In Situ Exploration of Thermal-Induced Domain Evolution with Phase Transition in LiNbO₃-Modified K_0.5Na_0.5NbO₃ Single Crystal

Temperature dependent domain evolution and domain-wall motion of ferroelectric materials have attracted dramatically increased interest due to the crucial influences on piezo- and dielectric properties and the performances of related nanoapplications. Here, the unambiguous evolutions of 90°/180° domain walls accompanied by phase transition have been discovered first in LiNbO₃ doped (K_0.5Na_0.5)NbO₃ single crystal during heating and cooling processes by in situ coupling characterizations of Raman spectra and piezoresponse force microscopy (PFM). Considering the vibrational modes of a NbO₆ octahedron, the stretching modes of A_1g(ν₁), E_g(ν₂), and the bending mode F_2g(ν₅) are sensitive to structural transition as well as the diversity of domain evolution. These modes are used to discuss the formation of 60° domain walls and the change of 90° domain-wall density. Under the precise control of the temperature gradient, homogeneous striped 90° domain boundaries movement and microscale out-of-plane polarization reversal can be observed. Simultaneously, as one of the most distinctive features from the ferroelectric first-order phase transition, a thermal hysteresis effect has been found in (K_0.5Na_0.5)NbO₃-0.05LiNbO₃ single crystal. The present study focuses on the correlation of crystal structural transformation and thermal domain evolution. It affords an effective opportunity to explore the stable performance in a broad temperature range for domain-wall dependent ferroelectric devices.