Absence of a PbTiO3 phase transition in PbTiO3/BaTiO3 superlattices

We have performed X-ray diffraction and Raman spectroscopy on three PbTiO3/BaTiO3 superlattices (PTO/BTO) from room temperature (RT) to 600°C. The superlattices were 4000 Å thick and the modulation wavelengths Λ, with equally thick layers of PTO and BTO, were 136, 187 and 357 Å. The samples were laser deposited onto single crystal MgO substrates buffered with 50 Å of SrTiO3 (STO). We observe no X-ray evidence of a phase transition in the PTO layers of these three superlattice samples up to 600°C. This result was reinforced by X-ray modeling of the samples. We also monitored the E(1TO) or “soft “ mode by Raman spectroscopy up to 550°C and found only a linear decrease of mode frequency with temperature for all three superlattices: another sign that no phase transition is taking place in this temperature range.