Large piezoelectricity in NaNbO₃-based lead-free ceramics via tuning oxygen octahedral tilt

The development of high-performance lead-free piezoceramics for replacing Pb-based perovskites has attracted lots of attention, and comparable room-temperature piezoresponse has been realized in (Na,K)NbO₃ and BaTiO₃-based ceramics with local structure heterogeneity via adjusting polymorphic phase boundary. In this work, a new method of oxygen octahedron tilt design is used in NaNbO₃-based lead-free ceramics, which usually shows complex oxygen octahedron tilt information inherited from NaNbO₃. The substitution of Ba(Fe_0.5Nb_0.5)O₃ and BaTiO₃ with high tolerance factor into NaNbO₃ leads to a gradual elimination of anti-parallel cation displacement and anti-phase tilt along the three axes; as a result, a single tetragonal phase with P4bm space group and only in-phase tilt along c axis is achieved in 0.88NaNbO₃-0.04Ba(Fe_0.5Nb_0.5)O₃-0.08BaTiO₃ ceramic. Accompanying the decreased oxygen octahedral tilt degree, a drastically improved d₃₃ up to 367 pC N^-1 (over ten times that of NaNbO₃ ceramic) is obtained, reaching a record high in NaNbO₃-based lead-free ceramics. Together with temperature insensitive piezoresponse originating from thermally stable oxygen octahedral tilt structure, the studied NaNbO₃-based materials show large potential for replacing Pb-based ceramics in some electronic devices. The oxygen octahedron tilt engineering would be used for designing more high-performance lead-free ceramics with high piezoresponse and excellent thermal stability simultaneously.