Structural evidence for the polymorphic phase boundary in (Na,K)NbO3 based perovskites close to the rhombohedral-tetragonal phase coexistence zone

The multi-phase coexistence is a technologically important concept in high-performance piezoelectric ceramics owing to the ability of achieving extremely increased electromechanical properties. However, in fact it cannot always produce desirable thermal stability of small-field piezoelectric properties relevant to the device application. In this work, a typical lead-free piezoelectric composition of (Na0.52K0.4)(Nb0.84Sb0.08)O-3-0.055LiTaO(3)-0.025BaZrO(3) coexisted with rhombohedral (R) and tetragonal (T) phases at room temperature was investigated as a case study by means of ex/in-situ synchrotron x-ray diffraction, Rietveld structure refinement and measurements of temperature-dependent electrical properties. The refined crystal structure proves to be a three-phase coexistence of R, orthorhombic (O) and T at virgin state, and evolves into the R+monoclinic (M-C)+T phase coexistence at poled state owing to an electric field induced irreversible O-Mc phase transition. Particularly, the field induced M-C phase can stably exist and maintain its concentration as long as the annealing temperature is below the Curie temperature. An interesting finding is that the poled sample still retains the polymorphism of phase transition like its virgin state featuring the temperature driven R-O(M-C)-T phase transition, leading to seriously degraded piezoelectric coefficients after thermal cycles due to domain de-texturing of not only M-C phase, but also R and T phases during repeated phase transition. The present study would benefit to understanding of the nature of polymorphic phase boundary and the structure origin of temperature-dependent piezoelectric properties in R-T coexisted (Na,K)NbO3-based lead-free solid-solution ceramics. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.