Tuning the Covalency of A–O Bonds to Improve the Performance of KNN-Based Ceramics with Multiphase Coexistence

Although a room-temperature multiphase coexistence (MPC) strategy improves the piezoelectric coefficient ( d 33 ) of potassium sodium niobate ((K,Na)NbO 3, KNN) ceramics, it still suffers from the dependencies on composition and temperature, making it remain challenging to further improve d 33 and temperature stability of strain for an already-built MPC. Here, we proposed a new route to resolve this issue, that is, tuning the covalency of A–O bonds in an already-built MPC. We chose 0.96(Na 0.60 K 0.40 )(Nb 0.955 Sb 0.045 )O 3 –0.04(Bi 0.5 Na 0.5 )ZrO 3 ceramics as an already-built MPC and replaced (Bi 0.5 Na 0.5 ) 2+ with Ba 2+ to tune the covalency of A–O bonds. Thus, we synthesized 0.96(Na 0.60 K 0.40 )(Nb 0.955 Sb 0.045 )O 3 –0.04(Bi 0.5 Na 0.5 ) 1– x Ba x ZrO 3 ceramics. We not only improved d 33 values from 450 pC/N (at x = 0) to 500–505 pC/N (at x = 0.05–0.10) but also obtained the enhanced temperature stability for strain at x = 0.10, outperforming that of samples with x = 0 and other KNN-based ceramics. The increased d 33 is attributed to the well-preserved MPC and the repaired long-range ordering, and the improved temperature stability of strain is due to shifting the MPC to a slightly higher temperature than room temperature. Therefore, the new route is useful to further improve the performance of an already-built MPC, benefiting to the future design of MPC and the practical application of KNN-based ceramics.