Thickness-dependent cation order and disorder in PbSc_0.5Ta_0.5O₃ thin films grown by pulsed laser deposition

We report on thickness-dependent cation ordering and ferroelectric properties of (001)-oriented epitaxial PbSc_0.5Ta_0.5O₃ (PST) thin films grown by pulsed laser deposition on SrTiO₃ (001) and Si (001) substrates. The PST film thickness was varied from 30 to 200 nm. Only films thicker than 40 nm reveal (partial) cation ordering, which increases with thickness as confirmed by the appearance and intensity of superstructure reflections in x-ray diffraction and transmission electron microscopy. In accordance with the two-state thermodynamic model, temperature-dependent dielectric constant investigations showed the presence of two kinds of phase transitions belonging to the normal ferroelectric and the relaxor state, respectively, both being present in the PST films. The influence of cation ordering on the phase transition and ferroelectric properties is discussed in detail. (paper)