Superstructures at low spin - high spin transitions

In many transition metal compounds, in particular those containing Fe and Co, there occur spin-state transitions between low-spin and high-spin (or intermediate-spin) states. We show that typical interactions between similar spin-state ions are short-range repulsion, and long-range interaction which can have a different sign depending on the elastic anisotropy of the lattice and on the direction between respective ions. Due to such character of effective interactions there may occur at spin-state transitions different superstructures — ordered arrangement of different spin-states, which in particular may have the form of stripes. The properties of the system TlSr2CoO5 for which such a superstructure was recently observed experimentally, are discussed from this point of view. Typeset using REVTEX