Optimising Ferroelectric Hybrid Frameworks through Tuning Electronegativity

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Hybrid framework materials, which combine metal ions and organic components as building blocks in an extended network, have attracted worldwide attention over the last two decades for the diversity of structures and functional properties they exhibit. They have long been of interest for the porous channels in their structures, which enables a wide range of molecular gases to be separated and stored. More recently, however, they have begun to show tremendous promise for their electronic and magnetic properties, which are critical to underpinning many modern technologies and have long been associated with metal oxide ceramics. These properties include:1. Ferroelectricity which is, the switchable polarisation of charge in a material such that it has a positively and negatively charged end. Ferroelectrics have applications in many industries, including medical and defence ultrasound technologies based on piezoelectric responses, where a material expands and contracts when in an applied electronic field.2. Multiferroicity, where a material has both ferroelectric and magnetic order, which has great potential for ultra-high density memory storage in information technologies. Analogues of some functional oxide structures remain important for hybrid frameworks with electronic functions. This includes perovskite related structures, in which small octahedral metal cations are bridged by anions at their corners, with a larger cation, an organic molecule in hybrids, sitting in the spaces between this extended framework. The electronic properties of hybrid frameworks can, however, arise from fundamentally different origins than found in oxides; for example charged molecular cations in hybrid perovskites can exhibit ferroelectric order because of relatively weak supramolecular interactions, such as hydrogen-bonding, between the molecule and the anionic framework. This enables them to be modified in unique ways, for example the ferroelectric response to applied electric fields in hybrids can be optimised to be consistent over a wide temperature range in a way typically associated with relaxor oxide materials, without introducing the complex mixture of cations needed for such oxides. The details of how such relaxor-like properties occur in hybrid frameworks is still unclear and it will be important to develop an understanding of this occurs in-order to be able to modify hybrids to optimise their physical properties.As the interactions responsible for ferroelectric order in hybrid frameworks are much weaker than in oxides it is critical to increase the strength of these contacts. This will enable the molecular cations to adopt the ordering patterns required for ferroelectricity at higher temperatures, leading to the development of materials that operate at useful conditions, above room temperature. One route to achieving this is to maximise the difference between the electronegativity, the strength of attraction between an ion and an electron, of the smaller metal cation and the anions, which indirectly strengthens the supramolecular interactions responsible for ferroelectric order of the molecular cations. This proposal will probe two routes for achieving this goal by making hybrid perovskite frameworks where either the octahedral cation is a less electronegative magnesium, calcium or strontium cation, or the anion is more electronegative i.e. chlorine, bromine or iodine. These materials will then be screened for their ability to feature molecular cations with ferroelectric ordering patterns above room temperature and the physical properties of materials meeting this criteria will then be characterised to establish optimised ferroelectric performance. A combination of experimental and computational techniques will then be used to understand the link between atomic level structure and physical properties in these materials, with a particular focus on the unique way in which relaxor-like responses arise in hybrid frameworks.