DIELECTRIC PROPERTIES IN ORIENTED THIN FILMS OF TUNGUSTEN-BRONZE-TYPE FERROELECTRIC Ba_6Ti_2Nb_8O_<30>

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(1) In order to investigate the origin for the extraordinary broad peak in the dielectric constant vs. temperature characteristics in Ba_6Ti_2Nb_8O_<30> ceramics, the temperature dependence of the dielectric constant in oriented thin films grown on (111)Pt/Ti/Si0_2/Si substrates by MOD method. Although the MOD films do not have enough orientation and their phase transition temperature shifts by about 13℃ to a higher temperature in comparison with the case of crystalline materials (245℃), it is observed no broad peak ; this suggests that the dielectric properties of ceramic samples are strongly influenced by internal stresses with random directions.(2) It is confirmed that the films grown on (100)SrTiO_3 and (100)MgO substrates align well along c-axis and that the a-axis of the grown films is tilted by ±18.5° with respect to the a-axis of the substrate. This tilted nature is closely related to the peculiar arrangement of oxygen octahedra in the unit cell of the teragonal tungsten-bronze structure.(3) The polarization-electric field characteristics are measured in the first time in ceramic samples as a function of temperature. It is found that the remanent polarization decreases with increasing temperature and tends to vanishe at 160〜175℃ this temperature is significantly lower than the phase transition temperature of single crystals (245℃) or the peak temperature of the dielectric constant in ceramic samples (180〜200℃). Moreover, the magnitude of the spontaneous polarization is as low as 1μC/cm^2, suggesting that only a small fraction of domains can probably make polarization inversion.(4) A polarized-light optical microscope observation is carried out on 100〜150 μm thick ceramics to check the domain structure along the thickness diraction. It is found that the size and the portions having birefringence are quite small in accordance with the experimental fact of very small spontaneous polarization.