Oxygen octahedron tilting, electrical properties and mechanical behaviors in alkali niobate-based lead-free piezoelectric ceramics

In this work, we present a new piezoelectric solid solution consisting of two typical alkali niobate-based materials, K 0.5 Na 0.5 NbO 3 (KNN) and Li 0.15 Na 0.85 NbO 3 (LNN). Although KNN and LNN have the same perovskite structure, they exhibit extremely different electrical properties and mechanical behaviors. The phase structures, electrical and mechanical evolutions of the new lead-free piezoelectric materials with different ratios of KNN and LNN are comprehensively and theoretically investigated. According to the X-ray diffraction patterns and curves of permittivity versus temperature, a series of complicated phase transitions can be found with varied LNN content. Rietveld refinement results based on XRD patterns reveal an oxygen octahedron tilting in the LNN-rich crystal structure, and simultaneously the reasons for octahedron tilting are discussed. The distorted crystal structure is accompanied by extremely decreased electric properties but increased mechanical properties, which reveals electrical and mechanical properties of alkali niobate-based piezoelectric ceramics strongly depend on their inner structures, and the enhancement of intrinsic hardness results in the deterioration of piezoelectric properties. Our work exhibits the detailed evolutions of structure, electrical and mechanical properties from KNN to LNN, which provides experimental and theoretical basis for development of new alkali niobate-based piezoelectric materials.