Composition-driven phase boundary and piezoelectricity in potassium-sodium niobate-based ceramics

The piezoelectricity of (K,Na)NbO₃ ceramics strongly depends on the phase boundary types as well as the doped compositions. Here, we systematically studied the relationships between the compositions and phase boundary types in (K,Na) (Nb,Sb)O₃-Bi_0.5Na_0.5AO₃ (KNNS-BNA, A = Hf, Zr, Ti, Sn) ceramics; then their piezoelectricity can be readily modified. Their phase boundary types are determined by the doped elements. A rhombohedral-tetragonal (R-T) phase boundary can be driven in the compositions range of 0.035 ≥ BNH ≥ 0.040 and 0.035 ≥ BNZ ≥ 0.045; an orthorhombic-tetragonal (O-T) phase boundary is formed in the composition range of 0.005 ≥ BNT ≥ 0.02; and a pure O phase can be only observed regardless of BNS content (≥0.01). In addition, the phase boundary types strongly affect their corresponding piezoelectricities. A larger d₃₃ (∼440-450 pC/N) and a higher d₃₃∗ (∼742-834 pm/V) can be attained in KNNS-BNA (A = Zr and Hf) ceramics due to the involvement of R-T phase boundary, and unfortunately KNNS-BNA (A = Sn and Ti) ceramics possess a relatively poor piezoelectricity (d₃₃ ≥ 200 and d₃₃∗ < 600 pm/V) due to the involvement of other phase structures (O-T or O). In addition, the underlying physical mechanisms for the relationships between piezoelectricity and phase boundary types were also discussed. We believe that comprehensive research can design more excellent ceramic systems concerning potassium-sodium niobate.