Soft mode dynamics and the reduction of Ti4+ disorder in ferroelectric/relaxor superlattices BaTiO3/BaTi0.68Zr0.32O3

We used pulsed laser deposition to grow a series of [BaTiO3](Lambda/2)/[BaTi0.68Zr0.32O3](Lambda/2) (BTZ/BT) superlattices (SL's) with a modulation period Lambda that varies between 16 A <=Lambda <= 1008 A; the total thickness is kept constant at about 4000 A. We determine the out-of-plane lattice parameters of the SL constituents by modeling the x-ray diffractograms of the SL's. The results indicate that the polar c axis of the BT layers lies in the plane of the film. The Raman data reinforces this interpretation. The Raman spectra of SL's give evidence of coupling between BT and BTZ layers and a narrowing of the Raman peaks suggest a reduction of the disorder of the Ti4+ ions due to the strain. This strain resulting from the lattice mismatch between the constituent layers is responsible for the upward frequency shift of the soft modes, especially the E(1TO) mode, which is markedly altered with respect to its analogs in BT-bulk crystals and BT film. This soft-mode behavior as a function of Lambda indicates that the crystal structure of all SL's is more rigid than in BTZ and BT single thin films.