Mechanisms of Oxidation of NdNiO3−δ Thermochromic Thin Films Synthesized by a Two-Step Method in Soft Conditions

Considering it is somewhat difficult to crystallize thermochromic nickelates through a soft process, a major challenge in NdNiO3 crystallization is the stabilization of the Ni3+ oxidation state against Ni2+. The aim of the present work was to understand why a soft annealing is sufficient while an oxygen high pressure (200 bar) is usually considered as required. Thin films of 300 nm have been deposited by reactive magnetron sputtering at room temperature and working pressure of 0.5, 4, and 7 Pa. The oxygen content of as-deposited and annealed films was estimated by Rutherford backscattering spectrometry, whereas the nickel oxidation state evolution was followed by X-ray photoelectron spectroscopy. The measurements showed that the Ni3+ state is reached after deposition whatever the deposition pressure but that an oxygen loss occurs during air annealing with a magnitude depending on the deposition pressure. X-ray diffraction measurements, four-point probe electrical measurements, and infrared optical properties vs temperature were consistent with these results. Thin films deposited at 0.5 Pa and annealed in air at 820 K lead to an orthorhombic structure which becomes rhombohedral when the deposition pressure reaches 7 Pa. The oxygen loss causes a deterioration of the electrical resistivity, which was also evidenced by the thermochromic behavior measured using Fourier transform infrared spectroscopy.