Please cite this article in press as: König J, Suvorov D. Evolution of the electrical properties of K 0.5 Bi 0.5 TiO 3 as a result of prolonged sintering. J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.04.003 ARTICLE IN PRESS +Model JECS-10076; No. of Pages 9 Available online at www.sciencedirect.com ScienceDirect Journal of the European Ceramic Society xxx (2015) xxx–xxx Evolution of the electrical properties of K 0.5 Bi 0.5 TiO 3 as a result of prolonged sintering J. König ∗ , D. Suvorov Advanced Materials Department, Joˇ zef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Received 5 February 2015; received in revised form 28 March 2015; accepted 2 April 2015 Abstract Lead-free piezoelectrics are materials exhibiting great potential, particularly because of their extended strains under high fields. However, the reported electrical properties remain inconsistent due to difficulties related to sample preparation. Here, we describe peculiarities of the synthesis procedure and their influence on the electrical properties of K 0.5 Bi 0.5 TiO 3 ceramics. This synthesis is characterized by the formation of K- and Bi-rich secondary phases and a reaction with humidity, which has a strong impact on the dielectric, ferroelectric and piezoelectric properties. These are very much improved after prolonged sintering, ascribed to the elimination of the hygroscopic K 2 O-rich phase. This is reflected in a higher resistivity, density and grain size. The electrical properties are improved in terms of a decrease in the dielectric losses, a two-fold increase in the permittivity value at the maximum, an increase in the remanent polarization to 29 C/cm 2 , and an increase in the small signal d 33 value from 40 to 120 pC/N. © 2015 Elsevier Ltd. All rights reserved. Keywords: K 0.5 Bi 0.5 TiO 3 ; Piezoelectric properties; Solid state synthesis; Dielectric properties 1. Introduction After a decade and a half of intensive searching for a lead- free piezoelectric material that can replace the Pb(Zr x Ti 1−x )O 3 (PZT) family, no such alternative material has been found. Suitable lead-free materials were only found for specific appli- cations [1–4]. These research efforts were largely focused on alkali-niobate- [3] and bismuth-based perovskites [4]. A com- mon feature of these investigations is that the most promising properties were observed in complex systems and over a very narrow compositional range [3,5–8]. In the past few years, however, there was a considerable improvement in our under- standing of the unusual phase transitions of promising lead-free compounds and the morphotropic compositions of their solid solutions [1,9–14]. Nevertheless, the reported properties of mor- photropic compositions, as well as pure compounds, remain ∗ Corresponding author. Tel.: +386 1 477 3762; fax: +386 1 251 9385. E-mail address: jakob.konig@ijs.si (J. König). inconsistent, presumably due to the difficult sample-preparation procedures [15–17]. Regarding the bismuth-based compounds, most attention was given to Na 0.5 Bi 0.5 TiO 3 (NBT) [18], whereas the potassium analogue was generally overlooked. The processing of K 0.5 Bi 0.5 TiO 3 (KBT) ceramics is charac- terized by the demanding synthesis conditions [19,20], which are believed to be the main reason for the scarcity of papers on KBT as well as the reporting of inconsistent results [16,21–23]. The main obstacle to obtaining dense, predominantly single- phase samples is the narrow sintering range limited by thermal decomposition [20]. The synthesis-related phenomena, unless carefully controlled, result in an off-stoichiometry material, an increased content of secondary phases, a low density, partial melting of the sample, and poor electrical properties due to the high conductivity [16,22–25]. The investigation of the forma- tion mechanism of KBT [20] showed that during the calcination stage the K 2 Ti 4 O 9 phase is formed, which at sintering tem- peratures transforms to K 2 Ti 6 O 13 . The reaction also releases a K 2 O-rich phase [30]. The potassium polytitanate phases are very stable and cannot be removed once formed. The optimal http://dx.doi.org/10.1016/j.jeurceramsoc.2015.04.003 0955-2219/© 2015 Elsevier Ltd. All rights reserved.