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sis, Microstructure, Dielectric and Ferroelectric Properties of (Na,Bi,K)TiO3 Ceramics G. M. Kalevaa, *, E. D. Politovaa, A. V. Mosunovb, S. Yu. Stefanovichb, A. B. Loginovb, D. R. Kashapovc, A. A. Efremovad, T. A. Koksharovac, Yu. V. Khripunovd, and B. A. Loginove aSemenov

Institute of Chemical Physics, Moscow, 119991 Russia Moscow State University, Moscow, 119991 Russia c Sirius Educational Center, Sochi, 354003 Russia d Oryol State University, Oryol, 302026 Russia e National research University MIET, Moscow, 124498 Russia *e-mail: [email protected] b

Received November 23, 2019; revised December 14, 2019; accepted January 12, 2020

Abstract—Single-phase ceramic samples with the perovskite structure [(Na0.5Bi0.5)1 – xKx] TiO3 (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) are prepared by the method of solid-state synthesis and their structure, microstructure and dielectric and ferroelectric properties are studied. An increase in the unit-cell parameters of the perovskite lattice as a result of an increase in the K-cation content in accordance with the ratio of the radii of cations in the A sublattice of the perovskite structure is proved. The method of second-harmonic generation confirms that the introduction of potassium cations in the A positions of the perovskite lattice promotes enhancement of the ferroelectric properties of the studied samples. Dielectric-relaxation effects are revealed, which indicate the presence of vacancies in the oxygen sublattice. Keywords: sodium bismuth titanate, ceramics, perovskite structure, microstructure, ferroelectrics, dielectric properties DOI: 10.1134/S1027451020040102

INTRODUCTION Oxide-ion conductors are widely used in many important technological instruments and devices, such as solid oxide fuel cells, oxygen sensors, pumps, and oxygen-permeable membranes [1–5]. The last decade has been marked by extraordinary progress in the study of new oxide-ion conductors that meet the requirements of creating environmentally friendly energy sources, sensors and other devices. Scientists are faced with the task of finding lead-free technological materials in the field of creating eco-friendly ceramic piezoelectric materials for applications in sensors, actuators and transducers in view of the current acute need to replace lead zirconate titanate PbZr1 – xTixO3 (LZT), which has been most widely used for several decades. The toxicity of lead as an element that is part of almost all widely used piezoelectrics, which causes significant harm to the environment, has motivated the search and development of lead-free materials over the last decade. Therefore, a new family of oxide-ion conductors based on a ferroelectric with the perovskite structure, i.e., sodium bismuth titanate Na0.5Bi0.5TiO3 (SBT), is one of the most promising candidates for the substitution of PZT and the formation of solid solu-

tions with other ferroelectric materials, such as BaTiO3 and K1 – xNaxNbO3, and piezoelectric and high-temperature dielectric capacitor applications [6–8]. High ion conductivity was found in the non-