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 <i>d</i>₃₃ (∼440–450 pC/N) and a higher <i>d</i>₃₃* (∼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 (<i>d</i>₃₃ ≤ 200 and <i>d</i>₃₃* < 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