Grain size induced monoclinic (Cm) to rhombohedral (R3c) transformation in sodium potassium bismuth titanate ceramics

Grain size effect on structure, remnant polarization and piezoelectric constant of lead-free Na0.4K0.1Bi0.5TiO3 ceramics with grain sizes of 0.58, 0.85, 1.19 and 1.83 μm was investigated. Synchrotron X-ray diffraction in combination with Rietveld refinement was used to examine the structural changes. We have reported an existence of monoclinic (Cm) phase at room temperature and discussed the grain size (1.83 → 0.58 μm) induced structural transformation (Cm→ R3c) through both intrinsic polarization rotation and adaptive phase theory. Polarization vector in the (11¯0)pc monoclinic plane rotates towards [111]pc with the reduction of grain size. Better domain alignment and large polar variants results enhanced remnant polarization and piezoelectric constant in 1.83 μm sample.