Theoretical studies

Introduction

I do not consider myself to be a theoretical physicist in the traditional sense of developing mathematical theories, and I do not consider simulation science to be real theory, but in the sense that I think about  theoretical ideas and develop thinking, well, I guess it can be called "theory".

The main theoretical work in this sense has concerned the flexibility of network structures. These are materials where you can define bonds between neighbouring atoms that connect together to form a near-infinite network. Usually it is possible to define small groups of atoms forming fairly rigid polyhedra, like SiO4 tetrahedra and TiO6 octahedra, which are connected to neighbouring polyhedra at corners. 

Rigid Unit Mode model

The main thing I have been involved in developing is the Rigid Unit Mode model. This concerns the low-energy dynamics of network structures. The starting assumptions are that there is a high energy cost for deforming the atomic polyhedra, and a low energy cost for changing the angle that two polyhedra form around a shared vertex. 

Displacive phase transitions

The headline application of the RUM model is to understand the origin and properties of displacive phase transition in network structures. The main idea is that the RUM, as a low-energy vibration, can act as the soft mode for a displacive phase transition. The simplest – but trivial – example is that of the octahedral tilting phase transitions in perovskites. The more challenging example is the family of aluminosilicates, composed of corner-sharing SiO4 and AlO4 tetrahedra.

We examined ...

Negative thermal expansion

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Pressure-induced softening

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