Direct observation of Ni³⁺ and Ni²⁺ in correlated LaNiO_3−δ films

School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics Beijing 100083, PRC

^a bixf@buaa.edu.cn

Received: 23 September 2010
Accepted: 7 February 2011

Abstract

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 Ni³⁺/Ni²⁺ ratio dependent transport is clarified by the combination of transport measurements and first-principle calculations. At lower Ni³⁺/Ni²⁺ ratio of 0.39, the LaNiO_2.64 film exhibits semi-conductive behavior with carriers mainly being hopping polarons, while higher Ni³⁺/Ni²⁺ 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 Ni³⁺/Ni²⁺ ratio.