Piezoelectrics have widespread use in today’s sensor and actuator technologies. However, most commercially available piezoelectric materials, e.g., Pb [ZrxTi1−x] O3 (PZT), are comprised of more than 60 weight percent lead (Pb). Due to its harmful effects, there is a strong impetus to identify new lead-free replacement materials with comparable properties to those of PZT. This review highlights recent developments in several lead-free piezoelectric materials including BaTiO3, Na0.5Bi0.5TiO3, K0.5Bi0.5TiO3, Na0.5K0.5NbO3, and their solid solutions. The factors that contribute to strong piezoelectric behavior are described and a summary of the properties for the various systems is provided.

Bismuth-based ferroelectric ceramics are currently under intense investigation for their potential as Pb-free alternatives to lead zirconate titanate-based piezoelectrics. Na0.5Bi0.5TiO3 (NBT), one of the widely studied compositions, has been assumed thus far to exhibit the rhombohedral space group R3c at room temperature. High-resolution powder x-ray diffraction patterns, however, reveal peak splitting in the room temperature phase that evidence the true structure as monoclinic with space group Cc. This peak splitting and Cc space group is only revealed in sintered powders; calcined powders are equally fit to an R3c model because microstructural contributions to peak broadening obscure the peak splitting.

The crystal structure of sodium bismuth titanate and related compounds is of great interest, as these may form part of a new generation of ferroelectric materials used in a multitude of piezoelectric applications. This work examines the short and long range structure of sodium bismuth titanate in different states of synthesis using X-ray and neutron pair distribution function studies. The average structure of NBT was modeled using the monoclinic Cc space group through a combined structural refinement of X-ray and neutron diffraction data via the Rietveld method. A small box approach was used to model the local structure based on the average structure of the material, as determined from the Rietveld structural refinement, and rule out the presence of local A-site ordering in NBT. A 'box-car fitting' method used to analyze the neutron PDF showed that bond environments change when averaged over different length scales and the calculated bond valence of Bi 3+ , in particular, is different locally from its average value. A model calculated using the Reverse Monte Carlo method allowed the positions of Na + and Bi 3+ to move independently, allowing the determination of their distinctive bonding environments with O 2-. This method revealed that Na + and Bi 3+ have slightly different atomic positions, an effect that may be the origin of the large atomic displacement parameters calculated for the A-site from the * Corresponding Author: email jjones@mse.ufl.edu, telephone 352-846-3788

Mn doping-induced structural and magnetic transformations in the antiferroelectric phase of the Bi1−xNdxFeO3 perovskites J. Appl. Phys. 112, 064105 (2012) Neutron diffraction study of stability and phase transitions in Cu-Sn-In alloys as alternative Pb-free solders J. Appl. Phys. 112, 053520 (2012) Communication: From graphite to diamond: Reaction pathways of the phase transition High resolution powder x-ray diffraction patterns of Na 0.5 Bi 0.5 TiO 3 at selected temperatures were examined to compare structural changes with observed piezoelectric thermal depoling temperatures. The depoling temperatures do not correlate with discrete phase transition temperatures, and therefore, a structural transition is not the origin of thermal depoling. Rather, a correlation is made with an increase in volume fraction of material which does not obey the long-range Cc space group. The origin of the thermal depoling behavior may be the loss of long-range ferroelectric order by a decreasing proportion of the Cc phase or the associated percolation of disordered nano-scale platelets.

Sodium bismuth titanate (NBT) ceramics are among the most promising lead-free materials for piezoelectric applications. This work reports the crystal structure and phase evolution of NBT and Fe-modified NBT (from 0-2 at% Fe) using synchrotron X-ray diffraction and Raman spectroscopy, both at ambient and elevated temperatures. The crystallographic results are discussed with reference to permittivity and piezoelectric thermal depolarization measurements of the same compositions. Changes in the depolarization temperature due to Fe substitution were detected by Raman spectroscopy, and were found to correlate closely with depolarization temperatures obtained from converse piezoelectric coefficient and permittivity measured in situ. The depolarization temperatures obtained from direct piezoelectric coefficient measured ex situ as well as the phase transition temperatures obtained from synchrotron X-ray diffraction were found to be at higher temperatures. The mechanisms underlying the relationship between permittivity and piezoelectric depolarization to structural transitions observed in Raman spectroscopy and X-ray diffraction are discussed.