Ferroelectric phase transition and low-temperature structure fluctuations in Ba4Nd2Ti4Nb6O30 tungsten bronze ceramics.

Dielectric characteristics of Ba4Nd2Ti4Nb6O30 tungsten bronze ceramics have been investigated over a broad range of temperature and frequency. The real part of the dielectric constant ([variant_greek_epsilon]′) shows a sharp peak at 417 K indicating the ferroelectric phase transition. Two dielectric relaxations are observed in the lower temperature range, which are very obvious in the dielectric loss (tan δ) and the imaginary part of the dielectric constant ([variant_greek_epsilon]″). The differential scanning calorimetry (DSC), Raman scattering, and ultrasonic studies of the present ceramics have also been conducted in a broad temperature range covering the ferroelectric transition and/or the dielectric relaxation temperatures. The endothermal peak in the DSC curve around the ferroelectric transition temperature confirms the first-order structural transition with the transition enthalpy of 5.72 kJ/mol. Steplike anomalies in the wave number of the Raman vibrations and abnormal drop of the intensity of the Raman spectra are observed accompanying the ferroelectric phase transition. The dielectric relaxations in the temperature range below Tc are attributed to the structure fluctuations associated with the coupling between the polarizability in the ab plane and the c axis displacement, as well as the remaining random field effects, which are suggested as the possible origin of the anomalous reduction of the Raman intensity below Tc as well as the anomalies in the ultrasonic velocity and attenuation measurements. Freezing of these structure fluctuations are detected by the ultrasonic anomaly at about 120 K and the increasing Raman intensity below 150 K. [ABSTRACT FROM AUTHOR]