Revealing the Origin of Property Discrepancy in KNN-Based Ceramics with Extreme K/Na Ratio for Sensing Application

Potassium-sodium niobate (KNN) ceramics are critical lead-free piezoelectric materials, offering eco-friendly alternatives with high performance for sustainable sensor applications. However, how to overcome the theoretical framework of conventional K/Na ratio limitation and achieve property enhancement in extreme composition remains to be fully understood. Herein, by combining density function theory calculation, Rayleigh analysis, and ferroelectric scaling behavior, the origin of property discrepancy in KNN-based ceramics with extreme K/Na ratio is unveiled. Compared with Na-rich sample, 2.3-fold enhanced piezoelectricity can be achieved in K-rich ceramics, superior to those with similar high K concentration. The deteriorated property in Na-rich sample comes from the existence of in-phase oxygen octahedron tilting (M₂ ⁺) mode, suppressing the polar ( ${\Gamma }_4^{-}$ ) mode and leading to a higher energy barrier. Nevertheless, the absence of M₂ ⁺ mode and the multiphase coexistence with a maze-like domain, promote polarization rotation and domain switching, resulting in improved piezoelectric response in K-rich ceramics. A compression-type accelerometer based on KNN with extreme K/Na ratio is designed and the sensitivity of K-rich ceramics is also much higher than that of Na-rich ones, highest in reported KNN-based piezoelectric accelerometers. The study provides a new paradigm to boost electrical properties and reveals the underlying mechanism of property discrepancy induced by extreme K/Na ratio, beneficial to the development of sensor applications.