Internal field effects on phase transitions in high permittivity displacive dielectrics

It is demonstrated that using probes of the electron and nuclear configuration within the room temperature distorted oxygen octahedra for the high permittivity displacive ferroelectric (dielectric) BaTiO₃ , the crystalline field at the Ti⁴⁺ site undergoes a discontinuity at the Curie point. It may now be seen why the spontaneous polarisation is reversible, mainly along the c-axis only, on application of an external field enabling a `rattling´ of this ion which disappears at the Curie point despite the slight residual distortion at right angles to this above T_c