Journal of Alloys and Compounds 484 (2009) 233–238 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Relaxor behavior and dielectric properties of (La, Ta)-modified (K 0.5 Na 0.5 )NbO 3 lead-free ceramics Jigong Hao a , Zhijun Xu a,c,∗ , Ruiqing Chu a,c , Wei Li a,c , Guorong Li b , Qingrui Yin b a College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China b The State Key Lab of High Performance Ceramics and Superfinemicrostructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China c Liaocheng University Renewable Energy and Environment Materials Research Center, China article info Article history: Received 10 February 2009 Received in revised form 26 March 2009 Accepted 29 March 2009 Available online 7 April 2009 Keywords: Ferroelectrics Phase transitions Relaxor Dielectric properties Piezoelectricity abstract Lead-free relaxor ceramics with composition (Na 0.5 K 0.5 ) 1-3x La x Nb 0.95 Ta 0.05 O 3 (KNTN-La x ) were prepared by the conventional solid-state route. The ferroelectric relaxor behavior and dielectric properties of KNTN- La x ceramics were systemically investigated. X-ray diffraction analysis showed that the crystal structure of the ceramics changed from orthorhombic to pseudo-cubic with increasing La-doping level. Dielectric study revealed that KNTN-La x ceramics underwent a transition from a normal ferroelectric to relaxor-like ferroelectric. Furthermore, P–E hysteresis loops, which were of slim-loop type with small remnant polar- ization values in the composition range of 0.015 ≤ x ≤ 0.03 confirmed the ferroelectric relaxor behavior of KNTN-La x ceramics. The samples of KNTN-La x ceramics in the composition range from x = 0 to 0.015 exhib- ited good electrical properties, piezoelectric constant d 33 of 98–119pC/N and electromechanical coupling coefficients k p of 33.2–37.0%, respectively. The results show that the series of KNTN-La x ceramics is one of the promising lead-free materials for electromechanical transducer applications. © 2009 Elsevier B.V. All rights reserved. 1. Introduction In recent years, relaxor ferroelectrics with complex perovskite structure have received much attention, as these materials can be widely used in fabrication of multilayer ceramic capacitors, electrostrictive actuators, and electromechanical transducers [1–6]. Until now, the main work for relaxors has being focused on com- plex lead-based perovskite relaxors, such as (Pb, La)(Zr, Ti)O 3 , Pb(Zn 1/3 Nb 2/3 )O 3 –PbTiO 3 and Pb(Co 1/3 Nb 2/3 )O 3 [2–4] because of their excellent dielectric, piezoelectric and electrostrictive prop- erties which are useful in actuating and sensing applications. However, these compounds have the obvious disadvantages associ- ated with the volatility and toxicity of PbO which can contaminate the environment and damage human health. Therefore, there is a great need to develop lead-free relaxor ferroelectrics with excellent piezoelectric properties for replacing the lead-containing ferro- electrics in various applications. (K 0.5 Na 0.5 )NbO 3 is a ferroelectric having K + and Na + complex on the A-site of ABO 3 perovskite structure with a rhombohedral sym- metry. Recently, considerable attention for lead-free piezoelectric ceramics has been given to (K 0.5 Na 0.5 )NbO 3 (KNN)-based ceramics ∗ Corresponding author at: College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China. Tel.: +86 635 8230923; fax: +86 635 8230923. E-mail address: zhjxu@lcu.edu.cn (Z. Xu). because of their relatively strong piezoelectric properties and high Curie temperature of 420 ◦ C [7–11]. However, difficulty in sinter- ing of the ceramics and control of stoichiometry leads to deviation from excellent properties. Now, introducing modification additions to form solid solutions with KNN by replacing the A and/or the B- site cation of the ABO 3 perovskite structure has been widely used to obtain dense ceramics with excellent properties [1,7–13]. Recently, it is reported that high dense KNN-based ceramics with excellent electrical properties have been found in some sys- tems such as (Na 0.5 K 0.5 )NbO 3 –SrTiO 3 [7], (Na 0.5 K 0.5 )NbO 3 –BaTiO 3 [1,12] and (Na 0.5 K 0.5 )NbO 3 –LiSbO 3 [13]. In addition, these modified KNN-based systems also exhibit a typical dielectric relaxor behav- ior. In our work, a new high dense KNN-based relaxor ferroelectric doped with La and Ta has been synthesized successfully by the conventional solid-state reaction method. The ferroelectric relaxor behavior of (La, Ta)-modified (K 0.5 Na 0.5 )NbO 3 ceramics was stud- ied in detail. Meanwhile, the effects of the additives on the phase transition behavior, microstructures and electrical properties of the KNTN-La x ceramics were studied. 2. Experimental (Na0.5K0.5)1-3xLaxNb0.95Ta0.05O3 (KNTN-Lax, x =0, 0.01, 0.015, 0.02 and 0.03) ceramics were prepared by the conventional solid-state reaction method. High purity Na2CO3,K2CO3, Nb2O5, La2O3 and Ta2O5 (purity over 99%) powders were used as starting materials. For each composition, the starting materials were weighed according to the stoichiometric formula and ball-milled for 8 h in ethanol. After dry- ing, the calcination was carried out at 850 ◦ C for 4 h and ball-milled again for 8 h. The 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.174