Donor doping of K0.5Na0.5NbO3 ceramics with strontium and its implications to grain size, phase composition and crystal structure

In this study, the particular effects of A-site donor doping, such as crystal-structure change, the secondary-phase formation and the grain-size decrease, in a lead-free piezoceramic material K0.5Na0.5NbO3 (KNN) doped with Sr2+, were investigated. Extended X-ray absorption fine structure (EXAFS) analyses proved that Sr occupies the perovskite A-sublattice, and locally modifies the KNN monoclinic structure to cubic. Introducing Sr into the A-sublattice, as well as accounting for the charge-compensating A-site vacancies in the starting composition, causes increasing lattice disorder and microstrain, as determined from a Rietveld refinement of the synchrotron X-ray diffraction data. Above 2% Sr the system segregates the A-site vacancies in a secondary phase in order to release the chemical pressure, as revealed by Raman spectroscopy. All these effects result in an increasing number of low-angle grain boundaries that limit the grain growth and finally lead to a significant grain-size decrease.