Structural transition and giant strain induced by A- and B-site concurrent donor doping in Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO 3 –SrTiO 3 ceramics Rizwan Ahmed Malik a , Ali Hussain a , Jamil Ur Rahman a , Adnan Maqbool a , Tae-Kwon Song a , Won-Jeong Kim b , Sun-Youn Ryou c , Myong-Ho Kim a,n a School of Advanced Materials Engineering, Changwon National University, Gyeongnam 641-773, Republic of Korea b Department of Physics, Changwon National University, Gyeongnam 641-773, Republic of Korea c Department of Advanced Materials Science & Engineering, Sun-Moon University, Chungnam 336-708, Republic of Korea article info Article history: Received 30 September 2014 Accepted 20 December 2014 Available online 30 December 2014 Keywords: Ferroelectrics Piezoelectric materials Field-induced strain abstract Lead-free 0.96[{Bi 0.5 (Na 0.84 K 0.16 ) 0.5 } 1Àx La x (Ti 1 Ày Nb y )O 3 ]–0.04SrTiO 3 (BNKLTN–ST, with x, y ¼0.00–0.030) ceramics were synthesised by conventional solid state reaction (CSSR) method. Results showed that A- and B-site concurrent donor doping into BNKT–ST ceramics induces a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic phase with a significant disruption of the long-range ferroelectric order. At an optimal composition (x, y ¼0.010), a giant field- induced strain of S ¼0.38% was achieved at an applied field of 5 kV/mm, with a corresponding normalized strain (S max /E max ) of 760 pm/V. Besides this, at further low field of 4 kV/mm, the S max /E max value slightly decreased to 725 pm/V, which is still highest among those obtained in non-textured lead- free BNT-based ceramics at such a low field. & 2015 Elsevier B.V. All rights reserved. 1. Introduction In the last decade, bismuth sodium titanate Bi 0.5 Na 0.5 TiO 3 (BNT) ceramics have attracted particular attention as a promising lead- free piezoelectric materials due to environmental concerns [1,2].A number of compositional modifications made in BNT-based solid solutions to improve their piezoelectric properties [3–7]. Jo et al. suggested that the abnormally large strain in BNT-based ceramics under electric fields originates from the reversible phase transition of a nonpolar (NP) phase to a ferroelectric (FE) phase due to their comparable free energies [8], despite the fact that the NP phase has recently been identified as an ergodic relaxor (ER) [9,10]. Indeed, various analysis into the crystalline phases and microstructure of BNT-based ceramics have successfully demonstrated reversible electric field-induced phase transitions [11–14]. Recently, it has been revealed that the large strain behaviour is related to the nucleation of a mixture of nanodomains, which disturb the long- range ferroelectric order rather than creating a long-range phase transition [15,16]. It has also been known that BNT-based ceramics require a relatively high electric field of over 7 kV/mm for phase transition to achieve a large strain [2–8], which limits the practical application of BNT-based ceramics. Previous studies on BNT-based materials [2–10] suggest that enhancement of the electric field-induce strain (EFIS) is accompanied by a disruption of the ferroelectric order by: the incorporation of a suitable A-site [17] or B-site, [5,6] concurrent doping of both the A- and B-sites [18], or by end members of ABO 3 type perovskites [19]. However, effect of A- and B-site concurrent donor doping in BNT- based ceramics has not been explored yet. In this letter, we report large strain response in novel BNKLTN–ST system through A- and B- site concurrent doping. 2. Materials and methods The BNKLTN–ST (x ¼ 0.00–0.030) piezoelectric ceramics were synthesised by CSSR technique using Bi 2 O 3 , Na 2 CO 3 , TiO 2 ,K 2 CO 3 , SrCO 3 , La 2 O 3 (99.9% Sigma Aldrich Co. St. Louis, MO) and Nb 2 O 5 (99.95%, Cerac Specialty Inorganicas) as starting materials. These powder mixtures were ball-milled for 24 h in ethanol with zirconia balls as milling media. The slurries were dried and calcined at 850 1C for 2 h. Disk-shaped ceramic specimens of 10 mm diameter were prepared by compacting the calcined power at 98 MPa. These were sintered at 1160 1C for 2 h in covered alumina crucibles. Electrical characterizations were performed according to our previously pub- lished methods [19]. 3. Results and discussion The X-ray diffraction patterns obtained from each of the BNKLTN– ST samples within a 2θ range of 20–701 are shown in Fig. 1. BNKLTN–ST Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.12.104 0167-577X/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ82 55 213 3711. E-mail address: mhkim@changwon.ac.kr (M.-H. Kim). Materials Letters 143 (2015) 148–150