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Large piezoelectricity in ternary lead-free single crystals

Since industry-standard piezoelectric materials contain toxic lead, such as lead zirconium titanate ceramics, a rapid surge in research on lead-free piezoelectric materials is occurring with regards to environmental safety and human health. Here, we report a ternary lead-free (Na0.5Bi0.5)TiO3-BaTiO3-(K0.5Na0.5)NbO3 single crystal with a large field induced strain of ~0.9% and a giant piezoelectric coefficient d33 of 840 pC N-1 at room temperature. Our results reveal that the electric field induced phase transition and phase boundary between energically comparable polar states (i.e., R3c and P4mm) plays an important role in achieving these ultrahigh piezoelectricity. Transmission electron microscopy and scan probe microscopy were employed to verify their weak-polar ferroelectric R3c and P4bm structure in the ground state. These unprecedentedly high piezoelectric properties make lead-free ternary single crystals leading candidate for piezoelectric-based device applications, especially towards the era of smart homes and implantable medical devices.

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Figure 1. Piezoelectric coefficient and structure of [001]-oriented NBT-6BT-KNN crystals under various fields. (a) Comparison of piezoelectric constant d33 at 300 K among ternary NBT-6BT-KNN single crystals, previously reported other lead-free piezoelectrics and market dominating PZT family. The lead-free piezoelectrics includes NBT-6BT-2KNN and NBT-6BT-3KNN ceramics, Bi-layer (bismuth-layered ferroelectrics), BaTiO3 single crystals, NBT-based Mn-doped NBT-6BT single crystal, textured KNN based (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 ceramics, KNN-based (1−x)(K1yNay)(Nb1zSbz)O3−xBi0.5(Na1wKw)0.5ZrO3 and (1−xy)K1−w Naw Nb1−zSbzO3−yBaZrO3−x-Bi0.5K0.5HfO3 ceramics,  and BZT-BCT [Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3] ceramics. (b) The corresponding piezoelectric constant d33 as a function of poling field Ep. The region in color mixture indicates the phase boundary and phase coexistence between weak polar ferroelectric R3c/P4bm and strong polar ferroelectric P4mm tetragonal phases. (c) The ground state with coexistence of weak polar rhombohedral R3c and tetragonal P4bm structures, and (d) the poled state along [001]PC with weak polar rhombohedral R3c and ferroelectric tetragonal P4mm structures (i.e., the volume fraction of R3c is much lower than that of P4mm). 



Yaojin Wang (汪尧进) Laboratory of Advanced Sensitive Materials and Devices
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Department of Materials Science and Engineering.Nanjing University of Science and Technology