Highly (100)-oriented lead-free Na0.5K0.5NbO3 (NKN) + 40 mol% Na(I) and K(I) thin films are fabricated on Pt/Ti/SiO2/Si substrates via a sol-gel processing method. A two-stage post-annealing process consisting of rapid thermal annealing (RTA) at various temperatures in the range of 650 similar to 850 degrees C followed by tube furnace (TF) treatment at a temperature of 700 degrees C is then employed to modify the microstructure and chemical bonds of the NKN films in an attempt to improve their dielectric and ferroelectric properties. It is shown that the optimal values of the dielectric constant (epsilon = 658 at 100 kHz), dielectric loss (tan delta = 0.113 at 100 kHz), remnant polarization (2P(r) = 17.1 mu C/cm(2) at 1 kHz), and coercive field (2E(c) = 372 kV/cm at 1 kHz) are obtained when using an RTA temperature of 750 degrees C. The superior electrical properties of the NKN film are the results mainly of an improved crystallization, a higher film density and a denser grain structure. It is shown that the ferroelectric properties of the NKN film are maintained for more than 25000 cycles in thermal environments of less than 150 degrees C. Hence, it is inferred that NKN has good thermal stability and endurance for ferroelectric devices. (C) 2017 Elsevier B.V. All rights reserved.