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 8kV∕mm. Polarization in this composition was not stable in that the piezoelectric coefficient d33 at zero electric field was only about 30pm∕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.