Structure and temperature-dependent phase transitions of lead-free Bi 1/2 Na 1/2 TiO 3 –Bi 1/2 K 1/2 TiO 3 –K 0.5 Na 0.5 NbO 3 piezoceramics Eva-Maria Anton a) Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany Ljubomira Ana Schmitt Institute of Applied Geosciences, Technische Universität Darmstadt, 64287 Darmstadt, Germany Manuel Hinterstein Institut für Werkstoffwissenschaft, Technische Universität Dresden, 01069 Dresden, Germany Joe Trodahl MacDiarmid Institute of Advanced Materials and Nanotechnology, Victoria University, Wellington, New Zealand Ben Kowalski Department of Materials Science & Engineering, University of Florida, Gainesville, Florida 32611 Wook Jo Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany Hans-Joachim Kleebe Institute of Applied Geosciences, Technische Universität Darmstadt, 64287 Darmstadt, Germany Jürgen Rödel Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany Jacob L. Jones Department of Materials Science & Engineering, University of Florida, Gainesville, Florida 32611 (Received 26 November 2011; accepted 21 May 2012) Structure and phase transitions of (1 À y)((1 À x)Bi 1/2 Na 1/2 TiO 3 –xBi 1/2 K 1/2 TiO 3 )–yK 0.5 Na 0.5 NbO 3 (x; y) piezoceramics (0.1 # x # 0.4; 0 # y # 0.05) were investigated by transmission electron microscopy, neutron diffraction, temperature-dependent x-ray diffraction, and Raman spectroscopy. The local crystallographic structure at room temperature (RT) does not change by adding K 0.5 Na 0.5 NbO 3 to Bi 1/2 Na 1/2 TiO 3 –xBi 1/2 K 1/2 TiO 3 for x 5 0.2 and 0.4. The average crystal structure and microstructure on the other hand develop from mainly long-range polar order with ferroelectric domains to short-range order with polar nanoregions displaying a more pronounced relaxor character. The (0.1; 0) and (0.1; 0.02) compositions exhibit monoclinic Cc space group symmetry, which transform into Cc 1 P4bm at 185 and 130 °C, respectively. This high temperature phase is stable at RT for the morphotropic phase boundary compositions of (0.1; 0.05) and all compositions with x 5 0.2. For the compositions of (0.1; 0) and (0.1; 0.02), local structural changes on heating are evidenced by Raman; for all other compositions, changes in the long-range average crystal structure were observed. I. INTRODUCTION Lead-free piezoceramics have attracted wide interest in recent years due to upcoming environmental and health con- cerns against the widely used lead-based piezoceramics. 1–3 Bi 1/2 Na 1/2 TiO 3 (BNT)-based compositions are of partic- ular interest for application due to relatively easy process- ing compared to K 0.5 Na 0.5 NbO 3 (KNN)-based lead-free ceramics. Since the piezoelectric properties and strain under electric field of pure BNT are poor (d 33 ;70 pC/N, 4 unipolar strain S ;0.1% at 8 kV/mm 5 ), modifications are commonly made, e.g., with BaTiO 3 (BT), 6 Bi 1/2 K 1/2 TiO 3 (BKT), 7,8 KNN, 5 or combinations of these systems like BNT–BKT–BT, 9 BNT–BT–KNN, 10 or BNT–BKT–KNN 11 to improve properties. Among those, (1 À y)((1 À x) BNT–xBKT)–yKNN ((BNT–xBKT)–yKNN) ceramics have recently shown promising electrical properties both at and off the morphotropic phase boundary (MPB) at x 5 0.2 of the basic system BNT–xBKT. 11–13 The room temperature (RT) structure of the binary system BNT–xBKT has been widely studied. Like all the other mentioned compositions, it crystallizes in a distorted perovskite structure. Jones et al. 14 performed x-ray diffrac- tion (XRD) and neutron diffraction on BNT–xBKT single a) Address all correspondence to this author. e-mail: anton@ceramics.tu-darmstadt.de DOI: 10.1557/jmr.2012.195 J. Mater. Res., Vol. 27, No. 19, Oct 14, 2012 Ó Materials Research Society 2012 2466