PHASE-TRANSITIONS IN FERRIMAGNETIC AND FERROELECTRIC CERAMICS BY AC MEASUREMENTS

Ac conductivity measurements were carried out on polycrystalline sampl es of a ferrimagnetic spinel (Zn0.44Mn0.56Fe2O4) and a ferroelectric p erovskite (Sr0.25Bi4Ti3.25O12.75), in the temperature range 20-160 and 20-660 degrees C, respectively, and in the frequency range 5 Hz-13 MH z. The impedance response in both cases could be resolved into two con tributions, associated with the bulk (grains) and the grain boundaries . An analysis by means of the ac conductivity power law showed evidenc e of a critical temperature of 132 and 536 degrees C, for the ferrimag netic and the ferroelectric samples, respectively, which corresponds t o the Curie temperature for each type of material. These results are i nterpreted in terms of the disorder increase approaching the phase tra nsition. (C) 1998 American Institute of Physics. [S0003-6951(98)04640- 3].