Sodium potassium niobate [(K (x) Na1-x )NbO3] films with high sodium composition (x = 0.06 and 0.24) were fabricated using a low-temperature (240 A degrees C) hydro-thermal method on (001) niobium-doped strontium titanate (Nb-STO) substrate. The films were annealed subsequently at 600 A degrees C. Thicknesses of the films were maintained in the range of similar to 800-1000 nm. Transmission electron microscopy-selected area electron diffraction studies revealed the appearance of super-spots, thereby confirming the tilting of the oxygen sub-lattice at both high Na compositions. The bright-field imaging and scanning transmission electron microscopy-energy dispersive spectroscopy elemental mapping revealed KNN film with K-rich interfacial regions and Na-rich top region of the film at both the high Na composition, whereas the potassium niobate (KNbO3, x = 1) film showed no such oxygen sub-lattice tilting, and a sharp substrate-film interface was observed. The observed tilting of oxygen sub-lattice is a direct consequence of the reduced structural stability in high Na compositions in the KNN solid solutions.