Ambient oxygen pressure in a pulsed laser deposition process has been observed to have a critical influence on the compositional, crystalline, and electrical properties of Na 0.5 K 0.5 NbO 3 ͑NKN͒ thin films grown onto polycrystalline Pt 80 Ir 20 and SiO 2 ͑native oxide͒/Si͑111͒ substrates. Films prepared at high oxygen pressure ͑ϳ400 mTorr͒ were found to be single phase and highly c-axis oriented. X-ray diffraction -2 scans and rocking curve data show a strong effect of NKN film self-assembling along the ͓001͔ direction regardless of the substrate texture. The high dielectric permittivity of 550, low dissipation factor of less than 3%, and high remanent polarization of 12 C/cm 2 indicate the high ferroelectric quality of the fabricated film. The role of the high-energy component of the erosion products has been proven to be crucial to film performance. On the other hand, films grown at low oxygen pressure ͑ϳ10 mTorr͒ have been found to be mixed phases of ferroelectric NKN and paraelectric potassium niobates. These films have shown superparaelectric behavior: 5% tunability at an electric field of 100 kV/cm, losses as low as 0.3%, and excellent stability to temperature and frequency changes.