Outstanding Piezoelectric Performance in Lead-Free 0.95(K,Na)(Sb,Nb)O ₃ -0.05(Bi,Na,K)ZrO ₃ Thick Films with Oriented Nanophase Coexistence

Lead-free 0.95(K _0.48 Na _0.52 )(Nb _0.95 Sb _0.05 )O ₃ -0.05Bi _0.5 (Na _0.82 K _0.18 ) _0.5 ZrO ₃ (KNSN-BNKZ0.05) piezoelectric films with preferred crystal orientation and enhanced thickness are fabricated on silicon substrates from a chemical solution approach. Adequate K excess is introduced to obtain a single perovskite phase in the resulting thicker films. The effects of thickness, crystal orientation, and structure of the films on the performance are investigated. Outstandingly large effective piezoelectric strain coefficient up to 250 pm V ⁻¹ is demonstrated over a macroscopic scale using a laser scanning vibrometer in the [100]-KNSN-BNKZ0.05 film with an enhanced thickness of 2.7 µm, competitive to the benchmark oriented lead zirconate titanate films on silicon. Atomically resolved electron microscopy reveals the coexistence of oriented ferroelectric rhombohedral (R) and tetragonal (T) phases at the nanometer scale with gradual polarization rotation, which can lower the domain wall energy and facilitate the large piezoelectric response. The increased film thickness reduces the in-plane mechanical clamping to enable more free deformation in the thickness direction and improve domain wall mobility, both further contributing to enhanced piezoelectric response.