Na0.5Bi0.5TiO3 phase relations: Thermodynamics and phase equilibria in the systems Bi2O3 - TiO2, Na2O - TiO2, and Na2O - Bi2O3 - TiO2

The lead-free piezoelectric material sodium bismuth titanate (NBT, Na0.5Bi0.5TiO3) has attracted considerable attention owing to its promising dielectric, piezoelectric, and electrical properties. However, the literature on the binary subsystems is contradictory and there are only limited data for the ternary system. The present work surveys all of the reports of the binary subsystems Bi2O3 - TiO2 and Na2O - TiO2 and synthesizes these data into inclusive revised versions. The compatibilities for the ternary system Na2O - Bi2O3 - TiO2 were determined experimentally, thus enabling the construction of a complete isothermal section at 800 degrees C. The compatibilities associated with the problematic binary subsystem Na2O - Bi2O3, which experiences extreme volatilisation, were determined through the generation of the absent standard-state thermodynamic functions for the relevant binary and ternary phases, thus providing a full suite of thermodynamic data for this system. The thermodynamic stability diagrams for Na2O, Bi2O3, and TiO2 thus were calculated. The isothermal section also addresses the contradictions in the literature concerning the formation of solid solutions of Bi12TiO20-x / Bi12-xTi1+xO20+0.5x, pyrochlore (Bi2Ti2O7 / NawBi2-xTi2-yO7-z), BTO (Bi4Ti3O12 / NaxBi4Ti3O12+0.5x), and NBT (Na0.5Bi0.5TiO3 / Bi1 +/- xNaxTiO3.5 +/- x). Further, it was observed that the congruent melting point of NBT, which was determined to be 1225 degrees C, was preceded by the onset of gradual structural destabilization at 940 degrees C. Also, the NBT rhombohedral -> tetragonal phase transformation was observed at an onset temperature of -250 degrees C. The present work thus provides platform data for the fabrication and reactivities of materials in the ternary system Na2O - Bi2O3. TiO2 and its binary subsystems.