Ab-initio study of structural, vibrational and non-linear optical properties of (TiO2)-(Tl2O)-(TeO2) glasses

TL;DR

This work uses ab-initio molecular dynamics to dissect how TlO0.5 modifiers depolymerize TeO2 networks and how TiO2 additions in ternaries counteract depolymerization by promoting repolymerization and higher Te–O coordination. The authors validate large structural models against experimental X-ray PDFs, analyze bonding with maximally localized Wannier functions, and quantify ring statistics, coordination environments, and Te/NBO trends. Vibrational and nonlinear optical properties are computed, showing Raman features align with experiments and ⟨χ^(3)⟩ remains robust at low TiO2 levels but can decrease with higher TiO2 content, while small TiO2 fractions in ternaries preserve high nonlinearity and network connectivity. Collectively, the results provide a predictive framework to tailor atomic structure and third-order optical responses in TeO2-based glasses through controlled TlO0.5 and TiO2 modifications.

Abstract

This paper reports on a systematic first-principles molecular dynamics investigation of binary (TlO$_{0.5}$)$_{y}$-(TeO$_2$)$_{1-y}$ and ternary $(TiO$_{2}$)$_{x}$-(TlO$_{0.5}$)$_{y}$-(TeO$_2$)_{1-x-y}$ tellurite glasses. The obtained structural models are validated against available measured X-ray pair distribution functions. In the binary system, increasing TlO$_{0.5}$ content induces network depolymerization through the reduction of Te coordination number, the substitution of Te-O-Te linkages with Te=O$^{-}$...Tl$^{+}$ units, and the proliferation of non-bridging oxygens. In addition, rings analysis demonstrates a loss of the network connectivity via the opening of small n-membered rings. In contrast, TiO$_2$ acts as a network former in ternary glasses, preserving Te coordination number, and promoting a high fraction of bridging oxygens. Ti atoms induces a network repolymerization that manifests through the formation of smaller Ti-containing n-membered rings thereby balancing the strong effect of Tl$_2$O modifier. Beside the structural analysis, we also computed Raman spectra and non-linear optical properties on the obtained large periodic models. Our results reproduce experimental trends in Raman band shifts with composition, while nonlinear optical calculations show that <$χ^{(3)}$> remains stable with TlO$_{0.5}$ addition in binary glasses, consistent with experiment. In the case of ternary systems, we find that the inclusion of a small fraction of TiO$_2$ preserves the high optical nonlinearity of the TeO$_2$ network while maintaining the overall network connectivity. These results establish a predictive framework for tailoring the atomic structure and nonlinear optical response of tellurite glasses through the controlled interplay of modifiers nature and concentration.

Figures (17)

• Figure 1: Comparison of total X-ray scattering PDF G($r$) between experiments (dashed lines) torzu2020 and FPMD simulations (solid lines) for $\mathrm{(TlO_{0.5})}_{y}-\mathrm{(TeO_2)}_{1-y}$ binary and $\mathrm{(TiO_2)}_{x}-\mathrm{(TlO_{0.5})}_{y}-\mathrm{(TeO_2)}_{1-x-y}$ ternary glasses. A vertical shift is applied for clarity.
• Figure 2: Partial PDFs g$_\text{O-O}$($r$), g$_\text{Te-O}$($r$), g$_\text{Tl-O}$($r$), g$_\text{Te-Te}$($r$), g$_\text{Te-Tl}$($r$) and g$_\text{Tl-Tl}$($r$) in binary $\mathrm{(TlO_{0.5})}_{y}-\mathrm{(TeO_2)}_{1-y}$ glasses with various $y$ concentrations.
• Figure 3: Distribution of bond-distances in Te (Top panel) and Tl (bottom panel) chemical groups with bridging oxygen (dashed lines) and non-bridging oxygen (solid lines). The distribution was normalized to the number of cations in each system.
• Figure 4: (Top panel) Running coordination number $n_{\text{TeO}}$(r) as a function of Te$-$O pair distance for all considered $\mathrm{(TlO_{0.5})}_{y}-\mathrm{(TeO_2)}_{1-y}$ binary glasses, computed using the MLWF formalism. Running coordination numbers (middle panel) $n_{\text{TeO}}$(r) computed using the MLWF formalism, and (bottom panel) $n_{\text{TiO}}$(r) computed as the integral of the corresponding partial PDF, for $\rm {(TiO_{2})}_{\textit{x}}-{(TlO_{0.5})}_{\textit{y}}-{(TeO_2)}_{1-\textit{x}-\textit{y}}$ ternary glasses. Vertical dashed lines indicate distance cut-offs used to compute the average coordination numbers.
• Figure 5: Tl running coordination numbers for the binary $\mathrm{(TlO_{0.5})}_{y}-\mathrm{(TeO_2)}_{1-y}$ glasses. The top panel shows the coordination number $n_{\text{Tl}}$ as a function of distance, obtained via integration of the partial Tl–O PDF. The middle panel displays the running $n_{\text{Tl-NBO}}$ for non-bridging oxygen (NBO), while the bottom panel illustrates the running $n_{\text{Tl-BO}}$ for bridging oxygen (BO).