Intrinsic ferroelectric properties of the nonstoichiometric perovskite oxide Ba1-xTi1-yO3-x-2y

The properties of many electronic materials are governed by defect type, distribution, and concentration. In this paper, we demonstrate the influence of equilibrated nonstoichiometry on the ferroelectric properties of the perovskite BaTiO3. It is shown from measurements of the transition temperature, thermal hysteresis, latent heat, and transformation strain that the concentration of partial and/or full Schottky defects significantly alters the nature of the weak first-order transition between paraelectric and ferroelectric phases. The coefficients of a conventional 2-4-6 Ginzburg-Landau polynomial, determined from the measured transition parameters, were also found to vary as functions of the defect type and concentration. These results illustrate for the first time the strong sensitivity of the Landau coeffecients characterizing the properties of the monodomain ferroelectric state to nonstoichiometry involving partial Schottky disorder reactions in the perovskite-structured oxides. The strong dependence of the Landau coefficients on the defect concentration points to the fact that most classical descriptions of the ferroelectric state for these materials, as found in the literature, are based on materials that are nonstoichiometric. The results underscore the importance of attaining an equilibrium state through controlled high-temperature processing in furthering a fundamental understanding of ferroelectricity in perovskite materials. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3109210]