Review of Phase Equilibria of the System BaO–TiO₂

The system BaO–TiO₂ plays a significant role in the field of functional ceramics owing to the existence of multiple dielectric, piezoelectric, and ferroelectric intermediate phases. Although the first binary phase diagram for this system was introduced in 1951, more than 70 years passed before construction of a complete phase diagram. The present work provides a comprehensive and critical review of the 12 (1951–2024) previously published binary phase diagrams (BaO–TiO₂), covering raw materials, processing conditions, characterization techniques, intermediate phases detected, and invariant points observed. Each diagram is examined in terms of its strengths and weaknesses. These data are supplemented with a brief chronological survey of ancillary ternary systems (BaO–TiO₂–M_xO_y), all of which include the subsystem BaO–TiO₂. These data allow the confirmation of the existence of phase assemblages and their temperature ranges of stability, thereby suggesting the conditions commensurate with metastable and equilibrium phase assemblages. The present review also addresses the ambiguities concerning the homogeneity regions of BaTiO₃, thus clarifying the solubility limits for BaTiO₃ hypostoichiometric and hyperstoichiometric solid solutions. The present work aims to survey the relevant published binary and ternary phase equilibria literature for the purpose of clarifying the existing intermediate phases, their temperature ranges of stability, and the solid solubility homogeneity regions of BaTiO₃.