Impedance Analysis and Conduction Mechanisms of Lead Free Potassium Sodium Niobate (KNN) Single Crystals and Polycrystals: A Comparison Study

Abstract

Impedance spectroscopy (IS) is used to compare defects and charge transport in (001) oriented K0.5Na0.5NbO3 (KNN) single crystals and its polycrystalline counterpart. KNN single crystals show lower conductivity than polycrystalline counterpart from room temperature up to 200 °C, but above this temperature polycrystalline KNN displays lower conductivity. The low temperature (T < 200 °C) behavior reflects the different processing conditions of ceramics and single crystals, which account for less defects prone to charge transport in the case of single crystals. As temperature increases (T > 200 °C) single crystals become more conductive than polycrystalline samples in which grain boundaries act as barriers to charge transport. For even higher temperatures, the conductivity difference between both is increased because of the contribution of ionic conduction in single crystals. Indeed the values of activation energy calculated to the high temperature range (T > 300 °C) were 1.60 and 0. 97 eV, confirming the charge transport because of ionic conduction and ionized oxygen vacancies in single crystals and polycrystalline KNN, respectively. It is suggested that single crystals with low defects content and improved electromechanical properties could be a better choice for room temperature applications, though at high temperatures less conductive ceramics may be the choice, depending on the targeted use.