Diffuse ferroelectric transition and relaxational dipolar freezing in : III. Role of order parameter fluctuations

Results of a high-temperature x-ray powder diffraction study of the phase transition between cubic and tetragonal phases of are presented for x = 0.08 and 0.12 which exhibit sharp and diffuse dielectric anomalies, respectively. Experimental evidence and arguments are advanced to show that the cubic and tetragonal phases coexist over a wide range of temperatures and that this coexistence is due to the fluctuation of the order parameter which is coupled electrostrictively to strains. For x = 0.08, the phase coexistence disappears below the dielectric anomaly temperature , indicating the critical nature of the fluctuations. For x = 0.12, these fluctuations are non-critical since the phase coexistence persists even below . The complete conversion to the tetragonal phase occurs at which is nearly lower than ). It is shown that the spontaneous polarization also increases gradually below and levels off at . The existence of structural and polarization anomalies well below the frequency-independent dielectric anomaly temperature cannot be rationalized either in terms of the Landau-like theories for ferroelectric transitions which predict or in terms of dipole glass transitions and/or relaxor ferroelectric transitions for which should be frequency dependent.