Abstract
This paper reports the accurate measurement of dielectric constant, dielectric loss and DC conductivity on two crystal samples, cut from the same boule, over a wide range of temperatures (from liquid-nitrogen temperature to +400 degrees C). Dielectric anomalies were observed at three transitions. The application of Devonshire's phenomenological theory of ferroelectrics reveals that the ferroelectric-paraelectric transitions, observed at -1 degrees C and -4 degrees C for the two crystal samples, are very close to second order in character. The peak in conductivity corresponds to a peak in the dielectric constant in the region of the Curie temperature. The spontaneous polarization was obtained from the measured values of pyroelectric current below +30 degrees C. Variation of residual polarization with applied field was studied at a fixed temperature from hysteresis loop data. Results for the frequency dependence of dielectric constant and loss, and for the field dependence of conductivity, are also presented. An attempt has been made to explain the observed physical properties on the basis of donor concentrations found to be present, though in small quantities, in the candidate samples.
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