, to study structural evolution and Na⁺ cation diffusion inside (TeO₂)_1-x-(Na₂O)_x (x = 0.10-0.40) glasses. Experimental and modeling results show a fair quantitative agreement in terms of total X-ray structure factors and pair distribution functions, thereby setting the ground for a comprehensive analysis of the glassy matrix evolution. We find that the structure of (TeO₂)_1-x-(Na₂O)_x glasses deviates drastically from that of pure TeO₂ glass. Specifically, increasing the Na₂O concentration leads to a reduction of the coordination number of Te atoms, reflecting the occurrence of a structural depolymerization upon introduction of the Na₂O modifier oxide. The depolymerization phenomenon is ascribed to the transformation of Te-O-Te bridges into terminal Te-O non bridging oxygen atoms (NBO). Consequently, the concentration of NBO increases in these systems as the concentration of the modifier increases, accompanied by a concomitant reduction in the coordination number of Na atoms. The structure factors results show a prominent peak at ∼1.4 Å that becomes more and more pronounced as the Na₂O concentration increases. The occurrence of this first sharp diffraction peak is attributed to the growth of Na-rich channels inside the amorphous network, acting as preferential routes for alkali-ion conduction inside the relatively stable Te-O matrix. These channels enhance the ion mobility.