Direct observation of Ni ³⁺ and Ni ²⁺ in correlated LaNiO 3−δ films

The X-ray photoelectron spectroscopic deconvolution analysis is applied to quantitatively distinguish valence states of Ni and to obtain the δ value in LaNiO 3−δ films on Si substrates. The mechanism of the Ni 3+ /Ni 2+ ratio dependent transport is clarified by the combination of transport measurements and first-principle calculations. At lower Ni 3+ /Ni 2+ ratio of 0.39, the LaNiO 2.64 film exhibits semi-conductive behavior with carriers mainly being hopping polarons, while higher Ni 3+ /Ni 2+ ratio transfers the LaNiO 2.84 film to an electronic conductor. The observed reduction of the electron-phonon interaction, shortened mean free path, and increased electron-electron coupling are suggested to be correlated to high Ni 3+ /Ni 2+ ratio.