Graduate Student Seminar Series - 2020

February 20, 2020

Topological Origins of the Mixed Alkali Effect in Glass

Arron Potter, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

The mixed alkali effect, the deviation from expected linear property changes when alkali ions are mixed in a glass, remains a point of contention in the glass community. While several earlier models have been proposed to explain mixed alkali effects on ionic motion, models based on or containing discussion of structural aspects of mixed-alkali glasses remain rare by comparison. However, the transition-range viscosity depression effect is many orders in magnitude for mixed-alkali glasses, and the original observation of the effect (then known as the Thermometer Effect) concerned the highly anomalous temperature dependence of stress and structural relaxation time constants. With this in mind, a new structural model based on topological constraint theory is proposed herein which elucidates the origin of the mixed alkali effect as a consequence of network strain due to differing cation radii. Discussion of literature models and data alongside new molecular dynamics simulations and experimental data are presented in support of the model, with good agreement.

Unit-Cell-Thick Oxide Synthesis by Film-Based Scavenging

Saloni Pendse, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

With the influx of flexible electronics as well as the emergence or prediction of unique phenomena in two dimensional forms of materials, epitaxy at weakly-coupled interfaces is gaining momentum as a feasible technique to develop nanostructured and two-dimensional materials. The weak substrate-film chemical interaction expected in this method of epitaxy has been believed crucial in enabling not only the formation of sharp heterostructures but also the mechanical exfoliation of the epilayer. Via growth of VO2 on a layered Dion-Jacobson perovskite, we unravel an unconventional understanding of epitaxy at weakly-coupled interfaces, entailing ions of the van der Waals substrate being scavenged by the growing film, resulting in the formation of a distinct and uniform unit-cell-thick interfacial layer. We show that VO­2 scavenges ions from the substrate and forms an epitaxial vanadate compound. Additionally, the crystal anisotropy of the substrate significantly modifies the energy landscape for diffusion of ions and leads to the creation of a unit-cell-thick epitaxial Aurivillius phase at the interface, predicted to exhibit the ferroelectric Rashba-Dresselhaus effect. The scavenging effect, interfaced with an anisotropic low-dimensional substrate, opens a new window to develop two-dimensional flexible components for future electronics.