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Phase structures and electrical properties of lead-free piezoelectric (1
x)(Bi0.5Na0.5)TiO3–x (K0.5Na0.5)NbO3 (BNT–xKNN) ceramics with 0.08 # x # 0.19 were systematically investigated. Results showed that a phase transition from a tetragonal to a pseudocubic phase occurred in this system, as KNN content increases. The addition of KNN shifted both the depolarization temperature Td and rhombohedral–tetragonal phase transition temperature TR-T to lower temperatures and tended to enhance the relaxor behavior of the ceramics, which was well explained by the microdomain–macrodomain transition theory with calculating criterion K. At x 5 0.10–0.11, Td reached room temperature (RT), which accordingly induced an enhancement of the unipolar strain that peaks at a value of 0.22% was obtained. Furthermore, as the compositions (x 5 0.12–0.15) have Td below RT, samples exhibited high electrostrictive response with large electrostrictive coefficient Q33 (0.017–0.019 m4/C2) and good thermostability comparable with that of traditional Pb-based electrostrictors.

I. INTRODUCTION

Much recent research has focused on the development of Bi-based piezoelectrics as environmentally compatible alternatives to lead-containing materials such as Pb(ZrxTi1
x)O3 (PZT) since the Bi31, like the Pb21 ion, is highly polarizable due to a 6s2 lone electron pair.1,2 Compounds such as (Bi0.5Na0.5)TiO3 (BNT) and its solid solutions with BaTiO3 (BT), (Bi0.5K0.5)TiO3 (BKT), (K0.5Na0.5)NbO3 (KNN), and other perovskites exhibit usable piezoelectric properties3–11 and are considered as candidates to replace Pb-based materials.6,9,11 One of the criteria for piezoelectrics intended for actuator applications is the amount of strain S that can be induced with moderate electric fields. Recently, large strain response even surpassing that of standard PZT compositions was found in BNT-based binary and ternary system,10,12–19 which provides new perspectives on the development of lead-free piezoceramics. As these systems have a lot of potential, a good understanding of their phase diagrams is crucial for further development and improvement, especially, since a large part of the observable strain is not due to the normal piezoelectric effect but to a phase transition from a ferroelectric (FE) phase to an antiferroelectric (AFE) (the so-called “AFE” has been clarified with a “nonpolar” phase20; the existence of “AFE” phase proposed based on the observation of a double P-E loop is merely a consequence of an electric-field-induced relaxor-to-FE transition21) induced by external electric fields.5 When the a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.328 J. Mater. Res., Vol. 27, No. 23, Dec 14, 2012

phase boundary between FE and “AFE” phases can be achieved at ambient temperature, the strain always reaches its maximum value at the FE–“AFE” phase transitions.13 Based on the above, BNT-based binary systems with large strain, e.g., BNT–KNbO3,10 BNT–Ba(Al1/2Sb1/2)O3,14 were reported. On the other hand, high-performance lead-free el