Support-Promoted Stabilization of the Metastable PZT Pyrochlore Phase by Epitaxial Thin Film Growth

Abstract

Thin films of lead zirconium titanium oxide with the Zr/Ti ratio close to 52/48 have been grown by pulsed laser deposition on epitaxial (100)CeO2 buffered R-plane sapphire substrates. Instead of the expected perovskite structure, these films are pure cubic metastable pyrochlore phase. From X-ray diffraction in θ–2θ mode and θ-scans it appears than the films are fully {100} oriented with a mosaicity in the range 0.8°–0.9°. In-plane characterizations, including RHEED photographs, electron-channeling patterns, XRD ϕ-scans, and near grazing incidence XRD, are indicative of high-quality epitaxial growth, cube-on-cube, on the CeO2 sublayer. RBS analyses show that increasing the deposition temperature in the range 560–650°C does not affect the Zr/Ti ratio, while the lead content drops significantly from Pb/(Zr+Ti)=0.7 to 0.3 (a stoichiometry close to the composition of “Pb(Zr0.52Ti0.48)3O7”); simultaneously, the unit-cell constant decreases monotically from 10.40 to 10.15 Å. A comparison with results obtained on a variety of other substrates suggests that the driving force that imposes the growth of the pyrochlore phase at the expense of the perovskite-like one is not related to the misfit, but to the nature of the interface at the atomic scale, due to the close structural relations between fluorite and pyrochlore. This hypothesis is confirmed by the obtention of the usual perovskite variant when a (111)CeO2 sublayer is used.