Transition metal oxides
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Transition metal oxides display a range of interesting electronic and magnetic properties that derive from their electronic structure. The closeness in energy of the d orbitals on transition metal ions and p orbitals in oxide ions means that significant σ and π interaction is possible between the two species. This allows stabilisation of a wider range of oxidation states than would at first be expected in such materials, giving new, exploitable properties. Examples include Cu (III) and Mn (III/IV) species, which provide some of the best-studied superconducting and colossal magnetoresistive materials.
Though there has been much research into many first row transition metal containing oxides; relatively little work has been carried out on the higher oxidation states of cobalt. We are currently undertaking a programme of solid state chemistry aimed at extending cobalt (III) chemistry analogous to existing transition metal oxides. More information on Co (III) can be found in the spin crossover section.
Ruddlesden-Popper Phases
Ruddlesden-Popper (RP) phases are layered Perovskites with the general formula AO(ABO3)n. Corner sharing BO6 octahedra form layers, with A atoms occupying the 9 and 12 coordinate interstitial sites. The first characterised RP phases were the SrO(SrTiO3)n series, fully characterised by Ruddlesden and Popper for n = 1, 2 and 3. n = ∞ corresponds to the standard Perovskite structure. These materials crystallise in the tetragonal space group I4/mmm (number 139) with an a0 lattice parameter of ~ 3.9Å
Ruddlesden-Popper structures for n = 1, 2 and 3. A atoms shown in green, BO6 in blue/red
These are the ideal RP structures obtained when the lattice not under strain. As the size of B increases relative to A, the octahedral network becomes compressed and may distort to relieve the strain. Distortions typically involve rotation or twisting of octahedra to prevent the distortion of the rigid octahedral body, giving a √ 2a0 x √ 2a0 supercell. Two possible distortions caused by 1) rotation in the ab plane and 2) tilting of octahedra are shown below (A atoms omitted for clarity). Work is currently being carried out to incorporate Co (III) ions into these structures, with a view to exploiting spin crossover to induce lattice distortion, hence a change in material properties.
Rotation of octahedra
Twisting of octahedra
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