Ferroic glass behavior in (Bi,Na)TiO3-based lead-free electroceramics

Ferroic glass materials, that is, relaxors, spin glasses, and strain glasses attract special attention because of their intriguing physical properties and potential for novel technological applications. The ferroic glass state emerges from a gradual freezing process of the disordered ferroic state, resulting in the so-called ferroic glass transition, which has unique features absent in a typical ferroic transition [1]. For instance, the strain glass behavior of materials can be identified by a peculiar thermal and frequency evolution of the mechanical response around the phasetransition region, measured from their anelastic properties. This mechanical phenomenon has been reported mainly for metallic systems [2,3], and recently, also in a few ceramics systems [4,5]. In the last two decades, Bi0.5Na0.5TiO3-based (BNT) lead-free electroceramics have received considerable attention, particularly the binary xBi0.5Na0.5TiO3- yBaTiO3 (BNT-BT) solid solutions because of their technological potential [6]. However, the ternary xBi0.5Na0.5TiO3-yBi0.5K0.5TiO3-zBaTiO3 (BNT-BKT-BT or BNBK) system has been less studied despite its possible technological applications and the interesting phenomenology behind a more complex system. Relaxor characteristics have been also reported for several (Bi,Na)TiO3- based ferroelectric compositions [7], however, despite ferroic glass states manifest manly in disordered systems, the possible presence of these states (relaxor and/or strain glass) in ternary BNBK compositions has not been still investigated. In this work, several characterization techniques are used to demonstrate the occurrence of simultaneous relaxor and strain glass states in the ternary lead-free BNBK system. Hysteresis and current density loops measurements at different temperatures for a BNBK ceramic showed typical characteristics observed in BNT-based ceramics containing the tetragonal P4bm phase. Above 400 K, slim loops like those of ferroelectric relaxors are observed. For temperatures between 400 and 450 K, a phase transition with simultaneous relaxor and strain glass character is revealed by the dielectric and anelastic characterizations (Fig. 1). The minimum of elastic modulus shifts to higher temperatures as the frequency increases, obeying the Vogel-Fulcher relation, which clearly shows the occurrence of a strain glass state in this material. Similar characteristics are observed in the imaginary part of the dielectric permittivity, which demonstrates the relaxor character of this phase transition. The origin of this phenomenon was elucidated with the help of structural characterizations, such as Raman spectroscopy and high-resolution synchrotron Xray diffraction, at different temperatures. Coexistence of the tetragonal P4mm and P4bm phases occurs at room temperature with approximate phase fractions of 75 and 25%. As the temperature increases, the P4mm tetragonal phase begins to transform into cubic Pm-3m near 330 K until about 450K, where it disappears. From this temperature, the material is composed of P4bm PNRs embedded in a non-polar cubic matrix, which is the origin of the observed relaxor state. The typical octahedra tilts of the P4bm PNRs result in spontaneous strain and hence, a tilt strain glass state also arises. Knowing the origin of the phenomenon observed in this BNT-based ternary system, new lead-free ferroic glass materials with improved properties can be designed.