Defect formation and structural distortion of (K,Na)NbO₃ single crystal induced by Mn addition

In order to clarify the intrinsic effects due to Mn addition on the piezoelectricity, the existing state of defects including Mn ions and its effects on the crystal structure were investigated using Mn-doped (K₀.₅Na₀.₅)NbO₃ (KNN) single crystals, which were reproducibly prepared by a rapid solid-state crystal growth (RSSCG) method. From X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) measurements, it is suggested that the most abundant form of Mn ions is Mn 2+ substituting for Nb⁵⁺ and that its ratio to Mn3+ and Mn4+ increases with increase in the additional amount of Mn. While the density of oxygen vacancies forming defect complexes with Mn 2+ increased, that of isolated oxygen vacancies decreased by Mn addition. This decrease should be one of the reasons for the suppression of leakage current. SXRD analysis showed an increase in the off-center displacement of B-site ions by Mn addition.