Research - SENN GROUP

Research

Our focus is on 5 main areas currently:

Molecular-Like Ordering in the Solid State

Magnetic Order

Microscopic Mechanisms of Ferroelectric and Multiferroics Materials

Negative Thermal Expansion in Layered Perovskites

Order-Disorder Phase Transitions

I am a crystallographer with a strong interest in studying electronic and magnetic ordering phenomena in the solid state. The purpose of our work is to elucidate the microscopic mechanisms involved in these processes and correlate these with a material’s observed physical properties. Our work involves performing experiments at central facilities to study subtle crystallographic distortions. Modelling and interpreting the resulting complex structures is often the most challenging part of our work. Symmetry analysis, correlation of local ordering parameters and electronic structure calculations are all used to aid us in this task.

‘Molecular-like’ Ordering in the Solid State

Our research in this area involves understanding the often complex crystal structures that arise as a result of phase transitions due to electronic ordering phenomena. The complexity of the resulting structures is often due to the competition between local (ion centred) and global (lattice) degrees of freedom.

In magnetite Fe+3Fe+2.52O4 we have shown that charge ordering (ion centred) in the low temperature (Verwey) phase is only stabilised by a complex orbital ordering involving three site distortions.[1, 2] This cooperative ‘Molecular-like’ ordering is also found to explain the unusual near integer charge ordering of Ru5.5+ to Ru5+ and Ru6+in Ru2O9 dimers of Ba3NaRu2O9.[3]

Investigating how molecular like interactions may stabilise unusual forms of charge ordering remains an active area of research.

Magnetic Order

We study magnetic ordering in metal oxides by neutron powder diffraction and single crystal magnetic x-ray scattering. The theoretical prediction of the structure of magnetically ordered phases remains at best unreliable. The reason for this is that the energy scales involved in magnetic exchange interactions are very small, and the ground state is often a result of a complex completion between many different exchange interactions. Experimental determination of magnetic structures and rationalisation of the sign and relative magnitude of exchange interactions is hence vitally important to improve the development of reliable theoretical models.

Understanding how the change in the electronic configuration of an ion may affect the sign (FM / AFM) of the magnetic exchange interaction, and the effect this has on the global magnetic structure is of particular interest to us. [4, 5]

Negative Thermal Expansion in Layered Perovskites

When you heat a material up, its constituent atoms start to vibrate more; intuitively we expect this to give rise to positive thermal expansion – the process by which a material expands in its physical dimensions on heating. However, some materials undergo negative thermal expansion due to certain kind of transverse lattice vibrations which, by a tensioning effect, cause a contraction in a direction perpendicular to their oscillation. Here our work is focused on using symmetry analysis to understand the occurrence of these lattice vibrations, and to gain insight into how we can engineer new materials with enhanced negative thermal expansion properties. [7]

Microscopic Mechanisms of Ferroelectric and Multiferroic Materials

As part of a 3 year RC1851 fellowship investigations into the “microscopic mechanisms in multiferroics materials” are being carried out. Multiferroic materials which exhibit both magnetism and ferroelectricity are an intriguing subgroup of materials. We perform crystallographic studies and symmetry analysis to understand how these two phenomena may be coupled together in the solid state. Understanding how these physical properties interact and can act to “switch” each other is vitally important work if the technological implications of these materials are to be realised. A particular area of interest for us is in understanding the “Hybrid Improper Ferroelectrics” mechanism in Ruddlesden-Popper oxides. [6]
By conducting in situ crystallographic studies at central facilities the switching mechanisms in these materials will be studied as a function of temperature, pressure and applied electric and magnetic fields.

Order-disorder phase transitions

In most phase transitions, the global symmetry breaking caused by the rearrangements of atoms, is the same as the local symmetry when viewed at the microscopic level on the unit cell length scale. However, some materials exhibit order-disorder phase transitions, where the local symmetry breaking is not the same as the global symmetry breaking, and hence a degree of disorder must exist. Rationalisation of the structure-property relationship in this class of phase transitions is particularly challenging. Using neutron PDF methods, symmetry analysis, and Monte Carlo simulations, we have been studying the rich phase diagram of the archetypal ferroelectrics Barium Titanate, gaining new insight into how global and local symmetry breaking can be reconciled with each other in these fascinating systems.[8]

Our focus is on 5 main areas currently: Molecular-Like Ordering in the Solid State Magnetic Order Microscopic Mechanisms of Ferroelectric and Multiferroics Materials Negative Thermal Expansion in Layered Perovskites Order-Disorder Phase Transitions

Our focus is on 5 main areas currently:

Molecular-Like Ordering in the Solid State

Magnetic Order

Microscopic Mechanisms of Ferroelectric and Multiferroics Materials

Negative Thermal Expansion in Layered Perovskites

Order-Disorder Phase Transitions