Inductive Effect of Nd for Ni3+ Stabilization in NdNiO3 Synthesized by Reactive DC Cosputtering

Abstract

This work focus on the oxidation states of nickel in NdNiO3, Nd2NiO4, and NiOx thin films deposited by reactive magnetron sputtering. Thin films with a high Ni3+ content have been reached in the ternary oxides using a soft subsequent annealing in air. In order to understand the presence of Ni3+ oxidation state, the present work establishes a comparative study between the NiOx and Nd–Ni–Ox thin films deposited at room temperature in reactive conditions using different oxygen partial pressures. Crystallographic phases have been systematically identified by X-ray diffraction showing crystallized NiOx and amorphous thin films in the case of Ni and Nd cosputtering. The ionization state of Ni ions has been tracked by X-ray photoelectron spectroscopy. Analyses confirm the presence of Ni2+ in NiOx films whereas Ni3+ is systematically observed in Nd–Ni–Ox films highlighting the key role of Nd element during the deposition. The electron donor character of Nd3+ on Ni3+ via O 2p states by an inductive effect is clearly evidenced. This effect is strengthened by a strong difference in electronegativity between the rare earth (Re) and the transition metal (M). This concept is enlarged and faced with other ReMO3 perovskites, and their thermodynamic stability is discussed.