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Tungsten-bronze type titanate BaNd2Ti4O12 ceramics were synthesized by solid state reactions.The conductivity and microwave dielectric loss of the samples that were thermally treated under various conditions and Ta-doped were investigated by electrochemical impedance measurement and microwave dielectric resonator measurement.The variation in conductivity with annealing atmospheres of air,O2,and N2 was consistent with the defect equilibriums 2O×O■2V·· O+O2↑+2e' and Ti× Ti+e'■Ti'Ti,suggesting n-type conductance for BaNd2Ti4O12.Thermal treatment in air/O2 was found to favor the elimination of the native defects V× O,Ti'Ti and weakly bound electrons thus decreasing the conductivity.Thermal treatment in a N2 atmosphere,which had a low oxygen partial pressure,increased the defect content and the conductivity.Thermal treatment in air/O2/N2 did not clearly affect the microwave dielectric loss,suggesting that native defects have negligible effects on this property.The air-annealed sample was found to have lower conductivity and lower microwave loss compared with the air-quenched sample.The change in conductivity was found to be related to the equilibrium of the native defects but the change in microwave dielectric loss might be explained by the release of thermally induced lattice strain.Ta doping reduced the conductivity but increased the microwave dielectric loss.This work shows that air-annealing may be an efficient way to improve the Q×f factor for BaNd2Ti4O12 ceramics,which was enhanced by ～12%.

BaxSr1-xTiO3 (0.40≤x≤0.70) ceramics have been prepared by the conventional solid-state reaction method. The changes of vibration modes in the pure BaxSr1-xTiO3 system have been investigated for an in-situ electric field by Raman spectra, and the transition from the cubic phase to tetragonal phases were obviously observed near the Curie temperature (Tc). Detailed results indicate that the A1(TO3) and E(TO4) modes correspond to the vibrations of O-Ti-O bond along a-axis in the ab plane, which are strongly related to the lattice structure of ABO3. With increasing electric field, the intensity of A1(TO3)/E(TO4) ratio is increased, suggesting that more and more paraelectric states are transformed into ferroelectric states, thus leading to the deformation of lattice, which results in the decrease of dielectric constant and the increase of tunability. The electric-induced-structure phase transitions responsible for the nonlinear dielectric properties are also discussed.