Topological Origins of the Mixed Alkali Effect in Glass

Influence of Magnesium on the Structure of Complex Multicomponent Silicates: Insights from Molecular Simulations and Neutron Scattering Experiments

A series of multicomponent glasses containing up to five oxides are studied using classical molecular dynamics simulations and neutron scattering experiments. The focus is on the role of magnesium in determining the structural properties of these glasses and the possible mixed effect during a sodium/magnesium substitution. Calculated structure functions (pair correlation function and structure factor) rather accurately reproduce their experimental counterpart, and we show that more fine structural features are qualitatively reproduced well, despite some discrepancies in the preferential spatial distribution between sodium and magnesium to aluminum and boron, as well as the nonbridging oxygen, distribution. The simulated systems offer a solid basis to support previous experimental findings on the composition-structure relationship, allowing for further analysis and property calculation. It is confirmed that the substitution of sodium by magnesium leads to the decrease of four-fold boron and a modification of the alkali coordinations with a significant change of the network structure. Specifically, magnesium coordination extracted from numerical simulations highlights a potential dissociation from penta- to tetra- and hexahedral units with increasing MgO contents along the glass series, which could not be resolved experimentally.

Competition Effects during Femtosecond Laser Induced Element Redistribution in Ba- and La-Migration Based Laser Written Waveguides

Fs-laser induced element redistribution (FLIER) has been a subject of intensive research in recent years. Its application to various types of glasses has already resulted in the production of efficient optical waveguides, tappers, amplifiers and lasers. Most of the work reported on FLIER-based waveguides refers to structures produced by the cross-migration of alkali (Na, K) and lanthanides (mostly La). The latter elements act as refractive index carrying elements. Herein, we report the production of Ba-based, FLIER-waveguides in phosphate glass with an index contrast > 10⁻². Phosphate glasses modified with the same amount of Na₂O and K₂O, and variable amounts of BaO and/or La₂O₃ were used to produce the FLIER-waveguides with Ba and or La acting as index carriers. Ba-only modified glasses show a waveguide writing threshold and light guiding performance comparable to that of La-based structures. However, mixed Ba-La glasses show a much higher element migration threshold, and much smaller compositionally modified regions. This behavior is consistent with a competition effect in the cross-migration of both elements (Ba and La) against the alkalis. Such an effect can be applied to inhibit undesired element redistribution effects in fs-laser processing applications in multicomponent glasses.

Mixed Alkali Effect in Silicate Glass Structure: Viewpoint of 29Si Nuclear Magnetic Resonance and Statistical Mechanics

The mixed alkali effect in glasses is the deviation from linear property changes when alkali cations are mixed. The extent of this effect and its structural origin remain topics of interest. In this work, we use a statistical mechanics approach to predict the composition-structure relationship in mixed modifier Na₂O-K₂O-SiO₂ glasses. This is achieved by accounting for the enthalpy between each pairwise alkali ion and silicate unit interaction. The initial enthalpy parameters are obtained based on experimental structural data for binary Na₂O-SiO₂ and K₂O-SiO₂ glasses, which can be transferred to predict the short-range order structure of mixed modifier glasses without additional free parameters. To this end, we have performed ²⁹Si magic angle spinning NMR spectroscopy measurements on y(xNa₂O-(1 - x)K₂O)-(100 - y)SiO₂ glasses with x = 0, 0.25, 0.5, 0.75, and 1 and y = 34, 42, and 50. Good agreement between experimental data and model predictions are observed. Finally, we use this information to discuss the relative entropic and enthalpic contributions to the mixed modifier effect in silicate glass structure.