Inorganic Materials Chemistry Research
The overall aim of the research in the Group is the synthesis and characterisation of functional inorganic materials. Such useful inorganic solids cover a wide range of types and applications including porous structures for catalysis and ion exchange, metal oxides for magnetic information storage and superconductivity, and synthetic analogues of natural minerals showing amazing and unique optical properties and colours. By making new inorganic solids, understanding their structures and properties we strive to design and synthesise ever improved materials for many important applications.
To synthesise inorganic materials an extensive range of methods, including direct solid state reactions, chemie-duce (soft chemistry), hydrothermal methods and high-pressure techniques are applied – these facets of the synthesis work result in considerable activity at the borders of solid state chemistry, physics and geochemistry.
To structurally characterise these inorganic compounds a full range of experimental techniques are been applied, including powder diffraction, (X-ray and neutron), single crystal X-ray diffraction, EXAFS, electron microscopy and MASNMR.
The effort can be summarised under four main research activties :- Optical Inorganic Materials, Magnetic and Electronic Materials, Functional Frameworks and New Structure Determination Techniques.
1. Optical Inorganic Materials
Inorganic Pigments.
Developing new inorganic pigments and optimising routes to known systems forms a major part of the Group's research portfolio. This includes projects on the determination of the structures of these intensely coloured materials, the development of new pigments and optimisation of commercial routes to pigments, with Holliday Pigments.
Work in collaboration with Merck concerns the formation of substrate based pigments including lustre pigments and nanotechnology in the control of pigment properties. Further work involves the development of completely new inorganic pigment systems such as colour changing pigments and optimisation of structural features for improved colours - e.g. a bright red replacement for the cadmium compounds currently used. Pigments are also the subject of a Partnership for Public Awareness grant entitled - "Creating a Colourful Life" - more information will soon be available on an associated website concerning the history and role of colourful pigments currently under construction.
Specialist Optically Functional Inorganics and Geomimetics
As well as researching inorganic pigments, where the colour arises from absorptions in the visible part of the spectrum, the Group is also interested in synthesising compounds with a broader range of optical properties. Examples include compounds that absorb or reflect infra-red or ultraviolet light - and that might be used in sunscreens, filters or coatings. By developing new materials and controlling their structures it becomes possible to tune the properties of an inorganic compound to absorb or reflect light of specific wavelengths.
Another important property of some inorganic compounds is the ability to absorb light of one wavelength and emit the energy as a photon of a lower energy - fluorescence. Such materials are widely used as phosphors in fluorescent lighting and as security features, e.g. inks printed on banknotes. The Group is currently developing new materials of this type and also materials that change colour when exposed to UV light - photochromics.
Upper. A mineral sample containing hackmanite (from Kangerlussaq, Ilimaussaq Complex, Greenland ) before and after exposure to UV radiation (254 nm, 10 s, 3400 µW/cm 2)
Lower. Synthetic samples of the equivalent photochromic sodalites before (centre) and after (left Na8[AlSiO4]6[Br2-x-ySx□y], right Na8[AlSiO4]6[Cl2-x-ySx□y]) excitation under the same UV light
The sodalite structure showing the framework as SiO4 (green) and AlO4 (purple) tetrahedra surrounding cages containing sodium ions (gold spheres) and anion sites (purple). Two sodalite cages are shown: the left hand one contains an ionisable anion (magenta) from which an electron is transferred to the previously vacant, neighbouring right-hand cage (small brown sphere) producing a colour centre.
2. Magnetic and Electronic Materials
Many materials with useful electronic and magnetic properties are complex oxides - that is oxides containing more than one metal ion. Example include the high temperature cuprate superconductors and magnetoresistive materials ( compounds whose resistance changes when a magnetic field is applied to them) used in information storage devices such as computer hard disks. Research within the Group concerns synthesising new oxides with a view to understanding and improving their electronic properties
Layered Oxides
Structures built from sheets of metal and oxygen ions include those of the high temperature superconductors and many useful magnetic materials. Research within the Group is currently focussed on two areas of study: (i) the so-called Ruddlesden-Popper phases (illustrated below) and (ii) structures where the metal oxygen layers are separated by halide anions.
In the former case the work is focussed on oxides containing the first row transition metals, particularly cobalt, nickel and iron, in high oxidation states. In such compounds unusual structural behaviours are coupled with changes in electronic properties so that materials may undergo a transition from metallic to insulating behaviour as it is heated and changes from one structure to another.
Ruddlesden-Popper structures for 1, 2 and 3 layers. A type ions are shown in green, BO6 octahedra shaded in red
Pyrochlores
Pyrochlores are a family of complex oxides of the formula A2B2O7. When B is a heavy transition metals such as rhenium, osmium or gold the electronic and magnetic properties exhibit by these compounds are strange and fascinating. We are currently synthesising new compounds of this type for characterisation in collaboration with Prof Brian Rainford ( Physics)
The pyrochlore structure with BO6 octahedra shaded in green. A cations are shown in red, and black represents non-framework oxygen positions
Battery Materials
Materials used in rechargeable lithium ion batteries are mainly complex oxides of the first row transition metals such as manganese and cobalt, e.g. LiCoO2. In collaboration with Prof John Owen we are currently investigating new high oxidation state compounds that could be used in such applications.
The structure of LiCoO2. CoO6 octahedra are shaded in red. Lithium ions are shown in yellow
3. Functional Frameworks
Inorganic framework materials are built from linked polyhedral units. The best know class are the zeolites whose applications include catalysis, ion exchange ( for water softening and removing toxic ions) and gas adsorption and separations. As well as studying the physical properties of many of these zeolite materials we are also undertaking research aimed at producing new framework structures. Some aims of this work include
Left: (Cs2[BAsO3OH]8[AsO4]2[CsCl4]Cl)2,a member of a new family of boroarsenate frameworks. AsO4 and AsO3OH tetrahedra are outlined in purple. BO4 tetrahedra are in red. Gold spheres represent caesium and green spheres are chlorine.
Right : Cs2(ZnAsO3OH)(ZnAsO4)2·H2O, a chiral, 16-ring channel framework. AsO4 and AsO3OH tetrahedra are outlined in blue and ZnO4 in orange. Grey spheres represent hydrogens pointing into the channel. Caesium and water in the channel are omitted.
4. New Structure Determination Techniques
The synthesis of a functional material generally requires a subsequent study of its structure to determine where it displays certain properties. For single crystals such a structure determination is often easily achieved (EPSRC National Crystalography Service at Southampton). For polycrystalline solids, which may also be solid solutions and contain defects and local disorder, such structure determination is often more difficult. As well as using techniques generally available for studying such materials ( e.g. powder X-ray diffraction) we are also involved in developing new structure investigation methods that allow a deeper insight into these structures to be gained.
Examples of methods that have developed include data analysis methods that combine information on long range and local order (diffraction and X-ray absorption spectroscopy) and the use of isotopically enriched materials in neutron diffraction. New methods of determining the hydrogen positions in key inorganic materials will be studied in the future.
The MFI framework structure of Silicalite showing the 12 crystallographically distinct silicon sites
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