Phase transitional behavior of potassium sodium niobate thin films

Combined X-Ray Diffraction (XRD) and Raman analyses were used to investigate the phase transitions of potassium sodium niobate (K0.5Na0.5)NbO3 thin films, prepared from the acetate–alkoxide based precursor solutions with different amounts of alkali acetate excess. Depending on the amount of sodium or potassium excess in the solutions, the microstructures of the about 240nm thick films on platinized silicon substrates consisted of columnar grains with the width of about 200nm or of equiaxed grains with the average size of about 50nm. Raman spectra and XRD patterns provided evidence of both monoclinic to tetragonal (TM–T) and tetragonal to cubic (TT–C) phase transitions in the films with different microstructures, similar as in the (K0.5Na0.5)NbO3 powder. In the films with columnar grains, the Curie temperature was decreased as compared to the value reported for the powder, which was connected to the presence of tensile stresses arising from the thermal expansion mismatch between the substrate and the film, while the TM–T in the films closely corresponded to the powder value which was related to the evolution of a pronounced b-axis orientation. Furthermore, the exact values of the TT–C depended on the chemical composition of the films which was also reflected in different sizes of respective unit cells. In the films with fine grains, the phase transitions had a diffuse character, but they were nevertheless also evidenced by dielectric spectroscopy.