Phase structure dependence of acceptor doping effects in (Bi0.5Na0.5)TiO3–BaTiO3 lead-free ceramics

Abstract

The introduction of acceptor dopants into perovskites is a common method for designing hard piezoelectric ceramics by forming defect dipoles. It is interesting to find that the doping effect of a few amount of MnO2 in (Bi0.5Na0.5)TiO3–BaTiO3 lead-free ceramics significantly depends on the local symmetry. A conventional hardening effect corresponding to an obviously enhanced mechanical quality factor and a decreased piezoelectric coefficient can be generated in the tetragonal phase rich zone, rather than in the rhombohedral-rich zone, where a typically amphoteric characteristic can be observed unexpectedly. The result was well interpreted by means of the formation of a local random field as a result of the local symmetry-breaking effect of point defects after doping MnO2 in addition to a traditional internal bias field as a result of the pinning effect of defect dipoles. Compared with tetragonal-rich ones, the obviously enhanced random field in rhombohedral-rich compositions tends to induce a weakening ferroelectricity, as evidenced by composition dependent normal ferroelectric-relaxor ferroelectric phase transition. This work provides a new understanding of how charged defects affect the domain stability in the matrix with different symmetries through the competition between random electric fields and internal bias fields.