Why do some materials shrink instead of expanding when heated?

Research Group: 
Centre for Condensed Matter and Material Physics
Full-time Project: 
Yes
Funding: 
QM Scholarship
EPSRC
CSC
Project Description: 
Our common experience is that most materials expand on heating. It may therefore come as something of a shock to learn that many materials instead shrink when heated, and do so over a very wide range of temperatures. Such materials have been known for less than 20 years. Thermal expansion is typically associated with the phonons (the quanta of lattice vibrations), and the reason for positive thermal expansion emerges naturally from an understanding of the thermal properties of phonons. Thus the origin of negative thermal expansion must arise from some unusual properties of phonons. The likely mechanism is associated with the topologies of crystal structures composed of fairly rigid bonds, with generated by large-amplitude transverse vibrations of atoms. Furthermore, but much less well understood, it is likely that negative thermal expansion is accompanied by another anomalous property, namely that materials become softer under compression. A lot of work is needed to turn some general ideas into a formal theory of negative thermal expansion and associated anomalous properties.

The project is concerned with taking a number of case studies and understanding the role of phonons by simulations of phonon dispersion curves and through larger-scale molecular dynamics simulations (a technique for virtual reality exploration of the atom-scale dynamics of materials). We will use quantum mechanical calculations of the interactions between atoms in several ways, including direct calculations of the phonons and through tuning of functional models. The focus will be on a select number of examples, including materials where the NTE is within one-dimensional chains (such as copper cyanide) and in families of materials where changes in composition can switch from negative to positive thermal expansion. Simulations will be used to calculate the key parameters in theory.

The student will gain a broad practical understanding in using computer simulation methods, including use of national high-performance computing. He/she will learn to apply for access to these facilities, and how to defend such applications at resource allocation meetings. The student will gain deeper knowledge of phonons (including anharmonic processes), phase transitions, and physical properties of materials.

1. Simulation study of pressure and temperature dependence of the negative thermal expansion in Zn(CN)2. H Fang, MT Dove, LHN Rummer and AJ Misqutta. Physical Review B 88, 104306, 2013. http://journals.aps.org/prb/abstract/10.1103/PhysRevB.88.104306
2. Framework flexibility and the negative thermal expansion mechanism of copper(I) oxide Cu2O. LHN Rimmer, MT Dove, B Winkler, DJ Wilson, K Refson and AL Goodwin. Physical Review B 89, 214115, 2014. http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.214115
3. Common origin of negative thermal expansion and other exotic properties in ceramic and hybrid materials. H Fang, MT Dove and AE Phillips. Physical Review B 89, 214103, 2014. http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.214103
Requirements: 
The student will need to be comfortable in using computers, and will be expected to learn some programming skills. He/she will also need to learn to engage with the consortium through whom we gain access to national supercomputing resources.
SPA Academics: 
Prof Martin Dove