The effect of uniaxial compressive stress on the crystal structure of a 6 mol. % Li-doped (K,Na)NbO3 (LKNN6a) ceramic was investigated using in situ synchrotron X-ray diffraction, revealing the stress-induced relative change in monoclinic Pm and tetragonal P4mm phases. As such, stress-induced phase transformations, in addition to the lattice deformation and domain switching, are the contributing factors for the observed macroscopic mechanical behavior of LKNN6a. The in situ stress-dependent diffraction data also demonstrates a method to mechanically modulate the polymorphic phase transition temperature (TPPT) to a higher temperature, as observed by the temperature-dependent permittivity measurements under a constant bias stress. The external uniaxial compressive stress increases the stability of the lower symmetry monoclinic phase, shifting TPPT to a higher temperature by 60 °C for the maximum uniaxial compressive stress of 300 MPa in the studied composition. Importantly, the stress-induced stabilization of the room-temperature ferroelectric phase can be useful to optimize the phase transition region, as well as increase the temperature stability of lead-free KNN.

(Na0.55K0.45)NbO3 (NKN) and acceptor-doped (Na0.55K0.45)(Nb1-xZr5x/4)O3 (NKNZ; x = 0.005 and 0.01) piezoelectric ceramics were fabricated by the conventional solid-state reaction method. Both ceramics were applied to a fatigue test for 106 cycles under an electric field at 3 kV/mm with unipolar drive. After the fatigue test, the maximum polarization of NKN and NKNZ (x = 0.005) was decreased by approximately 15 and 5%, respectively. Therefore, we found that the fatigue resistance properties of NKNZ ceramics, which substituted the Nb5+ ion with the Zr4+ ion, were improved. To study fatigue phenomena in piezoelectric ceramics, the chemical bond strength of both ceramics before and after the fatigue test was analyzed by soft X-ray emission spectroscopy (SXES), which has high energy resolution and is suitable for the evaluation of light elements in a local region with the micrometer order. We evaluated the formation of oxygen vacancies in as-sintered, poled, and fatigued ceramics by analyzing a chemical shift of the O Ka(3) peak appeared at approximately 175 eV in the SXES spectra. In NKN ceramics, the O Ka(3) peak was shifted to the higher energy side by the poling treatment and fatigue test. The peak position of fatigued NKN ceramics was close to that of the standard SrTiO3 sample. It was considered that the oxygen vacancies of the fatigued NKN ceramics had accumulated at grain boundaries instead of the oxygen moving. The improvement of the fatigue resistance for the NKNZ ceramics was attributed to the formation of a defect dipole connected with the oxygen vacancy and acceptor Zr ion.

Lead-free (Li,Na,K)NbO3-based multilayered piezoceramics were prepared, and their large-signal piezoelectric properties, under combined electrical and mechanical loadings, were characterized from 25 °C to 100 °C. Under zero stress, the multilayer exhibited a high large-signal piezoelectric constant d33∗ (=Smax/Emax) ≈ 350 pm/V with an applied unipolar field of 6 kV/mm. The stress-dependent d33∗, with a unipolar field of 6 kV/mm, featured a pronounced sensitivity to the electric field with an evolving peak at −80 MPa, which was not observed at 100 °C. The disappearance of the evolving peak with increasing temperature suggests a strong influence of the crystallographic phase on the electromechanical properties of (Li,Na,K)NbO3-based multilayers. Further investigations of the stress–strain loop and stress–polarization change revealed that the field-dependent peak below 100 °C was due to the non-180° domain orientation induced by the combined electric field and compressive stress.

Impedance spectroscopy for piezoceramics under high-power application was investigated. Impedance spectra under bipolar high electric field were completely different below and above TC. Below TC, high-power impedance spectra could be successfully fitted to a three resistance-constant phase element (3 R-CPE) equivalent circuit. Above TC, however, a 2 R-CPE equivalent circuit was sufficient to fit with high-power impedance spectra. For this reason, there was another relaxation factor caused by high-power application in the ferroelectric phase. From the high-power impedance spectra below TC, the nonlinear response of capacitance under a high electric field was calculated. Hence, domain wall motion would appear as a relaxation factor. Subsequently, high-power impedance spectra before and after cyclic unipolar fatigue treatment were compared. Considering the assumption that domain wall motion appeared as a relaxation factor, the capacitance caused by domain wall motion was decreased after cyclic fatigue. In conclusion, we believe that domain wall pinning could be evaluated by separating other electrical properties. In this work, the measurement possibility by impedance spectroscopy for high-power application for piezoceramics was investigated.