Nanoscale compositional segregation and suppression of polar coupling in a relaxor ferroelectric

A number of relaxor ferroelectric ceramics have been demonstrated to possess a near stable value of relative permittivity over very wide temperature ranges. This cannot be explained by conventional theories of relaxors. One such system is based on the perovskite solid solution series: (1-x) (Ba_0.8Ca_0.2)TiO₃-xBi(Mg_0.5Ti_0.5)O₃, giving stable relative permittivity from 150 to 500 °C. We show by scanning transmission electron microscopy and electron energy loss spectroscopic elemental mapping that nanoscale compositional segregation occurs in the temperature stable relaxor composition (x = 0.55), with Ba/Ti clusters some 2–4 nm in extent, separated by Bi-rich regions of comparable size. This nanomosaic structure is consistent with phase separation into a ferroelectrically active BaTiO₃ – type phase (Ba/Ti rich) and a weakly polar Bi/(Mg) rich perovskite solid solution. The possibility that nanophase segregation is the cause of weak dipole coupling and suppression of the dielectric relaxation peak is considered.