The Spin Crossover Phenomenon

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The spin crossover phenomenon was first discovered 60 years ago by Cambi et al. As a consequence of the splitting of the energy of d orbitals into the t_2g and e_g sets in a ligand field, octahedral complexes of TM ions with configurations of d⁴ to d⁷, may exist in high, intermediate or low spin states. In the case of d⁶ Co(III), the electronic configurations are HS(t_2g⁴e_g²), IS(t_2g⁵e_g¹) and LS(t_2g⁶ e_g⁰). When E≈K_BT application of external constraints i.e. temperature, pressure or electromagnetic radiation effects a change in state. Changes in colour, magnetic moment or structural properties are observed when SCO occurs.

Layered complex oxides of the transition metals that adopt the K₂NiF₄ structure demonstrate a number of key electronic properties such as superconductivity (e.g. La_1.8Ba_0.2CuO₄ and Sr₂RuO₄ ), oxide ion conductivity, low dimensional magnetic ordering. Such materials have been studied in depth for some transition metal elements, namely Cu, Fe, Cr and Ru but to a very limited extent for other metals e.g. Co(III) and Ni(III). Layered cobalt oxides with structures based on a CoO₂ square net demonstrate a number of unusual electronic and magnetic properties associated with the various spin states of the Co(III) ion and interactions between such centres. These properties appear to derive form the specific electronic states of Co(III) in octahedral environments and associated co-operative phenomena such as charge ordering, spin ordering (magnetism) and conductivity.

Many of these properties also appear to be a result of the closeness in energies of the high, intermediate and low spin states of Co(III) in the octahedral environment (d⁶, t_2g⁴ e_g² (HS), t_2g⁵ e_g¹ (IS), t_2g⁶ e_g⁰ (LS)). Thus redistributions of spins among these cobalt centres, sometimes concertedly, leads to major changes in the compound’s physical properties. HS and IS Co(III) systems are generally favoured at high temperatures and give rise to a high effective moment and metallic phases whereas the e_g electrons interact with the O 2p levels, in the low temperature insulating phases LS Co(III) is generally favoured. However, in practice the behaviours of Co(III) oxides are much more complex due to phenomena such as multiple sites and coordination environments leading to partial or complete spin ordering, low dimensionalities in the structure, dopant effects to name but a few. Materials that have been studied in detail include LaCoO₃ and the RBaCo₂O₅ family.

LaSrCoO₄ and some stoichiometrically closely related phases have been studied to some degree and show the spin-crossover behaviours albeit it weakly.

K₂NiF₄ Structure showing NiF₆ co-ordination (Left) and Sheets of Octahedra (Right)

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Ruddlesden-Popper Oxides

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