Ferroelastic behavior across the orthorhombic-to-tetragonal phase transition region of NKN-based lead-free ferroelectrics

In this study, the macroscopic mechanical behavior was characterized as a function of temperature (−150 °C to 400 °C) for polycrystalline (Na0.5K0.5)NbO3 with three dopant concentrations. Dopants can improve certain electromechanical properties and, in the case of NKN and Li+, shift the orthorhombic-to-tetragonal phase transition temperature to lower temperatures. In this study, the mechanical behavior of undoped NKN, LNKN6 with 6 mol. % Li+, and LNKN6 with additional dopants was characterized and compared with the temperature dependent dielectric response and crystal structure. During mechanical loading, the samples showed a nonlinear hysteretic response. At low temperatures, this is understood to be due to ferroelasticity. At temperatures in the vicinity of the orthorhombic-tetragonal phase transition temperature, a closed hysteresis behavior was observed, corresponding to a local maximum of the critical ferroelastic stress and a minimum in the remanent strain. The observed closed hysteresis behavior is...