Pressure- and Temperature-Induced Structural Phase Diagram of Lead-Free (K_0.5Na_0.5)NbO₃–0.05LiNbO₃ Single Crystals: Raman Scattering and Infrared Study

Ferroelectric lead-free KxNa1–xNbO3 (KNN) perovskite, whose piezoelectric properties can be comparable to those of traditional Pb-based systems, has aroused wide concern in recent years. However, the specific influences of the stress field on KNN’s structure and piezoelectric properties have not been well clarified and there are few descriptions about the temperature–pressure phase diagram. Here, we analyzed the phonon mode behavior and structural evolution of K0.5Na0.5NbO3-0.05LiNbO3 (KNN-LN) and MnO2-doped single crystals with pressure- and temperature-dependent phase structure variations by theoretical calculation, polarized Raman scattering, and infrared reflectance spectra. The different phase structures can be predicted at high pressure using the CALYPSO method with its same-name code. The rhombohedral → orthorhombic → tetragonal → cubic phase transition process can be discovered in detail by Raman spectra under different temperatures and pressures. The phase coexistence on the thermal phase boundary was confirmed by basic anastomosis. Meanwhile, it was found that the substitution of Mn in the NbO6 octahedron aggravates the deformation of high pressure on KNN-LN and the substitution of Mn at the B-site intensifies the structural evolution more severely than at the A-site. The present study aims at exploring octahedra tilt, phonon vibrations, and the internal structure on the general critical phase boundary in KNN-LN crystals. It provides effective help for the study of lead-free perovskite phase transformation and the improvement in piezoelectric properties under a high-pressure field.