Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3. I. Structure and room temperature properties

Lead-free piezoelectric ceramics (1-x-y)Bi0.5Na0.5TiO3-xBaTiO3-yK0.5Na0.5NbO3 (0.05≤x≤0.07 and 0.01≤y≤0.03), have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and piezoelectric properties of these ceramics were studied. Based on the measured properties, the ceramics were categorized into two groups: group I compositions having dominant ferroelectric order and group II compositions displaying mixed ferroelectric and antiferroelectric properties at room temperature. A composition from group II near the boundary between these two groups exhibited a strain as large as ∼0.45% at an electric field of 8 kV/mm. Polarization in this composition was not stable in that the piezoelectric coefficient d33 at zero electric field was only about 30 pm/V. The converse piezoelectric response becomes weaker when the composition deviated from the boundary between the groups toward either the ferroelectric or antiferroelectric compositions. These results were rationalized based on a field induced antiferroelectric-ferroelectric phase transition