Piezoelectric anomalies at the ferroelastic phase transitions of lead-free tungsten bronze ferroelectrics

This paper reports on the piezoelectric anomalies at the temperature or composition-induced ferroelastic phase transitions of tungsten bronze ferroelectrics. First, the temperature-dependent piezoelectric properties of Sr_1.9Ca_0.1NaNb₅O₁₅ (SCNN) ceramics were characterized using a resonance/anti-resonance method. SCNN has a ferroelastic phase transition manifested by a broad dielectric peak in the temperature range of -60°C to 20°C. The electromechanical coupling factor and elastic compliance showed the maximum at -40°C, increasing the transverse piezoelectric constant (d₃₁) by 38% compared with the room temperature value. Tungsten bronze ferroelectrics follow a trade-off relationship between the longitudinal piezoelectric constant (d₃₃) and the Curie temperature, while SCNN deviates significantly from the trend curve. This deviation is attributed to the ferroelastic phase transition close to room temperature. Second, the ferroelastic phase transition was investigated for epitaxial films of (1-x)(Sr₃Ba₂)Nb₁₀O _30-xBa₄Bi_2/3Nb₁₀O₃₀ as a function of the composition. A careful structural analysis by X-ray diffraction revealed that there is a ferroelastic phase boundary between tetragonal and orthorhombic crystals at x = 0.06-0.3. The electric field-induced strain and the relative dielectric constants characterized at 80K for the epitaxial films increased in the vicinity of the phase boundary composition. These results suggest that engineering the ferroelastic phase transition is an approach to improving the piezoelectric properties of lead-free tungsten bronze ferroelectrics.