Atomic-scale description of interfaces in rutile/sodium silicate glass-crystal composites.

Paul C M Fossati,William E Lee,Michael J D Rushton

doi:10.1039/c8cp00675j

Abstract

In this work interfaces between (Na2O)x(SiO2)1-x glasses (for x = 0.0, 0.1 and 0.2) and TiO2 crystals are simulated using molecular dynamics and empirical potentials. Interfaces are presented for the distinct terminating surfaces of TiO2 with Miller indices ≤2, the properties of which have been investigated using atomistic models. Simulations showed that partially ordered layers had been induced in the glass close to the interfaces, with successive oxygen-rich and cation-rich planes being noted. The first silicate layer in contact with the crystal tended to be highly-structured, with Si ions occupying well-defined positions that depend on the orientation of the crystal at the interface, and showing 2-dimensional ordering depending on glass composition. Finally, interface energies were calculated. These indicated that the interface formation may stabilise a crystal surface in comparison to maintaining a free surface. Results are presented suggesting that the structural flexibility of the glass network allows it to conform to the crystal, thereby providing charge compensation and avoiding large relaxation of the crystal structure close to the interfaces. Such interfacial properties could be crucial to improving phenomenological models of glass-crystal composite properties.

Highlights

Secondary phases are diverse in terms of their structure and composition, oxides with P42/mnm (rutile, e.g. (Ru,Rd)O2) or Fd3%m structures (spinel,[8] e.g. (Cr,Fe,Ni)3O4) have been observed
Interface position is defined as the z coordinate of the last crystalline layer. This layer can be composed of only Ti ions, only O ions, or a mixture of both
A time average of the count in each slice was taken over a 300 ps Molecular Dynamics (MD) run before being converted to a density by dividing by the volume of each slice

Summary

Introduction

Secondary phases are diverse in terms of their structure and composition, oxides with P42/mnm (rutile, e.g. (Ru,Rd)O2) or Fd3%m structures (spinel,[8] e.g. (Cr,Fe,Ni)3O4) have been observed. Secondary phases are diverse in terms of their structure and composition, oxides with P42/mnm Other phases commonly reported include molybdates and the so-called yellow phase.[5,9,10,11] The presence of these secondary phases may have significant and deleterious effects on material performance as a wasteform. Yellow phase is more soluble in water than the glass matrix, providing an efficient route for radionuclides to escape the wasteform. Secondary phases can change corrosion behaviour and overall durability of the wasteform.[7]. Some practical issues arise during processing, caused by secondary phases. It is important to understand the behaviour and properties of these secondary phases and the changes they cause in a wasteform’s characteristics

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