Solid State Chemistry
Research in the group focuses on the synthesis and study of novel solids which exhibit unusual electronic and magnetic behaviour. A particular area of interest is the utilization of unusual synthetic techniques and conditions to increase the degree of control and selectivity exhibited by solid state reactions.
Low Temperature Solid State Synthesis
In general, current preparative methods employ high temperatures to overcome the considerable diffusion barriers present in most solid-solid reactions. These high temperature treatments result in almost exclusive thermodynamic control of reaction products, thus precluding the formation of a large number of compounds with low thermal stability. There is therefore a considerable need to develop synthetic techniques that will allow much lower synthetic temperatures to be used and thus kinetic control to be exerted on product selection.
One approach to this problem is to utilize the observation that the mobility of the anions in most complex solids is significantly higher than that of the cations at a given temperature. This mobility difference allows us to manipulate the anion lattice (reductive deintercalation, anion exchange, oxidative insertion) at low temperatures, where the cations remain largely static. This enables the synthesis of a wide range of compounds with new and unusual metal oxidation states and metal co-ordination environments. In addition this approach allows us to investigate the diverse and extensive anion chemistry of these materials, which currently remains under explored.
Topotactic reduction using metal hydrides
Metal hydrides are extensively used as bases and reducing agents in organic chemistry. They can also be used as powerful low temperature reducing agents in the solid state to bring about the reductive deintercalation of oxide ions from complex transition metal oxides. These processes remove (deintercalate) oxide ions from the structures of complex materials whilst leaving the other atoms and ions in place. As a result materials with novel structures and highly unusual transition metal oxidation states can be prepared.
| Hydride reduction reactions can also be applied to mixed anion systems, such as the oxide-chloride Sr3Fe2O5Cl2 which on reduction forms Sr3Fe2O4Cl2 containing Fe(II) centres in square-planar coordination sites. |
Synthesis of oxide-fluoride phases
The selectivity observed in hydride reduction reactions can be used to prepare novel mixed anion materials. By following a reduce-then-fluorinate synthetic route the anion vacancy order in a topotactically reduced phase can be used to pattern the arrangement of fluoride anions in the subsequently prepared oxide-fluoride phase.
Cation Ordered Phases
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The coordination preferences of different metal cations can be utilized to encourage entropically disfavoured, cation ordered structures to form. Anion vacancies within the structure of the Ba4CaFe3O9.5 lead to a unique ordered arrangement of Ca2+ and Fe3+ cations within this phase which breaks the inversion symmetry of the cubic perovskite host lattice. This allows the material to exhibit properties such as second-harmonic generation (SHG) and piezoelectric behaviour forbidden to centric phases. In addition the phase exhibits magnetic order, making materials of this type good candidates for magnetoelectric and multiferroic behaviour.
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