The effect of size, strain, and long-range interactions on ferroelectric phase transitions in KTaO3/KNbO3 superlattices studied by X-ray, EXAFS, and dielectric measurements

Epitaxial, uniformly strained superlattices of ferroelectric KNbO3 and paraelectric KTaO3 are studied with respect to their structural and dielectric properties. For dielectric measurements, perfectly lattice-matched conducting Sr(Ru0.5Sn0.5)O-3 electrodes are used, and a broad, frequency-dependent maximum is observed in the capacitance-vs-temperature curves. Niobium K-edge glancing-angle EXAFS provides information regarding the crystal structure of KNbO3 films as thin as two unit cells in superlattices with equal KTaO3 and KNbO3 layer thicknesses, showing a clear difference between these thinnest-layer superlattices and films of the K(Ta0.5Nb0.5) O-3 solid-solution. X-ray diffraction measurements, on the other hand, indicate that these samples exhibit the same transition temperature T-c, indicating the importance of long-range electrostatic interactions. Analysis of the transition temperature for various structures leads to a clear identification of the effect of size and strain on T-c.