Long-range ferroelectric interactions in KTaO3/KNbO3 superlattice structures

Symmetric superlattice structures consisting of alternating atomic-scale layers of KTaO3 and KNbO3 with variable periodicity were grown on KTaO3 substrates by pulsed laser deposition. The in-plane structure of KNbO3 closely matches that of the KTaO3 substrate, resulting in KTaO3/KNbO3 heterostructures that are uniformly strained in-plane without misfit dislocations. This strain imposes an in-plane KNbO3 lattice spacing identical to that of the KTaO3 substrate for the temperature range 30 degrees C<T<700 degrees C, and a tetragonal-to-tetragonal transition is observed whose phase transition temperature T-c depends on the KNbO3 layer thickness. The in-plane strain results in a significant increase in this ferroelectric-paraelectric T-c for superlattices with relatively thick KNbO3 layers (T-c= 535 degrees C for a 17 nm thick layer, as compared to 435 degrees C for bulk KNbO3) and for K(Nb0.5Ta0.5)O-3 random-alloy thin films. As the superlattice period decreases, a reduction of T-c is observed. For superlattices with periodicities of 50 Angstrom or less, the Curie temperature is identical to that of the K(Ta0.5Nb0.5)O-3 random-alloy him, indicating significant long-range ferroelectric coupling across the KTaO3 layers. (C) 1998 American Institute of Physics. [S0003-6951(98)03720-6].