Atomic scale structure and dynamical properties of (TeO$_2$)$_{1-x}$-(Na$_2$O)$_{x}$ glasses through first-principles modeling and XRD measurements

TL;DR

This study combines first-principles molecular dynamics and XRD experiments to analyze how Na2O modifies the structure and enhances ion conduction in tellurite glasses, revealing structural depolymerization and Na-rich channels.

Contribution

It provides a detailed atomic-level understanding of structural changes and ion diffusion mechanisms in (TeO2)$_{1-x}$-(Na$_2$O)$_{x}$ glasses through combined modeling and experimental approaches.

Findings

Na2O reduces Te coordination number, causing depolymerization.

Formation of Na-rich channels facilitates ion mobility.

Structural deviations from pure TeO2 are quantitatively characterized.

Abstract

We resort to first-principles molecular dynamics, in synergy with experiments, to study structural evolution and Na$^+$ cation diffusion inside (TeO$_2$)$_{1-x}$-(Na$_2$O)$_{x}$ (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 (TeO$_2$)$_{1-x}$-(Na$_2$O)$_{x}$ glasses deviates drastically from that of pure TeO$_2$ glass. Specifically, increasing the Na$_2$O concentration leads to a reduction of the coordination number of Te atoms, reflecting the occurrence of a structural depolymerization upon introduction of the Na$_2$O 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 A, 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.