Volatilization of alkali ions and effects of molecular weight of polyvinylpyrrolidone introduced in solution-derived ferroelectric K_0.5Na_0.5NbO₃ films

Ferroelectric K_0.5Na_0.5NbO₃ (KNN) thin films were prepared by a chemical solution deposition approach with polyvinylpyrrolidone (PVP) of different molecular weights introduced in the precursor solutions. The volatilization of the alkali ions and the effects of the molecular weight of PVP were examined with x-ray diffraction (XRD), thermal analysis, mass spectrometry, and x-ray photoelectron spectroscopy (XPS). The results clearly showed that the volatilization of the alkali ions mainly happened at moderate temperatures before the crystallization of the KNN perovskite phase. Loss of Na was more significant than K ions during the heating process of KNN. The introduction of PVP with the appropriate molecular weight could effectively promote the crystallization of the KNN perovskite phase at reduced temperature and substantially suppress the loss of the alkali ions before crystallization. Therefore, a high dielectric constant, piezoelectric coefficient, and well saturated ferroelectric hysteresis loops were obtained in the KNN films in which PVP of the right molecular weight were introduced.