Fundamental Issues in the Synthesis of Ferroelectric Na_0.5K_0.5NbO₃ Thin Films by Sol−Gel Processing

In contrast to films fabricated by physical vapor deposition methods, thin films of Na 0.5 K 0.5 NbO 3 (NKN) made by metal alkoxide sol−gel routes generally fail to exhibit polarization-electric field responses typical of a ferroelectric. This study sets out to investigate the reasons for the problems in producing sol−gel NKN films by examining the thermochemistry of the gel to ceramic conversion. The NKN precursor gels displayed multiple heating DTA crystallization exotherms in the temperature range 480−550 °C, which are attributed to compositional segregation of NKN components. At higher temperatures, 800−850 °C, a heating DTA endotherm and cooling DTA exotherm are indicative of melting and recrystallization of sodium/potassium carbonate secondary phases. Additionally, repeated thermal analyses, after storage of the gel decomposition product under ambient conditions, revealed a tendency for hydration and carbonation on exposure to air. Together, these are critically limiting reasons for the absence of strong ferroelectricity in NKN films produced by standard sol−gel methods.