We used pulsed laser deposition to grow a series of ${\left[\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3\right]}_{\Lambda ∕2}∕{\left[\mathrm{Ba}{\mathrm{Ti}}_{0.68}{\mathrm{Zr}}_{0.32}{\mathrm{O}}_3\right]}_{\Lambda ∕2}$ (BTZ∕BT) superlattices (SL’s) with a modulation period $\Lambda$ that varies between $16\mathrm{\AA }⩽\Lambda ⩽1008\mathrm{\AA }$; the total thickness is kept constant at about $4000\mathrm{\AA }$. 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 ${\mathrm{Ti}}^{4+}$ 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 $E\left(1\mathrm{TO}\right)$ 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.

• Received 11 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.064107