Crystal structure and phase transition behavior in (K1−xNax)NbO3-based lead-free piezoelectric ceramic over a wide range of temperatures

Abstract

The phase transition temperature in a (K, Na)NbO3 (KNN) phase of a KNN-based composite lead-free piezoelectric ceramic with a KTiNbO5 system ((K1−xNax)0.86Ca0.04Li0.02Nb0.85O3−δ–K0.85Ti0.85Nb1.15O5–BaZrO3–Co3O4–Fe2O3–ZnO) is lower than that in an undoped KNN ceramic by approximately 200 °C. We have studied the structural changes around the phase transition by using synchrotron powder X-ray diffraction and transmission electron microscopy. The crystal system of the main KNN phase is assigned to tetragonal as a stable structure at room temperature and does not change to orthorhombic on lowering the temperature all at once. The crystal structure changes from tetragonal to orthorhombic through the successive transition state. The curve of the phase transition temperature from x = 0.33 to 0.75 has a V shape and reaches its lowest value of approximately 0 °C in the vicinity of x = 0.56. From selected-area electron diffraction patterns of the KNN phase, weak superlattice spots owing to the tilt-ordered NbO6 octahedra are observed for x ≥ 0.56. This tilt-ordered NbO6 octahedral phase is formed at the nanometer-scale (nanodomains) in the tetragonal and orthorhombic KNN matrices, regardless of the phase transition. The minimum x to generate the nanodomains is substantially equal to the Na fraction at which the starting temperature of the successive phase transition shifts to the lowest.