Functionality from local structure in conventional and hybrid Prussian blues

The family of metal-organic framework materials, in which metal ions are connected together into a network by organic linkers, has attracted worldwide attention over the past two decades for its diversity of both structures and functional properties. Yet these materials are being developed at a rate that far exceeds our ability to fully understand their behaviour. This research project will focus on the Prussian blue analogues, a particularly simple and elegant group of materials within the broader family of metal-organic frameworks that nonetheless displays a wide range of scientifically interesting and technologically relevant functionalities. As well as being of great interest in their own right, detailed studies of these materials will thus pave the way towards greater understanding of more complex structures.

The Prussian blue structure is a deceptively simple arrangement of ions to form a crystal lattice consisting of interlinked "cages". Many different molecular building blocks can be arranged in the same basic structure: thus although it has been known for many years, new materials of this type continue to be discovered. Judiciously selecting the right components can lead to materials with many remarkable properties. These include multiferroics, in which magnetic and electric ordering are interlinked, leading to applications in computing; photomagnets, whose magnetic behaviour changes when illuminated with light; and porous materials able to absorb gases or metal ions from their environment.

The ease with which the building blocks can be interchanged makes this a highly promising system for targeted materials design: if we can predict the effect any given component will have on a material's properties, we will be able to engineer a material with precisely the properties needed for a given application. In order to understand the link between composition and functionality, though, we must understand how both are related to the material's structure. Historically, the crystallographic methods used to determine the structure of materials have given very precise information about the average position of each atom, but only indirect indications of how far the structure might locally deviate from that average. However, total scattering experiments are sensitive to both the local and average structures of a material. Analysing these experiments using the reverse Monte Carlo algorithm can produce a model of the crystal structure that simultaneously reveals both the local and average structures, leading to an unprecedented understanding of these materials.

This research programme will investigate four carefully selected sets of materials with the Prussian blue structure. Using a combination of total neutron and X-ray scattering with computer simulation, the effects of three specific local features on these materials' functionality will be considered. Understanding the link between these features and the material properties will be an important step towards designing new materials in this family with specific functionality tuned to particular applications, and in turn towards a more comprehensive understanding of metal-organic framework materials in general.

The multidisciplinary approach of the proposed project links it closely to EPSRC Grand Challenges both in Physics (Nanoscale Design of Functional Materials) and Chemistry (Directed Assembly of Extended Structures with Targeted Properties). As such the outcomes of this research have the potential for substantial impact in the development of materials to address contemporary UK and global challenges.

First, the results of the project will be well placed for industrial development and exploitation. Several of the structure-function relationships identified as part of the proposed research will be relevant to industry, with applications including safe processing of nuclear waste and electronic data storage and manipulation. Our existing network of industrial links, particularly through the Materials Research Institute at QMUL, will ensure the effective dissemination and exploitation of relevant outcomes from this work, and ensure that the UK benefits from potential commercial developments.

Second, the methodology of this project will itself be relevant beyond an academic setting. We have already established a collaboration with PANalytical to distribute our reverse Monte Carlo program RMCprofile, including the code developed in this project, providing an effective pathway for the methodological advances to reach the widest possible range of users. Beyond the industries to which the specific materials investigated here are relevant, the total scattering methodology is already of considerable interest to the fields of pharmaceuticals (e.g., to "fingerprint" drugs), construction (e.g., to characterise cement and other structural materials), and nanotechnology (e.g., to guide the preparation of nanostructured materials). We have an established programme of tutorial meetings, typically as satellites to major conferences, to pass on the skills required to run reverse Monte Carlo simulations and interpret the results sensibly.

Finally, the project will have an impact on UK education through the development of simulations appropriate for the QMUL "Physics Academy" program. This annual event sees high school students visit the School of Physics and Astronomy for a week to work on a miniature research project under the guidance of an academic and undergraduate ambassadors. Previous molecular dynamics projects have proven both popular and successful. Some of the simulations proposed here will readily divide into small components suitable for high school students, providing them with a unique opportunity to make a genuine contribution to a cutting-edge research programme.