In situ combined stress- and temperature-dependent Raman spectroscopy of Li-doped (Na,K)NbO3

Abstract

External thermal, electrical, and mechanical fields can induce structural phase transitions in lead-free Li-modified Na0.5K0.5NbO3 ferroelectrics, which significantly influence the macroscopic electromechanical response. In particular, the relative stability of the polar monoclinic (or orthorhombic) and tetragonal phases under temperature and stress is critical to realize the ferroelectric and piezoelectric response. In this study, the effect of mechanical and thermal fields on the local structure in the vicinity of the monoclinic-tetragonal (M-T) phase boundary was investigated using a novel in situ combined uniaxial compressive stress- and temperature-dependent Raman spectroscopy experimental arrangement. Experiments were performed up to 300°C and −200 MPa, clearly demonstrating stress-induced M-T phase transition in Li-modified Na0.5K0.5NbO3. A stress-temperature phase diagram has been established based on the change in vibrational modes. It was possible to correlate the relative permittivity singularities previously observed to a given stage of the M-T phase transition using ratio between characteristic Raman band areas. In addition, the measurement method reported here can be applied to other functional ceramics to investigate the influence of mechanical fields on local structure.