Reversible phase transformation ofLaNiO3−xthin films studiedin situby spectroscopic ellipsometry

Abstract

An in situ spectroscopic ellipsometry study has been performed to investigate $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3\ensuremath{-}x}$ phase transition associated with oxygenation and/or reduction reversible treatments. Epitaxial thin films of $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3}$ were deposited onto $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ (001) substrates by pulsed laser deposition. Reduction of oxygen stoichiometry of these films under high vacuum, which leads to compound with the formula $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3\ensuremath{-}x}$, is realized in the deposition chamber and studied in situ by spectroscopic ellipsometry. Phases belonging to the series $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3\ensuremath{-}x}$ with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$ appear at high temperatures and lead to modifications of the optical properties, such as the refractive index and the extinction coefficient, which can be monitored by spectroscopic in situ ellipsometry. These phase changes are associated with a metal-insulator transition. Specifically, modifications occur in the nickel environment, in particular, part of the Ni-polyhedron change from an octahedral symmetry (with ${\mathrm{Ni}}^{3+}$) to partial squared plane symmetry (with ${\mathrm{Ni}}^{2+}$). The reversible oxygenation process is observed by in situ ellipsometry in the temperature range from $473\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The thermal variation of the refractive index exhibits a ``first-order-like'' transition whose amplitude depends on the oxygen pressure. The sensitivity of the oxygen deficient $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3\ensuremath{-}x}$ to the reoxygenation can be demonstrated not only through electrical properties, as commonly shown, but also through optical properties with a noncontact measurement. Ex situ x-ray photoelectron spectroscopy exhibits the presence of hydroxide, both La-OH and Ni-OH, at the surface, confirming the hygroscopic behavior of the lanthanum compound. In spite of this dominant surface reaction, a modification in the photoemission spectra can be observed for the oxygen deficient $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3\ensuremath{-}x}$ samples. This seems to indicate a loss of oxygen atoms in the LaO plane, leading to a modified perovskite structure. Furthermore, a too long reduction process drives to the irreversible creation of the highly insulating ${\mathrm{La}}_{2}\mathrm{Ni}{\mathrm{O}}_{4}$ phase with a segregation of NiO.