Ferroelectric phase transition and maximum dielectric permittivity of displacement type ferroelectrics (BaxSr1-xTiO3)

The main interest in the area of practical applications of ferroelectrics, particularly at the microwave frequency, is oriented now to room temperature. In this connection, ferroelectrics like BaxSr1-xTiO3 should be carefully studied. Such materials are characterized by the second order phase transition. In a perfect ferroelectric crystal, the phase transition takes place at temperature T-C, which is called the Curie temperature. Real (defected) crystals and ceramic samples are characterized by a presence of built-in electric field and mechanical strains. In the case of the real crystal, the temperature of the phase transition T-C', is displaced to lower temperature and the temperature of the maximum of epsilon(T)T-m is displaced to higher value with respect to T-C. Thus, for the real ferroelectric sample (not for an incipient ferroelectric), one has T-C'< T-C< T-m. Some investigators suppose by default that T-C'=T-C=T-m. Such a supposition is wrong and can lead to an incorrect treatment of experimental results. In the case of a thin film sample, the phase transition and the dielectric response of a ferroelectric sample are affected by the size of the sample, what is treated as a size effect. Experimental data obtained as a result of measurement of the dielectric constant as a function of temperature can be used for finding the Curie temperature and other parameters of the material. For this procedure, a suitable model of the dielectric response of the ferroelectric sample should be used. (C) 2000 American Institute of Physics. [S0021-8979(00)07622-2].