Large electrical strain in lead-free K_0.5Na_0.5NbO₃-based ceramics by heterovalent doping

Heterovalent doped (K_0.48-0.07xNa_0.52-0.43xBi_0.5x)(Nb_0.95-0.95xSb_0.05-0.05xZr_x)O₃ ceramics were fabricated using conventional solid-state reaction. Then, the phase structures, dielectric, ferroelectric, and electric-strain properties were investigated. The compositions were tuned to be located at polymorphic phase boundary with increasing heterovalent Bi³⁺ and Zr⁴⁺ doping levels. A large strain of 0.19% was obtained at relatively low electric fields of 30 kV/cm in the composition of x = 0.04. The normalized large-signal d₃₃∗ values were approximately 633 pm/V under a low driving electric field of 30 kV/cm, which were comparable or larger than the values reported for other lead-free families. The large strains obtained can be attributed to the formation of nanodomains and high-density domain walls, which were confirmed by the observations of domain morphology using transmission electron microscopy (TEM) technique. Excellent temperature stability of the strain properties of the x = 0.04 sample could be ascribed to the sluggish behaviour for the local structural heterogeneity in heterovalent-ion doped KNN ceramic. Theoretical simulations revealed that the Zr⁴⁺ produce the local stress at the BO₆ octahedra and Bi³⁺ could yield off-centering of AO₁₂ ployhedron due to the nature of Bi 6s lone pair electrons, which induced lattice expansion and local distortions in the sample. The local displacements are strongly anisotropic in heterovalent-ion doped system. It is believed that random local fields exist in these compositions owing to the eixstence of charge distribution. Such heterovalent doping of Bi³⁺ and Zr⁴⁺ could destory simultaneously the orthorhombic symmetry and the short-range ferroelecctric order, leading to the formation of complex nanodomains and local structral hetergenenity. Heterovalent doping may, therefore, offer a new avenve to design novel K_0.5Na_0.5NbO₃ (KNN) -based materials for their mutifunctional applications.