Effect of composition on the structure and piezoelectricity of Na0.5Bi4.5Ti4O15-based flexoelectric-type polar ceramics

► Na0.5Bi4.5Ti4O15-based flexoelectric-type polar ceramics is first fabricated. ► Effect of composition on the structure and piezoelectricity of ceramics is studied. ► We propose the possible poling mechanism of these polar ceramics. ► These polar ceramics possess depoling temperatures higher than 700°C. Flexoelectric-type polar ceramics might specify a class of heterogeneous ceramics that possess permanent, macroscopic, flexoelectric-type polarization and thus can exhibit both direct and converse piezoelectric effects without undergoing an electrical poling process. Here we show, by studying series of Na0.5Bi4.5Ti4O15-based nominal compositions, that flexoelectric-type polar ceramics can be synthesized by adding an appropriate amount of excess Bi2O3 to Na0.5Bi4.5Ti4O15, whereas the addition of only excess Na2CO3 or excess TiO2 to Na0.5Bi4.5Ti4O15 does not have this effect. The obtained Na0.5Bi4.5Ti4O15+xBi2O3 flexoelectric-type polar ceramics all contain Na0.5Bi4.5Ti4O15 and Bi12TiO20 phases, and their macroscopic polarization might originate from the partial alignment of distorted TiO6 octahedra and BiO5 polyhedra that are located within the grain boundary amorphous phases. Because the sodium-modified Na0.5Bi4.5Ti4O15+2.25Bi2O3 flexoelectric-type polar ceramics were found to have a piezoelectric coefficient, d33, larger than 8pC/N and a depoling temperature higher than 700°C, they are thought to have potential piezoelectric applications in the high-temperature region.