Análise da transição de fase difusa (TFD) em sistemas ferroelétricos relaxores: uma abordagem teórica

Abstract

The study of ferroelectric materials is of great importance due to their various technological applications, including non-volatile memories, piezoelectric sensors, and optoelectronic devices. A detailed understanding of the phase transition mechanisms in such systems, in particular in those showing relaxor characteristics, can lead to the development of materials with optimized properties, broadening their technological applications. Ferroelectric materials exhibit spontaneous polarization in the absence of an external electric field, and when cooled (or heated) the temperature at which the phase transition occurs is known as the Curie temperature (TC). For temperatures above TC, the material is in the paraelectric state and does not exhibit spontaneous polarization, whereas it is in the ferroelectric phase for temperatures below TC. However, this is not the case of relaxor ferroelectric materials, which exhibit the diffuse phase transition (DPT) phenomenon. The DPT is characterized by a range of phase transition temperatures rather than a well-defined temperature. The lack of a satisfactory model to explain and describe the DPT motivated this work, which proposes a revision of the different models existing in the literature to explain this phenomenon. The models were used to fit the dielectric permittivity curve as a function of temperature for the experimental data of the (Pb,La)(Zr,Ti)O3 (PLZT) system, previously synthesized in the research group. The obtained results of the fitting parameters for each model were discussed and compared, with special attention to Liu's model, which currently reveals to be the best one in the literature for the fitting the experimental curve. On the other hand, with the aim of obtaining additional microscopic information in the analyzed system, which is not present in the literature, this work proposes a new theoretical approach based on a canonical model for a better understanding of the DPT phenomenon. The validity of the model was debated and its validity limits analyzed. It is worth pointing out that the present work represents a pioneering research on the use of a theoretical model based on a canonical approach for a better description of the DPT.

Keywords: Diffuse Phase Transition, Relaxor Ferroelectrics, PLZT, Dielectric Properties.