Journal of the Korean Physical Society, Vol. 71, No. 12, December 2017, pp. 979∼985 Thermally-Driven Unequal Cation Vacancy Formation and Its Effect on the Dielectric Properties in K 0.5 Na 0.5 NbO 3 Ceramics Gowoon Kim, Hyeonjun Kong, Dongjin Kim, Hosu Lee and Hyoungjeen Jeen ∗ Department of Physics, Pusan National University, Busan 46241, Korea Hosun Lee and Mijung Jeen Pusan National University’s Core Research Facility, Pusan National University, Busan 46241, Korea Sungwook Mhin Korea Institute of Industrial Technology, Incheon 21999, Korea (Received 4 September 2017, in final form 18 October 2017) The formation of cation vacancies can be useful for electro-chemical devices. In this regard, an understanding of vacancy formation is an important subject for enhancing current electrochemical devices and for developing next generation energy devices. In this work, we chose the well-known lead-free ferroelectric (K0.5Na0.5)NbO3 (KNN) as a model system to understand both the formation of cation vacancies and the relationship between cation vacancies and the physical properties. We studied sintering-duration dependence of the dielectric properties and the cation contents of KNN ceramics at the temperatures near the melting point of KNN. The difference in sintering duration led to a drastic change in the dielectric property, as well as to the creation of cation vacancies. Interestingly, we observed unequal evaporation of cations during the sintering process, which was confirmed by the data obtained from laser-induced breakdown spectroscopy. In addition, we found more drastic changes in the imaginary dielectric constant, which were likely due to a decrease in ionic conducting species, such as K and Na, in KNN. PACS numbers: 77.84.Ek, 77.84.-s, 77.22.-d Keywords: Lead-free ferroelectrics, Cation vacancies, Laser-induced breakdown spectroscopy, Dielectric mea- surements DOI: 10.3938/jkps.71.979 I. INTRODUCTION Vacancy formation has usually been prevented, be- cause it leads us away from the desired materials’ proper- ties [1–3]. However, depending on the materials, vacant sites can be useful places not only to store mobile ions such as alkali metal ions but also to function as effective doping sites to tune electronic and/or magnetic ground states [4–6]. Recently, many research works have taken place especially by controlling the content of anions, i.e. oxygen ions [7–10]. However, not many reports for un- derstanding cation vacancies are available in the liter- ature. In this regard, (K 0.5 Na 0.5 )NbO 3 (KNN) would be an ideal model to be able to understand cation va- cancy formation and its effect on the physical properties, because it is a well-known material to form potassium (K) and sodium (Na) vacancies through the conventional ∗ E-mail: hjeen@pusan.ac.kr solid-state reaction [11–13]. The importance of KNN can be found in the fields of ferroelectrics, because it is one of the promising candi- dates to replace Pb(Zr,Ti)O 3 (PZT) [14–16]. However, concerns about harmful effects of PZT have recently been raised. This has led to a search for lead-free ferroelectric materials [17]. Among many candidate materials, KNN is not toxic, and its properties are comparable to those of PZT [18,19]. However, the creation of defect-free KNN has been a formidable task because of the high reactivity of KNN to moisture and the volatility of K and Na in high-temperature treatments [20]. In order to overcome these issues, some studies have tried to reduce the calci- nation temperature to reduce the volatilization of K and Na [21]. Also, excesses of K and Na have been added in the same amounts in the mixing process to compen- sate for K and Na evaporation [22,23]. These studies of- fer useful strategies for making stoichiometric KNN [11, 24]. However, because K and Na are different elements, their evaporation rates will be likely different. Thus, in- pISSN:0374-4884/eISSN:1976-8524 -979- ©2017 The Korean Physical Society