Phenomenological theory of uniaxial relaxor ferroelectrics

A phenomenological thermodynamic theory of uniaxial relaxor strontium barium niobate $\text{B}{\text{a}}_{0.5}\text{S}{\text{r}}_{0.5}\text{N}{\text{b}}_2{\text{O}}_6$ is developed using the Landau–Devonshire approach with two order parameters. The fourth-order thermodynamic potential allowed to explain the shape of the polarization hysteresis loops experimentally observed at different temperatures. We show that the broad maximum of the dielectric permittivity is not related to the phase transition and arise due to the coupling between polarization and true order parameter which has antiferroelectric nature. We found that the phase transition temperature is much higher than the maximum of the dielectric permittivity and very likely corresponds to so-called Burn's temperature. True order parameter has no simple relation with polar modes. (paper)