Orientation dependence of electrical properties of large-sized sodium potassium niobate lead-free single crystals

Abstract

Lead-free (K_0.25Na_0.75)NbO₃ (KNN25/75) single crystals with dimensions of Ø30 × 10 mm were successfully grown by a top seeded solution growth technique (TSSG). The X-ray diffraction pattern shows that the as-grown crystals possess an orthorhombic perovskite structure. The concentrations of K, Na, and Nb elements in the as-grown crystal were measured by X-ray fluorescence analysis. The dielectric, ferroelectric and piezoelectric properties of KNN25/75 crystals with different orientations, i.e., pseudocubic (100) and (110) ((100)_pc and (110)_pc), were investigated. A higher piezoelectric constant d₃₃ ~ 145 pC N⁻¹ and electromechanical coupling coefficient k_t ~ 69% were observed along the (100)_pc orientation compared with d₃₃ ~ 70 pC N⁻¹ and k_t ~ 51% along (110)_pc. However, when a bipolar electric field was applied, a two times higher strain value can be achieved in crystals oriented along (110)_pc than along (100)_pc. These orientation dependent physical properties can be explained in the framework of domain engineering and the switching effect.