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 work investigates how Na$_2$O modifier alters the atomic-scale structure and Na$^+$ diffusion in (TeO$_2$)$_{1-x}$-(Na$_2$O)$_x$ glasses ($x=0.10$–$0.40$) by combining first-principles molecular dynamics with XRD measurements. Using CP2K-based BOMD, melt-quench glass models are generated and analyzed with X-ray structure factors, PDFs, and Wannier-center (MLWF) bonding analysis to quantify Te–O depolymerization, TeO$_4$→TeO$_3$ transitions, and Na coordination changes; finite-temperature dynamics reveal Na-channel formation and diffusion pathways that correlate with a first sharp diffraction peak (FSDP). The study finds a pronounced Na$_2$O-driven depolymerization of the TeO$_2$ network, increasing non-bridging oxygens and creating Na-rich channels that enhance ion mobility, with structure factors and PDFs in fair quantitative agreement with experiments. The work provides detailed structural descriptors (Q$_m^n$ distributions, BO/NBO populations) and links intermediate-range order to ionic transport, offering insights for tuning glass conductivity via composition. Practical implications include improved understanding of alkali diffusion mechanisms in tellurite glasses and guidance for designing materials with targeted ionic conductivity and optical properties. All mathematical expressions are presented in $...$ format.

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.

Figures (21)

• Figure 1: The experimental (dashed lines) X-ray structure factors $S_{X}(q)$ of (TeO2)_1-x-(Na_2O)_x glasses with x= 0.0, 0.10, 0.1875 and 0.30 compared to the calculated total X-ray structure factors obtained via Fourier transform of the pair correlation functions in the real space (solid lines). The total X-ray structure factors of pure TeO2 taken from Ref.[raghvender2022structure] is also added. Vertical shifts were applied for clarity.
• Figure 2: Comparison between the calculated Faber–Ziman partial structure factors as a function of concentration x for the (TeO2)_1-x-(Na_2O)_x glasses with x= 0.10, 0.1875, 0.30 and 0.40. Vertical shifts are applied for clarity.
• Figure 3: The measured total X-ray pair correlation function G$_{\rm X}$(r) (dashed lines) for the (TeO2)_1-x-(Na_2O)_x glasses with x= 0.10, 0.1875, 0.30 and 0.40, compared to the obtained results from FPMD models (solid lines). The PDF of pure TeO2 taken from Ref.[raghvender2022structure] is also added. The curves are shifted vertically for clarity.
• Figure 4: Te-Te, Te-O and Na-O partial pair distribution functions g$_{\alpha\beta}$ for (TeO2)_1-x-(Na_2O)_x systems with x= 0.10, 0.1875, 0.30 and 0.40 obtained from FPMD. The curves are shifted vertically for clarity.
• Figure 5: The distribution of Te-O (top panel) and Na-O (bottom panel) distances, broken down into contributions of non-bridging oxygens (shown by dashed lines) and bridging oxygens (represented by solid lines) for (TeO2)_1-x-(Na_2O)_x systems with x= 0.10, 0.1875, 0.30 and 0.40 obtained from FPMD. All the distributions are normalized with respect to the number of Te and Na atoms in each system.