Macroscopic and local approaches of phase transition in sol-gel synthesized (Bi0.5Na0.5)TiO3-SrTiO3 thin films

High-quality (1 - x)(Bi0.5Na0.5)TiO3-xSrTiO(3) lead-free piezoelectric thin films (x = 0, 0.1, and 0.25) on Pt(111)/Ti/SiO2/Si(100) substrates were prepared by a sol-gel method. The microstructures of the thin films as a function of SrTiO3 doping level and temperature were investigated by X-ray diffraction and Raman spectroscopy. Their temperature-and frequency-dependent piezoelectric properties were studied on the nanoscale using switching spectroscopy piezoresponse force microscopy (SS-PFM). A rhombohedral ferroelectric to pseudocubic relaxor phase transition was observed when either ST content or temperature increased. The significant frequency dependence of both ferroelectric and piezoelectric properties was also disclosed by analyzing polarization hysteresis loops on the macroscopic scale and local switching dynamics at various frequencies. It was determined that the short-range order clusters came out through the long-range ferroelectric order, thus the nanoscale approaches are consistent with macroscopic data at elevated temperatures and various frequency ranges.