Complex oxide thin films: Pyrochlore, defect fluorite and perovskite model systems for structural, spectroscopic and catalytic studies

摘要

Well-ordered thin films of different defect fluorite and perovskite materials, namely lanthanum zirconate (La2Zr2O7), cerium zirconate (Ce2Zr2O7), lanthanum cerate (La2Ce2O7) and lanthanum strontium ferrite (La0.4Sr0.6FeO3), have been prepared by sputtering the respective powder targets onto vacuum-cleaved NaCl(0 0 1) single crystal facets. Characterization specifically also includes the sophisticated preparation of the initial target materials. For the defect fluorite materials, the target materials are the respective pyrochlore compounds, which reproducibly transform to the respective defect fluorite structures upon sputtering. At template temperatures of around 573 K, well-crystallized epitaxial thin films of the defect fluorite compounds La2Zr2O7 and Ce(2)Zr(2)O(7)result, whereas for the perovskite compound postannealing procedures at 973 K in air are necessary to obtain epitaxial ordering of orthorhombic structures. Epitaxial relations for the well-ordered defect fluorite thin films are determined to be La2Zr2O7(0 0 1)//NaCl(0 0 1) and Ce2Zr2O7(0 0 1)//NaCl(0 0 1), respectively. Structural and spectroscopic characterization of the films by (high-resolution) electron microscopy (HR-TEM), selected-area electron diffraction (SAED) and depth-profiling X-ray photoelectron spectroscopy (XPS) reveal that the structures and compositions of the initial target materials are well-preserved during the sputtering process. Following this preparation routine, access to well-ordered thin films of complex oxide materials with defined stoichiometry is therefore granted, which can subsequently be used as model systems for studies of their material's or catalytic properties, as presented for simpler systems earlier. As a primary example, we show that for the defect fluorite thin films of Ce2Zr2O7 the defect fluorite-pyrochlore phase transformation can be observed at around 1123 K as detected by in situ electron diffraction. (C) 2018 Elsevier B.V. All rights reserved.