Pressure-induced spin-state transition in BiCoO₃

The structural and electronic properties of BiCoO₃ under high pressure have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that the structure changes from a polar PbTiO ₃ type to a centrosymmetric GdFeO₃ type above 3 GPa with a large volume decrease of 13% at room temperature Revealing a spin-state change. The first-order transition is accompanied by a drop of electrical resistivity. Structural results show that Co³⁺ is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present. The pressure-temperature phase diagram of BiCoO₃ has been constructed enabling the transition temperature at ambient pressure to be estimated as 800-900 K.