Relaxor-ferroelectric transitions: Sodium bismuth titanate derivatives
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Introduction Interest in lead-free piezoelectrics and ferroelectrics has grown due to concerns about the toxicity of lead and resultant environmental legislation implemented in several countries1–3 (also see the Bell and Deubzer article in this issue).4 Sodium bismuth titanate, Na1/2Bi1/2TiO3 (NBT), and its solid solutions with other perovskites are some of the most heavily studied lead-free ferroelectric ceramic materials.5 NBT, discovered by Smolenskii et al.6 in 1961, is a relaxor ferroelectric. Relaxor ferroelectrics are a class of materials that exhibit a frequency dependence in the temperature of maximum permittivity, Tmax. NBT has a Tmax of 320°C with an additional phase transition at a rather low temperature of 200°C.5 NBT ceramics exhibit remanent polarization Pr = 38 µC/cm2, coercive field Ec = 73 kV/cm,* and piezoelectric coefficient d33 = 70 pC/N.7 NBT was originally assumed to have a cubic perovskite structure for purposes of calculating the lattice constant.6 Modern characterization techniques have revealed the structure to be much more complex. *Saturation was not reached.6
Modification of NBT-based systems improves many of their properties. For instance, alloying with BaTiO3 or (K1/2Bi1/2)TiO3 yield piezoelectric coefficients with nearly, or more than double, the values of unmodified NBT.5,8 Most optimized coefficients are found at a morphotropic phase boundary (MPB), which is a vertical or near-vertical boundary separating different polymorphic phases. In this article, we discuss the complex structure of NBT and its solid solutions, including their interesting relaxor and functional properties as well as their field-induced structural phase transitions, with a focus on polycrystalline materials.
The complex structure of Na1/2Bi1/2TiO3
The structural description of NBT has changed several times since its discovery 57 years ago. The structure is elusive because of several key features across multiple length scales that cannot be described in a single unit. Though the features of the NBT structure are central to the resultant properties of NBT and NBT-based solid solutions, some details of the structure are still under debate. At the smallest length scale, NBT owes much of its unique nature to the electronic structure of bismuth, as originally
Alisa R. Paterson, Department of Materials Science and Engineering, North Carolina State University, USA; [email protected] Hajime Nagata, Department of Electrical Engineering, Tokyo University of Science, Japan; [email protected] Xiaoli Tan, Department of Materials Science and Engineering, Iowa State University of Science and Technology, USA; [email protected] John E. Daniels, School of Materials Science and Engineering, University of New South Wales, Australia; [email protected] Manuel Hinterstein, Institute for Applied Materials, Karlsruhe Institute of Technology, Germany; [email protected] Rajeev Ranjan, Department of Materials Engineering, Indian Institute of Science, India; [email protected] Pedro B. Groszewicz, Fachbereich Chemie,
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