Phase evolution and correlation between tolerance factor and electromechanical properties in BNT-based ternary...

In this work, the structure of end-member Bi(Me0.5Ti0.5)O3 (Me = Zn, Ni, Mg, Co) was calculated through a first-principles method and lead-free piezoelectric ternary systems (0.94 -x)(Bi0.5Na0.5)TiO3-0.06BaTiO3-xBi(Me0.5Ti0.5)O3 (Me = Zn, Ni, Mg, Co) (BNT-BT-Bi(Me0.5Ti0.5)O3) were designed to achieve a large strain response for actuator applications. Composition-driven phase transition characteristics and the resulting associated piezoelectric and electromechanical properties were systematically investigated, and schematic phase diagrams were constructed. XRD measurements, Raman spectra analysis and temperature-dependent polarization and strain hysteresis loops indicate that Bi(Me0.5Ti0.5)O3 substitution induces a phase transformation from a ferroelectric rhombohedral to an ergodic relaxor pseudo-cubic phase, accounting for the large strain response (>0.3%) with a high normalized strain Smax/Emax (≥550 pm V(-1)) at around the corresponding critical composition in the vicinity of room temperature. In addition, correlations between the tolerance factor t of the added end-member, the calculated tetragonality and the morphotropic phase boundary (MPB) composition were sought. In comparison to other reported BNT-based systems, there is a noticeable correlation between the MPB composition and the calculated tetragonality of the end-member Bi(Me0.5Ti0.5)O3, and the t value corresponding to the formation of the MPB composition is approximately 0.981 in the present ternary system with low tolerance factor end-members. As a result, we believe that the general correlations and design principles obtained from the present comprehensive research will be effective to predict the approximate MPB region quickly in BNT-based ceramics with an excellent actuating performance.