[Paper Review] Structure, diffusion and orientational freezing in lithium metasilicate

Cristian Balbuena, Carolina Brito+1 more|arXiv (Cornell University)|Aug 7, 2013

Glass properties and applications21 references3 citations

TL;DR

This study uses molecular dynamics simulations to investigate the dynamic and structural transitions in lithium metasilicate (Li₂SiO₃), identifying two critical temperatures: Tc ≈ 1500 K, linked to mode-coupling theory and the cessation of relative tetrahedral motion, and Tg ≈ 1000 K, marking the glass transition where atomic positions within tetrahedra stop relaxing. The key finding is that orientational order parameters—measuring both relative tetrahedral orientation and local bond order—exhibit distinct changes at Tc and Tg, linking dynamic arrest to structural reorganization beyond standard mode-coupling theory.

ABSTRACT

We report on the dynamic and structural characterization of lithium metasilicate $Li_2SiO_3$, a network forming ionic glass, by means of molecular dynamics simulations. The system is characterized by a network of $SiO_4$ tetrahedra disrupted by $Li$ ions which diffuse through the network. Measures of mean square displacement of $Si$ and $O$ atoms allow us to identify a temperature at which tetrahedra stop moving relative to each other. This temperature $T_c\approx 1500\,K$ can be characterized within the framework of mode coupling theory. At a much lower temperature $T_g\approx 1000\,K$, a change in the slope of the volume versus temperature data allows to single out the glass transition. We find signatures of both transitions in structural order parameters, related to the orientation of tetrahedra. Going down in temperature we find that, around the mode coupling transition temperature, a set of order parameters which measure the relative orientation of tetrahedra cease to increase and stay constant below $T_c$. Another well known measure of orientational order, the bond orientational order parameter, which in the studied system measures local order within single tetrahedrons, is found to continue growing below $T_c$ until $T_g$, below which it remains constant. Our results allow to relate two characteristic dynamic transitions with corresponding structural transitions, as observed in two different orientational order parameters. Furthermore, the results indicate that the network of thetrahedra continue to relax well below the point where neighboring tetrahedra cannot rearrange relative to each other, and the glass is reached only upon a process of relaxation of atoms which form the thetrahedron, as quantified by the change in the bond orientational order parameters.

Motivation & Objective

• To understand the dynamic and structural transitions in network-forming ionic glass Li₂SiO₃.
• To investigate whether structural signatures accompany the dynamic slowdown near the glass transition.
• To examine the role of tetrahedral network order and orientational correlations in the dynamics of lithium metasilicate.
• To test the applicability of mode-coupling theory (MCT) in systems with strong short-range order, such as silicate networks.
• To explore the connection between dynamic arrest and structural reorganization using orientational order parameters beyond local bond order.

Proposed method

• Molecular dynamics simulations were performed on Li₂SiO₃ to study atomic motion and structural evolution across temperature.
• Mean square displacement and self-intermediate scattering functions were analyzed to extract diffusion coefficients and relaxation times, enabling identification of the mode-coupling critical temperature Tc.
• The glass transition temperature Tg was determined from a change in slope in the volume vs. temperature curve.
• Two orientational order parameters were employed: the Steinhardt bond-orientational order parameter to assess local tetrahedral order, and the Rey parameter to quantify relative orientation between neighboring tetrahedra.
• The behavior of these order parameters was tracked as a function of temperature to detect structural transitions corresponding to Tc and Tg.
• The results were interpreted within the framework of mode-coupling theory and compared to thermodynamic and dynamic observables.

Experimental results

Research questions

• RQ1 Does the mode-coupling transition at Tc ≈ 1500 K in lithium metasilicate correspond to a detectable structural change in the tetrahedral network?
• RQ2 Is there a structural signature associated with the glass transition at Tg ≈ 1000 K, beyond the dynamic arrest of atomic motion?
• RQ3 How do orientational order parameters—specifically those measuring relative tetrahedral orientation and local bond order—respond to temperature changes across Tc and Tg?
• RQ4 To what extent do the dynamics of tetrahedral rotation and internal relaxation decouple below Tc?
• RQ5 Can the two-step relaxation process observed in the system be linked to distinct structural transitions via orientational order parameters?

Key findings

• The mode-coupling critical temperature Tc ≈ 1500 K was identified from power-law scaling of diffusion and relaxation times, consistent with MCT predictions.
• At Tc, the relative orientation between neighboring SiO₄ tetrahedra, measured by the Rey parameter, ceases to increase and remains constant, indicating a structural arrest of tetrahedral reorientation.
• The glass transition temperature Tg ≈ 1000 K is marked by a change in slope of the volume vs. temperature curve and corresponds to the cessation of atomic relaxation within the tetrahedra.
• The bond-orientational order parameter continues to grow below Tc and only saturates at Tg, indicating that internal relaxation of tetrahedral atoms persists beyond the loss of relative motion between tetrahedra.
• The temperature window between Tc and Tg is large, suggesting that the system undergoes significant structural relaxation after the onset of dynamic arrest in tetrahedral rotation.
• The study establishes a direct link between dynamic transitions (Tc and Tg) and distinct structural changes in orientational order, demonstrating that both transitions have clear structural signatures in the tetrahedral network.