BaTiO₃-based piezoelectrics: Fundamentals, current status, and perspectives This review provides an overview of the fundamentals, current state of knowledge, and future directions for barium titanate-based piezoelectrics. The article discusses the crystallography, thermodynamics, and concepts necessary to understand piezoelectricity and ferroelectricity in BaTiO₃. Strategies for optimizing piezoelectric properties through microstructure control and chemical modification are introduced. The synthesis, microstructure, and phase diagrams of BaTiO₃-based piezoelectrics are systematically reviewed, along with their functional and mechanical properties. The most important materials treated include the (Ba,Ca)(Zr,Ti)O₃, (Ba,Ca)(Sn,Ti)O₃, and (Ba,Ca)(Hf,Ti)O₃ solid solution systems. The technological relevance of BaTiO₃-based piezoelectrics is discussed, along with potential market indicators. Perspectives on future research and promising applications of these materials are presented. BaTiO₃ (BT) was the first polycrystalline ceramic material discovered to exhibit ferroelectricity. During the 1950s, it was considered a serious candidate for piezoelectric transducer applications. However, PZT, which has better piezoelectric properties and a higher Curie temperature, was discovered soon after. This development reduced the interest in BT for piezoelectric applications. In 2009, Liu and Ren reconsidered the potential of BT-based materials for piezoelectric applications. The outstanding piezoelectric properties found in Ca- and Zr-modified BT resulted in a greater volume of work related to lead-free piezoelectrics. The discovery of other BT-based piezoelectrics with outstanding piezoelectric properties followed, albeit restricted to a limited temperature range. As of 2017, after 16 years of continuous worldwide research effort, the development of lead-free piezoelectrics is entering a mature stage. However, several challenges remain. No material currently encompasses the set of functional properties of PZT required for the broad range of piezoelectric applications in which it is implemented. Alkaline-niobate-based compositions possess piezoelectric properties generally lower than PZT, although the temperature stability of the piezoelectric properties is the best among lead-free materials. Alkaline-bismuth-titanate-based materials feature only moderate small signal piezoelectric properties with acceptable temperature stability. Thus, they have been thoroughly investigated for applications working in the large signal off-resonance regime due to their giant strain. Early works demonstrate, however, that the giant strain typically occurs at too high electric field for commercial applications. In contrast, BT-based materials have attracted attention dueBaTiO₃-based piezoelectrics: Fundamentals, current status, and perspectives This review provides an overview of the fundamentals, current state of knowledge, and future directions for barium titanate-based piezoelectrics. The article discusses the crystallography, thermodynamics, and concepts necessary to understand piezoelectricity and ferroelectricity in BaTiO₃. Strategies for optimizing piezoelectric properties through microstructure control and chemical modification are introduced. The synthesis, microstructure, and phase diagrams of BaTiO₃-based piezoelectrics are systematically reviewed, along with their functional and mechanical properties. The most important materials treated include the (Ba,Ca)(Zr,Ti)O₃, (Ba,Ca)(Sn,Ti)O₃, and (Ba,Ca)(Hf,Ti)O₃ solid solution systems. The technological relevance of BaTiO₃-based piezoelectrics is discussed, along with potential market indicators. Perspectives on future research and promising applications of these materials are presented. BaTiO₃ (BT) was the first polycrystalline ceramic material discovered to exhibit ferroelectricity. During the 1950s, it was considered a serious candidate for piezoelectric transducer applications. However, PZT, which has better piezoelectric properties and a higher Curie temperature, was discovered soon after. This development reduced the interest in BT for piezoelectric applications. In 2009, Liu and Ren reconsidered the potential of BT-based materials for piezoelectric applications. The outstanding piezoelectric properties found in Ca- and Zr-modified BT resulted in a greater volume of work related to lead-free piezoelectrics. The discovery of other BT-based piezoelectrics with outstanding piezoelectric properties followed, albeit restricted to a limited temperature range. As of 2017, after 16 years of continuous worldwide research effort, the development of lead-free piezoelectrics is entering a mature stage. However, several challenges remain. No material currently encompasses the set of functional properties of PZT required for the broad range of piezoelectric applications in which it is implemented. Alkaline-niobate-based compositions possess piezoelectric properties generally lower than PZT, although the temperature stability of the piezoelectric properties is the best among lead-free materials. Alkaline-bismuth-titanate-based materials feature only moderate small signal piezoelectric properties with acceptable temperature stability. Thus, they have been thoroughly investigated for applications working in the large signal off-resonance regime due to their giant strain. Early works demonstrate, however, that the giant strain typically occurs at too high electric field for commercial applications. In contrast, BT-based materials have attracted attention due