Soft Chemical Routes to Novel Quantum Materials

Quantum Materials can be broadly defined as those in which four fundamental degrees of freedom - lattice, charge, orbital and spin - are dynamically entwined to yield complex electronic or magnetic behaviour.

Magnetoelectric multiferroic materials - those which exhibit strong, coupled magnetic and electrical polarisations - naturally reside within this definition and are predicted to form a key part of future quantum technologies by providing a potential avenue to energy-efficient information storage.

This project, therefore, falls within the EPSRC Quantum devices, components and systems research area whilst supporting the EPSRC strategic priorities of quantum technologies and engineering net zero. The project builds on previous work in the Hayward group focused on hybrid-improper magnetoelectric materials.

In these systems directed chemical changes to the framework of an extended solid (typically a layered, transition-metal oxide) bring about well-defined changes to the crystal symmetry of a phase, to first induce an electrical polarization and then to couple it to a magnetic polarization or entangle it with local spin degrees of freedom, resulting in emergent complex behaviour which includes magnetoelectric coupling.

The ability to selectively lower the crystal symmetry of the system, via chemical changes administered under kinetic control, allows the many different possible coupling regimes to be explored in a controlled way.

The specific innovation for this project is to substitute some of the metal cations (Li+, Mn2+ etc.) in these systems with molecular cations (NH4+, NH3R+ etc.) to form ordered, metastable phases.

The inclusion of molecular species in this class of quantum material offers novel opportunities to control the breaking of the underlying crystal symmetry via a host of non-covalent interactions including hydrogen bonding, Van der Waals forces and shape selection.

Due to the versatility of these topochemical exchange reactions there exists a plethora of prospective guest molecules, careful selection of which could yield interesting quantum behaviour such as the possibility to introduce additional local spins by the inclusion of molecular radicals or light-excitable species.

This will provide new routes to control, induce and manipulate strongly coupled quantum behaviour in materials which are inaccessible by conventional, thermodynamically controlled solid-state synthesis.

The key to controlling structural distortions leading to complex magnetoelectric behaviour is a detailed understanding of the microscopic mechanism by which the distortion occurs.

In layered, transition-metal oxides symmetry lowering distortions occur by cooperative tilting of corner sharing MO6 octahedra (where M is a high oxidation state d0 transition metal).

Due to the poor scattering strength of oxide ions the accurate characterisation of these subtle tilts using powder X-ray diffraction techniques has proved challenging; even with complementary computational studies definitive structural solution has not been possible in most cases.

Therefore, a key component of this project will be the utilisation of powder neutron diffraction as the primary means of structural interrogation, as systems including molecular cations have not yet been probed using this method.

Only once the structures of materials have been determined can the structure directing effects of intercalated molecular cations be deduced and ultimately utilised to manipulate distortions conducive to magnetoelectric coupling and the development of novel quantum materials.